MESSENGER Mercury Flyby 1 Press Kit

8/7/2019 MESSENGER Mercury Flyby 1 Press Kit

http://slidepdf.com/reader/full/messenger-mercury-flyby-1-press-kit 1/45

Mercury Flyby 114 January 2008

A NASA Discovery Mission



Media Contacts

NASA HeadquartersPolicy/Program ManagementDwayne Brown

(202) 358-1726Dwayne.C.Brown @nasa.gov

The Johns Hopkins University Applied Physics LaboratoryMission Management, Spacecraft OperationsPaulette W. Campbell

(240) 228-6792 or (443) 778-6792

[email protected]

Carnegie Institution of WashingtonPrincipal Investigator Institution Tina McDowell

(202) 939-1120

[email protected]



NASA’s Mission to Mercury

General Release: MESSENGER Set or Historic Mercury Flyby ..................... 2A Close-up o Mercury ...................................................................................................... 2

Media Services Inormation ............................................................................ 4News and Status Reports .................................................................................................4

NASA Television .................................................................................................................. 4

MESSENGER on the Web ..................................................................................................4

Quick Facts ....................................................................................................... 5Spacecrat ............................................................................................................................5

Mission ..................................................................................................................................5

Program .................................................................................................................................5

Mercury at a Glance ......................................................................................... 6General ..................................................................................................................................6

Physical Characteristics ...................................................................................................6

Environment ........................................................................................................................6

Orbit .......................................................................................................................................6

Why Mercury? ................................................................................................. 7Key Science Questions ......................................................................................................7

Mercury’s Visitors .......................................................................................... 11Mariner 10 ...........................................................................................................................11

Future Missions .................................................................................................................11

Mission Overview .......................................................................................... 12Cruise Trajectory ...............................................................................................................12

Launch ..................................................................................................................................14

Earth Flyby Highlights ....................................................................................................15

Venus Gravity Assists .......................................................................................................17Flying by Mercury .............................................................................................................22

MESSENGER’s Deep Space Maneuvers .....................................................................25

Science Orbit: Working at Mercury .............................................................................26

Mission Operations ..........................................................................................................27

Science during Mercury Flyby 1 ................................................................... 28Probing Mercury’s Mysteries ........................................................................................28

The Spacecrat ............................................................................................... 31Science Payload ................................................................................................................32

Spacecrat Systems and Components .....................................................................37

Hardware Suppliers .........................................................................................................40

The MESSENGER Science Team ..................................................................... 41Program/Project Management..................................................................... 42

NASA Discovery Program.............................................................................. 42Other Discovery Missions ..............................................................................................42

Table o Contents

The inormation in this press kit was current as o January 10, 2008.For mission updates, visit http://messenger.jhuapl.edu/.




General Release: MESSENGER Set or Historic Mercury Flyby

NASA will return to Mercury or the rst time in almost 33 years on January 14, 2008, when the MESSENGERspacecrat makes its rst fyby o the Sun’s closest neighbor, capturing images o large portions o the planet neverbeore seen. The probe will make its closest approach to Mercury at 2:04 p.m. EST that day, skimming 200 kilometers(124 miles) above its surace. This encounter will provide a critical gravity assist needed to keep the spacecrat on trackor its 2011 orbit insertion around Mercury.

“The MESSENGER Science Team is extremely excited about this fyby,” says Dr. Sean C. Solomon, MESSENGERprincipal investigator, rom the Carnegie Institution o Washington. “ We are about to enjoy our rst close-up viewo Mercury in more than three decades, and a successul gravity assist will ensure that MESSENGER remains on thetrajectory needed to place it into orbit around the innermost planet or the rst time.”

During the fyby, the probe’s instruments will make the rst up-close measurements o the planet since Mariner 10’s

third and nal fyby o Mercury on March 16, 1975, and will gather data essential to planning the MESSENGER mission’sorbital phase. MESSENGER’s seven scientic instruments will begin to address the mission goals o:

• mappingtheelementalandmineralogicalcompositionofMercury’ssurface;• imaginggloballythesurfaceataresolutionofhundredsofmetersorbetter;• determiningthestructureoftheplanet’smagneticeld;• measuringtheplanet’sgravitationaleldstructure;and• characterizingexosphericneutralparticlesandmagnetosphericionsandelectrons.

A Close-up o Mercury

The cameras onboard MESSENGER will take more than 1,200 images o Mercury rom approach through encounter

and departure. “When the Mariner 10 spacecrat did its fybys in the mid-1970s, it saw only one hemisphere – a littleless than hal the planet,” notes Dr. Louise M. Prockter, instrument scientist or the Mercury Dual Imaging System, anda scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md. “During this fyby we willbegin to image the hemisphere that has never been seen by a spacecrat and at resolutions that are comparable to orbetter than those acquired by Mariner 10 and in a number o dierent color lters so that we can start to get an idea othe composition o the surace.”

One site o great interest is the Caloris basin, an impact eature about 1,300 kilometers (808 miles) in diameter andone o the largest impact basins in the solar system. “Caloris is huge, about a quarter o the diameter o Mercury, withrings o mountains within it that are up to three kilometers high,” says Prockter. “Mariner 10 saw a little less than hal oit. During this rst fyby, we will image the other side o Caloris. These impact basins act like giant natural drills, pullingup material rom underneath the surace and spreading it out around the crater. By looking through dierent colorlters we can start to understand what the composition o the Caloris basin may be and learn something about thesubsurace o Mercury.”

MESSENGER instruments will provide the rst spacecrat measurements o the mineralogical and chemicalcomposition o Mercury’s surace. The visible-near inrared and ultraviolet-visible spectrometers will measuresurace refectance spectra that will reveal important mineral species. Gamma-ray, X-ray, and neutron spectrometermeasurements will provide insight into elemental composition.




During the fyby, Doppler measurements will provide the rst glimpse o Mercury’s gravity eld structure sinceMariner 10. The long-wavelength components o the gravity eld will yield key inormation on the planet’s internalstructure,particularlythesizeofMercury’score.

The encounter provides an opportunity to examine Mercury’s environment in ways not possible rom orbit becauseo operational constraints. The fyby will yield low-altitude measurements o Mercury’s magnetic eld near the planet’sequator. These observations will complement measurements that will be obtained during the later orbital phase.

The fyby is an opportunity to get a jump start on mapping the exosphere with ultraviolet observations anddocumenting the energetic particle and plasma population o Mercury’s magnetosphere. In addition, the fybytrajectory enables measurements o the particle and plasma characteristics o Mercury’s magnetotail, which will notbe possible rom Mercury orbit.

MESSENGER is slightly more than halway through a 4.9-billion mile (7.9-billion kilometer) journey to Mercury orbitthat includes more than 15 trips around the Sun. It has already fown past Earth once (August 2, 2005) and Venus twice(October 24, 2006, and June 5, 2007). Three passes o Mercury, in January 2008, October 2008, and September 2009, willuse the pull o the planet’s gravity to guide MESSENGER progressively closer to Mercury’s orbit, so that orbit insertioncan be accomplished at the ourth Mercury encounter in March 2011.

“The complexity o this mission, with its numerous fybys and multitude o maneuvers, requires close and constantattention,” says MESSENGER project manager Peter D. Bedini, o APL. “MESSENGER is being driven by a team o extremely talented and dedicated engineers and scientists who are ully engaged and excited by the discoveriesbeore them.”

The MESSENGER project is the seventh in NASA’s Discovery Program o low-cost, scientically ocused spacemissions.Solomonleadsthemissionasprincipalinvestigator;APLmanagesthemissionforNASA’sScienceMissionDirectorate and designed, built, and operates the MESSENGER spacecrat. MESSENGER’s science instruments werebuiltbyAPL;NASAGoddardSpaceFlightCenter,Greenbelt,Md.;UniversityofMichigan,AnnArbor;andtheLaboratoryforAtmosphericandSpacePhysicsattheUniversityofColorado,Boulder;withthesupportofsubcontractorsacrossthe United States and Europe. GenCorp Aerojet, Sacramento, Cali., and Composite Optics Inc., San Diego, Cali.,respectively, provided MESSENGER’s propulsion system and composite structure.

The MESSENGER Science Team, recently augmented by an impressive array o experts participating in NASA’sMESSENGER Participating Scientist Program, includes 46 scientists rom 26 institutions. A complete list can be ound athttp://messenger.jhuapl.edu/who_we_are/science_team.html. Additional inormation about MESSENGER is availableon the Web at: http://messenger.jhuapl.edu.

http://messenger.jhuapl.edu/who_we_are/science_team.html

http://messenger.jhuapl.edu/


http://messenger.jhuapl.edu/who_we_are/science_team.html




Media Services Inormation

News and Status Reports

NASA and the MESSENGER team will issue periodic news releases and status reports on mission activities and makethem available online at http://messenger.jhuapl.eduand www.nasa.gov.

When events and science results merit, the team will hold media briengs at NASA Headquarters in Washington orthe Johns Hopkins University Applied Physics Laboratory in Laurel, Md. Briengs will be carried on NASA TV and theNASA Web site.

NASA Television

NASA Television is carried on the Web and on an MPEG-2 digital signal accessed via satellite AMC-6, at 72 degreeswestlongitude,transponder17C,4040MHz,verticalpolarization.ItisavailableinAlaskaandHawaiionAMC-7,at137degreeswestlongitude,transponder18C,at4060MHz,horizontalpolarization.ADigitalVideoBroadcast-compliant

Integrated Receiver Decoder is required or reception. For NASA TV inormation and schedules on the Web, visitwww.nasa.gov/ntv.

MESSENGER on the Web

MESSENGER inormation – including an electronic copy o this press kit, press releases, act sheets, missiondetails and background, status reports, and images – is available on the Web at http://messenger.jhuapl.edu. MESSENGER multimedia les, background inormation, and news are also available at www.nasa.gov/messenger.


http://www.nasa.gov/


http://www.nasa.gov/ntv


http://www.nasa.gov/mission_pages/messenger/main/index.html

http://www.nasa.gov/mission_pages/messenger/main/index.html


http://www.nasa.gov/ntv






Quick Facts

Spacecrat

Size: Main spacecrat body is 1.42 meters (56 inches) tall,1.85 meters (73 inches) wide, and 1.27 meters (50 inches)deep;afront-mountedceramic-fabricsunshadeis2.5meterstalland2metersacross(8feetby6feet);tworotatable solar panel “wings” extend about six meters (20eet) rom end to end across the spacecrat.

Launch weight: Approximately 1,107 kilograms (2,441pounds);included599.4kilograms(1,321pounds)ofpropellant and 507.9 kilograms (1,120 pounds) “dry”spacecrat and instruments.

Power: Two body-mounted gallium arsenide solarpanels;nickel-hydrogenbattery.Thesolar-panelpowersystem provided about 390 watts near Earth and willprovide 640 watts in Mercury orbit.

Propulsion: Dual-mode system with one bipropellant(hydrazineandnitrogentetroxide)thrusterforlargemaneuvers;16hydrazinemonopropellantthrustersforsmall trajectory adjustments and attitude control.

Science instruments: Wide- and narrow-anglecolorandmonochromeimager;gamma-rayand

neutronspectrometer;X-rayspectrometer;energeticparticleandplasmaspectrometer;atmosphericandsurfacecompositionspectrometer;laseraltimeter;magnetometer;radioscienceexperiment.

Mission

Launch: August 3, 2004, rom Launch Pad 17B at CapeCanaveral Air Force Station, Fla., at 2:15:56 a.m. EDTaboard a three-stage Boeing Delta II rocket (Delta II7925-H).

Gravity assist ybys: Earth(1)August2005;Venus(2)October2006,June2007;Mercury(3)January2008,October 2008, September 2009.

Enter Mercury orbit: March 2011.

Total distance traveled rom Earth to Mercury orbit:

7.9 billion kilometers (4.9 billion miles). Spacecrat circlesthe Sun 15 times rom launch to Mercury orbit.

Primary mission at Mercury: Orbit or one Earth year(equivalent to just over our Mercury years, or twoMercury solar days), collecting data on the compositionand structure o Mercury’s crust, its topography andgeologic history, the nature o its thin atmosphere andactive magnetosphere, and the makeup o its core andpolar materials.

ProgramCost: approximately $446 million (including spacecratand instrument development, launch vehicle, missionoperations and data analysis). This is just under $1.50 oreach current U.S. resident.

For comparison, Mariner 10 cost $135M in 1975 dollars,including all three fybys and the launch vehicle. Add“indirect” costs and account or infation, and the missionwould cost about $448 M today.




Mercury at a Glance

General

• Oneofveplanetsknowntoancientastronomers,in Roman mythology Mercury was the feet-ootedmessenger o the gods, a tting name or a planet thatmoves quickly across the sky.

• TheclosestplanettotheSun,Mercuryisnowalsothesmallest planet in the solar system.

• Mercuryhasbeenvisitedbyonlyonespacecraft;NASA’s Mariner 10 examined less than hal the suracein detail in 1974 and 1975.

Physical Characteristics• Mercury’sdiameteris4,880kilometers(3,032miles),aboutone-thirdthesizeofEarthandonlyslightlylarger than our Moon.

• Thedensestplanetinthesolarsystem(whencorrected or compression), its density is 5.3 timesgreater than that o water.

• ThelargestknownfeatureonMercury’spockmarkedsurace is the Caloris basin (1,300 kilometers or 810miles in diameter), likely created by an ancient asteroidimpact.

• Mercury’ssurfaceisacombinationofcraters,smooth

plains, and long, winding clis.• Therepossiblyiswatericeonthepermanently

shadowed crater foors in the polar regions.• Anenormousironcoretakesupmorethan60percent

o the planet’s total mass – twice as much as Earth’s.

Environment• Mercuryexperiencesthesolarsystem’slargest

swing in surace temperatures, rom highs above700° Kelvin (about 800° Fahrenheit) to lows near 90° K (about –300° F).

• Itsextremelythinatmospherecontainshydrogen,helium, oxygen, sodium, potassium, and calcium.

• TheonlyinnerplanetbesidesEarthwithaglobalmagnetic eld, Mercury’s eld is about 100 timesweaker than Earth’s (at the surace).

Orbit

• TheaveragedistancefromtheSunis58millionkilometers (36 million miles), about two-thirds closerto the Sun than Earth is.

• Thehighlyelliptical(elongated)orbitrangesfrom46 million kilometers (29 million miles) to 70 millionkilometers (43 million miles) rom the Sun.

• MercuryorbitstheSunonceevery88Earthdays,moving at an average speed o 48 kilometers (30miles) per second and making it the “astest” planet inthe solar system.

• Becauseofitsslowrotation–Mercuryrotatesonitsaxis once every 59 Earth days – and ast speed around

the Sun, one solar day on Mercury (rom noon tonoon at the same place) lasts 176 Earth days, or twoMercury years.

• ThedistancefromEarth(duringMESSENGER’sorbit) ranges rom about 87 million to 212 millionkilometers, about 54 million to 132 million miles.




Why Mercury?

Mercury, Venus, Earth, and Mars are the terrestrial

(rocky) planets. Among these, Mercury is an extreme:the smallest, the densest (ater correcting or sel-compression), the one with the oldest surace, the onewith the largest daily variations in surace temperature,and the least explored. Understanding this “endmember” among the terrestrial planets is crucial todeveloping a better understanding o how the planetsin our solar system ormed and evolved. To develop thisunderstanding, the MESSENGER mission, spacecrat, andscience instruments are ocused on answering six keyquestions.

Key Science Questions

Question 1: Why is Mercury so dense? Each o the terrestrial planets consists o a dense

iron-rich core surrounded by a rocky mantle, composedlargely o magnesium and iron silicates. The topmostlayer o rock, the crust, ormed rom minerals with lowermelting points than those in the underlying mantle,either during dierentiation early in the planet’s historyor by later volcanic or magmatic activity. The densityo each planet provides inormation about the relative

sizesoftheiron-richcoreandtherockymantleandcrust,since the metallic core is much denser than the rockycomponents. Mercury’s uncompressed density (what itsdensity would be without compaction o its interior bythe planet’s own gravity) is about 5.3 grams per cubiccentimeter, by ar the highest o all the terrestrial planets.In act, Mercury’s density implies that at least 60% o the planet is a metal-rich core, a gure twice as great asor Earth, Venus, or Mars. To account or about 60% o the planet’s mass, the radius o Mercury’s core must beapproximately 75% o the radius o the entire planet!

There are three major theories to explain why Mercuryis so much denser and more metal-rich than Earth, Venus,and Mars. Each theory predicts a dierent compositionor the rocks on Mercury’s surace. According to oneidea, beore Mercury ormed, drag by solar nebular gasnear the Sun mechanically sorted silicate and metalgrains, with the lighter silicate particles preerentiallyslowedandlosttotheSun;Mercurylaterformedfrommaterial in this region and is consequently enriched inmetal. This process doesn’t predict any change in the

composition o the silicate minerals making up therocky portion o the planet, just the relative amounts o metal and rock. In another theory, tremendous heat intheearlynebulavaporizedpartoftheouterrocklayerofproto-Mercury and let the planet strongly depleted in

volatile elements. This idea predicts a rock compositionpoor in easily evaporated elements like sodium andpotassium. The third idea is that a giant impact, aterproto-Mercury had ormed and dierentiated, strippedo the primordial crust and upper mantle. This ideapredicts that the present-day surace is made o rockshighly depleted in those elements that would havebeen concentrated in the crust, such as aluminum andcalcium.

MESSENGER will determine which o these ideasis correct by measuring the composition o the rockysurace. X-ray, gamma-ray, and neutron spectrometerswill measure the elements present in the surace rocksand determine i volatile elements are depleted or i elements that tend to be concentrated in planetarycrusts are decient. A visible-inrared spectrometer willdetermine which minerals are present and will permitthe construction o mineralogical maps o the surace.Analysis o gravity and topography measurements willprovide estimates o the thickness o Mercury’s crust. Together, these measurements will enable MESSENGER




to distinguish among the dierent proposed origins orMercury’s high density and, by doing so, gain insight intohow the planet ormed and evolved.

Question 2: What is the geologic history o Mercury? It is more than 30 years since Mariner 10 visited

Mercury and still only 45% o Mercury’s surace has beenimaged by a spacecrat. The part that has been seenappears cratered and ancient, with a resemblance to thesurace o Earth’s Moon. Slightly younger, less crateredplains sit between the largest old craters. A volcanicorigin has been suggested to explain the ormation o these plains, and slight color dierences between theplains and ancient cratered terrains are consistent withthe plains being volcanic. However, the typical resolution

o images rom Mariner 10 is not high enough to searchor diagnostic volcanic surace eatures to support ullythat idea, and thus the origin o the plains remainsuncertain.

Mercury’s tectonic history is unlike that o any otherterrestrial planet. On the surace o Mercury, the mostprominent eatures due to tectonic orces are long,rounded, lobate scarps and clis, some over a kilometerin height and hundreds o kilometers in length. Thesegiant scarps are believed to have ormed as Mercurycooled and the entire planet contracted on a global

scale. Understanding the ormation o these scarps thusprovides the potential to gain unique insight into thethermal history and interior structure o Mercury.

MESSENGER will bring a variety o investigations tobear on Mercury’s geology in order to determine thesequence o processes that have shaped the surace. TheX-ray, gamma-ray, and visible-inrared spectrometerswill determine the elemental and mineralogical makeupo rock units composing the surace. The camera willimage the previously unseen portion o the planet, andthe typical imaging resolution or MESSENGER will arsurpass that o most Mariner 10 pictures. Nearly all o the surace will be imaged in stereo to determine theplanet’sglobaltopographicvariationsandlandforms;thelaser altimeter will measure the topography o suraceeatures even more precisely in the northern hemisphere.Comparing the topography with the planet’s gravity eld,measured by tracking the MESSENGER spacecrat, willallow determinations o local variations in the thicknesso Mercury’s crust. This large breadth and depth o data

returned by MESSENGER will enable the reconstructiono the geologic history o Mercury.

Question 3: What is the nature o Mercury’smagnetic feld?

Mercury’s magnetic eld and the resultingmagnetosphere, caused by the interaction o Mercury’smagnetic eld with the solar wind, are unique inmany ways. Perhaps one o the most noteworthyobservations about Mercury’s magnetic eld is that thesmall planet has one. Mercury’s magnetic eld is similarto the “dipole” shape o Earth’s magnetic eld, whichresembles the eld that would be produced i therewas a giant bar magnet at the center o the planet.In contrast, Venus, Mars, and the Moon do not show

evidence or intrinsic dipolar magnetic elds, but theMoon and Mars have evidence or local magnetic eldscentered on dierent rock deposits.

Earth’s magnetic eld is very dynamic and constantlychanges in response to activity o the Sun, including thesolar wind and solar fares. We see the eects o thesedynamics on the ground as they aect power gridsand electronics, causing blackouts and intererencewith radios and telephones. Mercury’s magneticeld was shown by Mariner 10 to experience similardynamics;understandingthosevariationswillhelpus

understand the interaction o the Sun with planetarymagnetospheres in general.

Although Mercury’s magnetic eld is thought tobe a miniature version o Earth’s, Mariner 10 didn’tmeasureMercury’seldwellenoughtocharacterizeit. There is even considerable uncertainty in the strengthand source o the magnetic eld. Some theories haveproposed that Mercury’s magnetic eld is actually a reliand not actively being generated today. Advocates oran active global magnetic eld on Mercury, arising romfuid motions in an outer liquid portion o Mercury’smetal core, debate the molten raction o the core aswell as whether the eld is driven by compositionalor thermal dierences. However, these dierent ideasor the driving orce behind Mercury’s magnetic eldpredict slightly dierent eld geometries, so careulmeasurements by spacecrat can distinguish amongcurrent theories.




MESSENGER’smagnetometerwillcharacterizeMercury’s magnetic eld in detail rom orbit over ourMercury years (each Mercury year equals 88 Earth days)to determine its exact strength and how its strength

varies with position and altitude. The eects o the Sunon magnetic eld dynamics will be measured by themagnetometer and by an energetic particle and plasmaspectrometer. MESSENGER’s highly capable instrumentsand broad orbital coverage will greatly advance ourunderstanding o both the origin o Mercury’s magneticeld and the nature o its interaction with the solar wind.

Question 4: What is the structure o Mercury’s core? As discussed in Questions 1 and 3, Mercury has a

verylargeiron-richcoreandaglobalmagneticeld;this

inormation was gathered by the Mariner 10 fybys. Morerecently, Earth-based radar observations o Mercury havealso determined that at least a portion o the large metalcore is still liquid. Having at least a partially molten coremeans that a very small but detectable variation in thespin rate o Mercury has a larger amplitude because o decoupling between the solid mantle and liquid core.Knowing that the core has not completely solidied,even as Mercury has cooled over billions o years since itsormation, places important constraints on the planet’sthermal history, evolution, and core composition.

However, these constraints are limited due to the lowprecision o current inormation on Mercury’s gravityeld rom the Mariner 10 fybys, currently the only gravitymeasurements available or Mercury. Fundamentalquestions about Mercury’s core remain to be explored,such as what is the composition o the core? A core o pure iron would be completely solid today, due to thehigh melting point o iron. However, i other elements,such as sulur, are also present in Mercury’s core, even atonly a level o a ew percent, the melting point is loweredconsiderably, allowing Mercury’s core to remain at leastpartially molten. Constraining the composition o thecore is intimately tied to understanding what raction o the core is liquid and what raction has solidied. Is there just a very thin layer o liquid over a mostly solid coreor is the core completely molten? Addressing questionssuch as these can also provide insight into the currentthermal state o Mercury’s interior, which is very valuableinormation or determining the evolution o the planet.

Using the laser altimeter, MESSENGER will veriy thepresence o a liquid outer core by measuring Mercury’slibration. Libration is the slow 88-day wobble o theplanet around its rotational axis. The libration o the

rocky outer part o the planet will be twice as large i it isoatingonaliquidoutercorethanifitisfrozentoasolidcore. By radio tracking o the spacecrat, MESSENGERwill also determine the gravity eld with much betterprecision than accomplished with the Mariner 10fybys. The libration experiment, when combined withimproved measurements o the gravity eld, will provideconstraintsonthesizeandstructureofthecore.

Question 5: What are the unusual materials at Mercury’s poles?

Mercury’s axis o rotation is oriented nearlyperpendicular to the planet’s orbit, so that in the polarregions sunlight strikes the surace at a near constantgrazingangle.Someoftheinteriorsoflargecratersatthepoles are thus permanently shadowed and perpetuallyvery cold. Earth-based radar images o the polar regionsshow that the foors o large craters are highly refectiveat radar wavelengths, unlike the surrounding terrain.Furthermore, the radar-bright regions are consistentwith radar observations o the polar cap o Mars andthe icy moons o Jupiter, suggesting that the materialconcentrated in the shadowed craters is water ice. The

idea o water ice being stable on the surace o theplanet closest to the Sun is an intriguing suggestion.

The temperature inside these permanently shadowedcraters is believed to be low enough to allow water ice tobe stable or the majority o the observed deposits. Icerom inalling comets and meteoroids could be cold-trapped in Mercury’s polar deposits over billions o years,or water vapor might outgas rom the planet’s interiorandfreezeatthepoles.Afewcratersatlatitudesaslowas 72° N have also been observed to contain radar-brightmaterial in their interiors, and at these warmer latitudes,maintaining stable water ice or longer periods o timemaybemoredifcult;arecentcometimpact,inthelastew million years, delivering water to Mercury may berequired. Alternatively, it has been suggested that theradar-bright deposits are not water ice but rather consisto a dierent material, such as sulur. Sulur would bestable in the cold traps o the permanently shadowedcrater interiors, and the source o sulur could either bemeteoritic material or the surace o Mercury itsel. It has




also been proposed that the naturally occurring silicatesthat make up the surace o Mercury could producethe observed radar refections when maintained at theextremely low temperatures present in the permanently

shadowed craters.

MESSENGER’s neutron spectrometer will searchor hydrogen in any polar deposits, the detection o which would suggest the polar deposits are water-rich. The ultraviolet spectrometer and energetic particlespectrometer will search or the signatures o hydroxideor sulur in the tenuous vapor over the deposits. The laseraltimeter will provide inormation about the topographyo the permanently shadowed craters. Understanding thecomposition o Mercury’s polar deposits will clariy theinventory and availability o volatile materials in the inner

solar system.

Question 6: What volatiles are important at Mercury? Mercury is surrounded by an extremely thin envelope

o gas. It is so thin that, unlike the atmospheres o Venus,Earth, and Mars, the molecules surrounding Mercurydon’t collide with each other and instead bounce romplace to place on the surace like many rubber balls. Thisis called an “exosphere.”

Six elements are known to exist in Mercury’s

exosphere: (1) hydrogen, (2) helium, (3) oxygen, (4)sodium, (5) potassium, and (6) calcium. The observedexosphere is not stable on timescales comparable tothe age o Mercury, and so there must be sources oreach o these elements. High abundances o hydrogenand helium are present in the solar wind, the stream o hot,ionizedgasemittedbytheSun.Theotherelementsare likely rom material impacting Mercury, such asmicrometeoroids or comets, or directly rom Mercury’ssurace rocks. Several dierent processes may have putthese elements into the exosphere, and each processyieldsadifferentmixoftheelements:vaporizationofrocks by impacts, evaporation o elements rom the rocksdue to sunlight, sputtering by solar wind ions, or diusionrom the planet’s interior. Variability o the composition o Mercury’s exosphere has been observed, suggesting theinteraction o several o these processes.

MESSENGER will determine the composition o Mercury’s exosphere using its ultraviolet spectrometerand energetic particle spectrometer. The exospherecomposition measured by these instruments will

Learn More

For additional, detailed inormation

about the science questions driving the

MESSENGER mission, check out the

articles posted at:

http://messenger.jhuapl.edu/the_

mission/publications.html.

be compared with the composition o surace rocksmeasured by the X-ray, gamma-ray, and neutronspectrometers. As MESSENGER orbits Mercury, variationsin the exosphere’s composition will be monitored. The

combination o these measurements will elucidate thenature o Mercury’s exosphere and the processes thatcontribute to it.

http://messenger/the_mission/publications.html







Other Links

The Web and your local library (orbookstore) oer several sources o

inormation on the Mariner 10 mission

and Mercury, including:

The Voyage of Mariner 10: Mission to

Venus and Mercury (James A. Dunne

and Eric Burgess, Jet Propulsion

Laboratory, SP-424, 1978) – http://

history.nasa.gov/SP-424/sp424.htm

Mariner 10 Archive Project (Mark

Robinson, Arizona State University) –

http://ser.sese.asu.edu/merc.html

The SP-423 Atlas o Mercury On Line

(NASA History Web site) – http://

history.nasa.gov/SP-423/mariner.htm

Flight to Mercury by Bruce Murray

and Eric Burgess, Columbia

University Press, 1977 (ISBN

0-231-03996-4)

Mercury by Faith Vilas, Clark

Chapman, and Mildred Shapley

Matthews, University o Arizona

Press, 1988 (ISBN 0-8165-1085-7)

Exploring Mercury by Robert Strom

and Ann Sprague, Springer-Praxis

Books, 2003 (ISBN 1-85233-731-1)

Mercury’s Visitors

Mariner 10

Most o what we know about Mercury today comesrom Mariner 10’s three fyby visits in 1974 and 1975. Theybysweren’ttheNASAmission’sonlyhistoricmoments;Mariner 10 was also the rst spacecrat to use thegravitational pull o one planet (Venus) to reach anotherand the rst to study two planets up close.

Carrying two TV cameras with lter wheels, an inraredradiometer, a plasma experiment, a magnetometer,a charged particle detector, extreme ultravioletspectrometers, and a radio science experiment, Mariner10 was launched toward Venus by an Atlas Centaur 34 on

November 3, 1973. Ater fying past Venus on February5, 1974 – and snapping the rst close-up images o the planet’s upper clouds – the spacecrat headed orMercury in an orbit around the Sun. That trajectorybrought it past Mercury three times – on March 29 andSeptember 21, 1974, and March 16, 1975 – each aordingviews o the same side, and while the planet was at itsarthest point rom the Sun.

The spacecrat’s closest passes, respectively, occurredat 703 kilometers, 48,069 kilometers, and 327 kilometers(437 to 29,870 to 203 miles), giving it dierent vantages

on approach and departure. Mariner 10 mapped 45percent o Mercury’s surace at a scale o approximately1 kilometer (0.6 miles), revealing a landscape batteredwith impact craters and a ascinating mix o smooth androughterrain;discoveredaglobalmagneticeldandathinatmosphere;andconrmedthatMercury,thanksto a large, iron-rich core, has the highest uncompresseddensity o any planet.

Mariner 10’s reconnaissance whetted scientists’appetites to learn more about the innermost planet –and its results helped orm the questions MESSENGERwill try to answer three decades later.

Future MissionsMESSENGER will soon have company in its study

o Mercury. The BepiColombo mission, a collaborationbetween the European Space Agency (ESA) and theJapan Aerospace Exploration Agency (JAXA), will placea pair o spacecrat in orbit around Mercury, one to mapthe planet and the other to study the magnetosphere.

The mission is named or late Italian mathematician andengineer Guiseppe (Bepi) Colombo, who suggested toNASA the fight path that allowed Mariner 10 to fy byMercury three times. The two spacecrat are scheduledor launch in 2013 and or arrival at Mercury in 2019.

http://history.nasa.gov/SP-424/sp424.htm


http://cps.earth.northwestern.edu/merc.html

http://history.nasa.gov/SP-423/mariner.htm




http://cps.earth.northwestern.edu/merc.html






Getting a Boost

For a gravity assist, a spacecrat ies close to a planet and trades with the planet’s

orbital momentum around the Sun. Depending on the relative dierence in mass

between the planet and the spacecrat, as well as the distance between the two,

this exchange o momentum can impart a substantial change in spacecrat speed.

Since the spacecrat’s mass is negligible compared with the planet’s, this process

has a negligible eect on the planet’s orbit around the Sun. But the spacecrat

receives a great boost on the way to its next destination.

Gravity-assist maneuvers can be used to speed a spacecrat up or slow a

spacecrat down. Closest approach distance, direction, and the speed o aspacecrat relative to the planet’s speed all aect the acceleration and direction

change o the spacecrat’s trajectory. The greatest change in a spacecrat’s speed

and direction occurs when a slow-moving spacecrat approaches just above the

surace or cloud tops o a massive planet. The least change in a spacecrat’s speed

and direction occurs when a ast-moving spacecrat approaches a small planet rom

a great distance.

The combined eect o the six gravity assists romthree planetary bodies and ve deterministic Deep

Space Maneuvers (DSMs) – using the bipropellant LargeVelocity Adjust (LVA) engine o the spacecrat and theinfuence o the Sun – accelerates the spacecrat roman average speed around the Sun o 29.8 kilometers persecond (the Earth’s average speed around the Sun) to47.9 kilometers per second (Mercury’s average speedaround the Sun).

The cruise phase o the mission concludes in March2011, when the spacecrat will execute the Mercury orbitinsertion (MOI) maneuver, slowing the spacecrat roma heliocentric orbit and allowing it to be captured into

orbit around Mercury.

Cruise Trajectory The MESSENGER mission takes advantage o an

ingenious trajectory design, lightweight materials, andminiaturizationofelectronics,alldevelopedinthethreedecades since Mariner 10 few past Mercury in 1974 and1975. The compact orbiter, ortied against the searingconditions near the Sun, will investigate key questionsabout Mercury’s characteristics and environment with aset o seven scientic instruments.

On a 4.9-billion mile (7.9-billion kilometer) journeythat includes more than 15 loops around the Sun, thespacecrat’s trajectory includes one pass by Earth, two byVenus, and three by Mercury, beore a propulsive burn will

ease it into orbit around its target planet. The Earth fybyin August 2005, along with the Venus fybys in October2006 and June 2007, used the pull o each planet’s gravityto guide MESSENGER toward Mercury’s orbit. The Mercuryfybys in January 2008, October 2008, and September2009 adjust MESSENGER’s trajectory while also providingopportunities or the spacecrat to gather importantscience observations in advance o the mission’s orbitalphase.

Mission Overview




Multiple Flybys

Mariner 10 ew past Venus to reach Mercury, but the idea o multiple Venus/

Mercury ybys to help a spacecrat “catch” Mercury and begin orbiting

the planet came years later, when Chen-wan Yen o NASA’s Jet Propulsion

Laboratory developed the concept in the mid-1980s. MESSENGER adopts this

mission design approach; without these ybys, MESSENGER would move so

ast past Mercury (about 10 kilometers per second) that no existing propulsion

system could slow it down sufciently or it to be captured into orbit.

Earth to Mercury

MESSENGER cruise trajectory rom the Earth to Mercury with annotation o critical fyby and maneuver events.View looks down rom the ecliptic north pole.




LaunchMESSENGER launched rom complex 17B at Cape Canaveral

Air Force Station, Florida, on a three-stage Boeing Delta IIexpendable launch vehicle on August 3, 2004. The Delta II 7925-H

(heavy lit) model was the largest allowed or NASA Discoverymissions. It eatures a liquid-ueled rst stage with nine strap-onsolid boosters, a second-stage liquid-ueled engine, and a third-stage solid-uel rocket. With MESSENGER secured in a 9.5-meter(31-oot) airing on top, the launch vehicle was about 40 meters(or 130 eet) tall.

The launch vehicle imparted a launch energy per mass(usually denoted by C3 and equal to the excess over what isrequired or Earth escape) o approximately 16.4 km2 /sec2 to thespacecrat, setting up the spacecrat or a return pass by the Earthapproximately one year rom launch.

MESSENGER launch rom Cape Canaveral Air ForceStation, Florida, on August 3, 2004, at 2:15 a.m. EDT.

Team members in the MESSENGER Mission Operations Center at APL watch the spacecrat launch rom Cape Canaveral. The team began operating the spacecrat less than an hour later, ater MESSENGER separated rom the launch vehicle.




Earth Flyby HighlightsMESSENGER swung by its home planet on August 2, 2005, or a gravity assist that propelled it deeper into the inner

solar system. MESSENGER’s systems perormed fawlessly as the spacecrat swooped around Earth, coming to a closestapproach point o about 1,458 miles (2,347 kilometers) over central Mongolia at 3:13 p.m. EDT. The spacecrat used the

tug o Earth’s gravity to change its trajectory signicantly, bringing its average orbital distance nearly 18 million milescloser to the Sun and sending it toward Venus or gravity assists in 2006 and 2007.

Earth northern hemisphere

North ecliptic pole view o the trajectory between Earth and the rst Venus fyby.DashedlinesdepicttheorbitsofEarthandVenus.Timelinefadinghelpsemphasize

primary events.

Earth to Venus

View o the Earth fyby trajectory rom above northern Asia. Major country borders are

outlined in green on Earth’s night side. The yellow line marks the position o the day/night ordawn/dusk terminator.




were taken in ull color and at dierent resolutions, andthe cameras passed their tests.

Not only were these pictures useul or careully

calibrating the imagers or the spacecrat’s Mercuryencounters, they also oered a unique view o Earth. Through clear skies over much o South America, eaturessuchastheAmazon,theAndes,andLakeTiticacaarevisible, as are huge swaths o rain orest.

The pictures rom MESSENGER’s fyby o Earth include“natural” color and inrared views o North and SouthAmerica;apeekattheGalápagosIslandsthroughabreakintheclouds;andamovieoftherotatingEarth,takenasMESSENGER sped away rom its home planet.

MESSENGER’s main camera snapped several approachshots o Earth and the Moon, including a series o colorimages that science team members strung into a “movie”documenting MESSENGER’s departure. On approach, the

atmospheric and surace composition spectrometer alsomade several scans o the Moon in conjunction with thecamera observations, and during the fyby the particleand magnetic eld instruments spent several hoursmaking measurements in Earth’s magnetosphere.

The close fyby o Earth and the Moon allowedMESSENGER to give its two cameras, together known asthe Mercury Dual Imaging System (MDIS), a thoroughworkout. The images helped the team understand ullyhow the cameras operate in fight in comparison with testresults obtained in the laboratory beore launch. Images

Using various combinations o lters in the optical path, MESSENGER’s camera can obtain a mix o red,

green, and blue (RGB) light in various proportions to create a ull spectrum o colors. Inrared images arevisualizedbysubstitutingoneoftheRGBcomponents.Ontheleftisa“normal”colorimageoftheEarth.

On the right, the red component is the 750-nm (inrared) band, and green and blue are ormed rom the

630-nm and the 560-nm bands. Despite the substitution o only one band, the results are dramaticallydierent. Continental areas are mostly red due to the high refectance o vegetation in the near-inrared.

Short-wavelength light (blue) is easily scattered in Earth’s atmosphere, producing our blue skies, butalso obscuring the surace rom MESSENGER’s viewpoint. Inrared light is not easily scattered, so images

oftheEarthremainsharp.TheredcoloringinthecenteroftheimageisareectionoftheBrazilianrain

orests and other vegetation in South America.

Twins Image




During the rst Venus fyby on October 24, 2006, the spacecrat came within 2,987 kilometers (1,856 miles) o the surace o Venus. Shortly beore the encounter, MESSENGER entered superior solar conjunction, where it wason the opposite side o the Sun rom Earth and during which reliable communication between MESSENGER andmission operators was not possible. In addition, during the fyby the spacecrat experienced the mission’s rst andlongest eclipse o the Sun by a planet. During the eclipse, which lasted approximately 56 minutes, the spacecrat’ssolar arrays were in the shadow o Venus and MESSENGER operated on battery power.

Venus Gravity AssistsMESSENGER has fown by Venus twice using the tug o the planet’s gravity to change its trajectory, to shrink the

spacecrat’s orbit around the Sun and bring it closer to Mercury.

North ecliptic pole view o the trajectory between the rst Venus fyby and the rst Mercury fyby. Dashed lines

depicttheorbitsofEarth,Venus,andMercury.Timelinefadinghelpsemphasizeprimaryevents.

Venus to Mercury




View o the rst Venus fyby trajectory rom above the planet’s northern pole. The yellow line marks the position o

the day/night or dawn/dusk terminator. Closest approach time listed is in local spacecrat time, not accounting orthe one-way light time or the signals to reach the Earth.

Images o Venus on approach to MESSENGER’s rst Venus fyby on October 24, 2006.

As a result o this confuence o orbital geometry, the team did not make any scientic observations. But a ewweeks beore the fyby, MDIS snapped pictures o Venus (see below) rom a distance o about 16.5 million kilometers(10.3 million miles). Despite the low resolution o the images, it’s possible to see that Venus is shrouded in a thick blanket o clouds that hides its surace.

Venus Flyby 1

Approaching Venus




MESSENGER swung by Venus or the second time on June 5, 2007, speeding over the planet’s cloud tops at arelative velocity o more than 30,000 miles per hour and passing within 210 miles o its surace near the boundarybetween the lowland plains o Rusalka Planitia and the rited uplands o Aphrodite Terra. The maneuver sharpened thespacecrat’s aim toward the rst encounter with Mercury and presented a special opportunity to calibrate several o its

science instruments and learn something new about Earth’s nearest neighbor.

Venus Flyby 2

Venus 2 Approach

View o the second Venus fyby trajectory rom above the planet’s northern pole. The yellow line marks the positiono the day/night or dawn/dusk terminator. Closest approach time listed is in local spacecrat time, not accounting or

the one-way light time or the signals to reach the Earth.

Approach image taken through the 630-nm lter (stretched).

Global circulation patterns in the clouds are clearly visible.




The MASCS Visible and Inrared Spectrograph observed the Venus dayside near closest approach to gathercompositional inormation on the upper atmosphere and clouds, and the Mercury Laser Altimeter (MLA) carriedout passive radiometry at 1,064 nm and attempted to range to the Venus upper atmosphere and clouds or severalminutes near closest approach. The Gamma-Ray and Neutron Spectrometer (GRNS) observed gamma-rays andneutrons rom Venus’ atmosphere, providing inormation or planning the upcoming Mercury fybys and or calibrationrom a source o known composition.

All o the MESSENGER instruments operated during the fyby. The camera system imaged the night side innear-inrared bands and obtained color and higher-resolution monochrome mosaics o both the approachingand departing hemispheres. The Ultraviolet and Visible Spectrometer on the Mercury Atmospheric and SuraceComposition Spectrometer (MASCS) instrument obtained proles o atmospheric species on the day and night sides

as well as observations o the exospheric tail on departure.

Leaving Venus

MDIS departure sequence, 480-nm lter.




That the European Space Agency’s Venus Express mission was operating at the time o the fyby permittedthe simultaneous observation o the planet rom two independent spacecrat, a situation o particular valueforcharacterizationoftheparticle-and-eldenvironmentatVenus.MESSENGER’sEnergeticParticleandPlasmaSpectrometer (EPPS) observed charged particle acceleration at the Venus bow shock and elsewhere, and the

Magnetometer (MAG) measured the upstream interplanetary magnetic eld (IMF), bow shock signatures, and pick-upion waves as a reerence or energetic particle and plasma observations by both spacecrat. The encounter alsoenabled two-point measurements o IMF penetration into the Venus ionosphere, primary plasma boundaries, and thenear-tail region.

Color composite calibration rame o Venus’ clouds,centered at 17°N, 260°E.

Venus Clouds in Color

Interplanetary Golf

The MESSENGER spacecrat ew within

1.7 kilometers (1.05 miles) o the targeted aim

point during the approach to the second Venus

encounter, the interplanetary equivalent o hitting a

hole-in-one.




Flying by Mercury Three 200-kilometer (124-mile) minimum-altitude Mercury fybys are scheduled to put MESSENGER in position

to enter Mercury orbit in mid-March 2011. A pre-determined course correction maneuver – a Deep-Space Maneuver(DSM) using the main Large Velocity Adjust (LVA) engine – is scheduled approximately two months ollowing each

fyby to urther adjust spacecrat trajectory in preparation or the eventual capture into orbit around Mercury.

During the fyby the spacecrat will depart looking back toward sunlit views o the planet. MESSENGER’sinstruments will view each side o the hemisphere o Mercury not seen by Mariner 10 soon ater reaching minimumaltitude.

North ecliptic pole view o the trajectory between the rst Mercury fyby and Mercury orbit insertion. DashedlinesdepicttheorbitsofEarth,Venus,andMercury.Timelinefadinghelpsemphasizeprimaryevents.

Mercury Flybys




Ater the three fybys o Mercury, MESSENGER will have mapped nearly the entire planet in color, imaged most o the areas not seen by Mariner 10, and measured the composition o the surace, atmosphere, and magnetosphere. O the three fybys, the rst provides the most opportune geometry or scientic observation and discovery. In additionto returning the rst new spacecrat data rom Mercury in more than 30 years, this fyby will add signicantly to

the scientic record o the planet. In contrast to the orbital phase o the mission, the closest approach point to theplanet – a 200 kilometer altitude rom the surace – will occur near the planet’s equator and pass by longitudes notencountered at close range by Mariner 10. This approach provides unique vantages to gather high-resolution imageso the low-to-mid-latitude regions o the planet not previously seen in detail as well as measurements o the magneticand gravitational elds.

The second and third fybys o the planet oer similar fyby trajectories, also passing near low-to-mid latitudes, butat longitudes that have been previously mapped by the Mariner 10 spacecrat. These fyby opportunities, in addition toincreasing the heliocentric orbital velocity o the spacecrat, will expand the knowledge gained rom the rst fyby.

Mercury Flyby 1

View o the rst Mercury fyby trajectory rom above the planet’s northern pole. The yellow line marks theposition o the day/night or dawn/dusk terminator. Closest approach time listed is in local spacecrat time, not

accounting or the one-way light time or the signals to reach the Earth.

Mercury Flyby 2

View o the second Mercury fyby trajectory rom above the planet’s northern pole. The yellow line marks theposition o the day/night or dawn/dusk terminator. Closest approach time listed is in local spacecrat time, not

accounting or the one-way light time or the signals to reach the Earth.




Mercury Flyby 3

View o the third Mercury fyby trajectory rom above the planet’s northern pole. The yellow line marks the

position o the day/night or dawn/dusk terminator. Closest approach time listed is in local spacecrat time, notaccounting or the one-way light time or the signals to reach the Earth.

Terra Incognita

During its three ybys o Mercury in 1974-1975,

the Mariner 10 spacecrat imaged approximately

45% o the visible surace. Although radar and

visible images o portions o the planet not viewed

by Mariner 10 have been made rom Earth-based

observatories, MESSENGER’s frst Mercury yby

will reveal about 50% o the side o the planet never

beore seen in detail rom a spacecrat.




MESSENGER’s Deep Space ManeuversIn conjunction with the six planetary fybys, MESSENGER’s complex 6.6-year cruise trajectory includes more

than 40 anticipated trajectory-correction maneuvers (TCMs). These TCMs include ve deterministic Deep-SpaceManeuvers (DSMs), which use the spacecrat’s bipropellant Large Velocity Adjust (LVA) engine. In addition to imparting

a combined spacecrat change in velocity o more than 1 kilometer per second, MESSENGER’s DSMs are the primarymethod targeting the spacecrat beore each planetary fyby (except the second Venus fyby). Smaller velocityadjustment maneuvers that use the propulsion system’s monopropellant thrusters ne-tune the trajectory betweenthe main DSMs and the gravity-assist fybys o the planets.

Mercury Flyby 3

Timeline o deterministic Deep Space Maneuvers or the MESSENGER spacecrat over the entire cruise phase

o the mission. Additional statistical “ne-tune” maneuvers not listed.

On December 12, 2005, MESSENGER successully red its bipropellant LVA engine or the rst time, completingthe rst o the ve critical deep space maneuvers (DSM-1). The maneuver, just over eight minutes long, changedMESSENGER’s speed by approximately 316 meters per second (706 miles per hour), placing the spacecrat on target orthe rst Venus fyby on October 24, 2006.

This maneuver was the rst to rely solely on the LVA, the largest and most ecient engine o the propulsion system.ManeuversperformedwiththeLVAuseabout30%lesstotalpropellantmass–bothfuelandoxidizer–thantheotherthrusters,whichusemonopropellantfuelonly.Approximately100kilogramsofbipropellant(bothfuelandoxidizer),about 18 percent o MESSENGER’s total on-board propellant, was used to complete DSM-1.

On October 17, 2007, MESSENGER completed its secondcritical DSM – 155 million miles (250 million kilometers)rom Earth – successully ring the LVA again to changethe spacecrat’s trajectory and target it or its rst fyby o Mercury on January 14, 2008.

The maneuver, just over ve minutes long, consumedapproximately 70 kg o bipropellant (both uel andoxidizer),changingthevelocityofthespacecraftbyapproximately 226 meters per second (505 miles per hour).

The next two DSMs, one in March 2008 and the otherin December 2008, will be used to target the next twogravity-assist fybys o Mercury in October 2008 andSeptember 2009, respectively. The last DSM, in November2009, will be used to set the initial conditions or the nalMercury orbit insertion (MOI) maneuver on March 2011.

MESSENGER’s Large Gas Tank

To support the fve DSMs and fnal Mercury

orbit insertion maneuver, the propulsion

system was designed to impart more than

2.2 kilometers per second o velocity change

to the spacecrat over the lie o the mission.

To contain this amount o energy and still

be able to launch the spacecrat, over 54%

o the spacecrat’s launch mass was liquid

propellant – uel and oxidizer.




MESSENGER’s 12-month orbital mission covers twoMercurysolardays;oneMercurysolarday,fromnoontonoon, is equal to 176 Earth days. MESSENGER will obtainglobal mapping data rom the dierent instruments

during the rst day and ocus on targeted scienceinvestigations during the second.

While MESSENGER circles Mercury, the planet’s gravitythe Sun’s gravity, and radiation pressure rom the Sunwill slowly and slightly change the spacecrat’s orbit.Once every Mercury year (or 88 Earth days) MESSENGERwill carry out a pair o maneuvers to reset the orbit to itsoriginalsizeandshape.

For one year MESSENGER will operate in a highlyelliptical (egg-shaped) orbit around Mercury, a nominal200 kilometers (124 miles) above the surace at the

closest point and 15,193 kilometers (9,440 miles) at thearthest. The plane o the orbit is inclined 80° to Mercury’sequator, and the low point in the orbit (“periapsis”) comesat 60° N latitude. MESSENGER will orbit Mercury twiceevery 24 hours.

Orbit insertion occurs in March 2011. Using 30 percento its initial propellant load, MESSENGER will re itsLarge Velocity Adjust (LVA) engine and slow down byapproximately 860 meters per second (1,924 miles perhour), coming to a virtual stop relative to Mercury. Thisbraking maneuver will be executed in two parts: the

rst maneuver (lasting about 14 minutes) places thespacecrat in a stable orbit and the second maneuver, amuch shorter “cleanup” maneuver, is executed three dayslater near the orbit’s lowest point.

Science Orbit: Working at Mercury

Spacecraft Orbit at Mercury

MESSENGER operational orbit at Mercury.




Mission OperationsMESSENGER’s mission operations are conducted rom

the Space Science Mission Operations Center (MOC) atthe Johns Hopkins University Applied Physics Laboratory

in Laurel, Md., where the spacecrat was designed andbuilt. Flight controllers and mission analysts monitorand operate the spacecrat, working closely with themulti-institutional science team, the mission design teamat APL, and the navigation team at KinetX, Inc., basedin Simi Valley, Cali. Mission operators and scientistswork together to plan, design, and test commands orMESSENGER’s science instruments. Working with themission design and navigation teams, the operators build,test, and send the commands that re MESSENGER’spropulsion system to rene its path to and aroundMercury.

Like all NASA interplanetary missions, MESSENGERrelies on the agency’s Deep Space Network (DSN) o antenna stations to track and communicate with thespacecrat. The stations are located in Caliornia’s MojaveDesert;nearMadrid,Spain;andnearCanberra,Australia.All three complexes communicate directly with thecontrol center at NASA’s Jet Propulsion Laboratory,Pasadena, Cali., which in turn communicates with theMESSENGER Mission Operations Center. Typical DSNcoverage or MESSENGER includes three 8-hour contactsa week during the cruise phase and twelve 8-hour

sessions per week during the orbit at Mercury.

The Science Operations Center, also located atAPL, works with the mission operations team to planinstrument activities, as well as validate, distribute,manage, and archive MESSENGER’s science data.




Science during Mercury Flyby 1

On January 14, 2008, the MESSENGER spacecrat has

its rst encounter with its target destination, Mercury,and gets a taste or bigger events to come. This fybyprovides the rst opportunity to continue the scienticinvestigation o Mercury begun with the Mariner 10mission. Scientists have labored over the data romMariner 10’s three fybys or three decades, incorporatinginsights rom other planetary missions and Earth-basedobservations and ormulating new questions about theplanet nearest the Sun and the evolution o the terrestrialplanets as a group. Such labors led to the six questionsthat rame the MESSENGER mission and the initialproposal o the mission itsel in 1996.

The MESSENGER team will begin to answer thosequestions with the rst fyby, one o our encountersleading up to an orbital study o this enigmatic planet inMarch 2011. These fybys – driven by orbital mechanicsand technological and launch vehicle constraints – willprovide a unique perspective on Mercury, including viewso terrain not seen by Mariner 10 and other investigationsthat have been enabled by technological advancesin instrumentation. Data rom the fybys will lay thegroundwork or achieving the mission’s science goals.

The top priority o the rst Mercury fyby is the gravityassist required or eventual orbit insertion. But the uniquetrajectoriesofallthreeybys-characterizedbyalow-altitude pass at equatorial latitudes, in contrast to thehighly elliptical, nearly polar orbit that the spacecratwill eventually assume around the planet - provideopportunities or scientic measurements not possibleduring the orbital phase o this exciting mission. Whilenot “requirements” or mission success in an engineeringsense, the fybys provide special opportunities. On thisrst pass, or example, the entire science payload willmake measurements o the mission’s intended target. The data returned will give the science community aglimpse into the exciting secrets that will be explored inthe subsequent fybys and orbital phase, and allow theMESSENGERScienceTeamtooptimizefurtherthoselatermeasurements.

Probing Mercury’s Mysteries

The MESSENGER mission – rom its conception as avoyage o scientic exploration and discovery – ocuseson six key questions regarding Mercury’s nature andevolution:

Why is Mercury so dense?

What is the geologic history o Mercury?

What is the nature o Mercury’s magnetic feld?

What is the structure o Mercury’s core?

What are the unusual materials at Mercury’s poles?

What volatiles are important at Mercury?

Density . Perhaps the most undamental o the sciencequestions regarding Mercury is how it ormed with sucha large metal core, required to account or the planet’sanomalously high bulk density. Mercury’s core is sucha large raction o its mass that the acceleration dueto gravity is almost exactly the same at the surace o Mercury and Mars, even though the diameter o Mars is40% larger than that o Mercury. (Fact: a person would“weigh” the same on both planets – and about 2/5 theirweight on Earth.)

The clues to understanding how Mercury ormedmostly o metal are to be ound in the composition o therocky materials on Mercury’s surace. This rst fyby willprovide the rst measurements o the mineralogical andelemental composition o Mercury’s surace. Moreover,these measurements will be taken o regions near theequator at spatial resolutions not attainable at thoselatitudes during the orbital phase. Both the visible-nearinrared and the ultraviolet-visible spectrometers willbe observing the surace to provide refectance spectrathat will indicate mineral composition. The gamma-ray, X-ray, and neutron spectrometers will be makingmeasurements that will provide insight into elementalcomposition. These measurements will provide the rstindications o how this metal-rich planet likely ormed. Theories or Mercury’s unusually metal-rich compositioninclude: (i) loss o lighter rocky materials to the Sun in theearly circumsolar disk o gas and dust beore the planetsformed;(ii)vaporizationoftheouterrockylayerofanearlyMercurybythehotsolarnebula;or(iii)collisionofa large proto-planet with Mercury that removed most o Mercury’s outer rocky shell.




History . Additional clues to Mercury’s ormation

and evolution can be ound in the many landorms andgeological eatures on the surace. Mariner 10 imagedonly about 45% o Mercury’s surace, and while largeportions o the rest o Mercury have been imaged romEarth in both visible light and radar at comparativelycoarse resolution, MESSENGER will provide the rstdetailed global view.

During this fyby MESSENGER will collect the rstspacecrat-based images o the portion o the planetnotseenbyMariner10;thesewillincludehigh-spatial-resolution images near the equator that will not be

possible during orbital operations. The rst altimetricmeasurements will be returned rom the probe’slaser altimeter, again along an equatorial region thatwill not be viewed at low altitude during the orbitalphase. The rst stereo and gravity eld measurementswill also be part o the science returned during thisinitial encounter. The color images that are part o theobservation set, coupled with the inormation gatheredby the spectrometers, will provide deeper insightsinto Mercury’s crustal history. In addition to providingunprecedented detail on surace geology, including therst complete view o the 1300-km-diameter Caloris

impact basin, these measurements will provide thecontext or detailed interpretation o the elemental andmineralogical measurements.

Magnetism. One o the biggest surprises romMariner 10’s encounter with Mercury was the detectiono a global magnetic eld, which appears to be dipolar,as is Earth’s eld. The brie glimpses o the magneticeld o Mercury accorded by the rst and third Mariner10 fybys were insucient to provide details o eldorientation and structure. A planetary magnetic dynamo,thought to be the origin o Mercury’s magnetic eld, isa closed loop o electrical conductors in which currentis generated as the conductors are mechanically pulledacross a magnetic eld. This current, in turn, along withother motion involving planetary rotation, is responsibleor the eld across which the conductors move. Theprecise mechanism or generating such a dynamo diersamong the planets, but Mercury is the most Earth-like o the planets with internal elds, so detailed study o theeld at Mercury should provide insight into Earth’s owndynamo.

The mapping o the planet’s magnetic eld will

begin in January with low-altitude measurementsnear the equatorial region. These measurements willcomplement those obtained during the orbital phase. The magnetometer will also make observations artherrom the planet that will provide glimpses into theeld’s dynamical interaction with the solar wind, whichproduces a “magnetotail” and other magnetosphericphenomena.

Core. I Mercury’s core is pure iron, it should havesolidied long ago. However, the magnetic eldobservations and the ground-based radar detection

o Mercury’s orced libration in longitude indicate thatMercury has a molten outer core.

During this fyby, Doppler measurements takenwith the spacecrat’s telecommunication system willprovide a rst glimpse into Mercury’s gravity eldstructure. These measurements will rene the valueso the second-order gravity coecients, C

20and C

22,

representing the gravitational oblateness and equatorialellipticity, respectively. MESSENGER measurementsshould provide a signicant update to those taken byMariner 10 tracking. These measurements, combined

with recent ground-based radar measurements, willsignicantlyreneourknowledgeofthesizeandextento the planet’s liquid core.

Polar Deposits. Current expectations are that mosto the data that will determine the nature o polardeposits will come rom the orbital tour, but there canalways be surprises when measurements not possiblebeore are made. The rst Mercury fyby is equatorialand will not provide an opportunity to view the polarregions directly, but ultraviolet emissions and/or plasmacomposition changes detected during the fyby mayprovide important clues about these enigmatic eaturesprior to the orbital phase o the mission.

Volatiles. A signicant discovery by Mariner 10 wasdetection o the presence o a thin, tenuous atmosphereat Mercury comprised o the elements hydrogen,helium, and oxygen. Ground-based measurementshave extended the known elements in the exosphereto sodium, potassium, and calcium and have shownthat the abundances o these materials strongly vary




What the Future Holds

Space exploration is a challenge, but a challenge

the MESSENGER spacecrat is meeting. With

each Mercury yby, MESSENGER will provide new

insights and whet our appetite or more inormation,

until the spacecrat fnally enters orbit about

Mercury in March 2011. Such is the adventure that

awaits us.

both spatially and temporally. All o these species are“volatiles” - materials that have low boiling points and sotendtovaporizeatlowtemperatures.Intheoutersolarsystem, where temperatures are low, the term “volatiles”

reers to surace ices. For the terrestrials planets, volatilesreer to materials with low(er) melting points such assodium and potassium, as compared with high-melting-point, or reractory, materials.

MESSENGER’s rst encounter with Mercury providesan opportunity to get a jump start on the mapping o the exosphere with both ultraviolet observations andwith energetic particle and plasma measurements o theatmospheric regions. In addition, the unique trajectoryo the fyby enables ultraviolet measurements o themagnetotail - a region where many o these species are

removed rom the system. This region is not routinelyaccessible rom the orbital trajectory.




Ater Mariner 10’s visits to Mercury, the space science and engineering communities yearned or a longer and

more detailed look at the innermost planet – but that closer look, ideally rom orbit, presented ormidable technicalobstacles. A Mercury orbiter would have to be tough, with enough protection to withstand searing sunlight androasting heat bouncing back rom the planet below. The spacecrat would need to be lightweight, since most o itsmass would be uel to re its rockets to slow the spacecrat down enough to be captured by Mercury’s gravity. And theprobe would have to be suciently compact to be launched on a conventional and cost-eective rocket.

Designed and built by the Johns Hopkins University Applied Physics Laboratory – with contributions romresearch institutions and companies around the world – the MESSENGER spacecrat tackles each o these challenges.A ceramic-abric sunshade, heat radiators, and a mission design that limits time over the planet’s hottest regionsprotect MESSENGER without expensive and impractical cooling systems. The spacecrat’s graphite compositestructure – strong, lightweight, and heat tolerant – is integrated with a low-mass propulsion system that ecientlystores and distributes the approximately 600 kilograms (about 1,320 pounds) o propellant that accounts or 54 % o

MESSENGER’s total launch weight.

To t behind the 2.5-meter by 2-meter (roughly 8-oot by 6-oot) sunshade, MESSENGER’s wiring, electronics,systems, and instruments are packed into a small rame that could t inside a large sport utility vehicle. And the entirespacecrat is light enough to launch on a Delta II 7925-H ( “heavy”) rocket, the largest launch vehicle allowed underNASA’s Discovery Program o lower-cost space science missions.

The Spacecrat

MESSENGER spacecrat during integration and testing. The sunshade is visible on the let with thespacecrat undergoing testing in thermal-vacuum conditions at NASA’s Goddard Space Flight Center. On

the right, the solar panels and Magnetometer boom are in their position or fight during work at APL.




MESSENGER carries seven scientic instruments and a radio science experiment to accomplish an ambitiousobjective:returntherstdatafromMercuryorbit.Theminiaturizedpayload–designedtoworkintheextremeenvironment near the Sun – will image all o Mercury or the rst time, as well as gather data on the composition and

structure o Mercury’s crust, its geologic history, the nature o its active magnetosphere and thin atmosphere, and themakeup o its core and the materials near its poles.

The instruments include the Mercury Dual Imaging System (MDIS), the Gamma-Ray and Neutron Spectrometer(GRNS), the X-Ray Spectrometer (XRS), the Magnetometer (MAG), the Mercury Laser Altimeter (MLA), the MercuryAtmospheric and Surace Composition Spectrometer (MASCS), and the Energetic Particle and Plasma Spectrometer(EPPS). The instruments communicate to the spacecrat through ully redundant Data Processing Units (DPUs).

The process o selecting the scientic instrumentation or a mission is typically a balance between answering asmany science questions as possible and tting within the available mission resources or mass, power, mechanicalaccommodation, schedule, and cost. In the case o MESSENGER, the mass and mechanical accommodation issues werevery signicant constraints. Payload mass was limited to 50 kilograms (110 pounds) because o the propellant massneeded or orbit insertion. The instrument mechanical accommodation was dicult because o the unique thermalconstraintsfacedduringthemission;instrumentshadtobemountedwhereMercurywouldbeinviewbuttheSun

Science Payload

Spacecraft Instruments




would not, and they had to be maintained within an acceptable temperature range in a very harsh environment.Instrument details ollow. In each case the mass includes mounting hardware and thermal control components, andthepoweristhenominalaveragepowerconsumptionperorbit;actualvaluesvarywithinstrumentoperationalmode.

Mercury Dual Imaging SystemMass: 8.0 kilograms (17.6 pounds)

Power: 7.6 wattsDevelopment:Johns Hopkins University Applied Physics Laboratory

The multi-spectral MDIS has wide- and narrow-angle cameras (the “WAC” and “NAC,”respectively) – both based on charge-coupled devices (CCDs), similar to those oundin digital cameras – to map the rugged landorms and spectral variations on Mercury’ssurace in monochrome, color, and stereo. The imager pivots, giving it the ability to captureimages rom a wide area without having to re-point the spacecrat and allowing it to ollowthe stars and other optical navigation guides.

The wide-angle camera has a 10.5° by 10.5° eld o view and can observe Mercury through 11 dierent lters andmonochrome across the wavelength range 395 to 1,040 nanometers (visible and near-inrared light). Multi-spectralimaging will help scientists investigate the diversity o rock types that orm Mercury’s surace. The narrow-anglecamera can take black-and-white images at high resolution through its 1.5° by 1.5° eld o view, allowing extremelydetailed analysis o eatures as small as 18 meters (about 60 eet) across.

Gamma-Ray and Neutron SpectrometerGRNS packages separate gamma-ray and neutron spectrometers to collect complementary data on elements thatorm Mercury’s crust.

Gamma-Ray Spectrometer Mass: 9.2 kilograms (20.3 pounds)Power: 6.6 wattsDevelopment:Johns Hopkins University Applied Physics Laboratory, Patriot Engineering,Lawrence Berkeley National Laboratory, Lawrence Livermore National Laboratory

GRS measures gamma rays emitted by the nuclei o atoms on Mercury’s surace that arestruck by cosmic rays. Each element has a signature emission, and the instrument will look or geologically important elements such as hydrogen, magnesium, silicon, oxygen, iron,titanium, sodium, and calcium. It may also detect naturally radioactive elements such aspotassium, thorium, and uranium.

Neutron Spectrometer Mass: 3.9 kilograms (8.6 pounds)Power: 6.0 wattsDevelopment:Johns Hopkins University Applied Physics Laboratory, Patriot Engineering, LosAlamos National Laboratory

NS maps variations in the ast, thermal, and epithermal neutrons Mercury’s surace emitswhenstruckbycosmicrays.“Fast”neutronsshootdirectlyintospace;otherscollidewithneighboring atoms in the crust beore escaping. I a neutron collides with a light atom (likehydrogen), it will lose energy and be detected as a slow (or thermal) neutron. Scientists




can look at the ratio o thermal to epithermal (slightly aster) neutrons acrossMercury’s surace to estimate the amount o hydrogen – possibly locked up inwater molecules – and other elements.

X-Ray SpectrometerMass: 3.4 kilograms (7.5 pounds)Power: 6.9 wattsDevelopment:Johns Hopkins University AppliedPhysics Laboratory

XRS maps the elements in the top millimetero Mercury’s crust using three gas-lled detectors(MXU) pointing at the planet, one silicon solid-state detector pointing at the Sun (SAX), and theassociated electronics (MEX).The planet-pointing

detectors measure fuorescence, the X-ray emissions coming rom Mercury’ssurace ater solar X-rays hit the planet. The Sun-pointing detector tracks theX-rays bombarding the planet

XRS detects emissions rom elements in the 1-10 kiloelectron-volt (keV)range – specically, magnesium, aluminum, silicon, sulur, calcium, titanium, andiron. Two detectors have thin absorption lters that help distinguish among thelower-energy X-ray lines o magnesium, aluminum, and silicon.

Beryllium-copper honeycomb collimators give XRS a 12° eld o view, whichis narrow enough to eliminate X-rays rom the star background even whenMESSENGER is at its arthest orbital distance rom Mercury. The small, thermally

protected, solar-fux monitor is mounted on MESSENGER’s sunshade.

MagnetometerMass (including boom): 4.4 kilograms (9.7 pounds)Power: 4.2 wattsDevelopment: NASA Goddard Space Flight Center, Greenbelt, Md., and Johns HopkinsUniversity Applied Physics Laboratory

A three-axis, ring-core fuxgate detector, MAG characterizesMercury’smagneticeldindetail, helping scientists determine the eld’s exact strength and how it varies with positionand altitude. Obtaining this inormation is a critical step toward determining the source o Mercury’s magnetic eld.

The MAG sensor is mounted on a 3.6-meter (nearly 12-oot long) boom that keeps it away rom the spacecrat’sown magnetic eld. The sensor also has its own sunshade to protect it rom the Sun when the spacecrat is tiltedto allow or viewing by the other instruments. While in orbit at Mercury the instrument will collect magnetic eldsamplesat50-millisecondtoone-secondintervals;therapidsamplingwilltakeplacenearMercury’smagnetosphereboundaries.

Hot Space, CoolInstrument

To help it measure surace

gamma rays rom long distances

MESSENGER uses the most

sensitive detector available

– a high-purity germanium

semiconductor crystal. But while

MESSENGER moves through

one o the solar system’s hottest

environments, the crystal

must operate at cryogenic

temperatures. Instrument

designers addressed this

challenge by suspending the

detector on thin Kevlar strings

inside a high-tech thermos bottle

with a small, powerul rerigerato

(called a cryocooler) that

keeps temperatures at a rosty

–183° Celsius, or about –300°

Fahrenheit.




Mercury Laser AltimeterMass: 7.4 kilograms (16.3 pounds)Peak Power: 16.4 wattsDevelopment: NASA Goddard Space Flight Center

MLA maps Mercury’s landorms and other surace characteristics using an inrared lasertransmitter and a receiver that measures the round-trip time o individual laser pulses. The datawill also be used to track the planet’s slight, orced libration – a wobble about its spin axis –which will tell researchers about the state o Mercury’s core.

MLA data combined with Radio Science Doppler ranging will be used to map the planet’sgravitational eld. MLA can view the planet rom up to 1,000 kilometers (620 miles) away with an accuracy o 30centimeters (about one oot). The laser’s transmitter, operating at a wavelength o 1,064 nanometers, will deliver eightpulses per second. The receiver consists o our sapphire lenses, a photon-counting detector, a time-interval unit, andprocessing electronics.

Mercury Atmospheric and Surace Composition SpectrometerMass: 3.1 kilograms (6.8 pounds)Peak Power: 6.7 wattsDevelopment: Laboratory or Atmospheric and Space Physics, University o Colorado, Boulde

Combining an ultraviolet spectrometer and inrared spectrograph,MASCS will measurethe abundance o atmospheric gases around Mercury and detect minerals in its suracematerials.

The Ultraviolet and Visible Spectrometer (UVVS) will determine the composition andstructure o Mercury’s exosphere – the extremely low-density atmosphere – and study

itsneutralgasemissions.Itwillalsosearchforandmeasureionizedatmosphericspecies. Together these measurements will help researchers understand the processes that generate and maintain theatmosphere, the connection between surace and atmospheric composition, the dynamics o volatile materials on andnear Mercury, and the nature o the radar-refective materials near the planet’s poles. The instrument has 25-kilometerresolution at the planet’s limb.

Perched atop the ultraviolet spectrometer, the Visible and Infrared Spectrograph (VIRS) will measure the refectedvisible and near-inrared light at wavelengths diagnostic o iron and titanium-bearing silicate materials on the surace,such as pyroxene, olivine, and ilmenite. The sensor’s best resolution is 3 kilometers at Mercury’s surace.

Energetic Particle and Plasma SpectrometerMass: 3.1 kilograms (6.8 pounds)Peak Power: 7.8 wattsDevelopment:, Johns Hopkins University Applied PhysicsLaboratory and University o Michigan, Ann Arbor

EPPS will measure the mix and characteristics o chargedparticles in and around Mercury’s magnetosphere using anEnergetic Particle Spectrometer (EPS) and a Fast Imaging PlasmaSpectrometer (FIPS). Both are equipped with time-o-fight andEPS on let and FIPS

on right




energy-measurement technologies to determine particlevelocities and elemental species.

From its vantage point near the top deck o

the spacecrat, EPS will observe ions and electronsaccelerated in the magnetosphere. EPS has a 160° by 12°eld o view or measuring the energy spectra and pitch-angle distribution o these ions and electrons. Mountedon the side o the spacecrat, FIPS will observe low-energy ions coming rom Mercury’s surace and sparseatmosphere,ionizedatomspickedupbythesolarwind,and other solar-wind components. FIPS provides nearlyull hemispheric coverage.

Radio Science observations – gathered by trackingthe spacecrat through its communications system – will

precisely measure MESSENGER’s speed and distance romEarth. From this inormation, scientists and engineerswill watch or changes in MESSENGER’s movements atMercury to measure the planet’s gravity eld, and tosupport the laser altimeter investigation to determinethesizeandconditionofMercury’score.NASA’sGoddardSpace Flight Center leads the Radio Science investigation.




and the science orbit is designed to limit MESSENGER’sexposure to heat re-radiating rom the surace o Mercury. (MESSENGER will only spend about 25 minutes

o each 12-hour orbit crossing Mercury’s broiling suraceat low altitude.) The combination o the sunshade,thermal blanketing, and heat-radiation system allows thespacecrat to operate without special high-temperatureelectronics.

Power Two single-sided solar panels are the spacecrat’s

main source o electric power. To run MESSENGER’ssystems and charge its 23-ampere-hour nickel-hydrogenbattery, the panels, each about 1.5 meters (5 eet) by1.65 meters (5.5 eet), will support between 385 and485 watts o spacecrat load power during the cruisephase and 640 watts during the orbit at Mercury. Thepanels could produce more than two kilowatts o powernear Mercury, but to prevent stress on MESSENGER’selectronics, onboard power processors take in only whatthe spacecrat actually needs.

The custom-developed panels are two-thirdsmirrors (called optical solar refectors) and one-third

Thermal DesignWhile orbiting Mercury, MESSENGER will “eel”

signicantly hotter than spacecrat that orbit Earth. This

is because Mercury’s elongated orbit swings the planetto within 46 million kilometers (29 million miles) o theSun, or about two-thirds closer to the Sun than Earth. As aresult, the Sun shines up to 11 times brighter at Mercurythan we see rom our own planet.

MESSENGER’s rst line o thermal deense is a heat-resistant and highly refective sunshade, xed on atitanium rame to the ront o the spacecrat. Measuringabout 2.5 meters (8 eet) tall and 2 meters (6 eet) across,the thin shade has ront and back layers o Nextelceramic cloth – the same material that protects sectionso the space shuttle – surrounding several inner layerso Kapton plastic insulation. While temperatures onthe ront o the shade could reach 370° C (about 700°F) when Mercury is closest to the Sun, behind it thespacecrat will operate at room temperature, around 20°C (about 70° F). Multilayered insulation covers most o thespacecrat.

Radiators and diode (“one-way”) heat pipes areinstalled to carry heat away rom the spacecrat body,

Spacecrat Systems and Components

MESSENGER spacecrat in fight conguration, rear view




and during normal operations at least one o the twoantennas points at Earth.

High-gain antennas send radio signals through

a narrower, more concentrated beam than low-gainantennas and are used primarily to send larger amountso data over the same distance as a low-gain antenna. The anbeam and low-gain antennas, also located onMESSENGER’s ront and back sides, are used or lower-ratetransmissions such as operating commands, status data,or emergency communications. MESSENGER’s downlink rate ranges rom 9.9 bits per second to 104 kilobits persecond;operatorscansendcommandsat7.8to500bits per second. Transmission rates vary according tospacecraftdistanceandground-stationantennasize.

Command and Data HandlingMESSENGER’s “brain” is its Integrated Electronics

Module (IEM), a space- and weight-saving device thatcombines the spacecrat’s core avionics into a single box. The spacecrat carries a pair o identical IEMs or backuppurposes;bothhousea25-megahertz(MHz)mainprocessorand10-MHzfaultprotectionprocessor.Allfourare radiation-hardened RAD6000 processors, based onpredecessors o the PowerPC chip ound in some modelso home computers. The computers, slow by currenthome-computer standards, are state o the art or the

radiation tolerance required on the MESSENGER mission.

Programmed to monitor the condition o MESSENGER’s key systems, both ault protectionprocessors are turned on at all times and protect thespacecrat by turning o components and/or switchingto backup components when necessary. The mainprocessor runs the Command and Data Handlingsotware or data transer and le storage, as well as theGuidance and Control sotware used to navigate andpoint the spacecrat. Each IEM also includes a solid-statedata recorder, power converters, and the interacesbetween the processors and MESSENGER’s instrumentsand systems.

Intricate fight sotware guides MESSENGER’sCommand and Data Handling system. MESSENGERreceives operating commands rom Earth and canperorm them in real time or store them or laterexecution. Some o MESSENGER’s requent, criticaloperations (such as propulsive maneuvers) are

triple-junction solar cells, which convert 28 percent o the sunlight hitting them into electricity. Each panelhastworowsofmirrorsforeveryrowofcells;thesmall mirrors refect the Sun’s energy and keep the

panel cooler. The panels also rotate, so MESSENGER’sfight computer will tilt the panels away rom the Sun,positioning them to get the required power whilemaintaining a normal surace operating temperature o about 150° Celsius, or about 300° Fahrenheit.

PropulsionMESSENGER’s dual-mode propulsion system includes

a 660-newton (150-pound) bipropellant thruster orlargemaneuversand16hydrazine-propellantthrustersor smaller trajectory adjustments and attitude control.

The Large Velocity Adjust (LVA) thruster requires acombinationofhydrazinefuelandanoxidizer,nitrogentetroxide.Fuelandoxidizerarestoredincustom-designed, lightweight titanium tanks integrated into thespacecraft’scompositeframe.Heliumpressurizesthesystemandpushesthefuelandoxidizerthroughtotheengines.

At launch the spacecrat carried just under 600kilograms (about 1,320 pounds) o propellant, and it willuse nearly 30 percent o it during the maneuver thatinserts the spacecrat into orbit about Mercury. The small

hydrazinethrustersplayseveralimportantroles:four22-newton (5-pound) thrusters are used or small coursecorrections and help steady MESSENGER during largeengineburns.Thedozen4.4-newton(1-pound)thrustersare also used or small course corrections and serveas a backup or the reaction wheels that maintain thespacecrat’s orientation during normal cruise and orbitaloperations.

CommunicationsMESSENGER’s X-band coherent communications

system includes two high-gain, electronically steered,phased array antennas – the rst ever used on a deep-spacemission;twomedium-gainfanbeamantennas;andfourlow-gainantennas.Thecircularlypolarizedphasedarrays – developed by APL and located with the anbeamantennas on the ront and back o the spacecrat – arethe main link or sending science data to Earth. Forbetterreliabilitytheantennasarexed;they“point”electronically across a 45° eld without moving parts,




orientation, making sure the panels produce sucientpower while maintaining sae temperatures.

Five Sun sensors back up the star trackers,

continuously measuring MESSENGER’s angle to the Sun.I the fight sotware detects that the Sun is “moving”outofadesignatedsafezoneitcaninitiateanautomaticturn to ensure that the shade aces the Sun. Groundcontrollerscanthenanalyzethesituationwhilethespacecrat turns its antennas to Earth and awaitsinstructions – an operating condition known as“sae” mode.

programmed into the fight computer’s memory andtimed to run automatically.

For data, MESSENGER carries two solid-state recorders

(one backup) able to store up to 1 gigabyte each. Itsmain processor collects, compresses, and stores imagesand other data rom MESSENGER’s instruments onto therecorder;thesoftwaresortsthedataintolessimilartohow les are stored on a PC. The main processor selectsthe les with highest priority to transmit to Earth, ormission operators can download data les in any orderthe team chooses.

Antenna signal strength (and downlink rate) varieswith spacecrat-Earth distance and ground-stationantennasize.WhileorbitingMercuryMESSENGER

will store most o its data when it’s arther rom Earth,typically sending only inormation on its condition andthe highest-priority images and measurements duringregular eight-hour contacts through NASA’s Deep SpaceNetwork. The spacecrat will send most o the recordeddata when Mercury’s path around the Sun brings it closerto Earth.

Guidance and ControlMESSENGER is well protected against the heat, but

it must always know its orientation relative to Mercury,

Earth, and the Sun and be “smart” enough to keep itssunshade pointed at the Sun. Attitude determination– knowing in which direction MESSENGER is acing –is perormed using star-tracking cameras, digital Sunsensors, and an Inertial Measurement Unit (IMU, whichcontains gyroscopes and accelerometers). Attitudecontrolforthe3-axisstabilizedcraftisaccomplishedusing our internal reaction wheels and, when necessary,MESSENGER’s small thrusters.

The IMU accurately determines the spacecrat’srotation rate, and MESSENGER tracks its own orientationby checking the location o stars and the Sun. Star-tracking cameras on MESSENGER’s top deck store acompletemapoftheheavens;10timesasecond,oneo the cameras takes a wide-angle picture o space,compares the locations o stars to its onboard map, andthen calculates the spacecrat’s orientation. The Guidanceand Control sotware also automatically rotates thespacecrat and solar panels to the desired Sun-relative




Spacecrat Hardware SuppliersAntenna Waveguide: Continental Microwave, Exeter, N.H.Battery (with APL): Eagle Picher Technologies, Joplin, Mo.

Heat Pipes: ATK, Beltsville, Md.Inertial Measurement Unit: Northrop Grumman, Woodland Hills, Cali.Integrated Electronics Module (with APL): BAE systems, Manassas, Va.Launch Vehicle: Boeing, Huntington Beach, Cali.Precision Oscillator: Datum Timing Test and Measurement, Beverly, Mass.Propulsion: Aerojet, Sacramento, Cali.Reaction Wheels: Teldix GmbH, Heidelberg, Germany.Semiconductors: TriQuint, Dallas, Tex.Solar Array Drives: Moog, Inc., East Aurora, N.Y.Solar Arrays: Northrop Grumman Space Technology, Redondo Beach, Cali.Solid-State Power Amplifer Converters: EMS Technologies, Montreal, Canada.Star Trackers: Galileo Avionica, Florence, Italy.

Structure: ATK Composite Optics, Inc., San Diego, Cali.Sun Sensors: Adcole Corporation, Marlborough, Mass.Transponder:GeneralDynamics,Scottsdale,Ariz.

Instrument Hardware SuppliersMDIS: Integrator: APL, Laurel, Md.SSG, Inc. (NAC telescope), Wilmington, Mass.Atmel (CCD), San Jose, Cali.CDA Intercorp (lter wheel motor or WAC), Deereld, Fl.Starsys Research (pivot motor), Boulder, Colo.Optimax (WAC lenses), Chicago, Ill.Northrop Grumman Poly Scientic (twist capsule), Blacksburg, Va.

Optical Coating Laboratory, Inc. (heat lters), Santa Rosa, Cali.

GRNS: Integrator: APL, Laurel, Md.Ricor (cooler), Israel.Patriot Engineering (design, analysis, and subassembly o sensors), Chagrin Falls, Oh.Hammamatsu Corp. (photomultipliers), Japan.Lawrence Berkeley National Laboratory (GRS), Berkeley, Cali.Lawrence Livermore National Laboratory (GRS), Livermore, Cali.Space Science Laboratory, University o Caliornia, Berkeley, (GRS).

XRS: Integrator: APL, Laurel, Md.Amptek (components), Bedord, Mass.Metorex (X-ray sensor tubes), Espoo, Finland.

MAG: Integrator and digital electronics: APL, Laurel, Md.Goddard Space Flight Center (sensor and analog electronics), Greenbelt, Md.

MLA: Integrator: Goddard Space Flight Center, Greenbelt, Md.

MASCS: Integrator: LASP, University o Colorado, Boulder.

EPPS: Integrator, common electronics, and EPS subassembly: APL, Laurel, Md.University o Michigan (FIPS subassembly), Ann Arbor, Mich.Amptek (components), Bedord, Mass.Luxel (microchannel plate), Friday Harbor, Wash.Micron Semiconductor (solid-state detectors), UK.

Hardware Suppliers




The MESSENGER Science Team

MESSENGER’s Science Team consists o experts in all elds o planetary science, brought together by their ability tocomplete the science investigations conducted by MESSENGER. The team is divided into our disciplinary groups: (1)

Geochemistry, (2) Geology, (3) Geophysics, and (4) Atmosphere and Magnetosphere, with each team member givenresponsibility or implementation o a particular part o the mission’s science plan.

Principal Investigator: Sean C. Solomon, Director o the Department o Terrestrial Magnetism at the CarnegieInstitution o WashingtonProject Scientist: Ralph L. McNutt, Jr., Johns Hopkins University Applied Physics Laboratory (JHUAPL)Deputy Project Scientists: Brian J. Anderson and Deborah L. Domingue, JHUAPL

Science Team Members:

Mario H. Acuña NASA Goddard Space Flight Center

Daniel N. Baker University of Colorado

Mehdi Benna NASA Goddard Space Flight Center

David T. Blewett JHUAPL

William V. Boynton University of Arizona

Clark R. Chapman Southwest Research Institute

Andrew F. Cheng

JHUAPLDeborah L. Domingue JHUAPL

Larry G. Evans Computer Sciences Corporation and NASA

Goddard Space Flight Center

William C. Feldman

Planetary Science Institute

Robert Gaskell Planetary Science Institute

Jerey Gillis-Davis

University of Hawaii George Gloeckler University of Michigan and University of

Maryland

Robert E. Gold JHUAPL

Steven A. Hauck, II

Case Western Reserve University

James W. Head III

Brown University

Jörn Helbert Institute for Planetary Research, Deutsches

Zentrum für Luft- und Raumfahrt

Kevin Hurley

University of California, Berkeley

Catherine Johnson

University of California, San Diego, and

University of British Columbia

Rosemary Killen University of Maryland

Stamatios M. Krimigis JHUAPL and the Academy of Athens

David J. Lawrence Los Alamos National Laboratory

Jean-Luc Margot Cornell University

William McClintock University of Colorado

Tim McCoy Smithsonian Institution National Museum

of Natural History

Ralph L. McNutt, Jr.

JHUAPL

Scott L. Murchie

JHUAPL

Larry R. Nittler

Carnegie Institution of Washington

Jürgen Oberst

Institute for Planetary Research, Deutsches

Zentrum für Luft- und Raumfahrt

David Paige University of California, Los Angeles

Stanton J. Peale University of California, Santa Barbara

Roger J. Phillips Southwest Research Institute

Michael E. Purucker Raytheon at Planetary Geodynamics Lab,

NASA Goddard Space Flight Center

Mark S. Robinson

Arizona State University

David Schriver

University of California, Los Angeles

James A. Slavin


Ann L. Sprague

University of Arizona

David E. Smith NASA Goddard Space Flight Center

Richard Starr

The Catholic University of America

Robert G. Strom

University of Arizona

Jacob I. Trombka


Ronald J. Vervack, Jr. JHUAPL

Faith Vilas

MMT Observatory Thomas R. Watters Smithsonian Institution National Air and

Space Museum

Maria T. Zuber Massachusetts Institute of Technology




Program/Project Management

Sean C. Solomon o the Carnegie Institution o Washington (CIW) leads the MESSENGER mission as principalinvestigator. The Johns Hopkins University Applied Physics Laboratory (APL), Laurel, Md., manages the MESSENGER

mission or NASA’s Science Mission Directorate, Washington.

At NASA Headquarters, Alan Stern is the associate administrator or NASA’s Science Mission Directorate. JamesGreen is the director o that directorate’s Planetary Science Division. Anthony Carro is the MESSENGER programexecutive and Marilyn Lindstrom is the MESSENGER program scientist. Paul Gilbert is the Discovery Program manager,andAllenBacskayistheDiscoveryMissionmanagerforMESSENGER;theNASADiscoveryProgramismanagedoutofthe Marshall Space Flight Center.

At APL, Peter Bedini is the project manager, Ralph L. McNutt, Jr., is MESSENGER project scientist, Eric Finnegan ismission systems engineer, and Andy Calloway is mission operations manager.

NASA Discovery Program

MESSENGER is the seventh mission in NASA’s Discovery Program o lower cost, highly ocused planetary scienceinvestigations. Created in 1992, Discovery challenges teams o scientists and engineers to nd innovative andimaginative ways to uncover the mysteries o the solar system within limited cost-capped budgets and schedules.

Other Discovery MissionsNEAR (NearEarthAsteroidRendezvous)markedtheDiscoveryProgram’srstlaunch,inFebruary1996.TheNEAR

Shoemaker spacecrat became the rst to orbit an asteroid when it reached 433 Eros in February 2000. Ater collecting10 times the data initially expected during a year around Eros, in February 2001, NEAR Shoemaker became the rst

spacecrat to actually land on an asteroid and collect data rom its surace.

Mars Pathfnder launched December 1996 and landed on Mars in July 1997. The mission demonstrated severaltools and techniques or uture Mars missions – such as entering, descending, and landing with airbags to deliver arobotic rover – while captivating the world with color pictures rom the red planet.

Lunar Prospector orbited Earth’s Moon or 18 months ater launching in January 1998. The mission’s data enabledscientists to create detailed maps o the gravity, magnetic properties, and chemical makeup o the Moon’s entiresurace.

Stardust, launched in February 1999, collected samples o comet dust and provided the closest look yet at a cometnucleus when it sailed through the coma o Wild 2 in January 2004. It returned the cometary dust to Earth in January2006.

Genesis, launched in August 2001, collected solar wind particles and returned them to Earth in September 2004. The samples are improving our understanding o the isotopic composition o the Sun, inormation that will help toidentiy what the young solar system was like.

CONTOUR (Comet Nucleus Tour) was designed to fy past and study at least two very dierent comets as theyvisited the inner solar system. The spacecrat was lost six weeks ater launch, during a critical rocket-ring maneuver inAugust 2002 to boost it rom Earth’s orbit onto a comet-chasing path around the Sun.




Deep Impact, launched in January 2005, was the rst experiment to probe beneath the surace o a comet,attempting to reveal never beore seen materials that would provide clues to the internal composition and structureo a comet. A variety o instruments, both onboard the spacecrat and at ground-based and space-based observatoriesaround the world, observed the impact with the comet and examined the resulting debris and interior material.

Dawn, launched in September 2007 toward Vesta and Ceres, two o the largest main-belt asteroids in our solarsystem, and will provide key data on asteroid properties by orbiting and observing these minor planets.

Kepler, planned or a February 2009 launch, will monitor 100,000 stars similar to our Sun or our years, using newtechnologytosearchthegalaxyforEarth-size(orsmaller)planetsforthersttime.

The Gravity Recovery and Interior Laboratory, or GRAIL, mission, scheduled to launch in 2011, will fy twinspacecrat in tandem orbits around the Moon or several months to measure its gravity eld in unprecedenteddetail. The mission also will answer longstanding questions about Earth’s Moon and provide scientists a betterunderstanding o how Earth and other rocky planets in the solar system ormed.

Discovery also includes Missions o Opportunity – not complete Discovery missions, but pieces o a larger NASA ornon-NASA missions or creative reuses o spacecrat that have completed their prime missions. Those selected to dateor fight include:

• The ASPERA-3 (AnalyzerofSpacePlasmaandEnergeticAtoms)instrumentisstudyingtheinteractionbetweenthesolar wind and the Martian atmosphere rom the European Space Agency’s Mars Express spacecrat, which beganorbiting Mars in December 2003.

• The M3 (Moon Mineralogy Mapper), pronounced M-cube, is one o eleven instruments that will fy on boardChandrayaan-1, scheduled to launch in March 2008. Chandrayaan-1, India’s rst deep space mission, is a project o the Indian Space Research Organisation (ISRO). The goals o the mission include expanding scientic knowledgeo the Moon, upgrading India’s technological capability, and providing challenging opportunities or planetary

research or the younger generation.

• TheEPOXImissioncombinestwoscienceinvestigations–theExtrasolarPlanetObservationandCharacterization(EPOCh) and the Deep Impact Extended Investigation (DIXI). Both investigations will be perormed using the DeepImpact spacecrat, which nished its prime mission in 2005. EPOCh will use the Deep Impact spacecrat to observeseveral nearby bright stars or transits by orbiting planets, and DIXI involves a fyby o comet Hartley 2 in October2010.

• NExT (New Exploration o Tempel 1) will reuse NASA’s Stardust spacecrat to revisit comet Tempel 1, the cometarytarget o Deep Impact. This investigation will provide the rst look at the changes to a comet nucleus producedater its close approach to the Sun. NExT is scheduled to fy by Tempel 1 in February 2011.

For more on the Discovery Program, visit http://discovery.nasa.gov.

MESSENGER Mercury Flyby 1 Press Kit

Documents

Transcript of MESSENGER Mercury Flyby 1 Press Kit