Magellan Mission to Venus

8/8/2019 Magellan Mission to Venus

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INTRODUCTION

Figure I VenusThe National Aeronautics and Space Administration(NASA) mission to map the hidden surface of Venus withthe Magellan spacecraft is the capstone of a grand experi-ment to study the inner solar system. This research, span-ning 25 years, has sent spacecraft to study Mars, the Moon,Mercury and Venus-worlds which share a similar origin.Of these, Venus is most like Earth in size, age, composi-tion, and distance from the Sun. It also exhibits some strik-ing differences. Most importantly, it is the inner planet ofwhich we have the least information.On Venus lie many of the missing chapters in ou r under-standing of the solar systems and Earths evolution. Thegeologically stable worlds of Mars, Mercury and the Moonshow what the interior planets looked like shortly after theywere born. They represent the first 5% of solar systemhistory.

At the other end is the geologically dynamic Earth whichhas continued to evolve and is changing to this day. Itssurface has been completely recycled by plate tectonics 20times. Only the most recent 5% of its geologic record re-mains for us to study.It is like trying to understand an entire book from only thefirst and last chapters.To read the missing chapters on Venus, we must have a clearand detailed picture of the entire surface. That surface hasbeen hidden by a very dense atmosphere with opaqueclouds. Using Magellans radar imaging, we will be able tosee detail that matches or exceeds what we have from anyother planet.So why is it important that we learn about this missinggeological history? As we look at the planets of the solarsystem, we notice that the physical characteristics and prox-imity to the Sun of Venus, Earth and Mars suggest theyshould be far more alike. All began about the same timeunder similar conditions and made of the same materials.The three planets, however, followed very different evolu-tionary paths. Mars lost most of its water and atmosphere tobecome a frigid desert. On Venus, the atmosphere com-pressed and clouded over to cook the planet in an out-of-control greenhouse effect. Only Earth evolved to supportan incredibly rich variety of life.Why did these planets go different ways? That is a funda-mental question the 25-year exploration of the solar systemseeks to answer. It is critical to our understanding of Earthspast and, quite possibly, its future. The Magellan mission isexpected to provide us an incredible wealth of clues to theanswer.


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In the fading light of sunset and dawn, Venus often can beseen sparkling near the horizon as the brightest celestialbody traversing our night sky, excepting the Moon. Alterna-tively called the Evening Star or the Morning Star, it hasbeen a familiar object to poets, astronomers and navigatorsfor aeons. It was undoubtedly recognized by the explorerMagellan on his journey as one of the few familiar stars inthe skies of the southern hemisphere. But for all its familiar-ity; Venus has stubbornly remained an enigma.Of all the planets in the solar system, Venus is Earthsnearest neighbor with an orbit 25.7 million miles away. It isoften referred to as Earths twin or sister because of the twoplanets similarities. They are nearly the same in size, mass,density, composition, age, relative proximity to the Sun,and possession of an atmosphere with clouds.Such similarities once spawned imaginative concepts of asteamy, verdant world. We now know the measured similar-ities probably go no further.

IMean Distance from SunSidereal Period of RevolutionLengthof DayDiameterDensityMassGeologic CompositionSolar Radiant EnergySurface TemperatureAtmosphereAtmosDheric Pressure

Venus Earth67.2 Million Miles224.7 Earth Days 365.26 Days243 Earth Days 24.6 Hours7519 Miles 7926 Miles3.0oz/w in. 3.2 oacu in.0.81 5 x Earth 1Metal 8 Silicate Rock Metal8 Silicate Rock2.3 x Earths 1900 80Carbon Dioxide Nitrogen, Oxygen92 Bars 1 Bar

93 Million Miles

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1961 Veneral USSR1962 Mariner2 US1964 Zondl USSR1966 Venera283 USSR1967 Venera4 USSR1969 Venera 5 B 6 USSR1970 Venera7 USSR

Mariner5 US

Previous planetary missions have revealed a surface temper-ature of 900F, a carbon dioxide atmosphere with sulfuricacid rain, and an atmospheric pressure equal to that 2500 ft.deep in a terrestrial ocean. Venus has no water or watervapor, no moon, and no magnetosphere. The planets retro-grade rotation makes the Sun rise in the west and set in theeast. It turns so slowly that a Venusian day is longer than aVenusian year, with the demarcation between day and nightmoving at the speed of a steady walk.The perpetual cloud cover of carbon dioxide traps heat in agreenhouse effect gone berserk. It also has been an effec-tive barrier to telescopic study that has only been able toobserve the 225-mile-an-hour circulation of upper atmo-spheric clouds.During the space age, there have been 20 trips to Venus bythe U.S. and Soviet Union carrying low-resolution radarand other sensors. A few photographs taken during the brieflives of Soviet landers on the surface have shown tantalizingdetails of a tiny fragment of landscape.Radar from Earth and planetary probes has shown raisedland masses and the suggestion of peaks, volcanos, cratersand canyons. More specific knowledge of our neighbor re-mains veiled behind the drawn curtain of clouds.

1972 Venera8 USSR1974 Mariner10 us1975 Venera 9 8 10 USSR1978 PioneerVenus 1 8 2 USVenerall 812 USSR1982 Venera 13 8 14 USSR1983 Venera 15 B 16 USSR1985 Vega 1 8 2 USSR


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RFCIFPQ-id ~ X P R F ~ ~ T K ~ ~nnw EIH CCCP H t imeFigure 2 SuMace of Ven us Photographed by Soviet Venera 13 and 14 Landers


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the 1520s-a time when people still believed the Sun, plan-ets and stars revolved around an Earth comprised mostly ofland marked on maps as Terra Incognita. His journeyrevealed the vaster nature of Earth and the distribution ofbroad oceans and continents. Similarly, the spacecraft Ma-gellan is expected to provide a global understanding of thepoorly known surface of Venus.NASA began concept studies of a radar imaging probe toexplore the Venusian surface, called the Venus OrbitalImaging Radar (VOIR), in 1971 at Jet Propulsion Labora-tory and Martin Marietta. The project was cancelled in1982 for budget reasons, then reinstated in the 1984 NASAbudget as the Venus Radar Mapper on the understandingthat the spacecraft be built for about half the originallyestimated cost.

Figure 3 Magellan Spacecraft during Tests atMartin MariettaThe wealth of knowledge we expect to acquire about Venus,the inner solar system and Earth will be due, in large part,to a single, extraordinary spacecraft named Magellan carry-ing only one science instrument-radar. Yet it will transmitto Earth more data than all previous planetary missionscombined.In addition to its special contribution to science, Magellanhas a distinctive place in the current U.S. space program. Itis the first planetary mission to be launched by the spaceshuttle. It is the first to use a complex, Sun-circling Type4 trajectory to reach a planet. And it is the first of a seriesof missions resuming deep space research since the launchof the Pioneer Venus probes 11 years ago.Magellan is named after the Portuguese explorer FerdinandMagellan whose expedition circumnavigated the world in

Figure 4 Venus Orbital Imaging Radar (VO IR)Spacecraft

By then, the U.S. had accumulated an inventory of mission-proven technologies and spare components from the Viking,Voyager, Galileo and other planetary research projects.These were scoured for whatever they could provide theVenus project to save the costs of designing equipment fromscratch. The adjacent table shows Magellan subsystems thatare either spares or existing designs updated and modifiedfor Magellan.Also, advances in data processing and other software en-abled a simpler design to perform more complex tasks. Forexample, instead of separate antennas for mapping and te-lemetry, the craft was redesigned to make the high-gain


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Subsystems fr om Other SpacecrafrComponentMedium Gain AntennaHigh Gain AntennaEquipmentBusStar ScannerDesignRF Trawling Wave Tube AssembliesAttitudeControl ComputerCommand andDataSystemThruster RocketsElectric Power ConverterPowerControlUnitGyroscope DesignPyrotechnicControlSolid Rocket Motor DesignPropellant Tank Design

MariWf9MarsVoyagerVoyagerIUSulyssesGalileoGalileoVoyagerGalileoP-80VikingGalileoPAMSpaceShuttle A W

antenna do double duty. With only the loss of a few experi-ments and small compromises in telecommunications, Ma-gellan was on track again.Then the Challenger disaster in 1986 threw another hitch inthe journey to Venus. The explosion led to the re-evaluationand cancellation of the Centaur G-Prime booster for use on

the shuttle. The most powerful upper stage ever designed,Centaur was to have shot Magellan to Venus on a 4-monthtrajectory. Its explosive liquid oxygen and hydrogen propel-lants, however, were deemed toodangerousto be carried ina manned space vehicle.The less powerful Inertial Upper Stage ( IUS) replaced Cen-taur as the booster for Magellan, requiring some modifica-tions to designs and plans. The aluminum Centaur adapterstructurewas replaced with a lighter, graphite-epoxy frame.A lighter spring mechanism could also be used to separatethe less massiveIUS after burnout.The launch procedure was changed to deploy the solar ar-rays before ignition of the IUS because the booster's rollcontrol thrusters were too close to the ends of the solarpanels in their stowed position. Lastly, rather than subjectthe entire spacecraft to a repetition of full-up static tests inthe new IUS configuration, a mockup Magellan was con-structed for the tests. Fidelity was assured by using a realVoyager bus borrowed from a public display at the Smithso-nian Institution.

High Gain Antenna Mar Array

Propulsion ModuleSRMAdapter

x IUS

Figure 5 Magellan Spacecraft


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Figure 6 Voyag er Spacecraft

The loss of the Challenger vehicle and the 32-month sus-pension of shuttle missions delayed and reshuffled manyplanned space activities. The Galileo mission to Jupiter, forone, would have to launch on the date originally set asidefor Magellan or wait another two years for the necessaryalignment of planets to repeat. The result was a late Aprillaunch for Magellan and the first use of a complex, Sun-circling Type 4 trajectory.Thus, the $530 million mission and spacecraft that willlaunch in April are much different than NASA had planneda decade before, yet the scientific objectives that will beachieved remain almost unchanged.

OVERALL STRUCTUREThe Magellan mission vehicle as it is loaded in the spaceshuttle cargo bay is a stack consisting of1) Antennas-high-, medium- and low-gain, plus altimetry,2) Forward equipment module,3) Equipment bus,4) Solar panels,5 ) Propulsion module,6) Solid rocket orbit insertion motor,7) Adapter structure,8) Inertial Upper Stage.

Magellan1It7650 IbIUS I17 ft32,500 Ib

Figure 7 Magellan and IUSA s Carried in ShuttleCargo BayANTENNASThe large, parabolic, high-gain antenna is instrumental toall aspects of the mission:1) transmission and reception of the mapping radar pulses;2) transmission of science data to Earth;3) detection of radiant energy emitted by Venus; and4) reception from Earth of commands directing normal

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spacecraft activities.The dish is made of extremely strong, lightweight graphiteepoxy sheets mounted to an aluminum honeycomb forrigidity.


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Figure 8 Antennas

Suspended on graphite-epoxy struts above the center is thelow-gain antenna. It provides ground teamswith an alterna-tive means of commanding the spacecraft in case of anemergency that prevents use of normal frequencies. An ex-ample would be an unexpected solar flare that interfereswith the mission and command transmissions. Using thelow-gain frequency, mission control can command Magel-lan to suspend mapping and go into a safe attitude, solarpanels facing sunward, until normal communications linksresume.Mounted to one side of the high-gain dish is the altimeterantenna. During the mapping part of each Venus orbit, thisradar antenna is pointed vertically down at the surface toprovide onedimensional readings of the heights of geologicfeatures (the high-gain antenna is aimed obliquely to thespacecrafts direction of travel). The 5-foot-long graphite-epoxy structure has an aperture 2 by 1 feet and weighs 14pounds.At the bottom and one side of the forward equipment mod-ule is the conical medium-gain antenna. It is used for com-mands to and engineeringdata from Magellan during the15-month cruise from Earth.

FORWARD EQUIPMENT MODULEThe forward equipment module houses the synthetic aper-ture radar electronics, telecommunications, spacecraft navi-gation, batteries and p e r istribution controls. The box-

like housing, 5.3 by 3by 4 feet, is made of aluminum panelson a framework of square aluminum tubing that has beenchemically milled for weight reduction. Two sides of thebox have louvers for thermal conditioning. Solar reflectorcoverings shield the louvers from the intense sunlight atVenus.SyntheticApertureRadar (SAR) ensorThis method of seeing is the heart of the Magellan mis-sion. The visually opaque clouds of Venusaretransparent tothe 2.385-gigahertz radio frequency SAR ransmits and re-ceives through the parabolic antenna.In the same way a flashlight pierces the darkness of a roomand reflects off objects to reveal their position, shape andtexture, Magellans antennas are flashlights in an invisiblepart of the electromagnetic spectrum. The antennas alsoserve as eyes that enable the SAR sensor to receive thereflected electromagnetic waves.The altimetry radar receives a narrow angle of reflectedsignals to measure an objects altitude in the same waymilitary radar indicates a targets distance and speed. Pre-cise altitude is determined by the time lapse between a radartransmission pulse and the echos return. Since a radarpulse travels at light speed, the difference is a fraction of asecond. Magellans radar electronics adjust the timing ofradar pulses for spacecraft altitude, which is continuallychanging, to give measurements accurate within 150 feet.The radar used for images receives a much broader beamofreflected signals. Like the flashlight in the dark, radarpointed obliquely to a surface will show more shape andtexture than a head-on aim. As Magellan maps, the largeantenna will be aimed to the side of the spacecrafts trackover the planet, striking the surfice at an angle. The angleranges between 19and 52 with Magellans altitude.Imaging radar acquires twodimensional image data. Onedimension is provided by the time for radar echoes to re-turn, the other dimension is provided by Doppler effect.Doppler effect causes frequency and wavelength to shortenas a surface feature is approaching, and lengthen as itrecedes.Synthetic aperture refers to the size of the antenna for re-ceiving the echoes. The larger the antenna (its aperture), thebetter the image quality will be. Because the antenna movesa distance during the time it receives an echo, it simulatesthe reception of a much larger dish. The distance Magellantravels while a surface feature is within the SARs field ofview for receiving determines the functional size of theantenna.This synthetic aperture dimension varies as the spacecraftsvelocity varies along the eliptical orbit. The greatest speed


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MomentumWheels

thetic Aperture Radar

Figure 9 Forward Equipment Module and Solar Arrays

Figure 10 Altimetry and Imaging Radars

and, hence, greatest aperture and imaging accuracy are atthe periapsis north of the equator where images will have aresolution close to 300 feet.The SAR electronics must also compensate for Magellanschanging speed and altitude from Venus and for large varia-tions in the planet surface with adjustments in transmissionwavelength and frequency.In addition to radar altimetry and imaging, the sensor sys-tem detects radiant energy emitted by Venus and received bythe high-gain antenna.The electronics which perform SAR, altimetry and radiom-etry occupy nearly 20% of the forward equipment modulesvolume. Developed by Hughes Aircraft Company, the sys-tem controls the frequencies of the radar pulses, adjuststhem for movement of the spacecraft, and interprets thereturning reflections as points of brightness values that willconstitute a photo-like image.The sensor system is in 37 modules racked in a 5- by 3- by1-foot enclosure and weighing 340 pounds. It includes 177two-sided circuit boards and 28 multi-layered circuit boardswith 15,000 electronic parts and 22,000 other parts such asresistors and capacitors. The wiring harness has some4500terminations.


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All of the components comprising the system are in redun-dant pairs for insurance. During mapping, the system uses200 watts of electricity-the equivalent of a bright floorlamp. The data are stored in tape recorders located in thespacecraft bus until transmission.

. A Telecommunications

1 \ . p - - B a u e qPower Conditioner .

Figure I I Forw ard Equipment M odule Electronics

TelecommunicationsThe ability to acquire detailed knowledge of Venus surfacedepends as much on Magellans ability to send largeamounts of data to Earth as on the radar equipment. Most ofthe communications equipment for receiving, sending anddecoding radio signals is located in the forward end of theequipment module. Its NASA-standard transponder andtraveling wave tube assemblies enable Magellan to transmitat a peak rate of 268.8 kilobits per second. In comparison,the Viking orbiter in 1976 could transmit its detailed imagesof Mars at 2 kilobits per second.

Stellar navigationProjecting from one side of the forward equipment moduleis the barrel of the star scanner. This optical navigationdevice will locate two stars that pass through its field ofview at approximately 90 rom each other and the space-craft. These are compared to an on-board computer map ofpositions and luminances and adjust the gyroscopes to atriangulated position in space. The star scanner and attitudereference unit are mounted on a brazed beryllium opticalbench inside the equipment module.Attitude control-Magellan is a 3-axis stabilized craft re-quired to perform almost continuous changing of its orienta-tion in space as it circles Venus. This maneuvering is per-formed using gyros, thrusters and momentum reactionwheels controlled by a redundant pair of ATAC-16 com-puters in the spacecraft bus.On each orbit of Venus, Magellan will rotate four times:away from the planet to aim its antenna Earthward, towardspace to scan stars for calibrating exact position, again to-ward Earth to resume data transmissions, and back towardthe surface of Venus for mapping. Throughout each map-ping pass, the spacecraft continuously maneuvers in smallincrements to adjust the angle of the antenna to the changingcurvature of the planet below.

From Venus orbit, the system will send to Earth engineeringdata on the spacecrafts condition at 115.2kilobits per sec-ond through the medium-gain antenna, or at 1.2kilobits persecond through the high-gain antenna simultaneous with thescience data. Engineering data can be transmitted in realtime or from memory. Other telemetry downlink and com-mand uplink capabilities use different data rates and themedium-gain antenna. The low-gain antenna is reserved forreceiving emergency commands from Earth at a rate of 7.8bits per second.

Figure 12 Magellan O rbital Maneuvers fo rMapping and TransmissionIn the primary mission, the 1852 orbits require 7408majorattitude changes in 243 days. If these attitude changes weredone solely with thrusters, there would be more than 14,800thruster bums for each of the spacecrafts three control axes(one to start a maneuver, another to stop it, and periodicbums to control the rate). Magellan would need immensefuel tanks, indeed!


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However, Magellan is very miserly of the fuel in its single,small propellant tank. The repetitive attitude changes areaccomplished with reaction wheels that use a principlecalled transfer of momentum. To illustrate, imagine rapidlystirring a glass of iced tea with a spoon. When released, thespoon will begin to spin around the glass with the water.The water was given momentum by the stirring spoon, thentransferred some of that momentum back to the spoon.On Magellan, small reaction wheels spinning very fast im-part some of their rotational momentum to turn the space-craft. The spacecraft is stopped or reversed in its rotation bytransferring momentum back to the reaction wheels in theform of wheel speed. Three momentum wheels-one foreach axis of rotation-are located in the forward equipmentmodule.In theory, this system could work on its own forever. Butbecause of outside forces on Magellan, the stored momen-tum in the wheels will become inaccurate over time. Leftuncorrected, the probe would err increasingly in the aim ofits antenna. Therefore, thrusters are fired briefly once eachorbit to restore the momentum wheels to their proper speed.Gyroscopes in the attitude reference units (ARUs) providereference measurements for the amounts of rotation to becommanded, and tachometers on the reaction wheels deter-mine the amount of thruster firing to desaturate the mo-mentum buildup.Electrical power-The probe operates on 28 volts DC fedthrough a power conditioning unit in the equipment modulefrom either the solar arrays or a pair of nickel-cadmiumbatteries. Either battery could handle spacecraft power re-quirements alone should the other fail.Solar panels-The two square solar panels, 8.2 feet on aside, can supply 1200 watts. With the arrays deployed, Ma-gellan spans 30.6 feet from tip to tip of the panels. Thelight-colored lines visible on them are solar reflectors tokeep the temperature of the arrays below 239F even in fullsun. Approximately 35% of the surface is reflectivemirrors.The panels are hinged for stowage in the shuttle and de-ployed while in Earth orbit. At Venus, they rotate to followthe Sun. Solar sensors on the panel tips and a control pack-age in the multi-sided equipment bus maintain their sunwardorientation. The honeycomb aluminum backing structure,arms and oversized joints enable the panels to withstandforces up to seven times Earth gravity that will be producedby the Venus orbit insertion bum.BUSImmediately below the large box of the forward equipmentmodule is the 10-sided spacecraft bus built as a spare for theVoyager project. The bus is a bolted aluminum structure

with aluminum cover plates. The bus is 16.7 inches highand approximately 6 feet across. Each of its ten compart-ments has enclosures for electronics 16.4 by 18.7 by 7inches. The opening in the middle of the ring holds thehydrazine fuel tank for the liquid propulsion system.

Tape Recorder Tape Recorder,

Computer.Interface

/SolarArrayControls

nand CSyster

P&er Distribution,Conditioning

Figure 13 SpacecraftBus

The bus compartments contain computers, the input/outputinterface between the computers and Magellan subsystems,tape recorders, solar array controls, bulk data storage, andsolid motor separation controls.One minute, nonfunctional but intensely meaningful ele-ment affixed to the bus exterior is a microdot about the sizeof a large postage stamp. On it are the signatures of some10,OOO people who have been involved with the Magellanprogram during its life.Computers-Magellans brains are ATAC- 16 computersand the distributed command and data system. All com-puters are in redundant pairs as insurance against a break-down; all are fully reprogrammable; and all are modifiedGalileo equipment.The command and data system (CDS) decodes and distrib-utes commands received from Earth stored in its bulk mem-ory or tape recorders. It also prepares science data for trans-mission. Attitude and articulation commands received fromEarth feed through the data bus to the ATAC-16 computers.These regulate the position of Magellan and its back-and-forth changes between data gathering and transmitting.


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Data storage-Redundant tape recorders provide storagefor 1.8 gigabits of radar data. In addition to tape storage, abulk storage unit holds 5kilobytesof engineeringdata n theevent a real-time transmission of telemetry from Magellanis interrupted. All commandsto the spacecraftare stored forexecution at later, specified times. There are no real-timecommands.Pyrotechnical controls-Attached to the underside of onebus compartment is a box containing the control electronicsthat arm, disarm and fire various explosive bolts, pin-pullers and other mechanisms. These enable separation ofthe spent solid rocket motor and release of the solar panelsfrom their stowed position.PROPULSIONPropulsion equipment that is part of the Magellan spacecraftincludes the solid-fuel Star 48B rocket and a 24-thrusterliquid propellant system. The propulsion module structureprovides precisely aligned attachment of the solid motor, aswell as the liquid propellant thrusters and associatedplumbing.Solid motor-The Star 48B is the same motor that has beenused to send commercial communications satellites into

geosynchronous Earth orbit. The B denotes a motorusing the newer, light-weight, graphite-composite thrustcone. The motor weighs 4721 pounds, of which 4430pounds is propellant.The solid rocket motors 1.3 million pounds of thrust willdeflect Magellans trajectory into orbit around Venus. Priorto installation in the space shuttle, the motor is aligned withthe spacecrafts center of gravity to within 0.1 inch foraccurate direction control and to prevent the spacecraft frombeing tumbled.Liquid propellant system-The 24 multipurpose liquidpropellant thrusters provide guidance and attitude control.Pbsitioned in the middle of the multihceted spacecraftbusis the singletankcontaining 293 pounds of hydrazine mono-propellant. A helium tank is attached to the struts of thepropulsion module structure and will be used to offset adrop in the pressure of the hydrazine system that reducesthruster efficiency. The pressurant will be particularly im-portant if Magellans trajectory requires a major comctivefiring of the thrusters that drops the system pressure.At each of the four outboard tips of the propulsion structureare clusters of six thrusters: two 100-pound, one 5-pound,and three 0.2-pound thrust. The large motors, aimed aft,

PROPULSION MODULE

IUS Adapter

Figure 14 Propulsion Elements


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are used for steering in the midflight course corrections andthe Venus orbit insertion maneuver. The 5-pound motors,aimed sideways to Magellan's centerline, stabilize thespacecraft from rolling during the same maneuvers.For the duration of the mapping mission, the tiny 0.2-poundmotors provide thrusts to desaturate the momentum wheelsand can be used for attitude control if required. Eight pointaft and four are aimed for roll control.The propulsion module also provides the attachment pointsfor the IU S adapter structure. Both structures are made ofgraphite-epoxy trusses with sculptured titanium end fittings.Explosive bolts release the adapter after IUS burnout andthe Star 48B motor when it is spent.

THERMAL CONTROLMagellan will be subjected to sunlight 2.3 times that whichreaches Earth, potentially for several years. Shaded exteriorspacecraft temperatures can plunge to -400F Throughoutthe mission, the constant maneuvering of the probe willresult in every facet being subjected to the ranges of heatand cold. Special effort was required for thermal controlthat would keep electronics from frying and moving partsfrom freezing, while adding little weight or need for electri-cal power.Electronics housings are wrapped in multi-layered thermalblankets that insulate and reflect light (see detail). The outerlayer of the blankets is a material called astroquartz. It issimilar to glass fiber cloth, but more durable to withstandintense solar radiation. In fact, chemical binders normallyin astroquartz to control flaking had to be baked out whentests showed that the light intensity at Venus could discolorthem and eventually cause a buildup of heat.The high-gain antenna, low-gain antenna struts and propul-sion module structure are painted with a special, inorganicwater-based paint developed by NASA's Goddard SpaceFlight Center to withstand intense solar light without discol-oring. Electronics compartments in the forward equipmentmodule and equipment bus have louvers which can open orclose to regulate the dissipation of heat from inside thespacecraft. Covering these openings and strips of the solararrays are thin mirrors to reflect sunlight. The mirrors havebeen etched to diffuse reflections that could bake someother exterior part of the spacecraft.The net effect of these materials is that the probe will tendtoward cold temperatures rather than hot. To assure thatsome cold-sensitive components do not become too cold,electric heaters and flexible heater blankets have been in-stalled inside housings or wrapped around fixtures, such asthe solar panel articulation bearings.

Asboquam Glued toAluminized Kapton FilmThree Layers ofCrinkledAluminized Kapton

Eight Layers of Dacron Ne 'Alternated with SevenLayers ofAluminized Myla

Three Layers of CrinkledAluminized KaptonFilter ClothAluminized Kapton

Figure 15 Thermal Blanket Construction

I

Figure 16 Thermal Testing of Spacecraft


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SOFTWAREMagellan is a spacecraft that can do a lot of its own thinkingif problems arise. In the past, space missions often pro-duced huddled conferences of experts on the ground to de-termine ways of working around a malfunction. Taking ad-vantage of advances in fault detection software design,Magellan itself can analyze problems that occur and carryout a series of alternative remedies.Minor or slowly developing problems revealed by telemetrywill be managed by ground control. However, time-criticalor mission-critical malfunctions will be detected, analyzedand dealt with by twoon-board fault protection systems: onesoftware system for attitude control and the other for the restof the spacecraft.Problems in the attitude control are treated holistically ina full-system health analysis to ascertain the integral causeand remedy. Other spacecraft malfunctions are managed onan individual basis by software in the command and datasystem. Only if all efforts fail will Magellan turn, aim itsantenna Earthward and call for help.Although some of the spacecraft control software was inher-ited from the Galileo Jupiter mission, most is new because

of differences in the control system and mission. Ninetypercent of the 6OOO lines of attitude control software is new,including 2000 lines for fault protection. The 18,000 linesof code for the command and data system is 20% new and35% is modified Galileo code. Its fault protection softwaretotals 1500 lines.Spacecraftoperation is controlled for several days at a timeby commands sent from Earth and stored for playback. Thismethod requires very accurate navigational data that is up-dated frequently.Control of the radar system is performed with Radar Map-ping Sequencing Software (RMSS). This and attitude con-trol commands are in a simplified form on mission controlcomputers in Denver and at NASAs Jet Propulsion Labora-tory. For most commands, engineers simply select from alist and add parameters. The ground computers convert thisinto the Magellan system codes for transmission.During the interplanetary cruise, these commands will spanup to eight days of activity. During mapping, four days ofcommands are sent and updated every three days. The extraday provides a safety buffer for a delay in communications.

.

Figure 17 Magellan Spacecraft in Preflight Checkout at Kennedy Space Ce nter


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The Magellan mission is the first of a spectacular series ofevents marking the return to deep space exploration by theUnited States. Following it are NASA launches of the Gali-le0 Jupiter mission and the Hubble Space Telescope. In1992, the Mars Observer spacecraft will be launched. Thelast deep space exploration mission was the launch of thePioneer Venus probes in 1978.Magellan was previously scheduled for launch in April of1988, with amval at Venus four months later. However, thatdate came into conflict with the rescheduling of the Galileomission to Jupiter. Launching Magellan in April requires alonger flight to Venus but allows both missions to take ad-vantage of optimal planetary positions that will not be re-peated for two years.Magellan now will use a more complex '"Qpe 4" interplan-etary trajectory in which the probe circles the Sun one and ahalf times in a 15-month game of catch-up with Venus. Thelonger route to Venus requires less thrust to depart Earthorbit and delivers Magellan to Venus at a lower and moremanageable velocity.LAUNCHMagellan is scheduled for an April 28, 1989 liftoff. Thatdate is the first of 29 consecutive days available for launch,determined by the relative positions of Earth, Venus and themoving spacecraft over 15 months. Each day has a brief

launch window when Kennedy Space Center moves with theEarth's rotation into the position from which the shuttle canattain the orbit Magellan must use to reach Venus. Thiswindow grows from about 18 minutes to about 90minutesand shrinks again over the 29 days.

ai Venus At- rthOrbit /Arrival'h uaust 10.1990

VenusAt FirstOrbitRendezvousOctober198Z

Figure 19 Heliocentric "Type4" Trajectory

Open Cargo Tilt Up Deploy IUS ign. IUS sep.Shuttle Bay v v v v vI 2 I 3 I 4 I 5 I 6rbit No.Hour AfterLaunch I I I I I I I iI 2 3 4 5 6 7 8I v TrajectoryCorrectionBurns

v vVenusOrbitInsert StartMappingv v v

Launch p-nterpanetary Cruise1alibrationU' APr I May''I'pr I May I Jun I Jul I Aug i

1989 1990Venus Orbit SuperiorInsertion ConjunctionV V

Extended ConjunctionMission Gap RecoveryV Vp- Primary Mapping Mission-44 1

Figure 18 Mission Timelines


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Figure 20 Deploymentfrom Space ShuttleAlthough Magellan could be launched any day of the four-week period, its amval at Venus must fall within a 3-dayrendezvous window in early August, 1990. Midflightchanges of the spacecrafts trajectory will adjust for theactual launch date to time the rendezvous for August 10.Magellan is camed in the cargo bay of the orbiter Atlantisattached to an Inertial Upper Stage ( I U S ) that provides thevelocity necessary to escape Earths gravity and reach Ve-nus. At liftoff, the combined Magellan-IUS payload sits ona support cradle mounted to the walls at the aft end of thecargo bay. Umbilicals provide power and data links to theprobe while it is aboard the orbiter.Magellans deployment begins six hours after liftoff whenthe shuttle orbiter is in the fifth revolution of a 160-nautical-mile orbit inclined 28.85 O above the equator. The Magellan/IUS assembly is tilted on the cradle to a 58 angle. Groundand orbiter crews perform a final systems check, thensprings on the cradle eject the craft. As Magellan and Atlan-tis move apart, the spacecrafts solar panels unfold.About an hour later, when the orbiter has maneuvered a safedistance away, the two-stage IUS rocket is fired to begin thevoyage. The IUS-powered acceleration is completed 6.7

minutes later. The booster and Magellan then separate,coasting together for 8.75 minutes before guidance thrusterson the IUS fire to move it out of the Magellan orbit.

During the interplanetary cruise, Magellan will become aman-made planet circling the Sun in an eccentric orbit tran-secting the orbit of Venus. The orbits first intersect fourmonths after launch when Venus is elsewhere on its pre-scribed path. Eleven months later, when the orbits crossagain, Venus will be at th e rendezvous point.

Figure 21 Magellan Cruise Conjiguration

Throughout this phase of the mission, monitoring of thespacecrafts condition and control of its systems will beperformed at the Martin Marietta Astronautics facility inDenver. A mission control room there will be connectedthrough the Jet Propulsion Laboratory to NASAs DeepSpace Network (DSN) of radio communications dishes lo-cated in the Mojave Desert of California, at Canberra, Aus-tralia and at Madrid, Spain.A Magellan spacecraft simulation lab, built at the Denversite during the two-year hiatus of shuttle flights, will enablethe mission operations team to troubleshoot problems thatmight arise.


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During the long voyage around the Sun, there are threecritical bums of the probe's liquid-propellant guidance andattitude control thrusters. These correct the trajectory forthe rendezvous with Venus. The bums are directed by theDenver mission control center using stellar navigation mea-surements made by the spacecraft, combined with signalDoppler shift and vector analysis performed on the ground.The first bum occurs within a few days of launch; the sec-ond a year later; and the third occurs ten days before theVenus rendezvous.Four months after the Magellan launch, the Voyager 2 probewill shoot past Neptune, providing man's first close look atthat planet and Voyager's last look at our solar system.Although the DSN will be handling signals from both mis-sions, conflicts are unlikely because Magellan only requiresan engineering data exchange every several days.On arrival at Venus, August 10, 1990, the solid propellantrocket is fired to insert the spacecraft into its elliptical,polar mapping orbit and then is ejected. This critical eventoccurs when the spacecraft is on the side of Venus awayfrom Earth and communications are blocked by Venus. Justbefore the communications blackout, the flight control teamwill transmit the commands for automated firing of therocket. Even when the spacecraft is clear of Venus, the dataconfirming the success of the bum will take 20 minutes atthe speed of light to reach Earth.The final orbit comes to within 155 miles of Venus at itsclosest point (periapsis) 10" north of the equator, and is4977 miles away at its most distant point (apoapsis). Magel-lan completes one orbit every 3.15 hours.

For approximately 18 days, the spacecraft performs a seriesof position adjustments and instrument calibrations. Thisincludes a test of the synthetic aperture radar that will pro-vide the mission's first picture data of the Venusian surface,scheduled for transmission August 22, 1990. Mapping be-gins at the completion of this period.MAPPING MISSIONMapping for the initial mission lasts one Venusian day-243Earth days. As Magellan flies in its north-to-south polarorbit, Venus rotates slowly underneath. Each of the 1852passes of the spacecraft will allow the SAR to image a swathof landscape 10 to 17 miles wide and 10,OOO miles longduring a 37-minute mapping pass.As the spacecraft passes over the south pole, it stops map-ping and begins a maneuver to point its antenna at Earth.For about one hour, it transmits data back to the DSN anten-nas. A second maneuver at the orbit's apoapsis aims a starscanner to locate a pair of reference stars that are 90" to theprobe. The locations are compared to a star map in Magel-lan's memory to verify its position in space.After the star calibration, the spacecraft re-orients itselftoward Earth and resumes data transmissions for anotherhour. As it passes the north pole of Venus, yet anothermaneuver places it in mapping position again.Every three or four Earth days, the probe will receive up-dated instructions from mission control during the data

VOI Burn LocatioMapping Orbit Periapsis[lOoNorthLatitude)

Figure 22 Mapping Orbit Insertion

Sequence Start/ Turn (5.0 min)

IPlayback iurn (6.0 rnin)(56.6rnin)

Figure 23 Mapping Orbit Operations


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transmission periods. These instructions fine-tune mappingin the next four days based on what came before and what isknown from existing Venus information. There are 950commands in a typical upload of mapping instructions.The surface swaths scanned by the SAR overlap, particu-larly at the north pole. Therefore, the swaths will be stag-gered with every other one beginning slightly below thenorth pole and extending closer to the south pole. This willreduce excessive overlap and provide some surface informa-tion from the southernmost parts of the globe.When the radar system is in a passive state-not sendingsignals for SAR or altimetry-it can detect differences inthe thermal microwave energy radiated naturally by Venus.During the primary mission, radiometry measurements willbe taken after each SAR pulse to produce a global tempera-ture map. Surface temperature variations provide valuableclues to the composition of materials and chemicalprocesses.The map of Venus produced by the 243- primary missionwill have unavoidable gaps amounting to approximately10% of the planets surface. There will be a small, circularvoid around the south pole of Venus because Magellansorbit and the time requirements of alternating mapping anddata transmission result in that areas exclusion.A larger, pole-to-pole gap is caused by a superior conjunc-tion three months into the mission when Venus and Earthare on directly opposite sides of the Sun. The Sun willcompletely block or seriously degrade communications toand from Magellan for several days. The length of interfer-ence will depend on solar flare activity. During the superior

Figure 24 Superior Conjunction

conjunction, Magellan may be ordered to go into a safeposition where the spacecraft ceases maneuveringto lock itssolar arrays toward the Sun until it receives the command toresume mapping.An irregular area in the southern hemisphere will also beleft blank in the initial mission. This will be caused by theoccultation (blockage) of the data transmission by Venuswhen it lies between Magellan and Earth for several minuteseach orbit over a period of a b u t a week.In addition to these predictable gaps, scientists and missionplanners recognize the possible loss of another 20% due tothe unforeseen.EXTENDED MISSIONMost of the missing 10-30% of the Venusian map is tempo-rary because it may be filled on an extended mission. Dur-ing subsequent 243-day cycles, the relative positions of theplanets and the occultations will have changed sufficientlyfor Magellan to fill in the largest holes as it continuesorbiting.The extended mission will also permit stereoscopic imagingof areas that have drawn particular interest. The probe needonly scan a previously mappedarea from a slightly differentangle or angles to produce 3-dimensional pictures.Gravity measurements occur after the conclusion of map-ping since they require Magellan to transmit radio signals toEarth while the spacecraft is close to the planet-just theopposite of the mapping mission. Through a techniqueknown as radio interferometry, the signals received by theDSN are used to detect and map slight irregularities in theorbit caused by gravity variations. Large features with cor-responding mass cause the spacecraftto accelerate and gainaltitude. Surface depressions do the opposite.An extended mission could last six to nine Venusian days(four to six Earth years) or more. The limited commoditycontrolling its duration is the propellant necessary for theattitude control thrusters to aim the spacecraft for mappingand data relay.Magellan Missw n Events Schedule

1990

1991

April 23August 1August 10August28November 2August 10-28

April 18Julv 3

Course CorrectionCourse CorrectionVenus Orbit Insertioninstrument Calibration (LTestBeginMappingSuperior ConjunctionEnd of Nominal MissionStart ExtendedMissionRecoverSumriorConiunctionGao


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Superior Conjunction G ap28deg

Figure 25 Gap s in Coverage of Ven us

Figure 27 Valles Mann eris on Mars in Computer-Simulated 3-0 ro m Stereo Images

Figure 26 Planned Coverage of Ven us (SouthPolar)


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SCIENCESince Galileo's discovery of its clouds and umbral phaseslike those of Earth's moon, Venus has been very reluctant togive up her secrets. Earth-based observations revealed therapid movement of the clouds and an upper atmospherictemperature of 125"EIn recent years, a score of U.S. and Soviet space probesmeasured surface temperatures that would melt lead, sulfu-ric acid droplets at high altitudes and nearly pure carbondioxide of intense pressure at low altitudes, and what ap-pears to be lightning flashes possibly caused by volcanicactivity.Venus has been mapped by Pioneer 12 radar altimetry at 62miles resolution. For example, a mountain range lo00 mileslong and 100 miles across would be represented by 30evenly-space altitude measurements. Nonetheless, Pioneerproduced a map that revealed lowlands, highlands, andmountain ranges.Since greatest variations in surface shapes appear in thesehighlands north of the equator, the periapsis of Magellan'sorbit is north of the equator where it will provide its mostdetailed images.In its primary mission alone, Magellan will collect moreimage data than all previous deep space missions combined.SAR images and altimetry will cover 70-90%of the surface,with a high probability of 90% coverage in an extendedmission of mapping. The amount and resolution of the datawill produce a more detailed global picture of Venus thanexists for Earth, since much of Earth's geologic surfacefeatures are hidden or obscured by oceans.Magellan will collect four kinds of planet-wide data:1) Photograph-quality surface images revealing features as

small as 300 feet across,

Figure 28 Pionee r Altimetry M ap of Venu s

2) Altitude topography accurate to 100 feet,3) Gravity variations,4) Radiometry of surface temperatures.The completed Magellan spacecraft's capabilities exceedthe science requirements established at the start of the pro-gram that specified 70% coverage at image resolution of1640 feet and altimetry resolution to 164 feet.By itself, detailed, global imagery of Venus will tell vol-umes about the planet's geology, environment, and evolu-tion. In general, radar images will show smooth surfaces asvery dark, while surfaces of rough, fragmented rock will bebright.

Figure 29 Earth As Seen at Pioneer ResolutionPictures may indicate, as they have on Mars, whether wateronce existed on the surface. By counting meteor impactcraters, scientists can determine the surface age, the activityof any volcanic features and the history and character oferosion. The images should reveal what global forces, suchas plate tectonics or volcanism, prevailed in forming theplanet.By combining the various data, even more knowledge canbe acquired from the Magellan mission.SAR image data and the altimetry will be combined in largemosaics used to produce detailed topographic maps of theplanet. Initially, the entire planet will be charted on 20 mapsof low resolution. Areas of special interest will be selectedfor 220 high-resolution mosaics covering 15% of the Venu-sian surface. These maps will reveal surface features ofselected areas to the finest detail Magellan can provide.The altimetry data will be matched to a refined gravity mapof the planet, enabling scientists to equate surface features

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Figure 30 Oregon and Washington Area A s Seenat M agellan Rad ar Resolution

to variations in planet density that slightly affect its gravita-tional pull. From this, interior geological forces can beinterpreted.Combining radiometry data with surface imagery will pro-vide clues to the chemical composition of the surface andchemical processes occurring on and above the surface.Scattering of the radar signals by the surface will furtherindicate composition.This knowledge will fill a gap in the broader understandingof the inner solar system, including Earth. It will give us avastly clearer view of how and why planets are different, themechanisms that render worlds inhospitable, and the limitedconditions under which life has survived and evolved onEarth.

KEY PERSONNEL:Dr. Lennard Fisk-NASA Associate Administrator forSpace Science and ApplicationsDr. William Rotrowski-NASA Manager,Magellan ProgramJohn Gerpheide-Jet Propulsion Laboratory,Magellan Project ManagerCharles Brown-Martin Marietta Space Systems,Magellan Project Director

MANAGING AGENCY:National Aeronautics and Space AdministrationJet Propulsion LaboratoryPasadena, CA 91 103(213) 354-501 1

CONTRACTORS-Major Elements:Ball AerospaceBoeing AerospaceLEOSMorton ThiokolMotorolaOdeticsRalph M. Parsons Co.

Rocket Research Corp.SingerSpectrolabS p e wTRW-PSI

Star ScannerInertial Upper StageElectric Power SubsystemStar 48B Solid Rocket MotorBase Band Data ProcessingTape RecordersSolid Rocket Motor Adapter,Thruster Mounts, Solar PanelRocket Engine ModulesAttitude Reference UnitsSolar ArraysReaction WheelsPropellant Tank, Pressurant Tank

supports

CONTRACTORS-Prime:Martin Marietta Astronautics GroupP.O. Box 179Denver, CO 80201(303) 977-5364ResponsibilityOverall spacecraft design, development, assembly and test-ing; also launch operations and mission operations supportat control centers in companys Denver facility and the JPLcontrol center in California. Contract value to date: $216million.Hughes Aircraft CompanySpace and Communications GroupGroup CommunicationsP.O. Box 92919Los Angeles, CA(2 13) 648-0884ResponsibilityDesign, development assembly and testing of synthetic ap-erture radar electronics and altimeter antenna. Contractvalue to date: $99.7 million.

Magellan Mission to Venus

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