Lunette: An Affordable Canadian Lunar Farside...

2005 September 222005 International Lunar Conference

Toronto, Ontario1 2005 September 222005 International Lunar Conference

Toronto, Ontario1

Lunette: An Affordable Canadian Lunar Farside Gravity Mapping Mission

Kieran A. Carroll, Gedex Inc.Henry Spencer, SPSystems

Jafar Arkani-Hamed, University of TorontoRobert E. Zee, UTIAS Space Flight Laboratory


Toronto, Ontario2

Lunar Gravity Mapping Team

• Jafar Arkani-Hamed:– Lunette science team PI, University of Toronto Physics Dept

– Geophysicist/planetologist, Lunar morphology researcher, mascon specialist

• Gedex:– Geophysics exploration systems engineering company

– Developing a new-technology airborne gravity gradiometer for terrestrial mineral, oil and gas exploration

• UTIAS Space Flight Lab:– Bus contractor for MOST and NEOSSat

– Canadian pioneer in nanosat development: CanX-1 launched, CanX-2 nearly complete, CanX-3/4/5 in development

• Henry Spencer, SPSystems:– Software architect for MOST, NEOSSat microsats

– Mission architect for CRAFTI and PARTI small-body microsat-class missions


Toronto, Ontario3

Overview:Gravity Mapping for Lunar Exploration

• A valuable terrestrial geophysics exploration tool

• Has been pursued previously for Lunar exploration

• What’s new? – Lunar exploration is happening again!

– Technology advances enable higher resolution, lower mass/cost for Lunar gravity data collection:

• Practical terrestrial mobile gravity gradiometer instruments

• Microsat/nanosat technology


Toronto, Ontario4

Use of Gravity in Exploration

• Used by geophysicists to help understand what lies below the surface, e.g., mineral deposits, oil/gas bearing strata

• Deposits with density different from surrounding material produce anomalous gravity signature

• Example: a multi-billion-dollar diamond-bearing kimberlite pipe can be completely hidden by a few meters of over-burden, but easily detected with a suitable gravity instrument

• Optical instruments see only the surface; gamma-ray spectrometers and radar can see some depth below, but are attenuated by surface material

• Advantage: Nothing blocks gravity!

• (Disadvantage: Nothing blocks gravity…)

• Lunar application: “lumpy” sub-surface ice deposits, buried NiFemeteorites…


Toronto, Ontario5

Gravity of a Uniform Sphere

GMm/r2


Toronto, Ontario6

Gravity Anomalies


Toronto, Ontario7

Ground-Based Gravimetry

Good accuracy, but slow and expensive to collect!


Toronto, Ontario8

Airborne Gravimetry

• 1 km deep spherical deposit• 1x1011 kg (100 MT) excess mass • Fly-over at 100m altitude

• Gravity Units:– 1 Gal = 0.01 m/sec2 = 1 milliG– 1 mGal = 10-5 m/sec2 = 1 microG

• Peak anomalous vertical gravity component: 0.55 mGal (5.5x10-6

m/sec2)

Vertical Component of Anomalous Gravity (mGal)

0.000.100.200.300.400.500.60

-4 -3 -2 -1 0 1 2 3 4

Distance to mascon encounter (km)

Gra

vity

(m

Gal

)


Toronto, Ontario9

Airborne Gravity Gradiometry

• Gravity Gradient Units:– 1 Eo = 10-9 m/sec2/m

• Peak anomalous gravity gradient anomaly: ~10 Eo (1x10-8

m/sec2/m)

• 1 km deep spherical deposit• 1x1011 kg (100 MT) excess mass • Fly-over at 100m altitude

XX, ZZ and XZ Components of Anomalous Gravity Gradient

-12.0

-10.0

-8.0

-6.0

-4.0

-2.0

0.0

2.0

4.0

6.0

-4 -3 -2 -1 0 1 2 3 4


XX

an

d Z

Z G

ravi

ty G

rad

ien

t (E

o)

XX Gravity Gradient (Eo)

ZZ Gravity Gradient (Eo)

XZ Gravity Gradient (Eo)


Toronto, Ontario10

Types of Gravity Instruments

• Gravimeter/ Accelerometer– Approach #1: mass on a spring.

Measuring deflection measures one component of gravity force vector (out of 3).

– Approach #2: track a satellite from a distance (Clementine, LP, SELENE)

– In both cases, need to measure and subtract out base motion

• Gravity Gradiometer– Measures spatial rate of

change of one or more components of gravity vector

– Approach #1: pair of adjacent accelerometers (GOCE)

– Approach #2: orthogonal pair of “gravity gradient booms” on torsional springs

– Approach #3: track one orbiting satellite from another nearby satellite flying in formation with it (GRACE)


Toronto, Ontario11

Gedex Cross Component Gravity Gradiometer


Toronto, Ontario12

Lunar Gravity Mapping Background

• Robert L. Forward (Hughes Research Lab): – Proposed Lunar gravity gradiometer for Apollo, Lunar Polar Orbiter

(1965-75)

• Sjogren/Arkani-Hamed, JPL: – Apollo 18 sub-satellites

• Apollo, Clementine, Lunar Prospector radio tracking• Discovery of MASCONs• Nearside gravity map developed to medium accuracy• Various proposals for multi-hundred-million-$ Lunar gravity

mapping missions (e.g., ESA’s MORO), none yet flown• JAXA’s SELENE to fly in a low Lunar orbit with a high-orbit

relay satellite


Toronto, Ontario13

Lunar Gravity Mapping for Science

• Fundamental questions regarding the Moon’s formation• “Nearside/farside dichotomy” (crust thin on nearside, thick on farside)• Nature of mass concentrations (mascons) below the maria and some craters• Enables improvement in topography modeling from laser altimeter data• Key to understanding Lunar geological formations:

– These in turn are key to prospecting for high-grade mineral deposits, by understanding if mare basalts originate near the surface or from the mantle

• NASA HQ Code S sees Lunar gravity mapping as a priority• Nearside maps complete to 10-20 mGal level• Farside maps all inferred from nearside tracking data:

– Using multiple eclipse entry/exit conditions– Questionable assumption: that farside spatial gravity variations are similar to

those on nearside– Resulting model is likely inaccurate


Toronto, Ontario14

Current Lunar Gravity Models(Lunar Prospector)

Anomalous Gravity (mGal at surface)

Crustal Thickness (km)

Topography (km)

• Accurate topographical mapping requires an accurate gravity model

• Geological models are derived from both topography and gravity models


Toronto, Ontario15

Lunar Gravity Mapping for Exploration

• Near-term:– Map farside mascons to allow precise navigation

during unmanned landings

– Improve Lunar geodetic model to enable precise topographical mapping from orbit

• Medium-Term:– Provide geological context to support base site

selection

– Help identify areas with potential high-concentration ice/mineral deposits

– Terrestrial analogy: “sovereignty mapping” (government surveys) with results disseminated

• Long-term:– Detailed site surveys to identify drilling/ excavation

targets: drilling/digging holes is very expensive!

– Terrestrial analogy: airborne geophysical surveys of claims

Iron Oxide

Titanium Dioxide


Toronto, Ontario16

Microsat/Nanosat Technology

• Several 10-100 kg microsats <$10M now carrying out major LEO science missions: MOST, CHIPSat, PROBA-1, …

• Enabled by modern commercial consumer electronic components, from cell-phone, laptop, PDA and digital camera markets

• Well-proven in space environments worse than that near the Moon

• Low cost requires very efficient programme definition and management---almost all successful efforts based on AMSAT model

• Nanosats/Cubesats following close behind: 1-10 kg, some <$1M

• U of T’s Space Flight Laboratory is a pioneer in advanced nanosats, via their CanX nanosat program

CanX-1


Toronto, Ontario17

Lunette

• Science mission: to map Lunar farside gravity field, to 10-20 mGal

• Free-flying nanosatellite, ejected from and flying in formation with a parent satellite, both in low Lunar orbit, measuring relative range rate using radio tracking

• Complements JAXA’s SELENE “high/low” mission:– More effective at measuring high-spatial-frequency

components of the gravity field

• ~5 kg, ~~$2-5M (if done as SFL nanosat)

• Science instrument: ranging radio transponder

• Bus needs 3-axis attitude control and propulsion

• Initially proposed as a subsatellite payload for ISRO’s Chandrayaan-1 lunar satellite:

– Was short-listed; complements ISRO LIDAR topography payload

• Suitable for flight with any of several upcoming Lunar polar orbiting missions


Toronto, Ontario18

Baseline Design: Based on SFL’s CanX-3/BRITE Nanosat Bus

• Bright Star Photometry nanosat (“Nano-MOST”)

• 15x15x15 cm, <4 kg• Two fixed S-Band monopole

uplink antennas• Two S-Band patch downlink

antennas• Two fixed UHF monopole

beacon antennas• Body-mounted solar panels,

26% efficiency TJ cells, 4.1 W nominal power

• 3 nano reaction wheels

BSP Instrument

Li-ion Battery

GPS Antenna (2)

Star Tracker

Reaction Wheels (3)

UHF Beacon Antenna (2)

S-Band Uplink Antenna (2)

S-Band Downlink Antenna (2)

ACS Computer

Main OBC

Power Subsystem

S-Band TX/RX UHF Beacon GPS Receiver

TJ Solar Cells

Aluminium Panel and Sub-frame

5 cm


Toronto, Ontario19

Lunette Technology

• Basic Nanosat Bus and Ejection System:– U of T/Space Flight Laboratory CanX nanosat program– One satellite built and flown, funding secured for next 2 missions

• Reaction Wheel: – Prototype built, will be test-flown on CanX-2

• Star Tracker: – Baseline: software from MOST star tracker, CMOS camera initial design/imager

testing under BRITE mission studies

• Nanosat propulsion (25-75 m/sec):– Baseline vendors identified, flight hardware built for 25m/sec, breadboard testing

done for 75m/sec– Alternative is SFL-developed nanosat propulsion

• Low-power transponder:– Baseline: based on MOST S-band transmitter

• Processing of tracking data to extract gravity models:– Baseline: use NASA GSFC/JPL code via US team members– Alternative is to adapt GRACE software


Toronto, Ontario20

Lunette Measurement Sensitivity Calibration

• Calibration analysis scenario:• Spherical Mascon:

– 20 km deep– 35 km diameter– 1.5E16 kg excess mass

• Fly-over of mascon:– From -300 km to +300 km horizontally– 50 km altitude– 1.655 km/sec velocity

• Peak gravity anomaly: – 20 mGal (2e-4 m/sec^2)

• Peak inter-satellite speed variation:– Horizontal: 4 mm/sec– Vertical: 7 mm/sec

• Lunette target speed measurement sensitivity: 1 mm/sec after 10 seconds of averaging

Vertical and Horizontal Speed Variation Due To Mascon, 90km Inter-Satellite Horizontal Distance

-10

-5

0

5

10

15

20

0 50 100 150 200 250 300 350 400

Time (sec)

Sp

eed

Var

iati

on

(m

m/s

ec)

X Speed (Leading Satellite)

Z Speed (Leading Satellite)

X Speed Difference

Z Speed Difference

Vertical Component of Gravity Anomaly (mGal)

0

5

10

15

20

25

-400 -300 -200 -100 0 100 200 300 400


Gra

vity

(m

Gal

)

Gravity (mGal)


Toronto, Ontario21

A Gravity Gradiometer in Lunar Orbit:10x Improvement

• Gradiometer sensitivity calibration analysis scenario:

• Spherical Mascon:– 20 km deep– 11 km diameter– 1.5E15 kg excess mass (1/10 the mass of

previous example)• Fly-over of mascon:

– From -300 km to +300 km horizontally– 50 km altitude– 1.655 km/sec velocity

• Peak gravity anomaly: – 2 mGal (2e-5 m/sec^2)

• Peak gravity gradient anomaly: – ~0.6 Eo (5.8e-10 m/sec^2/m)

• Gedex target airborne gravity gradient anomaly sensitivity: <0.3 Eo after 10 seconds of averaging

Vertical Component of Gravity Anomaly (mGal)

0.0

0.5

1.0

1.5

2.0

2.5

-400 -300 -200 -100 0 100 200 300 400


Gra

vity

(m

Gal

)

Gravity (mGal)

XX and ZZ Components of Gravity Gradient

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

-400 -300 -200 -100 0 100 200 300 400

Distance to m ascon encounter (km )

XX

an

d Z

Z G

ravi

ty G

rad

ien

t (E

o)

XX Gravity Gradient (Eo)

ZZ Gravity Gradient (Eo)

Gsx = (Gzz-Gxx)/2


Toronto, Ontario22

MASCON Gravity Gradient Signature

• MASCONs are not point-masses– Shallow caps covering impact basins

• More-realistic MASCON model:– 5km thick circularly-symmetric

spherical shell– 10 degree solid angle (similar to

Imbrium MASCON): 300 km radius– Density difference: 500 kg/m3

• Simulated circular-orbit flyover, 50km above MASCON cap

• Calculated gradients in Tangential and Radial directions

10 deg

50km

T

R

5

1735km


Toronto, Ontario23

gTT

gRR

gTR

gRT

MASCON Gravity Gradient Signature

Colatitude (deg)

Gravity Gradient (Eo)

-30 -20 -10

Gravity gradient is low near the center of the mascon

Gravity gradient is high near the edges of the mascon, with largest peaks from diagonal gradients


Toronto, Ontario24

What Type of Gradiometer to Fly?

• GOCE type: – Room temperature pair of electrostatically levitated accelerometers– Suitable for orbital use– Unsuitable for use in Lunar surface application: too sensitive to angular

accelerations of base

• Gedex current type:– Superconducting cross component gradiometer– Suitable for orbital and ground use– Needs liquid helium cryostat

• Gedex next-generation gradiometer:– Room-temperature cross component type– Suitable for orbital and ground use– Mass should be microsat-compatible

Lunette: An Affordable Canadian Lunar Farside...

Documents

Transcript of Lunette: An Affordable Canadian Lunar Farside...