Lunette: An Affordable Canadian Lunar Farside Gravity Mapping

Lunette: An Affordable Canadian Lunar
Farside Gravity Mapping Mission
Kieran A. Carroll, Gedex Inc.
Henry Spencer, SPSystems
Jafar Arkani-Hamed, University of Toronto
Robert E. Zee, UTIAS Space Flight Laboratory
11
2005 International
Conference
2005
InternationalLunar
Lunar
Conference
Toronto,
Toronto,Ontario
Ontario
2005
22 22
2005September
September
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
2
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
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
3
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
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 NiFe
meteorites…
4
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
Gravity of a Uniform Sphere
GMm/r2
5
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
Gravity Anomalies
6
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
Ground-Based Gravimetry
Good accuracy, but slow and
expensive to collect!
7
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
Airborne Gravimetry
Vertical Component of Anomalous Gravity (mGal)
Gravity (mGal)
0.60
0.50
0.40
0.30
0.20
0.10
0.00
-4
-3
-2
-1
0
1
2
3
4
Distance to mascon encounter (km)
• 1 km deep spherical deposit
• 1x1011 kg (100 MT) excess mass
• Fly-over at 100m altitude
8
• 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)
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
Airborne Gravity Gradiometry
XX, ZZ and XZ Components of Anomalous Gravity Gradient
XX and ZZ Gravity Gradient
(Eo)
6.0
-4
4.0
2.0
0.0
-3
-2
-1
-2.0 0
1
2
3
4
-4.0
-6.0
XX Gravity Gradient (Eo)
-8.0
ZZ Gravity Gradient (Eo)
-10.0
XZ Gravity Gradient (Eo)
-12.0
Distance to m ascon encounter (km )
• 1 km deep spherical deposit
• 1x1011 kg (100 MT) excess mass
• Fly-over at 100m altitude
9
• Gravity Gradient Units:
– 1 Eo = 10-9 m/sec2/m
• Peak anomalous gravity gradient
anomaly: ~10 Eo (1x10-8
m/sec2/m)
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
Types of Gravity Instruments
• Gravimeter/ Accelerometer • Gravity Gradiometer
– 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
10
– 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)
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
Gedex Cross Component Gravity Gradiometer
11
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
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
12
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
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
13
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
Current Lunar Gravity Models
(Lunar Prospector)
Anomalous Gravity (mGal at surface)
Topography (km)
•
•
Accurate topographical mapping
requires an accurate gravity model
Geological models are derived
from both topography and gravity
models
Crustal Thickness (km)
14
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
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
•
Iron Oxide
Medium-Term:
– Provide geological context to support base site
selection
– Help identify areas with potential highconcentration ice/mineral deposits
– Terrestrial analogy: “sovereignty mapping”
(government surveys) with results disseminated
•
Titanium
Dioxide
Long-term:
– Detailed site surveys to identify drilling/ excavation
targets: drilling/digging holes is very expensive!
– Terrestrial analogy: airborne geophysical surveys of
claims
15
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
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
16
2005 International Lunar Conference
Toronto, Ontario
CanX-1
2005 September 22
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
•
17
Suitable for flight with any of several upcoming Lunar
polar orbiting missions
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
Baseline Design: Based on
SFL’s CanX-3/BRITE Nanosat Bus
TJ Solar Cells
Aluminium Panel
and Sub-frame
S-Band Downlink
Antenna (2)
Power Subsystem
BSP Instrument
Main OBC
Li-ion Battery
UHF Beacon
Antenna (2)
S-Band TX/RX
UHF Beacon
GPS Receiver
Star Tracker
Reaction
Wheels (3)
ACS Computer
5 cm
S-Band Uplink
Antenna (2)
• 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
GPS Antenna (2)
18
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
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
19
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
Lunette Measurement Sensitivity Calibration
Vertical Component of Gravity Anomaly (mGal)
– 20 km deep
– 35 km diameter
– 1.5E16 kg excess mass
•
25
Calibration analysis scenario:
Spherical Mascon:
20
Gravity (mGal)
•
•
5
•
20
Lunette target speed measurement
sensitivity: 1 mm/sec after 10 seconds
of averaging
-200
-100
0
100
200
300
400
Vertical and Horizontal Speed Variation Due T o Mascon, 90km
Inter-Satellite Horizontal Distance
20
X Speed (Leading Satellite)
Z Speed (Leading Satellite)
15
Speed Variation (mm/sec)
– Horizontal: 4 mm/sec
– Vertical: 7 mm/sec
-300
Dista nce to m a scon e ncounte r (km )
Peak gravity anomaly:
Peak inter-satellite speed variation:
Gravity (mGal)
0
-400
– 20 mGal (2e-4 m/sec^2)
•
10
Fly-over of mascon:
– From -300 km to +300 km horizontally
– 50 km altitude
– 1.655 km/sec velocity
•
15
X Speed Difference
Z Speed Difference
10
5
0
0
50
100
150
200
250
300
350
400
-5
-10
2005 International Lunar Conference
Toronto, Ontario
Tim e (se c)
2005 September 22
A Gravity Gradiometer in Lunar Orbit:
10x Improvement
Vertical Component of Gravity Anomaly (mGal)
2.5
•
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)
•
2.0
Gravity (mGal)
•
1.5
1.0
0.5
0.0
-400
Fly-over of mascon:
-300
Peak gravity gradient anomaly:
– ~0.6 Eo (5.8e-10 m/sec^2/m)
•
-100
0
100
200
300
400
Gedex target airborne gravity gradient
anomaly sensitivity: <0.3 Eo after 10
seconds of averaging
XX and ZZ Components of Gravity Gradient
0.4
0.3
0.2
XX and ZZ Gravity Gradient (Eo)
Peak gravity anomaly:
– 2 mGal (2e-5 m/sec^2)
•
-200
Dista nce to m a scon e ncounte r (km)
– From -300 km to +300 km horizontally
– 50 km altitude
– 1.655 km/sec velocity
•
Gravity (mGal)
0.1
0.0
-400
-300
-200
-100
-0.1
0
100
200
300
-0.2
-0.3
-0.4
-0.5
-0.6
XX Gravity Gradient (Eo)
ZZ Gravity Gradient (Eo)
Gsx = (Gzz-Gxx)/2
-0.7
Dis tance to m as con e ncounte r (k m )
21
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
400
MASCON Gravity Gradient Signature
• MASCONs are not point-masses
R
– Shallow caps covering impact basins
T
• 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
50km
5
1735km
10 deg
• Simulated circular-orbit flyover,
50km above MASCON cap
• Calculated gradients in Tangential
and Radial directions
22
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
MASCON Gravity Gradient Signature
Gravity gradient is low
near the center of the
mascon
Gravity Gradient
(Eo)
gRT
gTT
Gravity gradient is
high near the edges of
the mascon, with
largest peaks from
diagonal gradients
-30
-20
gRR
gTR
-10
Colatitude (deg)
23
2005 International Lunar Conference
Toronto, Ontario
2005 September 22
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
24
2005 International Lunar Conference
Toronto, Ontario
2005 September 22