Transcript Site Survey: Free Space (eg L2) and the Moon

Science Enabled by the
Exploration Architecture (and
return to the Moon)
John Mather
NASA Goddard Space Flight Center
STScI, Nov. 29, 3006
Nov. 29, 2006
Mather STScI Lunar Astrophysics
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Key Strategic Questions
• What scientific questions are ripe for the next
few decades?
• What scientific questions are worth the
money to do in space?
• Site surveys: advantages of the lunar surface
and free space?
• Robots or astronauts: which goals need
which systems?
• For given requirement, what are cost
differences between sites?
• How much does it all cost?
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Possible Hardware for Human
Space Exploration
• Orion (Crew Exploration Vehicle, CEV, under design/
construction)
• Ares 1 (Crew Launch Vehicle, CLV, under design/ construction)
• Ares 5 (Cargo Launch Vehicle, CaLV much larger)
• Lunar Surface Access Module (LSAM)
• Earth Departure Stage (EDS, cryo upper stage of Ares 5)
• Advanced space suits
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Hardware (2)
• Advanced servicing capabilities
– Remote robotic
– Local astronaut-controlled robots/manipulators
– EVAs
• Advanced habitat equipment
– Astronaut safety: centrifuges, shields, possibly from local
materials
– Life support: food production, recycling
– Solar and nuclear power and communication
– Service stations at Earth-Moon L1, Sun-Earth L2 (later)
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Hardware Enabling New
Astrophysics
• CEV and CLV, under design for construction
– New sites on Moon
– Servicing at new locations not on Moon
• Advanced servicing capabilities - TBD, very important to
astrophysics
– Very remote robotic (e.g. operated from ground)
– Local astronaut-controlled robots/manipulators
– EVA - depends on airlocks and many details
• Ares 5 (Cargo Launch Vehicle, CaLV)
– Larger payloads, farther away or faster
• Advanced habitat development
– Solar and nuclear power and communication
– Service stations at Earth-Moon L1, Sun-Earth L2 (later)
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Important Astro- & Solar System
Physics from the Moon
• Lunar geology: sample recognition, analysis,
excavation, return to Earth
• Lunar structure: mapping, gravity, surface and
interior chemistry and physics
• Lunar origin
• Solar system archeology, by interpretation of
samples
• Laser ranging from Earth, to test Einstein
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Payload Mass
• For JWST, launch vehicle cost ~ 3-4% of life
cycle cost, but launcher imposes strict mass
limit
• If same mass were landed on the Moon,
would need ~ 3x launcher capability, perhaps
rocket cost would scale in proportion?
• Cost estimation algorithms for observatories
say cost and mass are ~ proportional, so
6000 kg is about the maximum for a JWSTclass telescope anywhere
– Does this apply to observatory alone, or including
landing equipment?
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Stiffening a Big Telescope for 1/6 g
• No way to make a passively stable system
highly precise, ==> need active control loops
re-adjusted for each elevation angle
• Like adaptive optics on ground, but much
slower - OK but complicated
• Strength not an issue, since launch loads are
much larger
• For R. Angel concept of spinning liquid mirror,
gravity is required, but there is no possibility
of changing its axis from vertical.
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Dust
• Lunar dust is hazardous - sharp, small, sticky,
covers astronauts, requires cleaning to get
vacuum seals on suits
• Lunar dust levitates due to electrostatic
forces, seen by astronauts as a haze
• Laser retroreflectors may be contaminated by
dust - more info needed
• A serious engineering challenge to manage
dust around telescopes
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Optical Interferometers
• On Earth or Moon, complicated optical
systems with path length equalization
systems and huge rooms filled with trolleys
and mirrors
• Servicing might be necessary - ground based
equipment is hard to adjust
• Free-space version optically much simpler
– Path equalization by formation flying
– May still need servicing?
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Radio Telescopes
• Long wavelength (> 30 m) needs space
• Very little is known in this band, wide open for
exploration and surprise, but so far not recognized by
NAS as top scientific priority
– New generation ground-based observatories will allow
extrapolation from higher frequencies
• Need large array of dipoles to image large areas of
sky
• High angular resolution needs huge array
–  = /d
– 1 arcsec at 30 m means 6000 km span
• Reconfigure array to match required 
• TBD how quiet the environment must be
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Servicing Possibilities
• Lunar surface advantages
– Can’t get lost on lunar surface, but must travel by car or on
foot
– Tools can’t escape
– Astronauts could have permanent safe home (far future),
always available to service complex observatories
• Free space advantages
– Can be anywhere the telescope is, or can go
• LEO to EM L1 to SE L2 to …
–
–
–
–
Equipment is weightless - no lifting fixtures
No dust to contaminate telescope & tools
Extensive experience with HST, Space Station
Astronauts can come home from EM L1 in a flash if bad
solar weather
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Possible Servicing Uses
• CEV
– How far can it go to do servicing?
– Quick astronaut trip to SE L2? (too risky if EM L1 would be
enough, but maybe later…)
• Robotic servicing, e.g. using astronaut tools and
manipulator arms, to reduce risk or enable upgrades
– Beyond Einstein probes - servicing probably not needed,
but …?
– Interplanetary missions, robot explorers?
– Future Great Observatories
• Chandra, LISA, SIM, TPF-C, TPF-I, TPF-Occulter, SAFIR…
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Future Large Observatories from
Decadal Survey
• Chandra X-ray observatory
– Lunar surface bad for very precise optics, free space good,
servicing possibly valuable
• LISA gravity wave observatory
– Lunar site impossible, remote servicing possible by replacing a
member of the triangle with a new one (no robot or astronaut visit
needed)
• SAFIR far IR telescope
– Lunar surface much too hot except possibly in dark crater - don’t
know this yet, need ~ 4 K cooling for ~ 10 m telescope
• SPECS and SPIRIT, far IR interferometers
– ~ 4 K telescopes at all possible spacings in (u,v) plane
– Lunar surface not possible - too hot, telescopes not mobile
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Planet Finders
•
Kepler: transit search, 2008 launch
– Continuous monitoring of Cygnus region, declination ~ 40o +/- 23o
– Dark crater at North lunar pole? target elevation ~ 40o +/- 23.5o
•
Microlensing Planet Finder (Discovery proposal)
– Requires continuous monitoring of Galactic Center
– GC is in Ecliptic Plane, ~ on horizon from Lunar poles
•
Nearest Star Planet Transit Survey (extends ground-based surveys
with better photometry)
– Like Kepler, but all-sky survey, to find nearest and brightest, best candidates
for follow-up by JWST, etc.
– Lunar pole locations possible; need 2 for all-sky
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Planet Finders (2)
•
SIM
– Requires complete thermal stability and wide sky view
– Dark crater potential site, but loses > half of targets
•
TPF-Coronagraph
– Lunar surface probably impossible - optical system must be /3000 and
perfectly stable, and extremely clean (no dust at all!)
•
TPF-Interferometer
– Lunar surface probably impossible - but worth some study
– Filling (u,v) plane much easier in space than on surface of Moon
•
New Worlds Observer - remote occulter
– Lunar surface impossible - formation flight configuration with ~25,000 km
spacing
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Site Survey: the Moon and Free Space (e.g. L2)
Item
Lunar Surface
Free Space (e.g. Sun -Earth L2)
Delivered payload mass per launch
(implies launch cost difference)
~ 1/3 (depends on Isp of propulsion, many details)
1
Gravity
g/6, causes sag of optical system vs. pointing, needs stiff
structures, added mass. Enables spin-formed parabolic
mirror with vertical axis (R. Angel, P. Worden)
0
Servicing, repair, upgrade
Six Apollo missions; 1969 -- 1972; few days trip each way,
limited radiation exposure to astronauts.
Shuttle missions for HST, ISS, CGRO; robotic arms;
numerous robotic designs. Sun-Earth L2 much farther from
Earth than Moon. Possible service center at Earth-Moon L1.
Dust
Sticky, small, charged, naturally levitated above surface;
activated by astronauts, rovers, and retrojets; seen by
astronauts; evidence of accumulation on retroreflectors
0
Solar power duty cycle
14 days/29, except polar peaks (1) or dark craters (0), may
require storage for lunar night
1
Communications duty cycle
1 on front, needs relay on backside or deep crater
1
Temperature variation of environment
Variable solar direction (except in dark craters) requires
complex sunshield designs
Constant solar direction permits simple sunshield designs.
Observing duty cycle
Depending on stray light shields, power, thermal protection
and stability, and comm
1
Field of Regard
Depends on lunar latitude and horizon shape inside
thermal shields
Whole sky
Interferometer baseline maintenance
Passive, can’t get lost. Fixed positions, or movement
across challenging terrain
Active servos, full (u,v) coverage. Requires station keeping
and propulsion.
Path length compensation
Long range (comparable to spacing of collectors), to obtain
field of view and (u,v) coverage
Short range (few cm), as part of formation flying servo
control loop
Maximum baseline
Size of flat region on Moon
Optics limited, huge
Radio quiet
Far from Earth; back side is protected for now
Can be much farther from Earth
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What would I do?
• Coordinate with manned program to assess capabilities needed
by both manned program and science
• Understand approach of manned program to manage dust, and
what equipment and infrastructure they will develop and when
• Study how much dust contaminates lunar optics, and how to
mitigate it
• Study how to design astronomical equipment ON Moon
– AFTER manned program is defined, lunar sites and habitats are
selected, and infrastructure is known
– Lunar Astronomy is NOT a driver for the manned program - plenty
of other ways, currently easier, to do science
• Present to NAS review for comparison to other sites
• Offer new observing sites and infrastructure in competitive AO’s
for science
• Astronomers are ingenious: they’ll find a way to use the
infrastructure or the lunar surface!
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In the meantime
• Assess possible augmentations to Exploration
Architecture with joint benefits to science and
manned program
• Study potential radio astronomy at  > 30 m: does it
justify space equipment?
• Study (with AAAC) what equipment matches the
scientific goals for exoplanets - if very complex or
risky, servicing may be appropriate
• Study (with NAS) what has priority in next decade for
space and ground-based astronomy
– If top priorities could benefit from the VSE infrastructure, do
needed studies
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Summary and Conclusions
• Exploration Architecture & infrastructure (heavy lift vehicles,
CEV, robotic servicing) could enable much more powerful large
observatories, in free space, with much longer useful lifetimes,
than are possible today
• Since we’re going to the Moon, then study the Moon itself
• Lunar surface not best use of money for most telescopic
astronomy, but when manned program is defined, then offer
lunar sites and infrastructure in AO’s
• Astronomy is NOT a driver for manned program requirements too many other ways to do most science, and conflicting
program requirements drive up costs
• For specific science, e.g. gravity studies by laser retroreflector,
lunar placement is very important
• Need to know whether (expensive, fragile) human presence is
required on-site for astrophysics missions
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