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Spacecraft Gravitational
Wave Detectors
Wei-Tou NI
Center for Gravitation and Cosmology
Purple Mountain Observatory
Chinese Academy of Sciences
Nanjing, China
2009.10.26
Spacecraft Gravitational Wave Detectors
W-T Ni Galilio-Xu, Shanghai
1
OUTLINE
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2009.10.26
Introduction – Why in space?
LISA and LISA Pathfinder
General Concept of ASTROD --ASTROD I, ASTROD, ASTROD-GW,
Super-ASTROD
Primordial Gravitational Waves
Two potential frequency regions to
detect primordial GWs
Outlook
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Drag-free requirement makes the whole
spacecraft a detector
The spacecraft is the isolation system
for spurious forces
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Why go to Space?
•
Complementary to ground-based observatories
that are sensitive to high frequency GWs
0.1mHz-1 Hz
2006.03.18.
~10Hz-kHz
Gravitational Wave Detectors in Space: LISA, ASTROD and Later Missions
W.-T. Ni
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Hubble Deep Field, HST.WFPC2, NASA
GW sources in the high
frequency band
Gravity gradient
noise
on the Earth
RAS / IOP Meeting 14/02/03 B. Schutz
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Why in space?
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Minimal Spurious Perturbations
Longer Measurement Times
Experiments in space are able to
explore the GW universe and to
challenge our understanding of the
universe and look for slight deviations
that lead to grand unification theories
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Low Frequency GWs from:
2006.03.18.
Gravitational Wave Detectors in Space: LISA, ASTROD and Later Missions
W.-T. Ni
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LISA
LISA consists of a fleet of 3 spacecraft 20º behind earth in solar
orbit keeping a triangular configuration of nearly equal sides (5 × 106 km).
Mapping the space-time outside super-massive black holes by measuring the
capture of compact objects set the LISA requirement sensitivity between 102-10-3 Hz. To measure the properties of massive black hole binaries also
requires good sensitivity down at least to 10-4 Hz. (2017)
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Massive Black Hole Systems:
Massive BH Mergers &
Extreme Mass Ratio Mergers (EMRIs)
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Spacecraft
Gravitational Wave Detectors
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Space
craft
Gravit
ational
Wave
Detect
ors
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Ni
Galilio
-Xu,
Shang
hai
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Space
craft
Gravit
ational
Wave
Detect
ors
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Ni
Galilio
-Xu,
Shang
hai
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Space
craft
Gravit
ational
Wave
Detect
ors
W-T
Ni
Galilio
-Xu,
Shang
hai
Catalogs of GW sources
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Typical binaries: sky positions, distance, orbit
orientation, orbit separation, chirp mass for
the system, spin magnitude and orientation,
merger time (if appropriate)
Sources with subtantial orbital evolution:
masses of the individual objects
Most favorable cases: masses, spins and
distances to 1 %
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LISA Instrument & Sciencecraft
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LISA Pathfinder
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Paul McNamara for the LPF Team
LISA Pathfinder Project Scientist
GWADW
10th - 15th May 2009
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Drag-free AOC requirements
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Atmospheric (terrestrial) air column exclude a resolution of better than 1 mm
This reduces demands on drag-free
AOC by orders of magnitude
Nevertheless, drag-free AOC is needed for
geodesic orbit integration.
Thruster requirements
Thrust noise
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Proof mass
Proof massS/C coupling
Spacecraft Gravitational Wave Detectors
Control loop
gain
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LISA
Pathfinder
in Assembly
Clean Room
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LISA Orbit
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Problems on the Orbit Optimization for
the LISA Gravitational Wave Observatory
G. Li et al. (IJMPD 2008)
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Interlocking Developments
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Satellite/Lunar Laser Ranging in 1960s
Drag-free navigation for geodesy in
1970s
Concept of Laser Interferometry in Space
for GWs in 1980s
Concept of ASTROD and Interplanetary
Laser-Pulse Ranging in 1990s
Pulse and CW Optical Communication in
Space
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The General Concept of ASTROD
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The general concept of ASTROD (Astrodynamical
Space Test of Relativity using Optical Devices) is
to have a constellation of drag-free spacecraft
navigate through the solar system and range
with one another using optical devices
to map the solar-system gravitational field,
to measure related solar-system parameters,
to test relativistic gravity,
to observe solar g-mode oscillations,
and to detect gravitational waves.
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Gravitational Field in the
Solar System
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The solar-system gravitational field is determined by three
factors:
the dynamic distribution of matter in the solar system;
the dynamic distribution of matter outside the solar system
(galactic, cosmological, etc.)
and gravitational waves propagating through the solar system.
------------------------Different relativistic theories of gravity make different
predictions of the solar-system gravitational field.
Hence, precise measurements of the solar-system gravitational
field test these relativistic theories, in addition to enabling
gravitational wave observations, determination of the matter
distribution in the solar-system and determination of the
observable (testable) influence of our galaxy and cosmos.
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Common Science ---
Astrodynamic Equation
r ij
 ri (1PN )  2PN   r（i G - wave ) (i  0,1,  , n)
ri    
3
j  i j r ij
+ gal-cosmo term +non-grav term
ri (Post  Newton ) 
Aij 
1
r
3
ij
r
2
i
1
c
2
1   

r
3
  A r  B r 
j i
r ij 
2
ij
ij
j
3
2r
ij
ij
ij
rij r j   1 2        
2
5
ij
r
4
i
j
i
ij
 2    2   1 2  1 2  1.5

1

   3
 3
 3


3
3
3 r ij r ik
k
 r ij r ik

k i , j
r ij r jk r jk r ij r jk r ik 2r jk r ij


B
ij

r 2  1  
r r
r
ij
3
ij
j
ij
r（i G - wave )  Ri x  dotx  dotx
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ASTROD I (Cosmic Vision 2015-25)
submitted to ESA by H. Dittus (Bremen)
arXiv:0802.0582 v1 [astro-ph]
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Scaled-down version of ASTROD
1 S/C in an heliocentric orbit
Drag-free AOC and pulse ranging
Launch via low earth transfer orbit to
solar orbit with orbit period 300 days
First encounter with Venus at 118 days
after launch; orbit period changed to 225
days (Venus orbit period)
Second encounter with Venus at 336 days
after launch; orbit period changed to 165
days
Opposition to the Sun: shortly after 370
days, 718 days, and 1066 days
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Laser ranging / Timing: 3 ps
(0.9 mm)
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Pulse ranging (similar to SLR / LLR)
Timing: on-board event timer (± 3 ps)
reference: on-board cesium clock
For a ranging uncertainty of 1 mm in a distance of 3 ×
1011 m (2 AU), the laser/clock frequency needs to be
known to one part in 1014 @ 1000 s
Laser pulse timing system: T2L2 (Time Transfer by Laser
Link) on Jason 2
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Single photon detector
Jason 2 S/C
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Two GOCE sensor heads (flight models) of the
ultra-sensitive accelerometers (ONERA’s courtesy)
2 × 10^-12 m s^-2 Hz^-1/2 resolution in presence
of more than 10^-6 m s^-2 acceleration
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Summary of the scientific objectives in
testing relativistic gravity of the ASTROD I
and ASTROD missions
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ASTROD configuration (baseline
ASTROD after 700 days from launch)
Inner Orbit
Earth Orbit
1
.
Earth L1 point S/C
(700 days after 1*
launch)
Outer Orbit
-V1
L3
U2
n̂3
.
U1
Launch
Position
2*
S/C 2
2
.
Sun
n̂2
-V3
n̂1
L2
L1
U3
-V2
.
3 3*
S/C 1
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A comparison of the target acceleration noise
curves of ASTROD I, LISA, the LTP and ASTROD
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Anchoring
Dummy
telescope
Outgoing Laser beam
Proof
mass
LASER
Metrology
Capacitive
readout
Housing
Telescope
Optical readout
beam
Telescope
Incoming Laser
beam
Dummy telescope
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Proof mass
Large gap
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Solar oscillation modes
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Probing the sun’s core
as well as its internal
structure and dynamics
(with ASTROD only)
Solar gravity (g)-modes
have very small
amplitudes and
generate very small
radial velocities.
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GOLF Observation
Gough (theory)
Kumar, et al. (theory)
Comparison of surface radial velocity amplitudes for l=2 g modes (quadrupole modes)
(explanation in the text below) Theoretical estimates from [16] (thick dashed line) and [15]
(thick solid line); one sigma limit corresponding to an average of 50 modes observed by the
GOLF instrument with 10 years of data, derived from [10] (thin straight line); LISA one sigma
limit (grey solid line) assuming a one-year integration time and a strain sensitivity of 10-23
at 3000 μHz and a f-1.75 dependence [14]; ASTROD one sigma limit (thin solid line)
assuming a one year integration time and a strain sensitivity of 10-23 at 100 μHz and a f-2
dependence, with a spacecraft orbiting at 0.4 AU [14]. The surface velocity amplitudes for
ASTROD were derived using the most recent GW strain sensitivities in [14] and the equations
in [17]. The GW strain falls off as 1/R4, R distance to the Sun. The significant improvement
provided by ASTROD with respect to LISA is due to a combination of better strain sensitivity
and a smaller distance to the Sun.
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Test of relativistic gravity and
fundamental laws of spacetime
Measuring solar and planetary
parameters
and
Gravitational Waves
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ASTROD configuration (baseline
ASTROD after 700 days from launch)
Inner Orbit
Earth Orbit
1
.
Earth L1 point S/C
(700 days after 1*
launch)
Outer Orbit
-V1
L3
U2
n̂3
.
U1
Launch
Position
2*
S/C 2
2
.
Sun
n̂2
-V3
n̂1
L2
L1
U3
-V2
.
3 3*
S/C 1
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ASTROD’s GW gaols
-- dedicated to GW detection
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Larger Armlength  More Sensitivity to
Lower Frequency and Larger Wavelength
Better S/N to massive BH events  Better
accuracy for cosmic distance measurement
and probe deeper into larger redshift and
earlier Universe. Better probe to dark energy.
More sensitive to primordial gravitational
waves if foreground GWs can be separated.
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Time delay interferometry:
Technology
common to LISA and ASTROD
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Although the velocity in the Doppler shift
direction differ by 200-300 times, LISA and
ASTROD both need to use time delay
interferometry
The issue of large differences in frequency for
ASTROD is ideally solved by using optical comb
generator and optical frequency synthesizer
together with optical clock
Data analysis for ASTROD poses big challenges
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Interferometry Measurement System
(IMS): Main Constituents
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40 cm, f/1.5 transmit/receive telescope
Optical bench with interferometry optics, laser
stabilization
Gravitational reference sensor
1.064 μm Nd:YAG non-planar ring oscillator master
laser, 2 W Yb:YAG fiber amplifier, plus spare
Fringe tracking and phasemeter electronics, including
ultra-stable oscillator
Fiber link for comparing laser phase between two
arms
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ASTROD-GW Mission Orbit
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Considering the requirement for
optimizing GW detection while
keeping the armlength, mission
orbit design uses nearly equal
arms.
3 S/C are near Sun-Earth
Lagrange points L3、L4、L5，
forming a nearly equilateral
triangle with armlength 260
million km（1.732 AU）.
3 S/C ranging interferometrically
to each other.
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Spacecraft Gravitational Wave Detectors
S/C 1 (L4)
(L3)
S/C 2
Sun
Earth
60
球地
L1 L2
60
S/C 3 (L5)
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ASTROD-GW Mission Orbit
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Considering the requirement for
optimizing GW detection while
keeping the armlength, mission
orbit design uses nearly equal
arms.
3 S/C are near Sun-Earth
Lagrange points L3、L4、L5，
forming a nearly equilateral
triangle with armlength 260
million km（1.732 AU）.
3 S/C ranging interferometrically
to each other.
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Spacecraft Gravitational Wave Detectors
S/C 1 (L4)
(L3)
S/C 2
Sun
Earth
60
球地
L1 L2
60
S/C 3 (L5)
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Difference of Armlengths
in 10 years
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Angle between Arms in 10 Years
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Velocity in the Line-of-Sight
Direction (Men, Ni & Wang)
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Time delay interferometry:
Technology
common to LISA and ASTROD-GW
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2009.10.26
Although the velocity in the Doppler shift
direction for ASTROD-GW is smaller than LISA,
LISA and ASTROD-GW both need to use time
delay interferometry.
For ASTROD-GW, the Doppler tracking
technology developed in LISA could be used.
Telescope pointing of LISA could also be used.
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Motivation: Primordial Gravitational
Waves are probes to very early universe
--- after 10^(-43) s or even earlier
Primordial
Gravitational
Waves
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The Gravitational Wave Background from
Cosmological Compact Binaries
Alison J. Farmer and E. S. Phinney (Mon. Not. RAS [2003])
Optimistic (upper dotted), fiducial
(Model A, lower solid line) and
pessimistic (lower dotted)
extragalactic backgrounds plotted
against the LISA (dashed) singlearm Michelson combination
sensitivity curve. The‘unresolved’
Galactic close WD–WD spectrum
from Nelemans et al. (2001c) is
plotted (with signals from binaries
resolved by LISA removed), as
well as an extrapolated total, in
which resolved binaries are
restored, as well as an
approximation to the Galactic
MS–MS signal at low frequencies.
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Super-ASTROD
Region
Spacecraft Gravitational Wave Detectors
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DECIGO
BBO Region
Galilio-Xu, Shanghai
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BIG BANG OBSERVATORY
BBO; http://universe.gsfc.nasa.gov/be/roadmap.htm
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The Big Bang Observatory is a follow-on mission to LISA, a vision mission of
NASA’s “Beyond Einstein” theme.
BBO will probe the frequency region of 0.01–10 Hz, a region between the
measurement bands of the presently funded ground- and space-based
detectors. Its primary goal is the study of primordial gravitational waves from
the era of the big bang, at a frequency range not limited by the confusion
noise from compact binaries discussed above.
In order to separate the inflation waves from the merging binaries, BBO will
identify and subtract the signal in its detection band from every merging
neutron star and black hole binary in the universe. It will also extend LISA’s
scientific program of measuring wavesfrom the merging of intermediate-mass
black holes at any redshift, and will refine the mapping of space-time around
supermassive black holes with inspiraling compact objects.
The strain sensitivity of BBO at 0.1 Hz is planned to be ∼10−24, with a
corresponding acceleration noise requirement of < 10−16 m/(s2 Hz1/2).
These levels will require a considerable investment in new technology,
including kilowatt-power level stabilized lasers, picoradian pointing capability,
multi-meter-sized mirrors with subangstrom polishing uniformity, and
significant advances in thruster, discharging, and surface potential technology.
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6 S/C ASTROD optimized
for correlation detection
航天器S/C *3
航天器S/C2
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This configuration
is optimized for
the correlation
detection of GW
background
太 30
阳
60
航天器S/C *1
60
航天器S/C1
30
航天器S/C *2
航天器S/C3
地球
6 S/C ASTROD GW mission orbit
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Super-ASTROD (1st TAMA Meeting1996)
W.-T. Ni, “ASTROD and gravitational waves” in Gravitational Wave Detection,
edited by K. Tsubono, M.-K. Fujimoto and K. Kuroda
(Universal Academy Press, Tokyo, Japan, 1997), pp. 117-129.
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2009.10.26
With the advance of laser technology and the
development of space interferometry, one
can envisage a 15 W (or more) compact laser
power and 2-3 fold increase in pointing ability.
With these developments, one can increase
the distance from 2 AU for ASTROD to 10 AU
(2×5 AU) and the spacecraft would be in
orbits similar to Jupiter's. Four spacecraft
would be ideal for a dedicated gravitationalwave mission (Super-ASTROD).
Spacecraft Gravitational Wave Detectors
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Primordial Gravitational Waves
[strain sensitivity  (ω^2) energy sensitivity]
0.0
-2.0
-4.0
-6.0
bar-intf
2 intf
Nv = 3.2
(c) cosmic
strings
(b) String
-10.0
Log [h
Ωgw]
2
cosmology
-12.0
-14.0
-16.0
(single intf)
Nv = 4
-8.0
0
LIGO or VIRGO
ms pulsars
inflation
‘Average’
ASTROD
-18.0
*ASTROD
(correlation detection)
-22.0
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DECIGO/BBO-grand
(correlation detection)
Super-ASTROD
-20.0
-24.0
-18.0
LIGO II/LCGT/VIRGO II
(2 adv intf)
Extragalactic
Extrapolated
WMAP
(a)
LISA
* Super-ASTROD (correlation detection)
-14.0
-10.0
-6.0
-2.0
Log f
Spacecraft Gravitational Wave Detectors
[[[ [f(Hz)]
2.0
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6.0
10.0
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Primordial GW and Space Detectors
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For detection of primordial GWs in space. One may
go to frequencies lower or higher than LISA
bandwidth where there are potentially less
foreground astrophysical sources to mask detection.
DECIGO and Big Bang Observer look for gravitational
waves in the higher range
ASTROD-GW, Super-ASTROD look for gravitational
waves in the lower range.
Super-ASTROD: 3-5 spacecraft with 5 AU orbits
together with an Earth-Sun L1/L2 spacecraft and
ground optical stations to probe primordial
gravitational-waves with frequencies 0.1 μHz - 1 mHz
and to map the outer solar system.
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Sensitivity to Primordial GW
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2009.10.26
The minimum detectable intensity of a stochastic
GW background
is proportional to
detector noise spectral power density Sn(f) times
frequency to the third power
with the same strain sensitivity, lower frequency
detectors have an f ^(-3)-advantage over the
higher frequency detectors.
compared to LISA, ASTROD has 140,000 times
(52^3) better sensitivity due to this reason, while
Super-ASTROD has an additional 125 (5^3) times
better sensitivity.
arXiv:0905.2508
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Primordial Gravitational Waves
[strain sensitivity  (ω^2) energy sensitivity]
0.0
-2.0
-4.0
-6.0
bar-intf
2 intf
Nv = 3.2
(c) cosmic
strings
(b) String
-10.0
Log [h
Ωgw]
2
cosmology
-12.0
-14.0
-16.0
(single intf)
Nv = 4
-8.0
0
LIGO or VIRGO
ms pulsars
inflation
‘Average’
ASTROD
-18.0
*ASTROD
(correlation detection)
-22.0
2009.10.26
DECIGO/BBO-grand
(correlation detection)
Super-ASTROD
-20.0
-24.0
-18.0
LIGO II/LCGT/VIRGO II
(2 adv intf)
Extragalactic
Extrapolated
WMAP
(a)
LISA
* Super-ASTROD (correlation detection)
-14.0
-10.0
-6.0
-2.0
Log f
Spacecraft Gravitational Wave Detectors
[[[ [f(Hz)]
2.0
W-T Ni
6.0
10.0
Galilio-Xu, Shanghai
55
Summary
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2009.10.26
Introduction – Why in space?
LISA and LISA Pathfinder
General Concept of ASTROD --ASTROD I, ASTROD, ASTROD-GW,
Super-ASTROD
Primordial Gravitational Waves
Two potential frequency regions to
detect primordial GWs
Spacecraft Gravitational Wave Detectors
W-T Ni
Galilio-Xu, Shanghai
56
Thank you !
2009.10.26
Spacecraft Gravitational Wave Detectors
W-T Ni Galilio-Xu, Shanghai
57