On Technologies Common
to DECIGO, LISA and
ASTROD
Wei-Tou NI
Center for Gravitation and Cosmology
Purple Mountain Observatory
Chinese Academy of Sciences
Nanjing, China
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OUTLINE
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General Concept of --- ASTROD I, ASTROD Super-ASTROD
A comparison of technologies needed for DECIGO, LISA
and ASTROD with focus on common technologies.
Drag-free technology (including sensors and microthrusters), charge management system, thermal
diagonostic and thermal control, and laser optics are the
technology common to all.
Time delay interferometry is the technology common to
LISA and ASTROD.
Laser metrology is the technology common to DECIGO and
ASTROD. All three requires stringent suppression of
spurious noises. This commonness serves as a natural
basis for feedbacks and collaborations.
Outlook
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2008.11.12.
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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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
2008.11.12.
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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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.
2008.11.12.
Super-ASTROD
Region
First LISA-DECIGO Meeting: Common Technologies
DECIGO
BBO Region
W.-T. Ni
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The General Concept of ASTROD
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2008.11.12.
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.
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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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.
2008.11.12.
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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Summary of the scientific objectives in
testing relativistic gravity of the ASTROD I
and ASTROD missions
2008.11.12.
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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A Recent Development of LISA
Last talk in LISA7 Symposium:
General Relativistic treatment of
LISA optical links and TDI
S. Dhurandhar
 Ground interferometers also need to
de-convolute the orbit motion and
include GR when the precision increases
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2008.11.12.
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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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
2008.11.12.
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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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
2008.11.12.
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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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
2008.11.12.
Proof mass
Proof massS/C coupling
Control loop
gain
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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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
2008.11.12.
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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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
2008.11.12.
First LISA-DECIGO Meeting: Common Technologies
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)
2008.11.12.
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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Example 3 of Li et al. (IJMPD 2008):
the variations of the arm-lengths, trailing angle,
velocities in the measurement direction and the
angles between arms
2008.11.12.
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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Doppler Shifts
Doppler shift is due to the line-ofsight velocity between the
spacecraft
which we inspect in more detail in the next slides
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Line-of-sight velocity
between near-Earth and inner-orbit spacecraft
Peak velocities are of order 20 km/s, both ways
2008.11.12.
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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Line-of-sight velocity
between near-Earth and outer-orbit spacecraft
Peak velocities are of order 20 km/s, both ways
2008.11.12.
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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Doppler shift due to
line-of-sight velocity
With l.o.s. velocities of up to 20 km/s at certain
epochs, for wavelength 1.064 µm one would get a
Doppler shift of as much as 20 GHz, which needs to be
synthesised.
Attempt to pull laser frequency according to
Doppler shift (up to 0.01 %), might also be a
possibility.
2008.11.12.
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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Time delay interferometry:
Technology
common to LISA and ASTROD
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2008.11.12.
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 is
ideally solved by using optical comb
generator and optical frequency synthesizer
together with optical clock
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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Technology common to all
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2008.11.12.
Drag-free technology
(including sensors and micro-thrusters),
Charge management system,
Thermal diagonostic and thermal
control,
Lasers and optics.
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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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
2008.11.12.
Proof mass
Large gap
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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Laser Metrology
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2008.11.12.
Laser metrology is the technology
common to the needs of DECIGO
and ASTROD
LISA community is developing
various laser metrology methods
and various optical sensing methods
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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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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2008.11.12.
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).
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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Primordial GW and Super-ASTROD
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2008.11.12.
For detection of primordial GWs in space. One may
go to frequencies lower or higher than LISA/ASTROD
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
Super-ASTROD look for gravitational waves in the
lower range.
Super-ASTROD (ASTROD III) : 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.
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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Sensitivity to Primordial GW
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2008.11.12.
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 27,000 times
(30^3) better sensitivity due to this reason, while
Super-ASTROD has an additional 125 (53) times
better sensitivity.
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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Primordial Gravitational Waves
[strain sensitivity  (ω^2) energy sensitivity]
2008.11.12.
First LISA-DECIGO Meeting: Common Technologies
W.-T. Ni
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Thank you !
Especially to
Professor Tsubono and his group for
inviting me to this meeting and
organizers of this meeting for giving
me a chance to give this talk
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