Astrodynamic Space Test of Relativity using
Optical Devices (ASTROD I)
Hanns Selig
ZARM, University of Bremen, Germany,
ASTROD I – A class-M FP mission proposal for CV 2015-2025
in Exp. Astronomy 23 (2009)
H.Dittus, H.Krüger, S.Theil
Istitute of Space Systems DLR-RY, Bremen, Germany,
Claus Lämmerzahl, Hanns Selig
ZARM, University of Bremen, Germany,
Wei-Tou Ni, Antonio Pulido Patón
Purple Mountain Observatory,Nanjing, PR China
A.Rüdiger
MPI Grav., Hannover Germany
Laurent Gizon
MPI Solar System Research, Lindau-Katlenburg, Germany
Alberto Lobo
Inst. d´Estudis Espacials de Catalunya, Barcelona, Spain
Etienne Samain
OCA, Grasse, France
Pierre Touboul
ONERA DMPH, France
and the ASTROD I Study team
Paris, MG12 2009
ASTROD science goals
Astrodynamical Space Test of Relativity using Optical Devices
„ Testing relativistic gravity and the fundamental laws of spacetime with 5
order-of-magnitude improvement in sensitivity;
„ Improving the sensitivity in the 5 µHz to 5 mHz low frequency range for
gravitational-wave detection by several orders of magnitude as in LISA but
shifted toward lower frequencies;
„ Increasing the sensitivity of solar, planetary and asteroid parameter
determination by 3-4 orders of magnitude.
„ Detecting of solar g-mode oscillations
„ Demands post-post-Newtonian ephemeris framework to be established for
the analysis and simulation of data.
Paris, MG12 2009
ASTROD mission concept
„ 2 S/C on helio-centric orbits / 1 S/C on L1-orbit
„ Optical inter-satellite link
S/C 2
S/C 1
Laser Ranging
Inner Orbit
Sun
Launch Position
Outer Orbit
Earth Orbit
. L1 point
Earth (800 days after launch)
Paris, MG12 2009
ASTROD I
„ Scaled-down version of ASTROD
„ 1 S/C in an helio-centric orbit
„ Drag-free AOC
„ Launch via low earth transfer orbit to
solar orbit with orbit period 294 days
„ First encounter with Venus at 150 days
after launch;
„ Second encounter with Venus at 260
days after launch
„ Opposition to the Sun: shortly after 370
days, 718 days, and 1066 days
Paris, MG12 2009
ASTROD I science goals
β
γ
„ Needs 2PN (post-post-Newtonian) framework together with corresponding
ephemeris for data fitting
„ capable to detect the time delay (20 ps) due to the gravitomagnetic field
caused by the intrinsic rotation
„ Test of MOND theories
„ Test of gravitational Dark Matter / Dark Energy theories
Paris, MG12 2009
Cassini-Exp. / Shapiro Time Delay
Time delay for curved space-time due to grav. fields of Sun and Earth
∆t =
ρ ρ
rt C − rsC
c
ρS ρS
ρE ρE
2 ⎞
S
S
E
E
⎛
⎛
(
)
+
+
−
+
+
+
+
r
r
1
G
r
r
r
r
γ
m
/
c
r
(
1 + γ )GmS ⎜ s
t
t
s
S
t
t − rs
(
1 + γ )GmE ⎜ s
⎟
+
+
ln⎜ S
ln⎜ E E ρE ρE
ρS ρS
3
3
S
2 ⎟
c
c
r + rt − rt − rs + (1 + γ )GmS / c
r + rt − rt − rs
⎝ s
⎠
⎝ s
⎞
⎟
⎟
⎠
Cassini Conjunction Experiment 2002:
ƒ Satellit - Earth distance > 109 km
ƒ Ranging: X~7.14GHz & Ka~34.1GHz (dual band)
ƒ Result:
γ = 1 + (2.1 ± 2.3) × 10−5
Paris, MG12 2009
Shapiro time delays for ASTROD I
Shapiro Time Delay µs)
0.8
(B)
0.6
0.4
Y Axis (AU)
0.2
0.0
Sun
-0.2
-0.4
-0.6
-0.8
Venus
Mercury
spacecraft
-1.0
-0.8 -0.6 -0.4 -0.2
0.0
0.2
X axis (AU)
0.4
0.6
0.8
1.0
120
111.4 µs
107.2 µs
100
80
60
40
20
0
0
200
400
Mission Day
600
800
Simulation with:
(1) Uncertainty due to the imprecision of the ranging devices:
10 ps one way (Gaussian)
(2) Unknown acceleration due imperfections of the S/C drag-free AOC:
10-15m/s2
+ change direction randomly every 4 hr (~104s)
Equivalent to 10-13m/s2(Hz)-½ @ 10-4Hz
Paris, MG12 2009
ASTROD I orbit
718 day
Paris, MG12 2009
ASTROD I science goals
β
γ
„ Needs 2PN (post-post-Newtonian) framework together with corresponding
ephemeris for data fitting
„ capable to detect the time delay (20 ps) due to the gravitomagnetic field
caused by the intrinsic rotation
„ Test of MOND theories
„ Test of gravitational Dark Matter / Dark Energy theories
Paris, MG12 2009
Uncertainties of the solar quadrupol moment
„
δJ2/J2 as a function of mission time
Paris, MG12 2009
ASTROD I – Optics design
„
Ground station and the S/C communicate with each other via optical links.
„
Ranging with pulsed Laser measurements
„
S/C carries a telescope, 4 stabilized lasers and an optical bench:
– 2 (plus 2 spare) pulsed Nd:YAG lasers with timing system for recording
the receiving and emitting laser pulses from and to ground laser
stations.
– Quadrant photodiode detector
– 300 mm Ø Cassegrain telescope
– Sun light shield system
– Inertial sensor
– Atomic clock (cesium clock)
2 wavelengths (1064 nm and 532 nm):
Elimination of atmospheric and solar corona effects.
Paris, MG12 2009
Optical bench
„
„
„
„
„
„
„
„
Processes light collected by the telescope
Sends light back to earth
Pulsed laser light
Laser mean power: 1 to 2 W
Pulse width: 20 ps
Repetition rate: 100 Hz
In conjunction: 10-13 W received
Outgoing light polarized against incoming light
Paris, MG12 2009
Sunlight entering the optical bench
„
„
„
„
„
Telescopes point to each other in the plane of ecliptic
For a 30 cm telescope with 0.07 m2 aperture ca. 100 W of light power are
entering.
Sun light must be kept away from the optical bench.
Incoming laser light power is only 100 fW
Several stages of light blocking are needed to reduce background light
by 15 orders of magnitude.
Paris, MG12 2009
Background (sun) light elimination
„ Spectral filtering
„ Spatial filtering
„ Temporal filtering (timing)
„
Spectral filtering
– Use narrow band filter:
high standard, any wavelength < 1 nm (multi-layer dielectric filter)
– 1 nm out of 1064 nm is 10-4, so still have order of 0.1 W against ASTROD I
100 fW
„
Spatial filtering
– Pinhole
„
Temporal filtering
– 100 ns gate; 10 ms repetition rate;
– Filtering factor: 10-4
Paris, MG12 2009
Background (sun) light elimination - pinhole
Paris, MG12 2009
Background (sun) light elimination - pinhole
Paris, MG12 2009
Laser ranging / Timing
Pulse ranging (similar to SLR / LLR)
„ Timing: on-board event timer (± 2 ps)
reference: on-board cesium clock
„ For a ranging uncertainty of 3 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
„
– Single photon detector
Jason 2 S/C launched 2008
Paris, MG12 2009
Disturbance accelerations
„ Analysed with respect to:
– Gravity gradients
– Solar radiation pressure / solar wind
– Solar irradiance
– Micrometeorites
– Magnetostatic forces
– Lorentz force (due to test mass charging)
– Cosmic ray impacts
– Residual gas effects
– Radiometric effects
– Outgassing due to thermal effects
– Thermal radiation pressure
– Gravity gradients due to thermal distortions of the S/C
– Test mass sensor back action
– Capacitive fluctuations / patch effects
– Readout electronics
– Dielectric losses
Paris, MG12 2009
Disturbances and requirements
„ Estimated total acceleration disturbance @
0.1 mHz:
fp = 8.7 · 10-14 ms-2 Hz-1/2
Drag-free AOC requirements
„
„
„
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 to avoid contact between test mass
and cage.
Paris, MG12 2009
Conclusion
„ ASTROD I: Deep space Astrondynamics Mission with laser
ranging
„ Laser ranging on fW-scale
„ Drag-free AOCS to improve resolution
„ Reasonable experimental requirements
„ Enable gravitational experiments to determine Eddington
parameters and higher order PN parameters, solar J2, solar
system gravity, Pioneer anomaly
Paris, MG12 2009