Astrodynamic Space Test of Relativity usingOptical Devices (ASTROD I)

Hanns SeligZARM, University of Bremen, Germany,

Paris, MG12 2009

ASTROD I – A class-M FP mission proposal for CV 2015-2025in Exp. Astronomy 23 (2009)

H.Dittus, H.Krüger, S.TheilIstitute of Space Systems DLR-RY, Bremen, Germany,

Claus Lämmerzahl, Hanns SeligZARM, University of Bremen, Germany,

Wei-Tou Ni, Antonio Pulido PatónPurple Mountain Observatory,Nanjing, PR China

A.RüdigerMPI Grav., Hannover Germany

Laurent GizonMPI Solar System Research, Lindau-Katlenburg, Germany

Alberto LoboInst. d´Estudis Espacials de Catalunya, Barcelona, Spain

Etienne SamainOCA, Grasse, France

Pierre TouboulONERA 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-orbitOptical inter-satellite link

Sun

Inner Orbit

Earth Orbit

Outer OrbitLaunch Position

. Earth (800 days after launch)

L1 point

Laser Ranging

S/C 2

S/C 1

Paris, MG12 2009

ASTROD I

Scaled-down version of ASTROD1 S/C in an helio-centric orbitDrag-free AOCLaunch via low earth transfer orbit to solar orbit with orbit period 294 daysFirst encounter with Venus at 150 days after launch; Second encounter with Venus at 260 days after launchOpposition to the Sun: shortly after 370 days, 718 days, and 1066 days

Paris, MG12 2009

ASTROD I science goals

βγ

Needs 2PN (post-post-Newtonian) framework together with corresponding ephemeris for data fittingcapable to detect the time delay (20 ps) due to the gravitomagnetic field caused by the intrinsic rotationTest of MOND theoriesTest of gravitational Dark Matter / Dark Energy theories

Paris, MG12 2009

Cassini-Exp. / Shapiro Time Delay

Cassini Conjunction Experiment 2002:

Satellit - Earth distance > 109 kmRanging: X~7.14GHz & Ka~34.1GHz (dual band)Result: γ = 1 + (2.1 ± 2.3) × 10−5

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Time delay for curved space-time due to grav. fields of Sun and Earth

Paris, MG12 2009

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Venus Mercury spacecraft

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Shapiro time delays for ASTROD I

0 200 400 600 800

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Mission DaySimulation 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

βγ

Needs 2PN (post-post-Newtonian) framework together with corresponding ephemeris for data fittingcapable to detect the time delay (20 ps) due to the gravitomagnetic field caused by the intrinsic rotationTest of MOND theoriesTest 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 telescopeSends light back to earthPulsed laser lightLaser mean power: 1 to 2 WPulse width: 20 psRepetition rate: 100 HzIn conjunction: 10-13 W receivedOutgoing light polarized against incoming light

Paris, MG12 2009

Sunlight entering the optical bench

Telescopes point to each other in the plane of eclipticFor a 30 cm telescope with 0.07 m2 aperture ca. 100 W of light power areentering.Sun light must be kept away from the optical bench.Incoming laser light power is only 100 fWSeveral stages of light blocking are needed to reduce background lightby 15 orders of magnitude.

Paris, MG12 2009

Background (sun) light elimination

Spectral filteringSpatial filteringTemporal 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 clockFor a ranging uncertainty of 3 mm in a distance of 3 × 1011 m (2 AU), thelaser/clock frequency needs to beknown 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 mmThis reduces demands on drag-free AOC by orders of magnitudeNevertheless, drag-free AOC is needed to avoid contact between test massand cage.

Paris, MG12 2009

Conclusion

ASTROD I: Deep space Astrondynamics Mission with laserranging

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