ASTROD: a multipurpose mission for testing relativistic gravity in space

ASTROD: a multipurpose mission for testing relativistic

gravity in space

presented by

Antonio Pulido Patón

Center for Gravitation and Cosmology, Purple Mountain Observatory, CAS, Nanjing 210008

Relativistic Astrophysics workshop, Nanjing University, August 2006

Outline

• ASTROD mission concept.• Scientific objectives: measuring relativistic

parameters, gravitational waves, solar physics, etc.

• Crucial technology required: weak phase locking, sunlight shield, drag-free system.

• ASTROD I. ASTROD technology demonstrator.

• ASTROD I scientific objectives.

The road towards ASTROD• Laser Astrodynamics is proposed to study relativistic gravity and

to explore the solar system, 2nd William Fairbank conference (Hong Kong), and International workshop on Gravitation and Fifth Force (Seoul) 1993.

• A multi-purpose astrodynamical mission is reached (1994) in 7th Marcel Grossmann, July, 1994, Stanford (California).

• ASTROD (Astrodynamical Space Test of Relativity using Optical Devices) presented at 31st COSPAR Scientific Assembly July 1996.

• ASTROD and its sensitivity to dG/dt measurements presented in the Pacific conference on Gravitation and Cosmology. Seoul (Korea) February 1996.

• ASTROD and its related gravitational wave sensitivity presented at TAMA Gravitational Wave Workshop in Tokyo (Japan) 1997.

• The possibility of solar g-mode detection was presented in 3rd Edoardo Amaldi and 1st ASTROD Symposium (2001).

Current ASTROD Collaborators Purple Mountain

Observatory ,CAS

Wei-Tou Ni, Gang Bao, Guangyu Li, H-Y Li, A. Pulido Patón, F. Wang, Y. Xia, Jun YanCAST, L. Wang, J.Chang ,X. Hou, Z. Song, Q. Zhang,IP, CAS, Y-X Nie, Z. WeiYunnan Obs, CAS, Y.Xiong ITP, CAS, Y-Z Zhang

Nanjing U Tianyi Huang

Tsing Hua U Sachie Shiomi

Nanjing A & A U H. Wang

Nanjing N U, X. Wu, C. Xu

H S & T U, Ze-Bing Zhou

ZARM, Bremen Hansjörg Dittus Claus Lämmerzahl

Stephan Theil Imperial College Henrique Araújo Diana Shaul Timothy Sumner

CERGA J-F Mangin

Étienne Samain ONERA Pierre Touboul

Humboldt U, Berlin Achim Peters

U Düsseldorf Stephan Schiller

Andreas Wicht Max-Planck, Gårching Albrecht Rüdiger

Technical U, Dresden Sergei Klioner Soffel

U Missouri-Columbia Sergei Kopeikin

IAA, RAS George Krasinsky Elena Pitjeva

Nanyang U, Singapore H-C Yeh (Zhong Shan U aft

er July 1)

International collaboration period

• 2000: ASTROD proposal submitted to ESA F2/F3 call (2000)

• 2001: 1st International ASTROD School and Symposium held in Beijing; Mini-ASTROD study began

• 2002: Mini-ASTROD (ASTROD I) workshop, Nanjing• 2004: German proposal for a German-China ASTROD col

laboration approved• 2005: 2nd International ASTROD Symposium (June 2-3,

Bremen, Germany)• 2004-2005: ESA-China Space Workshops (1st &2nd, Noo

rdwijk & Shanghai), potential collaboration discussed• 2006(7): Joint ASTROD (ASTROD I) proposal to be submi

tted to ESA call for proposals• 2006: 3rd ASTROD Symposium (July 14-16, Beijing).

ASTRODynamical Space Test of Relativity using Optical Devices

Sun

Inner Orbit

Earth Orbit

Outer OrbitLaunch Position

. Earth (800 days after launch)

L1 point

Laser Ranging

S/C 2

S/C 1

ASTROD mission concept is to use three drag-free spacecraft. Two of the spacecraft are to be in an inner (outer) solar orbit employing laser interferometric ranging techniques with the spacecraft near the Earth-Sun L1 point. Spacecraft payload: a proof mass, two telescopes, two 1 W lasers, a clock and a drag-free system.

ASTROD scientific objectives

• Test Relativistic gravity with 3-5 orders of magnitude improvement in sensitivity. That includes the measurement of relativistic parameters β, γ, solar quadrupole moment J2, measurement of dG/dt, and the anomalous constant acceleration towards the Sun (Pioneer anomaly).

• Improvement by 3-4 orders of magnitude in the measurements of solar, planetary and asteroids parameters. That also includes a measurement of solar angular momentum via Lense-Thirring effect and the detection of solar g-modes by their changing gravity field.

• Detection of low frequency gravitational waves (5 µHz-5 mHz) from massive black hole and galactic binary stars.

ASTROD technological requirements

• Weak-light phase locking to 100 fW.• Heterodyne interferometry and data analysis for

unequal-arm interferometry.• Coronagraph design and development: sunlight in

the photodetectors should be less than 1 % of the laser light.

• High precision space clock and/or absolute stabilized laser to 10-17.

• Drag-free system. Accelerometer noise requirement: • (0.3-1)×10-15[1+10×(ƒ3mHz)2] m s-2 Hz-1/2 at 0.1 mHz <

ƒ < 100 mHz.• Laser metrology to monitor position and distortion of

spacecraft components for gravitational modeling.

ASTROD orbit design features

• The distance to the Sun of the inner spacecraft varies from 0.77 AU to 1 AU and for the outer spacecraft varies from 1 AU to 1.32 AU.

• The two spacecraft should go to the other side of the sun simultaneously to perform Shapiro time delay.

• To obtain better accuracy in the measurements of Ġ and asteroid parameters’ estimation, one spacecraft should be in inner orbit and the other in outer orbit.

• The two spacecraft at the other side of the sun should be near to each other for ranging in order to perform measurements of Lense-Thirring effect (measurement of solar angular momentum).

Testing Relativistic GravityThe Eddington-Robertson parameterization of the

metric,

• Parameterγ describes how much space-time curvature is produced by unit rest mass.

• Parameter β describes how much nonlinearity there is in the superposition law of gravity.

22 2

2 2 2 2 2 2

1 2 2 ...

1 2 ... sin

GM GMds dt

r r

GMdr r d r d

r

Testing Relativistic Gravity II

• In the context of studies of Einstein Equivalence Principle (EEP) the PPN formalism was proposed to distinguish between different gravitational theories.

• Different values for γ, β are predicted by different theories of gravity.

• In the case of General Relativity γ and β are equal to 1.

Tests of PPN space parameter γ• Values of parameter γ can be obtained by measuring

light deflection (by the sun for example), light retardation (Shapiro time delay), geodetic deviation.

• ASTROD will obtain γand β by measuring time retardation of light.

• ASTROD aims at an accuracy of 10-9 for parameter γ. This will require SECOND POST NEWTONIAN APPROXIMATION.

Relativistic parameter uncertainties for ASTROD

• The uncertainty of relativistic parameters (β,γ and J2) assuming 1 ps accuracy and ASTROD acceleration noise ~ 3×10-18 m s-2 (0.1 mHz) are, 1200 days after launch:

γ≈ 1.05×10-9; β≈1.38×10-9 and

J2≈3.8×10-11

Relativistic parameter uncertainty evolution

0 500 1000 1500 2000 2500 3000

10-9

10-8

10-7

10-6

10-5

10-4

10-3

Un

ce

rta

inty

Time (day)

1.0486167e-09 is achieved after observation 1200 days 6.8745917e-10 is achieved after observation 3000 days

Relativistic parameter

0 500 1000 1500 2000 2500 3000

10-9

10-8

10-7

10-6

10-5

10-4

10-3

Un

ce

rta

inty

Time (day)


Relativistic parameter

0 500 1000 1500 2000 2500 3000

10-11

10-10

10-9

10-8

10-7 3.79968e-11 is achieved after observation 1200 days 9.04288e-12 is achieved after observation 3000 days

Un

ce

rta

inty

Time (day)

J2 of Sun

γ β

J2

Solar angular momentum

• Lense-Thirring effect can be measured by taking the time difference between the light round trips SC1 - SC2 - Earth system basis and SC2 - SC1 - Earth System basis.

• The Newtonian time difference t1-t2 for 800-1034 days after launching gives about 10 ms. The Lense-Thirring effect has a totally different signature and for this period of time is about 100 ps.

• Assuming a laser stability of 10-15-10-13 one could achieve 10-5-10-7 level of uncertainty.

• Lense-Thirring effect is proportional to the solar angular momentum.

Evolution of ĠG and Pioneer anomaly uncertainties

0 500 1000 1500 2000 2500 3000

10-15

10-14

10-13

10-12

10-11

10-10


Un

ce

rta

inty

Time (day)

(dG/dt)/G

0 500 1000 1500 2000 2500 3000

10-17

10-16

10-15

10-14

10-13

10-12


Un

ce

rta

inty

Time (day)

Aa

Ġ measurement and anomalous acceleration towards the Sun

• ĠG≈2.82×10-15 yr-1 and the anomalous acceleration towards the sun Aa≈3.02×10-17 m s-2

(again assuming 1 ps and 3×10-18 m s-2).• By using an independent measurement of Ġ

ASTROD would be able to monitor the solar mass loss rate. The expected solar mass loss rate: a) electromagnetic radiation ~ 7×10-14 Msunyr, b) solar wind ~10-14 Msunyr, c) solar neutrino ~ 2×10-15 Msunyr and d) solar axions ~ 10-15 Msunyr.

Gravitational wave detection

l1

l2 θ1

θ2

SC 2

SC 1

ERS

Gravitational detection topology. Path 1: ERS-SC1-ERS-SC2-ERS. Path 2: ERS-SC2-ERS-SC1-ERS. To minimize the arm-length difference.

For example if a monochromatic gravitational wave with + polarization arrives orthogonal to the plane formed by SC1, 2 and ERS, then the optical path difference for laser light traveling through path 1 and 2 and returning simultaneously at the same time t, is given by

With 1=2l1c and 2=2l2c.

1 2 1 2 1 2 04 ( )(cos 2 cos 2 ) cos 2 ( ) cos 2 ( ) cos 2 ( )G G Gl h c f f f f t

Detecting Gravitational Waves

• To estimate gravitational wave strength sensitivity1/ 2 3/ 2

0 1/ 202 2 2

0

21 4( ) ,

sin (2 )Mh

t

AhcS f rss Hz

cu P D L f

Lasing power (Pt): LISA 1W, ASTROD 10 WShot noise level (ASTROD)≈ 1.2×10-21

Acceleration noise (A0) is the dominant source of noise at low frequencies.

0

Lwith u

c

Gravitational wave strain sensitivity for ASTROD compared to LISA

1E-25

1E-24

1E-23

1E-22

1E-21

1E-20

1E-19

1E-18

1E-17

1E-16

1E-15

1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02 1.00E-01

Frequency (Hz)

Gra

vita

tio

nal

Wav

e S

trai

n

LISA Bender extension

LISA, 1 yr int. time S/N=5

ASTROD, 1 yr int. time, S/N=5

Solar g-modes

• When the spacecraft moves in solar orbit the amplitude and direction of the solar oscillation signals are deeply modulated in addition to the modulation due to spacecraft maneuvering.

• Time constants for solar oscillations are about 106 yr for low-l g-modes and over 2-3 months for low-l p-modes. Close white dwarf binaries (CWDB) time constant are longer than 106 yr. Hence confusion background is steady in inertial space, only modulated by spacecraft maneuvering and not by spacecraft orbit motion.

• With this extra modulation due to orbit motion the solar oscillation signals can reach 5 orders lower than the binary confusion limit.

Technological requirements: ASTROD drag-free

1/ 2 215 2 1/ 23 10 1

3xfS f

m s Hzm mHz

410 0.1Hz f Hz

LISA acceleration noise of free fall test masses

ASTROD aims to improve LISA acceleration noise at 0.1 mHz by a factor 3-10, i.e., approx. 0.3-1×10-15 ms-2 Hz-1/2.

ASTROD bandwidth 5 µHz≤ƒ≤5mHz

Drag-free control concept

Thrusters

Spacecraft (SC)

Proof mass (PM)

SC-PM

Stiffness (K)

Spacecraft acceleration disturbances (ns)

Direct PM acceleration disturbances (np)

Thrusters

PM acceleration disturbance:

p -KXnr + np + (ns + TNt)Ku-1-2

ASTROD acceleration noise goal

1.00E-16

1.00E-15

1.00E-14

1.00E-13

1.00E-12

1.00E-11

1.00E-10

1.00E-06 1.00E-05 1.00E-04 1.00E-03

Frequency (Hz)

Acc

eler

atio

n n

ois

e u

nit

s m

s^(-

2)^

Hz^

(-1/

2)

LISA

LISA Bender

ASTROD

ASTROD acceleration noise goal compared to LISA and LISA extended version at low frequencies proposed by P. Bender [11].

ASTROD Gravitational Reference Sensor (GRS) preliminary concept

Move towards true drag-free conditions and improving LISA drag-freeperformance by a factor 3-10.

1. GRS provide reference positioning only. Laser beam does not illuminate directly the proof mass (avoiding cross coupling effects and pointing ahead problem) surface but the GRS housing surface.

2. Only one reference proof mass. GRS measures the center of mass position of the proof mass.

3. Optical sensing could replace capacitive sensing. Capacitance could still be used for control purposes.

4. Absolute laser metrology to measure structural changes due to thermal effects and slow relaxations.

LISA has adopted condition 1. Both conditions 1 and 2 avoid cross couplingdue to control forces aimed to keep the right orientation of the proof massmirror.

ASTROD will employ separate interferometry to measure the GW signal andthe proof mass-spacecraft relative displacement independently.

ASTROD GRS

Dummy telescope

Proof mass

Housing

Anchoring

Telescope

LASER Metrology

Telescope

Dummy telescopeProof mass

Outgoing Laser beam

Optical readoutbeam

Large gap Incoming Laser beam

Schematic of possible GRS designs for ASTROD: a) a cubical proof mass free floating inside a housing anchored to the spacecraft, b) a spherical (cylindrical) proof mass is also considered.

a) b)

Acceleration disturbances

• Position dependent (stiffness terms): a) sensor readout noise and b) external environmental disturbances affecting the spacecraft including thruster noise.

• Direct acceleration disturbances: a) environmental disturbances and b) sensor back action disturbances.

• For sensing proof mass-spacecraft relative displacement and control actuation both capacitive andor optical sensing will be considered.

Direct proof mass acceleration disturbances

•Magnetic interactions due to susceptibility () and permanent moment of the proof mass (Mr)

•Lorentz forces due to proof mass charging (Q).

•Thermal disturbances (radiometer, out gassing, thermal radiation pressure and gravity gradients).

•Impacts due to cosmic rays and residual gas.

Q, , MrV

BSC (Spacecraft

magnetic field)Bip (Solar magnetic field)

T (temperature fluctuations) induces P (pressure

fluctuations) and Gravity Gradients in the spacecraft

Cosmic rays impacts.

Residual gas

Environmental disturbances at 0.1 mHz

Capacitive sensing

V2 V1

Vg

Cg

Cx1Cx2

d-Δd d+Δd

•Capacitive sensing needs very close metallic surfaces to achieve good readout sensitivity.•Displacement readout sensitivity is proportional to d-1 or d-2 for different readout configurations.•By decreasing the gap, readout sensitivity increases but also back action disturbances and stiffness terms increase.

Back action disturbances at 0.1 mHz (Capacitive Sensing)

Low-frequency acceleration disturbances

• Magnetic interaction between the proof mass susceptibility and the interplanetary field.

• Capacitive sensing back action

2/3

16 2 1/ 22 6

0

2 1 10 0.10.20 10

3 10m sc ipm m

mHzf B B m s Hz

f

16 2 1/ 2,1 4

1 0.12 0.72 10

5 10x d

q dP

C V mHzf V q m s Hz

m d C f

Low-frequency acceleration disturbances II

TOB is Optical bench temperature fluctuations [11],1/ 2 1/ 2

4 5 1/ 2 15

5 5 1/ 2 14

5 3 1/ 2 13

6 2

1 1 0.110 , 3.0 10 , 1.2 10 ;

13 10 , 32 10 , 1.2 10 ;

110 , 12 10 , 4.6 10 ;

3 10 , 13 10

OB TRTS

OB TRTS

OB TRTS

OB

mHz mHzf Hz T K Hz f

f f

f Hz T K Hz f

f Hz T K Hz f

f Hz T K Hz

1/ 2 12 1, 4.9 10 ;TR

TS

f

ƒTR (Thermal Radiation Pressure) in units m s-2 Hz-1/2

Coherent Fourier components• Arise due to steady build up of charge on the test mass

Protons

Q(t) = 17.012t

0

20

40

60

80

100

120

140

0 2 4 6 8

Time(s)

net c

harg

e(e+

)

Q(t)

t

tQtQtQ

Coherent terms

( ) xk k d

T

QCe t t V t

mC d

22 2

2 2

2( )k k

T

Qf t t t d

C md

x x ip el t t QvB t m

Coulomb:

Lorentz:

Coherent Charging Signals (CHS) • CHS due to Coulomb forces are due to geometric (machining accuracy)

and voltage offsets (non-uniformity in the sensor surfaces, to minimise work function differences, patch effects, etc) in the capacitive sensor.

• These signals increase at low frequencies.

• The magnitude of these signals have been shown to compromise the target acceleration noise sensitivity of LISA (see D. N. A. Shaul [12])

• Ways of dealing and/or suppressing these signals are also discussed in [12].

1E-18

1E-17

1E-16

1E-15

1E-14

1E-13

1E-12

1E-11

1E-10

1E-09

1E-04 1E-03 1E-02 1E-01

f (Hz)

Acceleration spectral density

(ms

-2Hz -0.5)

Coulomb ~ t

Coulomb ~ t2

Lorentz ~ t

LISA

LISA PF

ASTROD I

ASTROD

Discharging schemes• Accumulation of charge in the test mass induces acceleration disturbances

through Lorentz and Coulomb interactions (if employing capacitive sensing).• Position dependent Coulomb forces also contributes to the coupling

between the proof mass and the spacecraft.• Discharging periodically the proof mass to maintain this signals under the

allowable limits, introduce coherent Fourier components as mentioned before.

• These CHS can spoil the sensitivity for a mission as LISA and therefore for ASTROD. Therefore there is a need for looking into continuous discharging schemes to suppress CHS.

• Recently, a deep UV LED as the promising light source for charge management was identified by Ke-Xun Sun et al. (Hansen Experimental Physical Laboratory, Stanford University, CA 94305-4085, USA). This system could have advantages over the more traditional mercury lamp-based system in three key areas: power efficiency, lower weight and flexible functionalities including AC operation out of the science measurement band

Optical sensing• A drawback of capacitive sensing is the need for close gaps between

metallic surfaces to increase sensitivity. The sensitivity is proportional to the difference between capacitance (C1-C2), therefore proportional to d-2.

• Optical sensing allows us using larger gaps between the PM and surrounding metallic surfaces.

• Optical sensing provides a way of sensing essentially free of stiffness.• Optical sensing sensitivity is limited by shot noise. Picometer

sensitivity can be achieved with W of lasing power and 1.5 m wavelength.

• Back action force, 2P/c, can be made negligible, with 1% compensation ~10-17 m s-2 Hz-1/2.

1/ 21

2nr

hcX

P

Efforts towards optical sensing I

• Optical lever. A test mass displacement induces a transversal beam displacement which is detected by the position sensor.

• Relevant noise sources: a) shot noise and b) amplifier current noise (ƒ-1/2).• Back action force disturbance depends on power fluctuations.• In Acernese et al. [10] they achieve displacement readouts of the order of 10-9 m Hz-1/2

down to 1 mHz.

Efforts towards optical sensing II

•Figure on the right side shows a GRS where the test masses are merged into a spherical proof mass [8]. •They consider all-reflective grating beam splitters, minimizing optical path errors due to temperature dependence refractive index. They demonstrate an optical sensing of 30 pm Hz-12.

Efforts towards optical sensing III• In Speake et al. [9] a prototype bench top polarization-based homodyne

interferometer based on wavelength modulation technique achieve a shot limited displacement sensitivity of 3 pm Hz-1/2 above 60 Hz (using 850 nm VCSEL with 60 nW optical power).

Picowatt and femtowatt weak light phase locking

• LISA needs to achieve weak phase locking of the order of 85 pW. Because of longer armlengths ASTROD I and ASTROD have to probe weak phase locking of the order of 100 femtowatts (assuming 1 W lasing power from far spacecraft).

uƒ (t) ud

(t)

(ω2)Output signal I2 (t)

Input signal I1

(t) (ω1) PD LF

Far-end Laser

~100 fw

VCO

Laboratory research on weak light phase locking for ASTROD14

Low Power Beam Intensity (measured using oscilloscope)

20 nW 2 nW 200 pW

20 pW

2 pW

High Power Beam

Intensity mW

2 2 0.2 0.2 0.2

Low Power Intensity Measured by Lock-in Amplifier

20.9 nW 2.15 nW

153 ~247 pW

N/A N/A

r.m.s. Error

signal Vrms mV

2.01 2.06 2.29 2.03 2.70

r.m.s Phase

error rad 0.0286 0.057 0.2 0.16 0.29

Phase-locking time

Longer than

observation duration

Longer than

observation duration

> 2 hours

> 2 hours

1.5 mins

Results on weak light phase locking for ASTROD (20 pW)

Error signal X: 20 µs/div Y:10 mV/div

LockedX: 50 ms/divY: 10 mV/div

FFT of the Error signal FFT of the Locked signal X: 125 kHz/divY:20 dB/div-70 dBm

Beat signal: 29.3 kHz

Weak light Phase Locking

• Requirement: phase locking to 100 fW weak light• Achieved: phase locking of 2 pW weak light with 200 µW local

oscillator• With pre-stabilization of lasers, improving on the balanced

photodetection and lowering of the electronic circuit noise, the intensity goal should be readily be achieved

• This part of challenge will be focussed on offset phase locking, frequency-tracking and modulation-demodulation to make it mature experimental technique (also important for deep space communication)

• Weak light phase locking experiment re-started at PMO

Sunlight Shield System

Narrow band filterFADOF filter

Sun shutter

Design of Sunlight Shield System

• The sunlight shield system consists of a narrow-band interference filter, a FADOF (Faraday Anomalous Dispersion Optical Filter) filter, and a shutter

• The narrow-band interference filter reflects most of the Sun light directly to space

• The bandwidth of the FADOF filter can be 0.6-5 GHz

• With the shutter, the Sun light should be less than 1 % of the laser light at the photodetector

FADOF

Faraday Anomalous Dispersion Optical Filter

A gas cell, of proper temperature, under proper magnetic field, can rotate the polarization of a beam of a particular frequency so that it the can be separated from other frequencies, in a very narrow-band fashion

This scheme has become standard technology, it has been applied to several problems, including a number of space missions

FADOF in ASTROD

50% of incoming sunlight has wrong polarization, is deflected; of remaining 50% very little is in the (narrow) FADOF bandwidth, so almost all of it is also deflected, only small portion (laser light) passes

ASTROD I

• Testing relativistic gravity with 3-order-of-magnitude improvement in sensitivity;

• Astrodynamics & solar-system parameter determination improved by 1-3 orders of magnitude;

• Improving gravitational-wave detection compared to radio Doppler tracking (Auxiliary goal).

Two-Way Interferometric and Pulse Laser Ranging between Spacecraft and Ground Laser Station

Relativistic parameter uncertainties for ASTROD I

• Assuming 10 ps timing accuracy and 10-13 m s-2 Hz-1/2 (ƒ = 0.1 mHz), a simulation for 400 days (350-750 days after launch) we obtain uncertainties forγ, β and J2 about 10-7, 10-7 and 3.8×10-9.

ASTROD I spacecraft: general features

1. Cylindrical spacecraft with diameter 2.5 m, 2 m height, and surface covered with solar panels.

2. In orbit, the cylindrical axis is perpendicular to the orbit plane with the telescope pointing toward the ground laser station. The effective area to receive sunlight is about 5 m2 and can generate over 500 W of power.

3. The total mass of spacecraft is 300-350 kg. That of payload is 100-120 kg.

4. Science rate is 500 bps. The telemetry rate is 5kbps for about 9 hours in two days.

ASTROD I spacecraft schematic design

Thermal Control

Black Surface

Black Surface

FEEP

Power Unit

Power Unit

Pulse Laser

CW LasersClockOptical Comb

Optical Cavity

FEEP

TIPO

Electronics

Telescope

Thermal Control

Black Surface

Black Surface

FEEP

Power Unit

Power Unit

Pulse Laser

CW LasersClockOptical Comb

Optical Cavity

FEEP

TIPO

Electronics

Telescope

Payload (1) Laser systems for interferometric and pulse ranging

(i) 2 (plus 1 spare) diode-pumped Nd:YAG laser (wavelength 1.064 m, output power 1 W) with a

Fabry-Perot reference cavity: 1 laser locked to the Fabry-Perot cavity, the other laser pre-stabilized by this laser and phase-locked to the incoming weak light. (ii) 1 (plus 1 spare) pulsed Nd:YAG laser with transponding system for transponding back the incoming laser pulse from ground laser stations. (2) Quadrant photodiode detector(3) 380-500 mm diameter f/1 Cassegrain telescope (transmit/receive), /10 outgoing wavefront quality

Payload II

(4) Sunlight Shield System (5) Drag-free proof mass (reference mirror can be separate):

50 35 35 mm3 rectangular parallelepiped; Au-Pt alloy of extremely low magnetic

susceptibility (<10-5); Ti-housing at vacuum 10-5 Pa ; six-degree-of- freedom capacity sensing.

(6) Cesium clock(7) Optical comb

ASTROD I drag-free requirement

1/ 2 215 2 1/ 23 10 1

3xfS f

m s Hzm mHz

410 0.1Hz f Hz

1/ 2 214 2 1/ 20.3

3 10 303

xfS mHz fm s Hz

m f mHz

410 0.1Hz f Hz

ASTROD I acceleration noise of free fall test masses

LISA acceleration noise of free fall test masses

Ground Station for the ASTROD I Mission at Yunnan Observatory

◆◆ Introduction of Yunnan Observatory 1.2m TelescopeIntroduction of Yunnan Observatory 1.2m Telescope

＆＆ Its Laser Ranging SystemIts Laser Ranging System

Key Requirements of ◆ Key Requirements of ◆ Ground Station for the Mission

Telescope Requirement: Pointing and Tracking Accurac◆ Telescope Requirement: Pointing and Tracking Accurac◆

yy

◆◆ Atmospheric Turbulence Effects on Laser Ranging

Yunnan Observatory 1.2 m telescopeYunnan Observatory 1.2 m telescope

Its Laser Ranging SystemIts Laser Ranging System

Coordinates:Coordinates:

Latitude Latitude

25.0299 25.0299 N N

Longitude Longitude

102. 7972 102. 7972 E E

Elevation Elevation

1991.83 m1991.83 m

Launcher and Mission Lifetime

• Launcher: Long March IV B (CZ-4B)

• Mission Lifetime:

3 years (nominal)

8 years (extended)

Outlook (ASTROD I)

• Testing relativistic gravity and the fundamental laws of space-time with three-order-of-magnitude improvement in sensitivity; gamma to 10-7 or better, beta to 10-7, J2 to 10-9, asteroid masses to 10-3 fraction

• Improving the sensitivity in the 5 µHz - 5 mHz low frequency gravitational-wave detection by several times;

• Initiating the revolution of astrodynamics with laser ranging in the solar system, increasing the sensitivity of solar, planetary and asteroid parameter determination by 1-3 orders of magnitude.

• Optimistic date of launch: 2015

Conclusions

• ASTROD is a multi-purpose space mission employing pulse and interferometric ranging to measure relativistic and solar system parameters, and low-frequency gravitational waves.

• A ten-fold improvement in acceleration noise would allow us to reach relativistic parameter uncertainties at the ppb level.

• A simple version of ASTROD, ASTROD I, aims to test the feasibility of crucial technology for ASTROD and yet to obtain important scientific results.

Thank you!

Documents

ASTROD: a multipurpose mission for testing relativistic gravity in space