1 Wei-Tou NI Center for Gravitation and Cosmology Purple Mountain Observatory Chinese Academy of Sciences Nanjing, China On Technologies Common to DECIGO, LISA and ASTROD

First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 2 OUTLINE 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 micro- thrusters), 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

First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 3 Common Science --- Astrodynamic Equation + gal-cosmo term +non-grav term

First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 4 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) single- arm 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. Super-ASTROD Region DECIGO BBO Region

First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 5 The General Concept of ASTROD 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 6 Gravitational Field in the Solar System 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.

First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 7 Summary of the scientific objectives in testing relativistic gravity of the ASTROD I and ASTROD missions

First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 8 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

First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 9 ASTROD I (Cosmic Vision ) submitted to ESA by H. Dittus (Bremen) arXiv: v1 [astro-ph] 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

First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 10 Laser ranging / Timing: 3 ps (0.9 mm) 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 × m (2 AU), the laser/clock frequency needs to be known to one part in s Laser pulse timing system: T2L2 (Time Transfer by Laser Link) on Jason 2 Single photon detector Jason 2 S/C

First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 11 Drag-free AOC requirements Thruster requirements Proof mass Proof mass- S/C coupling Thrust noise Control loop gain 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.

First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 12 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

First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 13 ASTROD configuration (baseline ASTROD after 700 days from launch)

First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 14 LISA LISA consists of a fleet of 3 spacecraft 20 º behind earth in solar orbit keeping a triangular configuration of nearly equal sides (5 × 10 6 km). Mapping the space-time outside super-massive black holes by measuring the capture of compact objects set the LISA requirement sensitivity between Hz. To measure the properties of massive black hole binaries also requires good sensitivity down at least to Hz. (2017)

First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 15 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

16 Doppler Shifts Doppler shift is due to the line-of- sight velocity between the spacecraft which we inspect in more detail in the next slides

First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 17 Line-of-sight velocity between near-Earth and inner-orbit spacecraft Peak velocities are of order 20 km/s, both ways

First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 18 Line-of-sight velocity between near-Earth and outer-orbit spacecraft Peak velocities are of order 20 km/s, both ways

First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 19 Doppler shift due to line-of-sight velocity With l.o.s. velocities of up to 20 km/s at certain epochs, for wavelength 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.

First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 20 Time delay interferometry: Technology common to LISA and ASTROD Although the velocity in the Doppler shift direction differ by 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 21 Technology common to all 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 22 Incoming Laser beam Proof mass Outgoing Laser beam Optical readout beam Large gap Telescope Dummy telescope Telescope Anchoring LASER Metrology Housing Proof mass Dummy telescope Capacitive readout

First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 23 Laser Metrology 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 24 Super-ASTROD (1 st 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 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 gravitational- wave mission (Super-ASTROD).

First LISA-DECIGO Meeting: Common Technologies W.-T. Ni 25 Primordial GW and Super-ASTROD 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 26 Sensitivity to Primordial GW 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 27 Primordial Gravitational Waves [ strain sensitivity  (ω^2) energy sensitivity]

28 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