ASTROD-GW: Overview and Progress
  In this paper, we present an overview of ASTROD-GW (ASTROD [Astrodynamical
Space Test of Relativity using Optical Devices] optimized for Gravitational
Wave [GW] detection) mission concept and its studies. ASTROD-GW is an
optimization of ASTROD which focuses on low frequency gravitational wave
detection. The detection sensitivity is shifted by a factor of 260 (52) towards
longer wavelengths compared with that of NGO/eLISA (LISA). The mission consists
of three spacecraft, each of which orbits near one of the Sun-Earth Lagrange
points (L3, L4 and L5), such that the array forms an almost equilateral
triangle. The 3 spacecraft range interferometrically with one another with an
arm length of about 260 million kilometers. The orbits have been optimized
resulting in arm length changes of less than 0.00015 AU or, fractionally, less
than 10^(-4) in twenty years, and relative Doppler velocities of the three
spacecraft of less than 3 m/s. In this paper, we present an overview of the
mission covering: the scientific aims, the sensitivity spectrum, the basic
orbit configuration, the simulation and optimization of the spacecraft orbits,
the deployment of ASTROD-GW formation, TDI (Time Delay Interferometry) and the
payload. The science goals are the detection of GWs from (i) Supermassive Black
Holes; (ii) Extreme-Mass-Ratio Black Hole Inspirals; (iii) Intermediate-Mass
Black Holes; (iv) Galactic Compact Binaries and (v) Relic Gravitational Wave
Background. For the purposes of primordial GW detection, a six spacecraft
formation would be needed to enable the correlated detection of stochastic GWs.
A brief discussion of the six spacecraft orbit optimization is also presented.
