ASTROD - An overview

The objectives of the Astrodynamical Space Test of Relativity using Optical Devices (ASTROD) Mission concept are threefold. The first objective is to discover and explore fundamental physical laws governing matter, space and time via testing relativistic gravity with 3-6 orders of magnitude improvement. Relativistic gravity is an important cornerstone of physics, astronomy and cosmology. Its improved test is crucial to cosmology and modem theories of gravitation including superstring theories. Included in this objective is the precise determination of the relativistic parameters beta and gamma, the improved measurement of (G) over circle and a precise determination of an anomalous, constant acceleration directed towards the Sun. The second objective of the ASTROD mission is the high-precision measurement of the solar-system parameters. This includes: (i) a measurement of solar angular momentum via Lense-Thirring effect and the detection of solar g-mode oscillations via their changing gravity field, thus, providing a new eye to see inside the Sun; (ii) precise determination of the planetary orbit elements and masses; (iii) better determination of the orbits and masses of major asteroids. These measurements give better solar dynamics and probe the origin of our solar system. The third objective is to detect and observe gravitational waves from massive black holes and galactic binary stars in the frequency range 50 muHz to 5 mHz. Background gravitational -waves will also be explored. A desirable implementation is to have two spacecraft in separate solar orbit carrying a payload of a proof mass, two telescopes, two 1-2 W lasers, a clock and a drag-free system, together with an Earth reference system. The two spacecraft range coherently with the Earth reference system using lasers. When they are near, they range coherently to each other. The Earth reference system could be ground stations, Earth satellites and/or spacecraft near Earth-Sun Lagrange points. In this overview, we discuss the payload concept, the technological requirements, technological developments, orbit design, orbit simulation, the measurement of solar angular momentum, the gravitational-wave detection sensitivity, and the solar g-mode detection possibility for this mission concept. A simplified mission, Mini-ASTROD with one spacecraft ranging optically with ground stations, together with Super-ASTROD with four spacecraft of 5 AU (Jupiter-like) orbits, will be mentioned in the end. Super-ASTROD is a dedicated low-frequency gravitational-wave detection concept. For Mini-ASTROD, the first objective of ASTROD will be largely achieved; the second objective will be partially achieved; for gravitational wave detection, the sensitivity will be better than the present-day sensitivity using Doppler tracking by radio waves.