ASTROD-GW: Overview and Progress

Abstract: 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.

ASTROD-GW: Mission Concept and Advancements in Low-Frequency Gravitational Wave Detection

In this paper, Wei-Tou Ni presents an in-depth analysis of the ASTROD-GW mission, emphasizing its optimization for low-frequency gravitational wave (GW) detection. This mission is a transformation of the original ASTROD mission, leveraging advancements in astrodynamical space tests of relativity using optical devices. The ASTROD-GW mission features three spacecraft positioned near the Sun-Earth Lagrange points (L3, L4, L5), forming an almost equilateral triangle with arm lengths of approximately 260 million kilometers. This configuration allows for interferometric ranging among the spacecraft with minimal arm length variation and very low relative Doppler velocities, thereby enhancing the sensitivity of GW detection.

Key Contributions

1. Scientific Goals: ASTROD-GW aims to detect GWs from various astrophysical sources, including supermassive black holes, extreme mass-ratio inspirals, intermediate-mass black holes, galactic compact binaries, and relic gravitational wave backgrounds. The mission's sensitivity in the frequency band 100 nHz to 1 mHz is unmatched, bridging a critical gap left by current and planned detectors such as LISA and NGO/eLISA.

2. Frequency Sensitivity Enhancement: Compared to LISA, ASTROD-GW shifts the sensitivity spectrum towards longer wavelengths by a factor of 52 to 260, addressing the low-frequency noise constraints primarily due to longer arm lengths. This enhancement enables a more profound exploration of GW sources in the lower frequency range, providing a pivotal advantage in studying cosmic history and galactic evolution with black holes.

3. Orbit Optimization: The proposed orbit configuration optimizes arm lengths and stabilizes relative velocities, reducing perturbations from planetary gravitational fields. Initial conditions are refined through numerical simulations, ensuring spacecraft stability and precise measurement capabilities over extended mission durations.

4. Time Delay Interferometry (TDI): The study outlines the implementation of TDI to suppress laser frequency noise by precisely matching optical path lengths. ASTROD-GW requires sophisticated TDI technology to achieve requisite sensitivity levels, benefiting from advancements and lessons from previous missions such as LISA and NGO/eLISA.

Future Implications

The ASTROD-GW mission concept potentially accelerates advancements in gravitational-wave astronomy, offering a novel vantage point through its superior low-frequency sensitivity. It is expected to contribute significantly to the understanding of galaxy co-evolution with black holes and the dynamics of dark energy. As preparation for the ASTROD-GW mission continues, further investigations will likely focus on improving weak light manipulation, minimizing low-frequency noise, and enhancing deployment strategies.

In summary, the ASTROD-GW project stands out for its optimized observation strategy, allowing a closer scrutiny of gravitational wave phenomena across broader cosmic timescales and distances. It will complement ongoing and future GW detection efforts, potentially paving the way for a deeper understanding of fundamental physics and cosmic evolution. The work on ASTROD-GW not only marks significant advancements in the technical design of space-based GW detectors but also enhances theoretical perspectives in gravitational wave research and its practical applications in cosmology and astrophysics.