The Low Frequency Sensitivity to Gravitational Waves for ASTROD

ASTROD is a relativity mission concept encompassing multi-purposes. One of its main purposes is to detect gravitational waves sensitive to low-frequency band similar to LISA, but shifted to lower frequencies. In this aspect, ASTROD would complement LISA in probing the Universe and study strong-field black hole physics. Since ASTROD will be after LISA, in the Cosmic Vision time-frame 2015-2025, a ten-fold improvement over LISA accelerometer noise goal would be possible, allowing us to test relativistic gravity to 1 ppb and improve the gravitational-wave sensitivity. In this paper, we address to this possible improvement, especially in the frequency range below 0.1 mHz. We look into possible thermal noise improvement, magnetic noise improvement, spurious discharging noise improvement and local gravitational noise improvement. We discuss various possibilities of lower-frequency gravitational-wave responses and their significance to potential astrophysical sources.