Ice Giant Missions as Gravitational Wave Detectors

The past year has seen many papers underlining the significance of a space mission to Uranus and Neptune. Proposed mission plans usually involve a ~10 year cruise time to the ice giants. This cruise time can be utilized to search for low-frequency gravitational waves (GWs) by observing the Doppler shift caused by them in the Earth-spacecraft radio link. We calculate the sensitivity of prospective ice giant missions to GWs in comparison to former planetary missions which searched for GWs. Then, adopting a steady-state black hole binary population, we derive a conservative estimate for the detection rate of extreme mass ratio inspirals (EMRIs), supermassive- (SMBH) and stellar mass binary black hole (sBBH) mergers. For a total of ten 40-day observations during the cruise of a single spacecraft, approximately 0.5 detections of SMBH mergers are likely, if Allan deviation of Cassini-era noise is improved by ~10² in the 10⁻⁵ − 10⁻³ Hz range. For EMRIs the number of detections lies between O(0.1) − O(100). Furthermore, ice giant missions combined with the Laser Interferometer Space Antenna (LISA) would improve the GW source localisation by an order of magnitude compared to LISA by itself. With a significant improvement in the total Allan deviation, a Doppler tracking experiment might become as capable as LISA at such low frequencies, and help bridge the gap between mHz detectors and Pulsar Timing Arrays. Thus, ice giant missions could play a critical role in expanding the horizon of gravitational wave searches and maybe even be the first to detect the first SMBH merger.