Outer Solar System Spacecraft to Probe the μHz Gravitational-wave Frontier

The microhertz frequency band of gravitational waves probes the merger of supermassive black holes, as well as many other gravitational-wave phenomena. However, space-interferometry methods that use test masses would require further development of test-mass isolation systems to detect anticipated astrophysical events. We propose an approach that avoids onboard inertial test masses by situating spacecraft in the low-acceleration environment of the outer solar system. We show that, for Earth–spacecraft and interspacecraft distances of ≳10 au, the accelerations on the spacecraft would be sufficiently small to potentially achieve gravitational-wave sensitivities determined by stochastic gravitational-wave backgrounds. We further argue, for arm lengths of 10−30 au and ∼10 W transmissions, that stable phase locks could be achieved with 20 cm mirrors or 5 m radio dishes. We discuss designs that send both laser beams and radio waves between the spacecraft, finding that, despite the ∼ 10 4 × longer wavelengths, even a design with radio transmissions could reach stochastic background-limited sensitivities at ≲0.3 × 10 −4 Hz. Operating in the radio significantly reduces many spacecraft design tolerances. Our baseline concepts require two arms to do interferometry. However, if one spacecraft carries a clock with Allan deviations at 10 4 s of 10 −17 , a comparable sensitivity could be achieved with a single arm. Finally, we discuss the feasibility of achieving similar gravitational-wave sensitivities in a “Doppler tracking” configuration where the single arm is anchored to Earth.