The science case for a broad program of gravitational wave (GW) detection across a wide range of frequencies is exceptionally strong. At present, the GW frequency band lying between the sensitivities of pulsar timing arrays and LISA, roughly 0.01-100 microhertz, is an open frontier in this rapid evolving field. I will discuss recent progress in ideas to access this band, and some associated challenges and opportunities.

I will outline a conceptual mission proposal in which a few carefully chosen asteroids which orbit in the inner Solar System can be employed as naturally occurring gravitational test masses. A GW detector can then be constructed by ranging between these asteroids using optical or radio links and atomic clocks. I will discuss how a newly estimated gravity gradient noise arising from the combined motion of the other ~million asteroids in the inner Solar System sharply cuts off the sensitivity of this proposal below ~microhertz frequencies. Sensitivity in the middle of this band is mostly limited by various solar perturbations to the asteroid test masses, while the high-frequency sensitivity is limited by noise in the ranging link and asteroid rotational motion. A mission of this type promises significant projected GW strain-sensitivity reach for 0.1--10 microhertz frequencies. I will also mention how it could be repurposed for detection of asteroid-mass-scale dark states transiting the Solar System.

Additionally, I will discuss a different proposal that would enable sub-microhertz GW detection by measuring the astrometric GW signal in a novel way. In contrast to previous studies of this signal based on large-scale astrometric survey data, we propose to monitor to extreme precision the angular separations of a small number of hot, distant, photometrically stable white dwarfs. I will discuss why these are good objects to monitor, and sketch out the parameters of the space-based stellar interferometery instrument that would be required to make the required measurements. While this mission would be ambitious, it may be one of the few ways to access this particularly challenging frequency band.