  A major challenge for gravitational-wave (GW) detection in the $\mu$Hz band
is engineering a test mass (TM) with sufficiently low acceleration noise. We
propose a GW detection concept using asteroids located in the inner Solar
System as TMs. Our main purpose is to evaluate the acceleration noise of
asteroids in the $\mu$Hz band. We show that a wide variety of environmental
perturbations are small enough to enable an appropriate class of $\sim 10$
km-diameter asteroids to be employed as TMs. This would allow a sensitive GW
detector in the band $\text{(few)} \times 10^{-7} \text{Hz} \lesssim
f_{\text{GW}} \lesssim \text{(few)} \times 10^{-5} \text{Hz}$, reaching strain
$h_c \sim 10^{-19}$ around $f_{\text{GW}} \sim 10 \mu$Hz, sufficient to detect
a wide variety of sources. To exploit these asteroid TMs, human-engineered base
stations could be deployed on multiple asteroids, each equipped with an
electromagnetic transmitter/receiver to permit measurement of variations in the
distance between them. We discuss a potential conceptual design with two base
stations, each with a space-qualified optical atomic clock measuring the
round-trip electromagnetic pulse travel time via laser ranging. Tradespace
exists to optimize multiple aspects of this mission: for example, using a
radio-ranging or interferometric link system instead of laser ranging. This
motivates future dedicated technical design study. This mission concept holds
exceptional promise for accessing this GW frequency band.
