  Gravitational-wave (GW) detectors that monitor fluctuations in the separation
between inertial test masses (TMs) are sensitive to new forces acting on those
TMs. Ultralight dark-photon dark matter (DPDM) coupled to $U(1)_B$ or
$U(1)_{B-L}$ charges supplies one such force that oscillates with a frequency
set by the DPDM mass. GW detectors operating in different frequency bands are
thus sensitive to different DPDM mass ranges. A recent GW detection proposal
based on monitoring the separation of certain asteroids in the inner Solar
System would have sensitivity to $\mu$Hz frequencies [arXiv:2112.11431]. In
this paper, we show how that proposal would also enable access to new parameter
space for DPDM coupled to $B$ [respectively, $B-L$] charges in the mass range
$5\ [9] \times 10^{-21} \text{eV} \lesssim m_{\text{DM}} \lesssim 2 \times
10^{-19} \text{eV}$, with peak sensitivities about a factor of 500 [50] beyond
current best limits on $\varepsilon_B$ [$\varepsilon_{B-L}$] at $m_{\text{DM}}
\sim 2 \times 10^{-19} \text{eV}$. Sensitivity could be extended up to
$m_{\text{DM}} \sim 2 \times 10^{-18} \text{eV}$ only if noise issues
associated with asteroid rotational motion could be overcome.
