Detectability of massive binary black holes with sub-mHz gravitational wave missions

Future space-based gravitational wave (GW) observatories beyond LISA aim to explore the sub-millihertz to microhertz frequency band, targeting massive binary black hole (MBBH) coalescences across cosmic distances. In this work, we evaluate the detection and localization capabilities of such missions, taking into account the full galactic foreground. We show that signal-to-noise ratios (SNRs) can reach several thousand over a wide redshift range. Using ASTROD-GW as a representative mission concept, we consider three orbital configurations---nonprecessing and precessing with varying inclinations---and assess their performance across different MBBH populations. In the absence of precession, we find that the dominant quadrupole mode alone results in a two-hemisphere degeneracy, which is resolved by including higher-order harmonics. These harmonics equalize localization performance across configurations, reducing the relative advantage of orbital precession. Operating in the $10\text{ }\text{ }\ensuremath{\mu}\mathrm{Hz}--10\text{ }\text{ }\mathrm{mHz}$ band, sub-mHz detectors partially overlap with LISA's sensitivity while extending significantly into lower frequencies, enabling more precise localization of high-mass MBBHs. The resulting high SNRs open new opportunities for multimessenger astronomy, cosmological inference, and tests of gravitational theory. Our results highlight the complementary strengths of sub-mHz missions in advancing the next frontier of GW astrophysics.