Looking for gravitational waves with orbital dynamics: GUEST mission and the $μ$Hz band of Gravitational Waves

`Adding colour'' to the gravitational-wave sky by probing unexplored frequency bands has the potential to unveil new and unexpected phenomena in the Universe. The microhertz band, between pulsar timing arrays and LISA, is a particularly attractive target: it should host the extension of the signal recently found by pulsar timing arrays (if it is generated by a family of sources of gravitational waves), and it may harbour signals from the early Universe, primordial black holes or dark matter. In these proceedings, we review how the orbits of gravitationally bound systems act as resonant detectors of microhertz gravitational waves and ultra-light dark matter. We then describe GUEST (Gravitational Universe Exploration with Satellite Tracking), a mission concept recently proposed to the ESA F-class call, consisting of two dense passive spheres covered with retroreflectors, placed in highly eccentric Earth orbits and tracked by a global laser-ranging network.