  The Double Asteroid Redirection Test (DART) mission will be the first test of
a kinetic impactor as a means of planetary defense. In late 2022, DART will
collide with Dimorphos, the secondary in the Didymos binary asteroid system.
The impact will cause a momentum transfer from the spacecraft to the binary
asteroid, changing the orbit period of Dimorphos and forcing it to librate in
its orbit. Owing to the coupled dynamics in binary asteroid systems, the orbit
and libration state of Dimorphos are intertwined. Thus, as the secondary
librates, it also experiences fluctuations in its orbit period. These
variations in the orbit period are dependent on the magnitude of the impact
perturbation, as well as the system's state at impact and the moments of
inertia of the secondary. In general, any binary asteroid system whose
secondary is librating will have a non-constant orbit period on account of the
secondary's fluctuating spin rate. The orbit period variations are typically
driven by two modes: a long-period and short-period, each with significant
amplitudes on the order of tens of seconds to several minutes. The fluctuating
orbit period offers both a challenge and an opportunity in the context of the
DART mission. Orbit period oscillations will make determining the post-impact
orbit period more difficult, but can also provide information about the
system's libration state and the DART impact.
