  The triple asteroids and triple Kuiper belt objects (collectively called the
triple minor planets) in the Solar system are of particular interest to the
scientific community since the discovery of the first triple asteroid system in
2004. In this paper, the Hill stability of the nine known triple minor planets
in the Solar system is investigated. Seven of the systems are of large size
ratio, i.e. they consist of a larger primary and two moonlets, while the other
two systems have components of comparable size. Each case is treated
separately. For the triple minor planets that have large size ratio, the
sufficient condition for Hill stability is expressed in closed form. This is
not possible for the systems with comparable size components, for which the
Hill stability is assessed by a combination of analytical and numerical means.
It is found that all the known triple minor planets are Hill stable, except
3749 Balam, for which the incomplete orbital parameters make the Hill stability
of the system uncertain. This suggests that there might be more such stable
triple minor planets in the Solar system yet to be observed. It is also shown
that the Hill stability regions increase as the mutual inclination between the
inner orbit and outer orbit decreases, the semimajor axis ratio of the inner
orbit with respect to the outer orbit decreases, and the mass ratio of the
outer satellite with respect to the inner satellite increases. This study
therefore provides useful information about dynamical properties of the triple
minor planets in the Solar system.
