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Abstract

We analyze a sample of 58 Jupiter family comets (JFCs) in near-Earth orbits, defined as those whose perihelion distances at the time of discovery were qdisc<1.3 au. In our definition JFCs have Tisserand parameters 2<T<3 and orbital periods P<20 yr. We integrated the orbits of these objects, plus 50 clones for each one of them, for 104 yr in the past and in the future. We find that most of them move on highly unstable orbits, having fallen in their current near-Earth orbits in the recent past, going from less than one hundred years to a few thousands years. They experience frequent close encounters with Jupiter down to distances ≲0.1 au. This is the expected behavior for comets whose limited physical lifetimes in the near-Earth region make them unlikely to survive there for more than about a few hundred revolutions. In this sense the orbits of most JFCs are typically “cometary”, and they should be regarded as newcomers in the near-Earth region. Yet, a minor fraction of JFCs (less than about one third) are found to move on stable orbits for the past ∼104 yr, and in some cases are found to continue to be stable at 5×104 yr in the past. They also avoid very close encounters with Jupiter. Their orbital behavior is very similar to that of NEAs in cometary orbits. While “typical” JFCs in unstable orbits probably come from the trans-Neptunian region, the minor group of JFCs in asteroidal orbits may come from the main asteroid belt, like the NEAs. The asteroidal JFCs may have a more consolidated structure and a higher mineral content than that of comets coming from the trans-Neptunian belt or the Oort cloud, which could explain their much longer physical lifetimes in the near-Earth region. In particular, we mention comets 66P/du Toit, 162P/Siding Spring, 169P/NEAT, 182P/LONEOS, 189P/NEAT, 249P/LINEAR, 300P/Catalina, and P/2003 T12 (SOHO) as the most likely candidates to have an origin in the main asteroid belt. Another interesting case is 207P/NEAT, which stays near the 3:2 inner mean motion resonance with Jupiter, possibly evolving from the Hilda asteroid zone.