Are There Many Inactive Jupiter-Family Comets among the Near-Earth Asteroid Population?

FERNANDEZ, J. A.; GALLARDO, T.; BRUNINI, A.

ICARUS

ACADEMIC PRESS INC ELSEVIER SCIENCE

Lugar: New York; Año: 2002 vol. 159 p. 358 - 368

0019-1035

We analyze the dynamical evolution of Jupiter-family (JF)comets and near-Earth asteroids (NEAs) with aphelion distancesQ > 3.5 AU, paying special attention to the problem of mixing ofboth populations, such that inactive comets may be disguised asNEAs. From numerical integrations for 2 × 106 years we find thatthe half lifetime (where the lifetime is defined against hyperbolicejection or collision with the Sun or the planets) of near-Earth JFcomets (perihelion distances q<1.3 AU) is about 1.5×105 years butthat they spend only a small fraction of this time (∼a few 103 years)with q < 1.3 AU. From numerical integrations for 5 × 106 years wefind that the half lifetime of NEAs in ?cometary? orbits (definedas those with aphelion distances Q > 4.5 AU, i.e., that approach orcross Jupiter?s orbit) is 4.2 × 105 years, i.e., about three times longerthan that for near-Earth JF comets.We also analyze the problem ofdecoupling JF comets from Jupiter to produce Encke-type comets.To this end we simulate the dynamical evolution of the sample ofobserved JF comets with the inclusion of nongravitational forces.While decoupling occurs very seldom when a purely gravitationalmotion is considered, the action of nongravitational forces (as strongas or greater than those acting on Encke) can produce a few Enckes.Furthermore, a few JF comets are transferred to low-eccentricityorbits entirely within the main asteroid belt (Q < 4 AU and q >2 AU). The population of NEAs in cometary orbits is found to be adequatelyreplenished with NEAs of smaller Q?s diffusing outward,from which we can set an upper limit of ∼20% for the putativecomponent of deactivated JF comets needed to maintain such apopulation in steady state. From this analysis, the upper limit forthe average time that a JF comet in near-Earth orbit can spend as adormant, asteroid-looking body can be estimated to be about 40%of the time spent as an active comet. More likely, JF comets in near-Earth orbits will disintegrate once (or shortly after) they end theiractive phases.