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The origin of Trojan asteroids, which librate about Jupiter's L$_4$ and L$_5$ points, presents a long-standing puzzle in planetary dynamics. Models of Trojan asteroid origin are usually based on the assumption that they were inserted into their present orbits as a result of collisions after the formation of Jupiter. However, Peale (1994, {\it Icarus} 106 , 308) has shown that the Lagrange points could be stable in the presence of the solar nebula, opening up the possibility that nebula gas drag played a role in inserting asteroids into the Lagrange points. We have recently performed a series of numerical simulations of the migration of planetesimals due to nebula drag. We find that a planetesimal can have a close encounter with a planet which inserts it into the 1/1 resonance. When the planet is on a circular orbit, the planetesimal has a second encounter which causes it to be ejected from the resonance. However, when the planet's orbit is eccentric then the resulting asymmetry in the planetesimal's orbital evolution prevents further close encounters with the planet. In the case of planetesimals approaching a $10^{-6}$M$_{\sun}$ planet with an eccentricity $e \ge 0.02$, then $\sim 10\%$ of the migrating bodies can be thrown into the 1/1 resonance and decay down to the L$_5$ point.
This work was supported in part through NASA Planetary Geology and Geophysics Grant \# NAGW-2061 at UCSB and \# NAGW-1107 at SUNYSB.
