Minor bodies that are denizens of the Inner Solar System are of course well-known. The most abundant are the asteroids, whose Main Belt extends between 2 and 3.5 Astronomical Units (AU). The asteroids are remnant planetesimals and it has long been believed that the Main Belt is perhaps the only zone in the Inner Solar System where planetesimals could survive in such long lived orbits. Work done with Serge Tabachnik, as part of his D.Phil dissertation, has directly challenged this cherished assumption. The work was published in Nature and caused a media feeding frenzy -- links to the media are given on Serge's homepage.
The equations of motion of over 1000 test particles in the Inner Solar System have been integrated for 100 Myrs. The test particles feel the gravitational forces of the Sun and eight planets (Pluto is neglected), together with the effects of the moon and post-Newtonian corrections. Test particles undergoing a close approach to a planet or the Sun are removed from the integration. After 100 Myrs, there remain narrow belts of particles with low inclinations and eccentricities, both between the Sun and Mercury, and roughly midway between the orbits of all of the terrestrial planets.
By re-simulating these four belts with greater resolution and extrapolating the numbers of surviving particles to 5 Gyrs, we suggest that there may be two locations in the inner Solar System in which planetesimals can survive. The first is the domain of the Vulcanoids (0.09-0.21 AU) between the Sun and Mercury, where remnant planetesimals with radii between 1-50 km can escape evaporation and persist on nearly circular orbits for extremely long times. The second is a belt between the Earth and Mars (1.1-1.25 AU). A search through the catalogues of Near-Earth Objects reveals a number of asteroids with rather low eccentricities and inclinations in this belt, exemplified by the recently discovered 1996 XB27.
Together with Serge Tabachnik, numerical surveys of Trojan orbits of the four terrestrial planets have been conducted. Ensembles of in-plane and inclined orbits in the vicinity of the Lagrange points are integrated for up to 100 million years. Mercury is the least promising planet, as it is unable to retain tadpole orbits over 100 million year timescales. Mercurian Trojans are unlikely to exist. Both Venus and the Earth are much more promising, as they possess rich families of stable tadpole and horseshoe orbits. Our survey of Trojans in the orbital planes of Venus is undertaken for 25 million years. Some 40 % of the survivors are on tadpole orbits. For the Earth, the integrations are pursued for 50 million years. The stable zones in the orbital plane are larger for the Earth than for Venus, but fewer of the survivors (20 %) are tadpoles. Mars has two known Trojans (5261 Eureka and 1998 VF31). Our integrations of trajectories in the vicinity of the Martian Lagrange points suggest that there is a large stable zone. The survivors occupy a band of inclinations between 15 degrees and 40 degrees and at the L4 and L5 Lagrange points. Both 5261 Eureka and 1998 VF31 lie deep within one of the stable zones, which suggests they may well be of primordial origin. The number of such undiscovered primordial objects around Mars may be as high as 50. Observational surveys are now being planned to follow up these theotetical advances.