BOX 1.1 Orbital Evolution of NEOs
All near-Earth objects (NEOs) are in chaotic, planet-crossing orbits; their orbits evolve as a consequence both of long-range (secular) perturbations, due chiefly to the gravitational attraction of Jupiter and Saturn, and of close-range perturbations due to infrequent close encounters with one or more of the terrestrial planets.* Long-range perturbations drive precession of the long axis of the orbit relative to the line of the nodes and related variations in the eccentricity and inclination of the asteroid's orbit. The orbits of NEOs that overlap Earth orbit can intersect Earth's orbit, typically four times, during a complete cycle of precession of the long axis. Also, many orbits that currently lie outside that of Earth (orbits of the Amors) can become overlapping as a result of secular changes in eccentricity and can intersect Earth's orbit during precession. An example is the orbit of the fairly large Amor asteroid (1580) Betulia, whose orbit can intersect Earth's eight times during one cycle of precession. NEOs whose orbits can intersect Earth's as a result of secular perturbations and thus can collide with Earth, therefore, are called Earth crossing. It should be noted, however, than many Earth crossers cannot collide with Earth because the phase symmetry of their free oscillations causes their perihelia to be outside Earth's orbital plane when their eccentricities are high enough for their perihelia to be inside 1 AU.
Occasional close encounters with one or another terrestrial planet lead to long-term chaotic evolution of the orbits of NEOs. Hence, over time, noncrossing Amors can become crossing or evolve into Apollos, Apollos can become Atens, and vice versa. Ultimately, many NEOs can become Jupiter crossing and then generally are ejected from the solar system, or they may evolve through perturbations into small, extremely eccentric orbits and be vaporized during close encounters with the Sun.
NEOs are thought to be derived primarily from fragments produced by collisions between asteroids in the main asteroid belt. Studies of the physics of collision and the observed disposition of orbital elements of asteroid families suggest that the changes in velocity imparted to kilometer-size fragments during catastrophic collisions generally do not exceed a few hundred meters per second. These changes are an order of magnitude smaller than those required to inject main-belt asteroid fragments into Earth-approaching orbits. In many cases, however, the small changes in velocity imparted to collisional fragments are sufficient to shift them into a dynamical resonance, such as a mean motion commensurable with the mean motion of Jupiter or a secular resonance. Resonant amplification of the orbital eccentricity of the fragment can then lead to a planet-crossing orbit. Synergistic interplay between resonant perturbations and perturbations due to encounters with Mars probably plays an important role in delivering NEOs to Earth-crossing orbits.
processes (e.g., collisional and thermal histories, surface alteration, fluid-rock interactions) can be assessed. However, such modified bodies may have made up a substantial portion of the planetesimals that accreted to form the terrestrial planets,7 thereby providing information related to the early stages of planet growth.
1. T. Gehrels, ed., Hazards Due to Comets and Asteroids, University of Arizona Press, Tucson, Ariz., 1994.
2. D. Morrison, ed., The Safeguard Survey: Report of the NASA International Near-Earth-Object Detection Workshop, Jet Propulsion Laboratory, Pasadena, Calif., 1992.
3. Solar System Exploration Division, Office of Space Science, Report of the Near-Earth Objects Survey Working Group, NASA, Washington, D.C., 1995.
4. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press , Washington, D.C., 1994, p. 63.
5. Space Science Board, National Research Council, Strategy for the Exploration of Primitive Solar-System Bodies—Asteroids, Comets, and Meteoroids: 1980–1990, National Academy Press, Washington, D.C., 1980, p. 47.