Asteroids In Unusual Orbits

Although most asteroids travel in fairly circular orbits, there are notable exceptions.

In addition to the near-Earth asteroids, discussed above, some objects are known to travel in orbits that extend far inside or outside the main belt. One of the most extreme is (3200) Phaethon, the first asteroid to be discovered by a spacecraft (the Infrared Astronomical Satellite in 1983). Phaethon approaches to within 0.14 AU of the Sun, well within the perihelion distance of 0.31 AU for Mercury, the innermost planet. By contrast, Phaethon's aphelion distance of 2.4 AU is in the main asteroid belt. This object is the parent body of the Geminid meteor stream, the concentration of meteoroids responsible for the annual Geminid meteor shower seen on Earth each December.

Because the parent bodies of all other meteor streams identified to date are comets, Phaethon is considered by some to be

The Difference Between Asteroids and Comets

Asteroids traditionally have been distinguished from comets by characteristics based on physical differences, location in the solar system, and orbital properties. An object is classified as a comet when it displays "cometary activity"—i.e, a coma, or tail, or any evidence of gas or dust coming from it. In addition, any object on a nonreturning orbit (a parabolic or hyperbolic orbit, rather than an elliptical one) is generally considered to be a comet.

Although these distinctions apply most of the time, they are not always sufficient to classify an individual object as an asteroid or a comet. For example, an object found to be receding from the Sun on a nonreturning orbit and displaying no cometary activity could be a comet, or it could be a planet-crossing asteroid being ejected from the solar system after a close encounter with a planet, most likely Jupiter. Unless such an object reveals itself by displaying cometary activity, there is usually no way to determine its origin and thus to classify it unequivocally. The object may have formed as an icy body, as comets do, but lost its volatile materials during a series of passes into the inner solar system. Its burned-out remnant of rocky material would presently have more physical characteristics in common with asteroids than with other comets.

a defunct comet—one that has lost its volatile materials and no longer displays the classic cometary features of a nebulous coma and a tail. Another asteroid, (944) Hidalgo, is also thought by some to be a defunct comet because of its unusual orbit. This object, discovered in 1920, travels sunward as near as 2.02 AU, which is at the inner edge of the main asteroid belt, and as far as 9.68 AU, which is just beyond the orbit of Saturn, at 9.54 AU.

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