# How do asteroids, meteors and comets disappear?

I recently read in the paper about asteroids, comets and meteors disappearing. Do you have any thoughts on the cause of the disappearances?

Yours is a very interesting question, I'll try to reply as briefly as possible. The mechanisms by which asteroids, comets and meteors disappear are different, so my reply would be in three parts.

Meteors

When we talk about meteors, we talk about small objects (usually less than 1 m in diameter) that are captured by the gravitational attraction of the Earth and fall into its atmosphere. The meteor itself is the luminous tail that is left behind the infalling object (meteoroid) and that is due to the ionization of the air and by the subsequent recombination ofthe ions. Most of the meteors actually disappear, since they are consumed by the attrition with the air. Some of the largest objects, however, manage to arrive to the ground. We call these objects meteorites. To summarize, most of the meteors actually disappear due to the attrition with the Earth's atmosphere.

Asteroids

When we talk about asteroids, we talk about objects whose range in diameters goes from ~10m to 963 km (1 Ceres). Most of these objects are on orbits that are stable for long periods of time (1 billion year or more). However, some of them may happen to be displaced, for several reasons, to special orbits that are "in resonance" with the orbit of Jupiter (or Saturn). When we say that an asteroid is in resonance we mean that its period is a simple fraction of the period of Jupiter. So, for example, if an asteroid revolves around the Sun with a period which is 2 times that of Jupiter, we say that it is in a 2:1 resonance with Jupiter. This is important because Jupiter is gravitationally perturbing the asteroid's orbit. If there is no special relationship between the two orbital periods, the perturbations pull the asteroid's orbit in different directions, and the net effect is null over a single orbit. But if there is a special relationship between the planet and asteroid period, the perturbations may have an average that is not null over an asteroid orbit, and the net effect is that the asteroid orbit is "distorted".

The orbit of the asteroid, which is an ellipse, may become more elliptical, and if the perturbations are strong enough, the asteroid may go in an orbit that is going to put it in a collision path with the Sun. That is how asteroids disappear.

Comets

Most of the brightest comets are object that are either on very eccentric orbits (elliptical orbits in which one of the axis is much longer than the other) or on unbound orbits (orbits that are not closed, they can be parabolyc or hyperbolic). What happens with these comets is that either they pass once close to the Sun, when they became active and therefore visible, with a next passage that is going to happen hundreds of thousand of years afterwards (comets on highly eccentric elliptical orbits), or either they pass close to the Sun once, and then never again (parabolic or hyperbolic comets). In this sense comets "disappear". Another way in which comets may disappear is by crushing into the Sun. There are at list ~30 comets per year that have this destiny: we call these objects "Sungrazers".

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