## How do asteroids, meteors and comets disappear? (Beginner)

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 will be in three parts.

Meteors

When we talk about meteors, we mean small objects (usually less than one meter 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 completely burn up in the atmosphere. 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 ablation in the Earth's atmosphere.

Asteroids

When we talk about asteroids, we talk about objects whose range of diameters goes from about ten meters to 975 kilometers (Ceres). Most of these objects are on orbits that are stable for long periods of time (billions of years). However, some of them may happen to be displaced, for several reasons, into 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 elongated, 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, or with a planet. Or, in some cases, the asteroid may be completely ejected from the Solar System. That is how asteroids disappear.

Comets

Most of the brightest comets are objects that are either on very eccentric orbits (elliptical orbits in which one of the axes is much longer than the other) or on unbound orbits (orbits that are not closed, they can be parabolic or hyperbolic). When a comet passes close to the Sun, it becomes active and therefore visible. Its next close passage may be many years afterwards. Or it may pass close to the Sun once, and then never again (parabolic or hyperbolic comets, on trajectories that escape the Solar System). In this sense comets "disappear". Another way in which comets may disappear is by breaking apart when they get too close to the Sun, or by crashing into the Sun. Over one thousand such objects have been observed; we call them "sungrazers".

#### Valerio Carruba

Valerio is currently a Professor at the São Paulo State University in Brazil (UNESP), and he mostly works with asteroid dynamics.  He went to college in Italy at the University "La Sapienza", took his Ph. D. in Qstronomy at Cornell University, and then went to Brazil in 2004 for various pos-docs that then "evolved" into his current permanent position.

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