JultikaUniversity of Oulu repository

Abstract

Asteroids and comets have been thought to be two separate kinds of objects in our solar system. In recent years, however, evidence has been found that there is a continuum of objects that have both asteroid-like and comet-like properties.

Active asteroids are objects with typical asteroid orbits that eject dust almost like comets, forming dust tails. They can lose mass by different mechanisms, which include ice sublimation, impact ejection, rotational instability, electrostatic forces, thermal stresses, dehydration, sodium volatilization, and radiation pressure.

There are some challenges in observing asteroid activity, but two dozen mass-losing asteroids have been found. Examples of active asteroids are the main-belt comet 133P/(7968) Elst-Pizarro where ice sublimates, the large main-belt asteroid (596) Scheila that collided with a smaller body, and the near-Earth asteroid (3200) Phaethon whose mass-loss is likely related to thermal stresses and sodium volatilization.

In the future, more research is needed in active asteroid observations, models, and laboratory experiments. Space missions with active asteroids as their targets have also been planned.