Main-belt comet
A main-belt comet is an object in the asteroid belt that shows comet-like activity from ice turning to gas.
A main-belt comet orbits within the main asteroid belt, like an asteroid, but occasionally grows a tail or coma, like a comet. The activity is thought to be caused by buried ice turning to gas when the body is warmest, near its closest approach to the Sun.
The class was recognised in 2006, when astronomers showed that 133P/Elst–Pizarro and two other belt objects repeatedly grew dust tails near perihelion. About twenty main-belt comets are now known. In 2023 the James Webb Space Telescope detected water vapour around one of them, 238P/Read, confirming that ice was responsible.
Main-belt comets show that water ice can survive in the asteroid belt today, below the surface. This supports the idea that asteroids could have delivered water to Earth and fits with the evidence from carbonaceous chondrites and the Ryugu and Bennu samples, which contain minerals formed by liquid water.
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Level: Expert Advanced