Further out in the Solar System, the Centaur asteroid 2060 Chiron (aka comet 95P/Chiron) intermittently displays a coma of comet-like outgassing and other activity. This asteroid is sporadically to persistently active (depending on what you read here or here) and has a clear tail thought to be composed of collisional dust. Active asteroids reside in the inner and outer Solar System and the image shown is the inner Solar System asteroid 6478 Gault. There are also number of objects which have been classified as both asteroid and comet or are currently ambiguous in nature and have been assigned to categories such as active asteroids, main belt comets, manx comets and extinct comets.Īt first glance, the stunning featured image at the very top of this article (from NASA/ESA/ESO) could be mistaken for that of a comet, but it is an active asteroid.

(Note that H is a way of estimating the relative size of the asteroid because, in general, smaller H means brighter and therefore a larger diameter.) These are categorised into Apollo, Aten, Atira and Amor types depending on whether their orbits are Earth-crossing or entirely within or entirely outside Earth’s orbit.Īsteroids which have a minimum orbit intersection distance ( MOID) with Earth of 0.05 AU or less (where AU is an astronomical unit and 1 AU is the Earth-Sun distance of roughly 150 million km) and that have an absolute magnitude (H) of 22.0 or less are deemed to be potentially hazardous asteroids. There are also a not insignificant number of small asteroids that reside closer to Earth (within the orbit of Mars at 1.5 AU) – the so-called near-Earth objects (NEOs). Almost half of the mass of the Main Belt is provided by four large asteroids: 1 Ceres (946 km), 4 Vesta (525 km), 2 Pallas (512 km) and 10 Hygiea (431 km). Most asteroids are on orbits that keep them in the inner Solar System orbiting in the Main Belt (at around 2.2–3.3 AU) between Mars (at 1.5 AU) and Jupiter (at 5.2 AU) and on orbits that are less elliptical (more circular) than comets.Īn estimate of the number of asteroids that exist in the asteroid belt is hard to give, but astronomers believe there may be up to 2 million asteroids larger than 1 km and millions of smaller ones. Cometary orbits cross the ecliptic (plane of Earth’s orbit) at almost any angle, unlike asteroid orbits which are inclined at significantly smaller angles.Īsteroids are composed of rock and metal, either as solid coherent bodies or loosely bound rubble piles. Comet orbital periods range from a few years to thousands and millions of years, or they may orbit the Sun once and never come back if they are on parabolic or hyperbolic orbits (more on that later). The coma and tail can extend for hundreds and thousands of kilometers from the comet’s nuclei which is only a fraction of the size of the glow of the comet’s coma observed from Earth.Ĭomets spend most of their time in the outer Solar System far from the Sun which is why they still contain volatile material, unlike the majority of asteroids. The A to Z of asteroids So what is the difference between an asteroid and a comet?Ĭomets are loose collections of ice, dust and rocky particles, usually on highly eccentric (elliptical) orbits that take them to the outer reaches of the Solar System and beyond, and in close to the Sun where the volatile ices and dust they contain “outgas” to produce a tail and a gravitationally unbound atmosphere known as a coma.So what is the difference between an asteroid and a comet?.This article contains the following sections: From active asteroids to dormant comets, rubbile piles to remnant cores, main belt comets and manx comets to ambiguous interstellar objects, this article provides a brief overview of the smaller objects in our planetary zoo and the main differences - and similarities - between asteroids and comets.īut if it’s meteors you’re looking for, read Meteoroids, Meteors and Meteorites.

Comet vs meteor