Asteroids and comets have a few things in common. They are both celestial bodies orbiting our Sun. They are both “leftovers” — made from materials from the formation of our Solar System 4.5 billion years ago. But there are a few notable differences between these two objects, as well. The biggest difference between comets and asteroids, however, is what they are made of.

While asteroids consist of metals and rocky material, comets are made up of ice, dust, rocky materials and organic compounds. When comets get closer to the Sun, they lose material with each orbit because some of their ice melts and vaporizes. Asteroids typically remain solid, even when near the Sun.

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When comets approach the Sun, some of their ices melt. This causes another notable difference between asteroids and comets: comets have “tails” while asteroids generally don’t. When the ices in comets begin to melt and other materials vaporize from the heat from the Sun, this forms a glowing halo that extends outward from the comet as it sails through space. The ice and compounds like methane and ammonia develop a fuzzy, cloud-like shell called a coma. Forces exerted on the coma by the Sun’s radiation pressure and solar wind cause an enormous, elongated tail to form. Tails always points away from the Sun.

Asteroids typically don’t have tails, even those near the Sun. But recently, astronomers have seen some asteroids that have sprouted tails, such as asteroid P/2010 A2. This seems to happen when the asteroid has been hit or pummeled by other asteroids and dust or gas is ejected from their surfaces, creating a sporadic tail effect. These so-called “active asteroids” are a newly recognized phenomenon, and as of this writing, only 13 known active asteroids have been found in the main asteroid belt, and so they are very rare.

Another difference between asteroids and comets is in their orbital patterns. Asteroids tend to have shorter, more circular orbits. Comets tend to have very extended and elongated orbits, which often exceed 50,000 AU from the Sun. (*Note: 1 AU, or Astronomical Unit, equals the distance from the Earth to the Sun.)

There is a big difference when it comes to numbers… although there is a caveat in that we don’t know precisely how many asteroids OR comets there are in our Solar System, since many have never been seen. Astronomers have discovered millions of asteroids – some as small as dust particles and others measuring hundreds of kilometers across. But as of this writing, astronomers have found only about 4,000 comets. However, some estimates say there could be one hundred billion comets in the Oort cloud.

The fact that asteroids and comets were both formed during the earliest days of our Solar System has scientists studying both with keen interest. By examining them up close with satellites and landers — such as the current Rosetta mission with the Philae lander to Comet 67P — scientists hope to learn more about what our Solar System looked like in its earliest days.

Scientists also study comets and asteroids to determine the likelihood of them hitting Earth and other planets, and what effect their flybys could have on planetary atmospheres. In November of 2014, a comet named Siding Spring flew very close to Mars, and scientists are still studying the encounter. One recent study says that Mars gets bombarded by 200 small asteroids or comets every year.