Asteroids and comets have things in common, but their differences aren’t few, and, distinguish between these two celestial bodies can lead to confusion.
The greatest difference between a comet and an asteroid is the material from which they are formed. While the asteroids compose themselves mainly of rocks, metals and minerals (which in the future could become a great source of resources and a great economical opportunity), the comets are formed by rocks, ice, and dust, which normally are situated in their upper layers.
This is the reason for which, when the comets approach the Sun during their orbits and they are exposed to more elevated temperatures, a part of the materials that compose it melt, leaving, behind him, a big tail that can easily be observed from the Earth.
Comets have the named comet tail, which is a cloud of gas, dust, and materials evaporated from the comet when approaching the Sun. When travelling through space, this tail extends, always pointing to the contrary direction from the Sun, either if it approaches or if it goes away, due to the solar wind. Additionally, this trail that leaves the comet during its orbit can coincide with the orbit of the Earth, giving rise, at that time, to the known meteor showers.
In contrast, normally the asteroids don’t any tails, as they are mainly composed of solid materials. However, in 2010, an asteroid with a tail was discovered, the named P/2010 A2. This phenomenon is the result of the previous collision between two asteroids, which gave rise to this asteroid with a tail of 50 000 kilometres
Some asteroids can have a thin ice layer on their surface, that when it melts causes these tails, even if never as long as those of the comets.
The asteroids and the comets are also different in what concerns their location in the solar system.
The majority of asteroids are located near the Sun, mainly in the asteroid belt, a region between the orbits of Mars and Jupiter in which are found millions of rocks of diverse sizes, from the millimetres of diameter to the hundreds of kilometres.
On the contrary, the comets are normally situated at the limits of the solar system, where the temperatures are lower and where the ice doesn’t melt. The regions where they concentrate the more are the Kuiper belt and the Oort cloud.
The orbits of the asteroids and the comets are also very different. Asteroids tend to have circular and short orbits. This is the case with the asteroid belt.
However, the orbit of the comets is much more extended and long. Many times their orbits surpass a distance of 50 000 astronomical units from the Sun. (An astronomical unit is the distance that separates the Earth from the Sun, that to say, 150 million kilometres). The comets situated at the Oort cloud have a longer orbit than the ones that have their origin at the Kuiper belt.
Some characteristics in common
Despite having a big quantity of differences, asteroids and comets also have some features in common:
- The two of them are bodies that orbit the Sun.
- Asteroids and comets were formed from the materials of the young solar system, about 4 500 million years ago.
- Sometimes astronomers find themselves with some difficulties when determining if a celestial body is an asteroid or a comet despite all their important differences. In fact, as I said, some asteroids possess a small tail, which can make them appear like comets. In the same way, with the comets a similar situation can take place: if a comet approaches the Sun many times, the ice on its surface can end up, for which it could easily be confounded with an asteroid.