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.
Here's a list of missions to asteroids and comets. We also know that both comets and asteroids are in other solar systems beyond our own. In , scientists using the Spitzer Space Telescope witnessed what they think was a crash between two huge asteroids orbiting another star 1, light-years. In , astronomers saw evidence of comets pummeling a planet orbiting the star Eta Corvi, which is about 59 light-years away from us.
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 , a comet named Siding Spring flew very close to Mars, and scientists are still studying the encounter. But this may happen more often that we think: one recent study says that Mars gets bombarded by small asteroids or comets every year.
How likely is it that our planet could be hit by a large asteroid or comet? We do know that Earth has been hit many times in the past by asteroids and comets whose orbits bring them into the inner Solar System. There is strong scientific evidence that cosmic collisions played a major role in the mass extinctions documented in Earth's fossil records. None at this time pose any threat to Earth.
Additionally, the possibility of mining both asteroids and comets someday is also becoming a source of interest for industrialists and commercial space ventures, such as Planetary Resources.
Want more resources on asteroids? Here's an infographic on the differences between asteroids, comets, meteors and meteoroids. And here's Hubblesite's news releases about asteroids. Explore further. More from Astronomy and Astrophysics.
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Share Twit Share Email. Artist view of an asteroid with companion passing near Earth. Credit: P. Most asteroids look like giant space potatoes, with their oblong shapes and surface that's pockmarked by numerous craters caused by collisions with other asteroids.
Only a small number of asteroids are large enough that their gravity forms them into spheres, such as Ceres. The composition of asteroids range from dark, rocky clumps of rubble consisting of clay and silicate rocks to bright and solid amalgamations of metals such as iron or nickel, according to NASA. Nearly all asteroids are found in a doughnut-shaped region between Mars and Jupiter, called the asteroid belt. The belt formed not long after the birth of Jupiter when the massive planet's gravity trapped planet-forming leftovers, causing them to collide with one another and form the millions of asteroids we see in the belt today.
For millennia, the sight of a comet elicited fear and awe. Ancient astronomers believed comets foretold the death of princes and the outcomes of wars. Modern astronomers know comets are the ice-clad leftovers from the material that formed our solar system billions of years ago. Astronomer Fred Whipple was the first to describe comets as dirty snowballs, or icy conglomerates of frozen gases and dust. The snowball makes up the central nucleus of a comet, which is often less than a few miles across, according to NASA.
When a comet nears the sun, the nucleus warms up and the ice begins to sublimate from solid to gas. This produces an atmosphere surrounding the comet that can grow to thousands of miles in diameter, called a coma. Radiation pressure from the sun blows away the dust particles in the coma to produce a long, bright dust tail. A second tail is formed when high-energy solar particles ionize the gas, creating a separate ion tail. The difference between the composition of asteroids and comets is likely due to how and where they were born, wrote Britt Scharringhausen , a professor of astronomy at Beloit College in Wisconsin.
The current number of known comets is:. Key Science Targets. Comets orbit the Sun just like planets and asteroids do, except a comet usually has a very elongated orbit.
As the comet gets closer to the Sun, some of the ice starts to melt and boil off, along with particles of dust. These particles and gases make a cloud around the nucleus, called a coma. The coma is lit by the Sun. The sunlight also pushes this material into the beautiful brightly lit tail of the comet.
Key Facts Comets Comets are frozen leftovers from the formation of the solar system composed of dust, rock, and ices. Kid-Friendly Comets Comets orbit the Sun just like planets and asteroids do, except a comet usually has a very elongated orbit. JPL's lucky peanuts are an unofficial tradition at big mission events.
It's suspected that about 5, years ago a comet swept within 23 million miles of the Sun, closer than the innermost planet Mercury.
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