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The difference between asteroids and comets goes back to the way they were first formed. Although they were ‘born’ at the same time, they formed under different circumstances.

• The solar system started out as a rotating cloud of gas and dust. At the center, the Sun formed through gravitational collapse, which released heat. At the center of this dusty gaseous nebula, the region was hot and dense, while the outer regions were cooler. Asteroids formed near the center where only rock and metal could remain solid. Comets formed at the outer reaches which was cold enough for water and gases to freeze. Comets, therefore, are “dirty snowballs” found at the outer reaches of the solar system, in the Kuiper Belt (named for astronomer Gerard Kuiper who postulated its existence) and the Oort Cloud (named for Dutch astronomer Jan Oort, ditto).

• Most asteroids are lumps of rock which circulate in the asteroid belt between Mars and Jupiter. The total mass of all the asteroids in the solar system combined would actually be less than that of the Moon.

• The word “asteroid” means “star-like.” In disaster movies, it’s always an asteroid that’s going to ruin Earth. An asteroid wiped out the dinosaurs.

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• The Rosetta spacecraft, carrying the Philae lander, rendezvoused with a comet called 67P/Churyumov–Gerasimenko in 2014. 67P indicates that this was the 67th periodic comet discovered. Churyumov and Gerasimenko are the names of the discoverers. The Philae lander landed on the comet almost 11 years after leaving Earth. It analyzed more than 35,000 escaping dust grains and found 45% of them to be carbon-based molecules like proteins, carbohydrates — organic matter which could help in the search for alien life. The remaining 55% of the dust is formed of inorganic minerals, mostly silicates. (cont)

Halley’s comet is known for reappearing every 76 years (it was last here in 1986, and will be back again in 2061). Many comets have a much longer orbit period, but some are shorter. Encke’s comet comes around every 3.3 years (catch it in October 2023).

• Whenever asteroids or comets slam into each other or slam into planets or moons, debris is released, and this is where meteors come from. Meteors are generally no bigger than 3 feet wide, and often are merely a speck of dust. They are simply bits and pieces of comets and asteroids.

• When comets pass close to the Sun, the icy materials start to melt off, leaving behind a long tail. The word comet originates with the Greek word meaning “long hair.”

• Meteor showers originate with the passing of a comet, when Earth passes through the debris field left in its wake. The Perseids each August and the Leonids in November are caused by the debris coming off comet Swift-Tuttle. The Orionids in October come off of Halley’s Comet.

• Meteoroids, meteors, and meteorites are names for space rocks at different stages. The ones floating around in space are meteoroids. The ones that enter Earth’s atmosphere and burn up are called meteors. The ones that don’t burn up entirely and instead crash into the surface of the Earth are called meteorites. Most of the meteors which we call “shooting stars” are merely the size of a small pebble.

• The “-ite” ending is often used in the names of rocks and minerals, such as anthracite. That’s a good way to remember that meteorites are the ones that make it to Earth’s surface as rocks.

• NASA’s Johnson Space Center has a collection of meteorites that acts as a sort of meteorite library. Studying them provides clues to how our solar system was formed.