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An asteroid is a small body that orbits the Sun (in our Solar System, that is) too small to have reached hydrostatic equilibrium (read: basically to be more-or-less spherical (i.e., spheroidal) and not an irregularly shaped flying space rock) and that unlike a comet suffers no outgassing that envelops it into a cloud called "coma".[note 1] Sizes range from pretty much whatever you want (car-sized rocks or even less) to Ceres. Ceres is also classified as a dwarf planet because it's spheroidal but resides in the asteroid belt; it has a size of around 1,000 kilometers.
A bit of history
The first one to be discovered, Ceres, back in the 19th century was at first considered a planet and the same happened with those few that were found in the next decades until the astronomer William Herschel proposed to name them asteroids, as most of them appeared star-like and did not resolve into disks unlike real planets in the telescopes of those years.[note 2] Despite that suggestion the term "asteroid" and "planet" were used interchangeably until someone noticed the mess that was to have a Solar System with hundreds of planets, as visual observation was replaced by photography to hunt them and the number of asteroids simply exploded (just imagine attempting to learn the name of all those "planets" in school), and "asteroid" was the term used for them until the IAU changed things somewhat in 2006 and placed asteroids, comets, and similar objects into the category "small Solar System bodies".
Origin and composition
It's currently thought that asteroids are remnants of the old days when the Solar System was forming and there were rocks galore flying elsewhere. The largest ones, such as Ceres or Vesta would have been formed like (terrestrial) planets by accretion of smaller bodies, being large enough to differentiate into layers of different compositions (core, mantle, crust)[note 3] while the small ones would be the result of impacts that would have shattered those large enough to have been able to acquire internal structure, with later impacts grinding down the results of those collisions and so on until there're just small rocks and dust.
Early theories suggested that the asteroids could have been what remains of a planet located in what's now the asteroid belt (see below) between Mars and Jupiter, that either went boom by itself of was destroyed by a collision with another large body and entering into crank and bad science-fiction territory, said planet would have become history thanks to stuff that include its inhabitants playing too much with nukes, the planet itself going nuclear, or even Nibiru paying it a visit. However the combined mass of all the asteroid is just a small fraction of the Moon's mass and more than a half is concentrated on the twelve largest ones, so it's instead considered that Jupiter's gravitational influence prevented them from forming a planet during the Solar System's early times and processes as gravitational interactions of the four largest planets, etc. expelled most of them from the belt, sending significants number into the inner Solar System while they'd have gone kamikaze while the dust produced by asteroid collisions would have ended spiralling into the Sun.
Broadly speaking asteroids can be classified into "carbonaceus" (with carbon in addition to rock, thus very dark and assumed to come from the crust of those shredded large ones), "stony" ('nuff said, coming from their mantles), "metallic" (also self-explanatory, remnants from their cores), and those of peculiar composition as Vesta. However nature is far more complex than that and much more elaborated classifications have been developed.
Where to find them
Most asteroids -you can spot the brightest of them as the already mentioned Ceres and Vesta with a pair of binoculars if you know where to look- live as mentioned above between the orbits of Mars and Jupiter in what's known as the "asteroid belt"[note 4] doing what asteroids do all time:
watching pr0n on the Galactic Internet, having a good time alone, or metallic ones listening to good heavy metal rotating, basking on the faint sunlight of those distances, waiting for the day bipedal apes who live on the third big rock from the Sun will send a probe to study them, and as commented above crashing among themselves. And no, controllers of spacecraft sent to that place or the outer Solar System have not to dodge them as Han Solo in The Empire Strikes Back while chased by TIE Fighters and Darth Vader's fleet as it's pretty much empty despite being up to millions of them.[note 5] However, as said crashes and/or gravitational interactions with the real planets can expel them from their orbits, one can find them in places such as Jupiter's orbit (trojan asteroids) or especially in orbits close to Earth which can be very nasty as discussed below.
That stray asteroids can pass close to Earth means that sooner or later they could collide with either the former or the Moon (they simply do not care), which not only is bad for them but can also be very hazardous for the health of planetary inhabitants, as in causing effects that begin from harmless ones (shooting stars) and scale up to the very bad luck of one making your day by falling into either your house or car or, worse, you, the equivalent of a nuclear weapon detonating in the atmosphere and ruining the day of quite a number of people, and finally causing planetary lifeforms not having to worry anymore about their debts (just ask the non-avian dinosaurs). Don't panic, as our technology allows us to detect the largest ones and there are many strategies discussed to either send dangerous asteroids elsewhere or destroy them — funding as usual is another thing.
Since asteroids (and comets) can ruin our day or worse, some woo-meisters will either exaggerate claims of space agencies about the probability of a given body deciding to go kamikaze on Earth, claiming to have information that They hide, or simply invent them — of course the end result is always that the asteroid/comet crashes into Earth and kills us/causes the NWO to rise from the ashes/signals the Second Coming/whatever. Just look for "asteroid" here and have fun.
- This is just an outline, to read more on this topic the other Wiki has an article waiting for you
- And modern ones too unless you use the Hubble, tricks like adaptive optics, non-telescopic methods such as a radar for those close enough, or better yet mount your scope into a spacecraft and chase one
- These ones are also known as protoplanets, as most of them would have coalesced forming the rocky planets
- The bodies of the Kuiper belt are not discussed here, as they are too far away and mainly composed of ice, even if there's the possibility of Ceres herself as well as other much smaller bodies of similar composition coming from there
- Scaling down things, scatter one or two handfuls of very fine sand in a few square kilometers and you'll be able to picture (in 2-D) how sparse is the belt