Asteroid



An asteroid is a small astral body (they are typically composed of ice, metal, and rock) 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 causes it to be enveloped in a cloud called a "coma". Sizes range from pretty much whatever you want (car-sized rocks or even less) to Ceres. Ceres is classified as a dwarf planet because it's spheroidal but resides in the asteroid belt; it is considered to be around 1,000 kilometers in diameter.

A bit of history
One of the biggest, Ceres, was discovered back in the 19th century and was at first considered to be a planet. 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. Despite that suggestion, the term "asteroid" and "planet" were used interchangeably until someone noticed what a mess it 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 (imagine making a diorama of 1.9 million asteroids), and "asteroid" was the term used for them until the IAU changed it a bit in 2006 and placed asteroids, comets, and other, similar objects into the category of "small Solar System bodies".

Nomenclature of these bodies uses a number to indicate its order of discovery plus a name (2 Vesta, for example), and began using female names taken from Greco-Roman mythology following the trend used for Solar System bodies. Once the pool was exhausted, astronomers began to use male names transformed into female ones (as 349 Dembowska), male ones mythological or not (eg: 624 Hektor), names female or not from other mythologies (ie: 2715 Mielikki), and even those from deceased people, fictional characters, etc (25924 Douglasadams, 2309 Mr. Spock…)

Origin and composition
It is currently thought that asteroids are remnants of the good ol' days when the Solar System was forming and there were rocks galore flying about. The largest of them (Ceres and ) would have been formed like terrestrial planets. That is, by accretion of smaller celestial bodies, being large enough to differentiate into layers of different compositions (core, mantle, crust), 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 they became small chunks of rock and bits of 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 would have become history thanks to stuff that include its inhabitants playing too much with nukes, the planet itself going nuclear, or even the very scary demon-planet Nibiru paying it a visit. However, the combined mass of all the asteroids 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 significant numbers into the inner Solar System where they'd have while the dust produced by asteroid collisions would have ended

Broadly speaking, asteroids can be classified into "carbonaceous" (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 rocky 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 numerous classifications have been developed.

Where to find them
As mentioned above, most asteroids live between the orbits of Mars and Jupiter in what's known as the "asteroid belt" doing what asteroids do all time: rotating, basking on the faint sunlight of those distances, waiting for the day those bipedal apes who live on the third big rock from the Sun will send a probe to study them, even if it in kamikaze fashion, and extremely rarely crashing among themselves. And no, spacecraft sent there (or the outer Solar System) do not have to dodge them like Han Solo did in The Empire Strikes Back when he was fleeing all those TIE fighters. Although there are millions of asteroids, they're all pretty far from each other, so it's practically empty. 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 or especially in  which can be very nasty as discussed below.

But you don't have to go into space to see them. In fact, you don't even need a telescope. You can spot at the very least the two brightest, Ceres and Vesta, with a pair of binoculars if you know where to look.

Hazards
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 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 discussed to either send dangerous asteroids elsewhere or destroy them — funding, as usual, is another thing.

Woo
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.