Asteroids

Asteroids 

When we look at the diagram of the solar system, we'll see a big gap between Mars and Jupiter. A few centuries ago, that bugged Astronomer; they really wanted to be a planet there. On the first day of the 19th century January-1, 1801 Italian Astronomer Guiseppi Piazzi found a point of light moving at just the right speed to be the desired planet, but it was just a dot, and too faint to physically to be a terribly big object.

He suspected it might be a comet, but follow up observations showed up it wasn't fuzzy. The object was given the name Ceres.

A little over a year in 1802, another one was found then in 1804, astronomer spotted a third one and the fourth one in 1807. It was becoming clear that a new class of solar system object had been discovered. Given they were all dots in the telescope of the time, points of light like stars. They were given the name "Asteroids" which literally mean star-like.

By the end of the 19th century, more than 450 had been found. The rate of discovery had been accelerated over the year and now we know thousand or millions probably billions larger than 100m across longer than 1km in size. 

Asteroids: It's a class of smaller bodies that are rocky or metallic that orbits the sun out to Jupiter. Most of them, about 3/4, are carbonaceous, which means they have a lot of carbon in the 75% Carbon based. 

About 1/6th are silicaceous - heavy on the silicon base materials (17% Silicon Based). The rest are lumped into one catch-all category but are dominated by metallic objects, Literally loaded with iron, Nickel and other metals (18% Miscellaneous).

Many of them orbit the sun between Mars and Jupiter that this region is called the Main Belt.

For example - There are very few asteroids about 425 million Kilometer from the sun. Asteroids at that distance would have an orbital period of about 4 years, a simple fraction of Jupiter's 12 year period.

Any asteroids there would feel a repeated tug from Jupiter's mighty gravity, pulling it out of that orbit, The resulting gap is called Kirkwood Gap and there are several such asteroid deserts, all with simple multiples of Jupiter's period.

In the way, the main belt like Saturn's rings, where gap is carved out by the gravity of the orbiting moons. Another way to group Asteroids is by orbit some have similar orbits and may have formed from a bigger parent asteroid that gets disrupted by an impact. These groups are called families, and there are over dozens known.

If we took all the asteroids in the Main Belt and lumped them together they'd be far smaller than our own Moon!. Ceres is the biggest, at about 900 kilometers across Its round, nearly spherical due to its own gravity crushing it into a ball.

Ceres probably has a rocky core surrounded by a water ice mantle. The amount of water in it is staggering - probably more than all the fresh water on Earth! It may be liquid under the surface, like the Oceans of Enceladus and Europa.

Images by dawn, as it approached the asteroid to show its surface is heavily crated, and some craters are very bright, they may be exposed ice under the surface or just fresher, brighter material. There is a tantalizing observation of localized water vapor on the surface which may be from sublimation; ice turning directly into a gas due to the sun's heat, or it might indicate cryovolcano.

Dawn also visited vesta, which is the third largest but second most massive asteroid known. Vesta is roundish, flattened a bit like a ball someone's sitting on it.

The Southern Hemisphere gets hammered by impacts long ago, leaving a huge basin there. Several other main-belt asteroids have been visited by spacecraft, mostly via flybys, Lutetia, Gaspra, Steins, Mathilde. Ida is another and was discovered to have a small moon orbiting it. 

In fact, a lot of asteroids have moons or are actually binary, with two similar sized bodies in orbit around each other.

Kleopatra, a weird dog bone shaped rock, has two moons. A few years ago scientist realized that asteroids have spent billions of years whacking into one another sometimes in high-speed collisions, sometimes more slowly slower hits can disrupt the asteroid, crack it, but not necessarily be strong enough to actually disrupt so that it breaks apart.  

Overtime enough hits like that can leave behind we call them Rubble Pile. 

Why is there even a main asteroid belt at all?. 

The solar system formed from a disk of material and overtime that material started to clump into bigger and bigger pieces. As planet formed, they swept up and pulled in lots more stuff and grew larger.
Jupiter consumed a lot of the material around it, but not al and left a lot of debris inside its orbit. Some of them clumped together to form middling sized objects, probably smaller than the planet, we have now, but big enough to, undergo differentiation; Heavy stuff like metals sank to the middle, and lighter stuff formed a mantle and crust.

There is probably a lot more material between Mars and Jupiter's billions of years ago, but it either got eaten by Jupiter, or by planets immense gravity altered the asteroid's orbit flinging them away. This may be : Why Mars is so small too; Jupiter rubbed it all of its food as it formed. While most asteroids live in the main belt not all of them do. Some have orbits that cross that of Mars, taking them closer to the Sun and this is known as Mars crossing Asteroids. Some have orbits that take them an even closer to the sun crossing Earth's orbit and Astronomer calls them Apollo Asteroids.

Some have orbits that are almost entirely in Earth's orbit, called Aten Asteroids. Aten and Apollo asteroids can get pretty close to Earth, so we call them Near-Earth Asteroids. Their orbits and the orbits of the Earth don't actually cross each other but some do have paths that literally intersect Earth's orbit. 

Originally, Asteroids were named after female goddesses Ceres, Vesta, Juno, and so many but after the discovery of lots of asteroids, the astronomer was short of names.

The astronomer who discovered Asteroids were allowed to name them.