Asteroids

Asteroids have been making quite a few headlines as of late. Recently, we had NASA announcing its intentions to capture and asteroid robotically and bring it back for study by a manned expedition. The ten-year, $2.6 billion project would partner with private companies to capture a 500-ton, near-Earth asteroid that would be bagged, brought back, and placed in a gravitational parking lot known as the Earth-Moon lagrangian point (L2). There, a manned outpost could study it and set up a mining station to harvest its resources, especially its trapped water. Considering that it currently costs $10,000 per pound to haul water into orbit, mining it from an asteroid could save a billion dollars at current launch prices. Add to that the ability to use water to create rocket fuel by splitting it into its elemental components hydrogen and oxygen, it's no surprise that water is also called "space gold".
   On February 15, 2013, a meteor exploded over Russia’s Ural mountains in the Chelyabinsk region, injuring about a thousand people, as the shockwave blew out windows and rocked buildings. On that same day, there was a close flyby of asteroid 2012 DA14, which passed within about 27,000 km of Earth which is closer than the orbits of television and weather satellites that surround our planet. The two events were unrelated. 

Ceres, the largest asteroid
and also a dwarf planet.

   In the weeks after these events, there’s been a renewed call for creating an asteroid detection system. As it stands now, all anyone could do if we discovered a large asteroid headed toward New York City or some other large metropolitan area is “pray,” according to NASA chief Charles Bolden. We only know the whereabouts of about 10% of the estimated 10,000 city-killer asteroids. The Chelyabinsk asteroid is the largest to hit Earth since the 1908 Tunguska asteroid exploded over Siberia, leveling 80 million trees over an area of around 2,100 sq km.
   The asteroid belt lies between the orbits of Jupiter and Mars. Even though there are asteroids in other parts of the solar system, most are found here. About half the mass of the belt is contained in the four biggest asteroids: Ceres, Vesta, Pallas and Hygiea. These have average diameters of more than 400 km, while Ceres, which is also a dwarf planet, has a diameter of about 950 km. The remaining asteroids are thinly distributed and range all the way down in size to dust particles.

Vesta as imaged by the Dawn spacecraft.

   Asteroids are rocky. Because they come from the inner solar system, any ice would have been baked off by the sun long ago. Their orbits are fairly predictable, so with good observations, we can track down the big ones and determine if they’re threats.
   There are more objects beyond Neptune. The Kuiper Belt extends more than 100 times farther from the Sun than Earth. Beyond that is the Oort Cloud which extends 10,000 times farther from the Sun than Earth. These collections of small, icy bodies are remnants from the formation of the solar system. When their orbits are disturbed by other objects they can move into the inner solar system, becoming comets. As they come close to the sun that ice evaporates and creates the comet’s tail. They are less dense than asteroids, and tend to be moving faster by the time they reach the inner solar system. Some comets, like Halley’s comet which returns every 76 years, have predictable, periodic orbits.
   Knowing where an object comes from is a good indicator as to whether it is an asteroid or a comet. It’s not all black-and-white—objects from the outer solar system might be rocky and some asteroids do have some ice. But overall this is good way of thinking about them.