If an asteroid with a diameter of more than 1 km struck the earth, it would result in permanent climate change and the likely eradication of human life. For this reason, scientists have attempted to map as many of them as possible. It's still not a perfect system, however. Last month the previously undiscovered asteroid 2009 DD45 whizzed past our planet, visible with the naked eye from the southern hemisphere, at 20 km per second only 66,000 km above the surface.
Asteroids get closer than this all the time, so scientists say there's nothing to fear from 2009 DD45.
The vast majority of asteroids are located in the belt between Mars and Jupiter. Their combined mass is less than that of the Earth's moon. There are 26 known asteroids larger than 200 km in diameter. We probably know more than 99% of the large asteroids, more than 100 km in diameter, but very few of the smaller ones. There are probably more than a million with diameters of 1 km or more. Most are in relatively circular orbits, but some are much more eccentric, which can sometimes bring their flight paths uncomfortably close to Earth.
The first asteroid was discovered on January 1, 1801, by Giuseppi Piazzi. He believed he had discovered a small planet, and named it Ceres, after the Roman goddess of grain. Within the next few years, asteroids Pallas, Vesta, and Juno were discovered. By the end of the 19th century, scientists had discovered thousands of asteroids.
Ceres is the largest known asteroid, with a diameter of 590 miles. This is large enough so that gravity has given it a roughly spherical shape. For this reason, astronomers have re-classified Ceres as a dwarf planet, like Pluto.
The photo to the right was taken by the Hubble telescope, and is the clearest image of Ceres to date, but the dwarf planet is scheduled to be explored by unmanned NASA probes within the next five years.
In 1991 the Galileo spacecraft gave us our first up close images of asteroids, photographing Ida and Gaspra on its way to Jupiter.
Ida is especially interesting because it has its own tiny satellite, the asteroid Dactyl (with a diameter of about 1.5 km, you can see it as a tiny dot on the right side of the photo). It is believed that several asteroids in the region were originally one big piece, which broke apart after a collision with another object. In fact, many asteroids may be part of binary systems or even small groups of satellites revolving around larger neighbors.
Perhaps the most notable asteroid of all is Apophis, whose orbit brings it very close to Earth once every few years. It is "only" 350 meters across, but will be in our vicinity in 2029. Scientists estimate a 1 in 45,000 chance that Apophis will collide with Earth in 2029. If this were to happen, the impact would release 880 megatons of energy. When Meteor Crater, Arizona was formed (pictured at the top of this post), only 3 - 10 megatons were released. If Apophis hit Earth on the land, the impact would kill millions. If it struck the water, it would create devastating tsunamis in all directions.
There's no need to panic. Space is filled with asteroids, comets, and other space debris that could impact the Earth at any time. NASA and ESA are developing plans to visit and possibly deflect such objects if they get too close to our planet, in addition to mapping as many objects as possible so that we can predict an impact with enough time to react. Learning about our rogue neighbors, the asteroids, is a critical step towards preventing future catastrophes.
For my previous posts on outer space, click here.
There's a great book, filled with beautiful pictures from around the Universe, called Astronomy: A Visual Guide, by Mark A. Garlick. It's essential reading for any outer space enthusiast.