In February 2013, a 60-foot-across (18-meter), 11,000-metric ton (12,125-ton) meteor exploded in the sky over the Russian city of Chelyabinsk, injuring more than 1,200 people [source: Yeager]. Coincidentally, that same day, an even bigger object--an asteroid half the size of a football field--passed about 17,200 miles (27,680 kilometers) from Earth. Had it struck, it would have exploded with a force of about 2.4 million tons (2.2 million metric tons) of dynamite, the equivalent of hundreds of Hiroshima-sized A-bombs [source: Reuters].
Both of those space rocks, though, are tiny compared to some of the other asteroids hurtling through space. And we know that if a big enough object slams into our planet, the results might be hellish. About 66 million years ago, an object 6 miles (10 kilometers) in diameter smashed into the Yucatan Peninsula in Mexico, trigging a cataclysm that killed off the dinosaurs and most other animal and plant life on Earth [source: Reuters].
That's why identifying and tracking asteroids that might venture within the vicinity of Earth is an especially high-priority NASA mission. The goal of NASA's Near Earth Objects program is to compile a database of near-Earth objects and track their movements [source: Messier].
Asteroids are made mainly of rocks and minerals and are formed in the warmer inner solar system, between Mars and Jupiter. They are leftover pieces from the formation of the inner planets of Mars, Earth, Venus and Mercury. Comets, on the other hand, are made up of water ice and dust and formed in the colder outer solar system. Comets are leftovers from the formation of the outer planets of Jupiter, Saturn, Uranus and Neptune. Comets and asteroids that are drifting near Earth (within 28 million miles or 45 million kilometers of Earth's orbit) are considered to be near-Earth objects [source: NASA].
To hunt for them, NASA has repurposed an existing satellite, the Wide-field Infrared Survey Explorer, or WISE, originally launched in 2009 to search for distant stars and galaxies. NASA envisions that WISE will discover about 150 previously unknown near-Earth objects and gather information about the size and other properties of about 2,000 more [source: NASA].
WISE and the NEO program hopefully will give NASA advance warning of an object on a collision course—and time to implement a defensive strategy, whether that means diverting the asteroid with gravity tractors, solar sails or other future technologies, or simply destroying it with a nuclear blast [source: Messier]. That might help us to avoid the worst natural disaster ever.