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2012 DA14's route
The image above shows how close 2012 DA14 got as it passed by Earth. It orbits the Sun and takes 368 days to complete one full cycle. The encounter with Earth will speed its orbit up to 317 days. It will be at least three decades before 2012 DA14 could possibly come this close to Earth again.
In 2004, a newly discovered a 250-yard wide asteroid named Apophis, made a splash in the media when it was predicted that it had a 2.5 percent chance of striking Earth in 2029. New instruments and calculations, which are sixty times better than the ones used previously, show that there's virtually no chance of hitting Earth in 2029. And just last month it was determined that there's no chance of it hitting Earth in 2036 either. Apophis is circled above.
One of the biggest asteroid threats found so far is from 500-yard wide "1999 RQ36." It's estimated that it has a 1-in-2,400 chance of impacting Earth in the late 22nd century. NASA plans to send the OSIRIS-REx to land on the asteroid and take samples for further study. OSIRIS-REx is expected to launch in 2016, arrive in 2018, and work until 2021 before returning back to Earth with soil samples.
History shows that recent (in scientific terms) asteroid impacts have changed the face of the Earth. An iron-based asteroid about this size of 2012 DA14 blasted out Meteor Crater in Arizona which is about 4,000 feet wide and 700-800 fett deep. Another in 1908, hit Tuguska, Siberia. The asteroid that made the impact event, now know as the "Tunguska Event," was just slightly smaller than 2012 DA14, approximately 100 – 130 feet or 30-40 meters across. It destroyed about 750 square miles of forest in and around the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai, Russia.