On 14 June 2012 the newly discovered asteroid 2012 LZ1 flew past the Earth at a distance of 5.3 million km (roughly 14 times as distant as the Moon). The asteroid had only been discovered four days earlier, and was estimated to be 500 km across, enough to cause quite a bang if it hit us, though this was never thought likely to happen.
The asteroid was on a course quite close to ours, so that it stayed in range of Earth-based telescopes for some time; this was not an opportunity to be wasted, and 2012 LZ1 was closely watched from a number of centers around the globe. One of these centers was the Planetary Radar System at the Arecibo Observatory, in Puerto Rico, which scanned 2012 LZ1 in some detail as it withdrew from the Earth.
Radar image of 2012 LZ1 produced by the Arecibo Observatory.
The results of this study came as a shock; 2012 LZ1 was twice as large as originally thought, 1 km across rather than 500 m; this may not sound like a lot, but for a roughly spherical object is 33.5 times the volume, which adds up to rather a lot of rock. Such a rock hitting the Earth could have serious global effects, so having a close encounter with one with only four days notice is fairly alarming.
The subject of objects from space impacting on the Earth is a controversial one. Everybody agrees that a significantly large object hitting the Earth is likely to have serious consequences, but scientists differ in their views on the extent to which this happens. Planetary scientists, who look for evidence that the Earth's history may have been influenced by such impacts find plenty of evidence for this, however geologists, who try to find the causes of events on Earth, find no proof that impacts have caused these events. While it is quite likely that large impacts do happen on Earth, and do have widespread consequences, we do not have examples of events where the most likely cause was extra-terrestrial. For example, the extinction event at the end of the Cretaceous could have been caused by a large object impacting the Yucatan Peninsula in Mexico, it is far more likely to have been caused by an outpouring of flood basalts in India called the Deccan Traps.
See also Asteroid 2012 LZ1 flies by the Earth, The origin of Comet P/2006 VW₁₃₉, Asteroid 2012 KP24 passes Earth at a distance of 51 000 km, Neptune's Trailing Trojans and The eta Aquarid Meteors.
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