Rocks from space hit Earth every day. The larger the rock, though, the less often Earth is struck. Many kilograms of space dust pitter to Earth daily. Larger bits appear initially as a bright meteor. Baseball-sized rocks and ice-balls streak through our atmosphere daily, most evaporating quickly to nothing.
Significant threats do exist for rocks near 100 meters in diameter, which strike the Earth roughly every 1000 years. An object this size could cause significant tsunamis were it to strike an ocean, potentially devastating even distant shores.
A collision with a massive asteroid, over 1 km across, is more rare, occurring typically millions of years apart, but could have truly global consequences. Many asteroids remain undiscovered.
In fact, one was discovered in 1998 as the long blue streak in the above archival image taken by the Hubble Space Telescope. Further, just last year the 50-meter wide asteroid 2012 DA14 was discovered that will pass inside the orbit of Earth’s geosynchronous satellites this coming Friday.
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