A 150-metre wide hunk of space rock, called 2003 DZ15, will be flying past the Earth tonight, making its second closest pass by our planet in 300 years.This will be the asteroid's third pass by Earth since it was discovered in February 2003, but astronomers have tracked its orbit so well that their records show all the undetected fly-bys it made before, going back to 1901, as well as all the future fly-bys it will make, all the way up to the year 2200. Of all of these 99 encounters, tonight's will be the second closest ever, as it passes within 3.5 million kilometres of us (about 9 times the distance to the Moon), at 8:37 p.m. Eastern Time.
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The last time 2003 DZ15 came anywhere near this close was, apparently, back on February 13th, 1976, when it got within 5.7 million kilometres of us, and the only time it will get closer (at least for the next 187 years) is on this same day (July 30) in the year 2063. However, even then, it will still miss us by over 6 times the distance from here to the Moon.
Apollo asteroids like 2003 DZ15 are among the most dangerous to us, though. There are a few different kinds of asteroids, based on the type of orbit they have. Apollo asteroids have orbits that are very similar to the size and shape of Earth's orbit, but their orbit isn't centered on the Sun like ours is. It's offset, so that the asteroid swings around close to the Sun, crosses Earth's orbit and heads out towards or beyond Mars before it swings back for another pass around the Sun. It's the fact that they cross Earth's orbit that makes them so dangerous.
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The next big fly-by will be in just over a week, on the night of August 8th/9th, when another Apollo asteroid named 2005 WK4 will cruise past us. 2005 WK4 is over two and a half times the size of 2003 DZ15, and it will come a bit closer to us, but it still stays about 3 million kilometres away (roughly 8 times the distance between us and the Moon), so we're still safe.