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Giant asteroid is about to give Earth a close call

Paul Sutherland, Feature writer
Jan 25, 2015, 3:44 UTC

Sen—One of the largest known asteroids to threaten the Earth will make a close pass on Monday night, 26-27 January, coming to just three times the distance of the Moon from us.

There is absolutely no danger of an impact from the mountain-sized rock, but the flyby, at a distance of 1.2 million km (745,000 miles) is the closest that any asteroid of its size will come until 2027. 

Though a number of other asteroids have come closer in recent years, they have all been considerably smaller.

The cosmic missile, labelled 2004 BL86 after the year in which it was discovered, is thought to be around 500 metres wide (a third of a mile). It orbits the Sun every 1.84 years, but usually crosses the Earth’s orbit when we are at a much greater distance from it.

Asteroid 2004 BL86 will be close enough to be visible in backyard telescopes at a magnitude (brightness) of about 9.5 as it rapidly crosses the sky, covering 2.5°, or five times the apparent width of the Moon an hour when at its nearest.

It will provide a challenge for astrophotographers with cameras capable of taking time exposures as it heads from the constellation of Hydra up through Cancer, passing close to the famous Beehive Cluster of stars, otherwise known as Praesepe or M44.

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A NASA diagram of their orbits around the Sun illustrates how close asteroid 2004 BL86 will pass to the Earth on Monday night. Image credit: NASA/JPL-Caltech

As well as being a target for amateur astronomers, 2004 BL86 will also be studied closely by professional scientists. NASA scientists will use radio dishes in California and Puerto Rico to bounce signals off the asteroid, allowing them to make radar maps of the chunk of space rock.

It was discovered in 2004 by an automatic sky survey telescope called LINEAR, based in New Mexico, during its hunt for potentially hazardous objects such as asteroids and comets.

Don Yeomans, manager of NASA’s Near Earth Object Program Office in California, said in a statement: “Monday, January 26 will be the closest asteroid 2004 BL86 will get to Earth for at least the next 200 years. And while it poses no threat to Earth for the foreseeable future, it’s a relatively close approach by a relatively large asteroid, so it provides us a unique opportunity to observe and learn more.”

Because the asteroid comes as close as it does, its exact path against the starry background will vary depending from where in the world it is observed, so it is not possible to provide a general chart. Amateur astronomers can obtain positions for their own locations by visiting this NASA website and searching for 2004 BL86.

Robin Scagell, of the Society for Popular Astronomy, told Sen: “Spotting the asteroid won’t be easy, but it is well within reach of amateur telescopes and will make a fun challenge. Though you won’t actually see it moving, its motion across the sky will become evident in just a few minutes.

“Keen photographers can capture the asteroid by mounting the camera, equipped with a telephoto lens and focused on infinity, and taking a number of time exposures of 30 seconds or more, depending on the darkness of their sky.”