Earth is about to be visited by the largest close-approaching asteroid on record.
Known as 2005 YU55, it is about a quarter mile (400 meters) across, round, and quite dark. When it comes closest to us, at 6:28 p.m. EST (23:28 Universal Time) on November 8th, it will be 198,000 miles (319,000 km) from Earth’s surface — closer than the Moon’s orbit. Professional astronomers around the world will closely follow the asteroid as it glides across the sky.
Weather permitting, backyard skywatchers also have a chance to spot this interloper. A few hours after passing closest to us, it will peak in brightness at magnitude 11.1, roughly 100 times fainter than the limit of human vision. “The good news,” says Alan MacRobert of SKY & TELESCOPE magazine, “is that you should be able to spot the asteroid with your telescope if it has an aperture of at least 6 to 8 inches.”
The asteroid’s track past Earth is especially favorable for western Europe and North America. But you’ll need to know exactly where and when to look: the object will traverse the 70 degrees of sky eastward across several constellations, from Aquila to Pegasus, in just 10 hours. At times it will zip among the background stars at 7 arcseconds per second, covering a Moon’s width of sky in under 5 minutes. And light from a nearly full Moon will brighten the entire sky somewhat, making faint stars and the asteroid somewhat more difficult to spot.
To determine where to look, SKY & TELESCOPE’s editors have prepared two detailed finder charts. The first gives a general sense of where to look, and the second provides a detailed view to use while outside with your telescope. Once you’ve aimed at exactly the right spot, you shouldn’t have much trouble telling which starlike point is 2005 YU55. It will be gliding fast enough to move along in real time as you watch using a moderately high-magnification eyepiece.
“As it passes Earth, the asteroid gets so close that its position among the stars will be significantly affected by your location,” explains MacRobert. So the magazine’s detailed finder chart takes this parallax effect into account, by including small upside-down maps of the United States that permit you to establish the correct path for your location.
Discovered nearly six years ago by Robert McMillan at Steward Observatory’s Spacewatch Telescope in Arizona, 2005 YU55 has been this way before. In April 2010, it ventured close enough for detailed radar probing by the giant radio dish at Arecibo, Puerto Rico. It also approached even closer in 1976, though it went by undetected.
Professional astronomers have recruited advanced backyard amateurs to do precision brightness measurements of the asteroid during its flyby.