"Long before it's in the papers"
November 08, 2011

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Largest near-Earth asteroid to pass by

Nov. 7, 2011
Courtesy of Sky & Telescope
and World Science staff

Earth is about to get a vis­it from the larg­est close-approaching as­ter­oid on rec­ord, Sky & Tel­e­scope Mag­a­zine is re­port­ing. Known as 2005 YU55, it is about a quar­ter mile (400 me­ters) across, round, and quite dark, said the mag­a­zine, cit­ing as­tro­no­mers.

When it comes clos­est to us, at 6:28 p.m. East­ern time (23:28 Un­iver­sal Time) on Nov. 8, it will be 198,000 miles (319,000 km) from Earth’s sur­face—clos­er than the Moon. As­tro­no­mers around the world are ex­pected to closely fol­low the ob­ject as it glides across the sky.

Weath­er per­mit­ting, back­yard sky­watch­ers al­so have a chance to watch. 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. “You should be able to spot the as­ter­oid with your tel­e­scope if it has an ap­er­ture of at least 6 to 8 inch­es,” said Sky & Tel­e­scope Ed­i­tor Al­an Mac­Rob­ert.

The in­ter­lop­er’s track past Earth is es­pe­cially favora­ble for west­ern Eu­rope and North Amer­i­ca. But you’ll need to know where and when to look: the ob­ject will cross the 70 de­grees of sky east­ward across sev­er­al con­stella­t­ions, from Aq­ui­la to Peg­a­sus, in just 10 hours. At times it will move fast enough to cov­er a Moon’s width of sky in un­der five min­utes. Light from a nearly full Moon is ex­pected to bright­en the sky some­what, mak­ing faint stars and the as­ter­oid a bit harder to spot.

To de­ter­mine where to look, Sky & Tel­e­scope 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. The chart takes this in­to ac­count by in­clud­ing small upside-down U.S. maps that let you es­tab­lish the cor­rect path for your loca­t­ion.

Discov­ered nearly six years ago by Rob­ert Mc­Mil­lan at Stew­ard Ob­ser­va­to­ry 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.

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Homepage image courtesy NASA Jet Propulsion Laboratory

Earth is about to get a visit from the largest close-approaching asteroid on record, Sky & Telescope Magazine is reporting. Known as 2005 YU55, it is about a quarter mile (400 meters) across, round, and quite dark, said the magazine, citing astronomers. When it comes closest to us, at 6:28 p.m. Eastern time (23:28 Universal Time) on Nov. 8, it will be 198,000 miles (319,000 km) from Earth’s surface—closer than the Moon. Astronomers around the world are expected to closely follow the object as it glides across the sky. Weather permitting, backyard skywatchers also have a chance to watch. 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. “You should be able to spot the asteroid with your telescope if it has an aperture of at least 6 to 8 inches,” said Sky & Telescope Editor Alan MacRobert. The interloper’s track past Earth is especially favorable for western Europe and North America. But you’ll need to know where and when to look: the object will cross the 70 degrees of sky eastward across several constellations, from Aquila to Pegasus, in just 10 hours. At times it will move fast enough to cover a Moon’s width of sky in under five minutes. Light from a nearly full Moon is expected to brighten the sky somewhat, making faint stars and the asteroid a bit harder to spot. To determine where to look, Sky & Telescope 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, but was undetected.