|Update: The flyby of asteroid 2005 YU55 generated a lot of interest among the news media and general public. Click here for details.|
Roll out the red carpet! Earth is about to be visited by the largest close-approaching asteroid on record. Known as 2005 YU55, it comes closest to us on November 8th at 23:28 Universal Time (6:28 p.m. EST), when it passes 198,000 miles (319,000 km) from Earth's surface — closer than the Moon's orbit. It will be visible from the Americas and Europe through much of the night.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 passed close enough for detailed radar probing by the giant radio dish at Arecibo, Puerto Rico.
The Arecibo observations showed this asteroidal emissary to be a quarter mile (400 meters) across and remarkably round. Given its size and dimness, its surface must be quite dark and thus likely carbon-rich. Its rotation period is relatively long, 18 to 20 hours.
In the grand scheme of things it's more micro-planet than minor planet, but we've never knowingly had something this big come this close before. Were it to strike Earth, 2005 YU55 would deliver a kinetic-energy punch equivalent to several thousand megatons of TNT. It's the kind of potential threat that outer-space sentries lose sleep over.But fear not: the Arecibo observations allowed dynamicists to recompute the big rock's orbit with enough accuracy to ensure that it won't strike Earth within the next 100 years. (That said, it will pass just 175,000 miles from Venus in 2029, close enough to alter its orbit slightly. This adds uncertainty to predictions for its next close encounter with Earth in 2041, when the minimum distance could be anywhere from 200,000 to 30 million miles.)
So we might as well just enjoy this month's show. The asteroid will approach Earth from the sunward direction, so it will be a daylight object until just before the time of closest approach. A few hours later 2005 YU55 should reach a visual magnitude of 11.1, within reach of backyard telescopes with apertures of at least 6 inches under fairly dark skies — though you'll be fighting light from the nearly full Moon. (By the way, that bright thing near the Moon tonight is Jupiter.)The pass's track is especially favorable for western Europe and North America. But you'll need to know exactly where to look at exactly what time: the object will traverse the 70° of sky eastward from Aquila to central Pegasus in just 10 hours, clipping along at 7 arcseconds per second. Use the chart here to get a sense of what part of the sky it's in, then download our detailed finder chart for use between 9 and 10 p.m. November 8th Eastern Standard Time (2:00 and 3:00 November 9th Universal Time).
If you don't have a suitable scope, or if it's cloudy tonight, check out the live video webcast of asteroid 2005 YU55 from the 25-inch telescope at Clay Center Observatory in Massachuetts (continuously from about 6 p.m. to 3 a.m. EST). Another live webcast is available from Bellatrix Astronomical Observatory in Italy.
Amateur asteroid sleuths Brian Warner and Robert Stephens have mounted a campaign to obtain detailed photometry (brightness measurements) of the asteroid. You'll need at least a 10- or 12-inch scope, a CCD camera, and ideally one or more of the standard photometric filters commonly used by professional astronomers. Details.Meanwhile, this visit by 2005 YU55 is providing an unprecedented opportunity for high-reolution radar study. Astronomers have lined up extensive radar campaigns with Arecibo and with NASA's Goldstone facility in California's Mojave Desert, using big radio dishes in West Virginia and elsewhere as receivers. "The signal-to-noise ratios will be more than 1 million for Goldstone observations on November 89," explains Lance Benner (Jet Propulsion Laboratory). This SNR is heady territory for radar work, high enough to yield thousands of pixels across the object and to achieve surface resolution "comparable to what can be obtained by a spacecraft flyby mission."
So I hope you all get a chance to spot 2005 YU55 as it zips past Earth.