How bright will an asteroid be?

Question

In the news, it has been stated that there will be a fairly large asteroid passing fairly close to Earth soon. I've been trying to find a good observation guide, including determining how bright it will be, with bad luck. Specifically, I want to find out if this is a naked eye visible object, and if so, how to find it. Thanks!

Answer 1

I think you're referring to asteroid 2005 YU55 which is making an approach on November 8.

This article on NASA JPL's Asteroid Watch site gives some details including:

The asteroid's surface is darker than charcoal at optical wavelengths. Amateur astronomers who want to get a glimpse at YU55 will need a telescope with an aperture of 6 inches (15 centimeters) or larger.

which to me implies that it would not be visible with the naked eye. It's estimated size is 1,300 feet (400 meters).

JPL has also posted an hour-long video here discussing the asteroid.

Another NASA site, the Near Earth Object Program, has better technical information which may help locate the asteroid. Some excerpts:

...the object will reach a visual brightness of 11th magnitude and should be easily visible to observers in the northern and southern hemispheres. The closest approach to Earth and the Moon will be respectively 0.00217 AU and 0.00160 AU on 2011 November 8 at 23:28 and November 9 at 07:13 UT.

...

The best time for new ground-based optical and infrared observations will be late in the day on November 8, after 21:00 hours UT from the eastern Atlantic and western Africa zone.

To get the exact coordinates, you can try NASA's Solar System Dynamics site. The page for asteroid 2005 YU55 has orbital elements with a link to generate ephemeris.

Answer 2

Don't bother checking exact locations until a few hours before closest passage (on Tuesday evening in North America) because the orbit will have last minute changes. The asteroid is predicted to be around 10th magnitude, but from personal experience with several earlier asteroid passages you will need at least a 10-inch aperture to see it. What has worked for me is to find a small star pattern somewhere ahead of the asteroids position on its track, concentrate on that and wait for the asteroid to move into view. In most cases, the asteroids have been ahead of their predicted positions by about 5 minutes, probably because of acceleration by the Earth's gravity. Good luck!