On Saturday night, August 17–18, a recently discovered asteroid will pass close enough to Earth to be easily spotted in small telescopes or even binoculars. The latest calculations by the Minor Planet Center in Cambridge, Massachusetts, indicate the flyby will bring it to within 527,000 kilometers (327,000 miles) of Earth, just outside the Moon’s orbital distance.
Astronomers first detected this object, designated 2002 NY40, on July 14th with the 1-meter LINEAR telescope in New Mexico. It was then only 19th magnitude and appeared nearly stationary in western Aquarius, brightening steadily on an in-bound path toward Earth. Finally this week, with the flyby fast approaching, the asteroid began moving northwest at an ever-increasing rate.
Judging by its brightness, this interloper is roughly 500 meters (0.3 mile) across. But detailed photometry by Lenka Sarounova (Czech Republic), Sergio Foglia (Italy), and David DeGraff, William Holliday, and Walter Cooney (USA) already shows it to have an irregular shape. Using their observations, Petr Pravec (Ondrejov Observatory) finds that 2002 NY40 rotates in 20 hours and varies by more than a magnitude. The light-curve amplitude “indicates that the asteroid is an elongated object with an equatorial-axis ratio greater than 2:1,” Pravec reported to the Minor Planet Mailing List August 9th.
Almost immediately after its discovery in July, astronomers determined that there was no danger of 2002 NY40 striking Earth during this flyby. In addition, both NEODyS, operated by the University of Pisa, and NASA’s Near-Earth Object Program quickly ruled out an impact during the coming century. But the encounter affords Earth-based astronomers a rare chance to study an asteroid at very close range, however briefly.
Where and How to Look
On the night of Saturday, August 17–18, 2002 NY40 should reach magnitude 10 or even 9 during the period when it is well placed for viewing from Europe, Africa, and the Americas. Although it might be spotted in binoculars, small telescopes should give a more satisfying view by magnifying the object’s apparent motion. Skywatchers should be able to perceive this motion any time it glides near a background star. When the asteroid is closest to Earth at around 7:47 Universal Time on the 18th, it will be traveling eastward at a breezy 8 arcminutes per minute!
For help in locating the asteroid that night, Sky & Telescope has prepared four finder charts (A, B, C, and D) that span a 60° arc across the heavens from Sagitta through Vulpecula, Cygnus, Lyra, and Hercules. Each chart is a PDF file; these are readable on any computer using Adobe's free Acrobat Reader software, version 3.0 or later, and can be printed out for use at the telescope.
Chart A is mainly of use to observers in Europe and Africa, with tracks plotted for the period 20:00 UT (on the 17th) to 01:30 UT on the 18th.
Chart B includes the start of tracks suitable for North American viewers, plotted from 1:30 to 4:30 UT.
Chart C shows the continuation of the asteroid’s path as viewed from Boston and Los Angeles between 4:10 and 6:10 UT.
Chart D shows the the final portion of the track for North American viewers and concludes at 7:40 UT.
Because of the parallax effect, the asteroid’s exact trajectory depends on your geographical location. Tracks for several widely separated cities are shown, and you can estimate the track for your own location relative to those shown. (For example, the track for Denver would lie between the tracks shown for Boston and Los Angeles, somewhat nearer the latter.) Each plotted track covers only the period when the asteroid is at least 10° above the horizon in a fully dark sky at that location. Our plots are based on astrometric measurements received by the Minor Planet Center through August 11th. Each individual track should be quite reliable, but the object’s arrival time at a specific point along a track is still uncertain by 1 or 2 minutes.
To catch sight of this fleet visitor, the best strategy is to pick out a star near which the asteroid will pass at a specific UT. About 10 minutes in advance, park your telescope on that star and watch for the asteroid to come by. If you miss it, find another plotted star farther down the track and try again.
Keep in mind that the asteroid will be slightly fainter than the stars shown in our plots, yet still quite easy to see in a small telescope. A mere 24 hours after it goes by, however, the object will plunge hopelessly beyond reach of Earth-based telescopes as it heads closer to the Sun. (We will then be viewing its unilluminated side, which explains why it becomes so faint, so fast.)