Stars twinkle, but usually not like this! On the morning of Nov. 20, asteroid 752 Sulamitis will be seen, from some places on Earth, passing in front of a star in the constellation Gemini, making the star fade away for up to 10 seconds. This will be long enough for amateur astronomers with home video cameras to contribute valuable information to studies on the size and shape of the asteroid.
Not much is known right now about 752 Sulamitis, except that it may be up to 105 kilometers (65 miles) in diameter. The path of this stellar eclipse may cut across the upper eastern half of the United States. "Being able to see an asteroid pass in front of, or occult, a star is rare," said Lance Benner, an expert on asteroids at NASA's Jet Propulsion Laboratory, Pasadena, Calif., and a member of the International Occultation Timing Association (IOTA).
This is the first time an asteroid occulting a star can be seen in North America since 1975, when such predictions became readily available. The star, known as Tejat, is bright enough to be seen with the naked eye. The asteroid is very faint -- about 18,000 times fainter than the star, and cannot be seen without a telescope.
The region where the eclipse is visible will be a narrow band slanting from North Carolina's coast northwest to Madison, Wis., to northern Alaska. Residents of Chicago, Minneapolis, Indianapolis and Cincinnati may also be able to see the asteroid as it fades out for about two seconds. The asteroid's path may be about 78 kilometers (48 miles) to the right or the left of the route drawn on the map.
The star, which is located in the southern corner in the constellation Gemini (just above Orion), will be intersected by the asteroid from 6:41 to 6:49 a.m. Eastern time. It will take eight minutes to move from the East Coast to the north boundary of the continental United States. Star-watchers can set up their cameras and telescopes, mark their location, and tune in to short-wave radio station WWV, Boulder, Colo. at 2.5, 5, 10, 15, 20 megahertz. By coordinating with the station's time signal, observers can mark when they see the star, which will be just one-third of the moon's brightness, disappear and reappear as the asteroid blocks it from view. The occultation will be unlike the normal twinkling of the star caused by Earth's atmosphere, because it will take about three seconds to fade out, remain obscured from view for 10 seconds and fade back in 3 seconds.
Observations of the shadow across the star can be reported to the International Occultation Timing Association. An analysis of the observations will help define the asteroid's shape and size. Reports from those who observe the star but don't see it disappear are still scientifically useful -- they can help determine the size of the asteroid and the precise location of the asteroid's track across the United States. And observers in outlying areas may see only a partial eclipse of the red supergiant star, since it doesn't have a sharp outline like younger stars.
Observations can be reported to the IOTA, online at http://www.lunar-occultations.com/iota/iotandx.htm. JPL is managed by the California Institute of Technology for NASA. For more information, and a partial map of viewing locations, see www.jpl.nasa.gov.