Asteroid to pass within viewing range of observers on Earth

Friday, August 16th 2002, 12:00 am

By: News On 6

WASHINGTON (AP) _ An asteroid will pass close enough to the Earth to be viewed with binoculars on Saturday night, but astronomers said there is no immediate danger that the half-mile-wide space rock will hit the planet.

The asteroid, known as 2002 NY40, was discovered July 14. Astronomers said Friday that it will zip by about 350,000 miles from the Earth, about 1.3 times farther away than the moon.

It is expected to be faintly visible by binoculars or by telescope after sunset on Saturday to about 3 a.m. EDT Sunday as it appears to pass near the star Vega and clip through the constellation Hercules.

Don Yeomans, director of NASA's Near-Earth Object Program at the Jet Propulsion Laboratory in California, said an asteroid passage within view of the Earth is uncommon.

``Flybys like this happen every 50 years or so,'' Yeomans said in a statement released by NASA. The last known occurrence was on Aug. 31, 1925, when a similar-sized asteroid, called 2001 CU11, passed by just outside the orbit of the moon. That flyby was unrecognized until 77 years later, when modern astronomers detected the space rock and backtracked its orbit.

Since astronomers know 2002 NY40 is coming, they are preparing a reception. A giant radar beacon at the Arecibo observatory in Puerto Rico will bounce signals off the asteroid as it whizzes past. The return radar signal will help astronomers learn the shape of the space rock and help plot its future path through space.

``At present we know there's little risk of a collision with 2002 NY40 for decades,'' said Jon Giorgini, a member of a JPL radar observation team. ``When the Arecibo radar measurements are done, the orbit uncertainties should shrink by more than a factor of 200. We'll be able to extrapolate the asteroid's motion hundreds of years into the past and into the future.''

At its closest approach to Earth early Sunday, the asteroid will shine at a stellar magnitude of 9, about one-sixteenth the brightness of the faintest star visible without a telescope.

Yeomans said asteroids are generally difficult to see because they are mostly black, like charcoal.

``The most common ones _ carbon-rich C-type asteroids _ reflect only 3 to 5 percent of the light that hits them,'' he said. ``Metallic asteroids, which are somewhat rare, reflect more, 10 to 15 percent.''

Astronomers are uncertain of the composition of 2002 NY40, but they should know after taking readings with ground-based telescopes, said Yeomans.

The path followed by 2002 NY40 will make it relatively easy to spot with binoculars or telescopes. To an Earth observer, it will appear to fly past Vega, the brightest star in the summer nighttime sky. It is expected to appear as a speck of light moving at about 8 degrees an hour, NASA said. The best viewing will be from the Northern Hemisphere, starting just after sunset on Saturday in North America and during the hours just before dawn on Sunday in Europe.

The bright side of the asteroid will face the Earth early in its passage, giving observers a look at its fullest sunlit phase. But as it moves on, the dark side will come into view and the asteroid will noticeably dim and then disappear from view, NASA said.