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Asteroid Flyby Of Earth August 18

by Tony Phillips
for NASA Space Science
Huntsville - Jul 31, 2002
A big space rock will soon come so close to Earth that sky watchers can see it through binoculars. But relax, there's no danger of a collision, but it will be close enough to see through binoculars: a big space rock, not far from Earth.

Astronomers discovered the nearby asteroid, named 2002 NY40--not to be confused with better-known 2002 NT7--on July 14th. It measures about 800 meters across, and follows an orbit that ranges from the asteroid belt to the inner solar system. On August 18th, the asteroid will glide past our planet only 1.3 times farther away than the Moon.

"Flybys like this happen every 50 years or so," says Don Yeomans, the manager of NASA's Near-Earth Object Program office at JPL. The last time (that we know of) was August 31, 1925, when another 800-meter asteroid passed by just outside the Moon's orbit. In those days there were no dedicated asteroid hunters--the object, 2001 CU11, wasn't discovered until 77 years later. At the time of the flyby, no one even knew it was happening.

2002 NY40 is different. We know the asteroid is coming, and astronomers have time to prepare.

One team of observers led by Mike Nolan at the giant Arecibo radar in Puerto Rico will "ping" 2002 NY40 with radio waves as it approaches Earth. Such data result in impressive 3D maps of asteroids, which have often surprised astronomers with their weird shapes. Some prove to be binary systems (one space rock orbiting another) and one even looks like a dog bone.

"Radar data will also improve our knowledge of the asteroid's orbit," adds Jon Giorgini, a member of the radar team from JPL. "At present, we know there's little risk of a collision with 2002 NY40 for decades. 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, too."

2002 NY40 is faint now. It shines by reflected sunlight like a 17th magnitude star. As it nears Earth, however, the space rock will brighten, soaring to 9th magnitude on August 18th. That's about 16 times dimmer than the dimmest star you can see without a telescope. But as asteroids go, it's very bright.

"Asteroids are hard to see," explains Yeomans, "because they're mostly black like charcoal. The most common ones--carbon-rich C-type asteroids--reflect only 3% to 5% of the light that hits them. Metallic asteroids, which are somewhat rare, reflect more: 10% to 15%."

"We don't know yet what this asteroid is made of," he continued, "but we'll have a much better idea by the end of August." Astronomers using ground-based telescopes will have little trouble recording the asteroid's spectrum and thus its composition.

On the date of closest approach, the asteroid will sail past Vega, the brightest star in the evening summer sky. Sky watchers with powerful binoculars or small telescopes can see it--a speck of light moving 8 degrees per hour. (Note: The flyby will be visible mostly from Earth's northern hemisphere; this is not a good opportunity for southern sky watchers. North Americans can see it best after sunset on Aug. 17th; Europeans should look during the hours before dawn on Aug. 18th.)

Something extraordinary will happen hours after 2002 NY40 passes Earth: the space rock will quickly fade.

Asteroids, like moons and planets, have phases. The sunlit side of 2002 NY40 is facing Earth now. It's full, like a full Moon. On August 18th, the asteroid will cross Earth's orbit on its way toward the Sun. Then the phase of the asteroid will change--from full to gibbous to half.... finally the night side will turn to face Earth. The asteroid will grow dark, like a new Moon.

It's not every day you can peer through binoculars and see a near-Earth asteroid--and then see it disappear. But 2002 NY40 has a lot to offer.

"Mother Nature is making it very easy for us to study this one," says Yeomans. That's good because "we need to know more about near-Earth asteroids in case we ever need to destroy or deflect one." What are they made of? How are asteroids put together? These are key questions that 2002 NY40 will help answer.

"Don't forget," adds Yeomans, "most asteroids pose no threat to Earth. But they do contain valuable metals, minerals and even water that we might tap in the future." When such asteroids come close (but not too close!) we have relatively easy access to them--both to study and, one day perhaps, to visit.

Or, to paraphrase Nietzsche, asteroids (like 2002 NY40) that do not hit us, make us stronger.

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