A Good Month for Asteroids

Asteroid hunters have enjoyed a close-up look at two new potentially hazardous space rocks as they passed close to the Earth in September.

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September 20, 2000 -- This has been a good month for astronomers studying Near-Earth asteroids (NEAs). No fewer than five sizable space rocks have flown past our planet since the beginning of September -- three of them in the last four days.

There was no danger of a collision at any time, say researchers. All of the asteroids missed our planet by comfortable margins of 11 to 75 lunar distances. Still, by cosmic standards, they were close at hand.

Among the parade were two asteroids, 2000 QW7 and 2000 RD53, that were brand-new discoveries. NASA's Near Earth Asteroid Tracking system spotted 2000 QW7 on August 26, 2000 -- less than a week before its closest approach. Then, 11 days later on Sept. 6th, MIT's LINEAR program detected 2000 RD53. Both were passing by our planet no farther away than 12 times the distance to the Moon. As news of the discoveries spread, astronomers rushed to their telescopes for a closer look.

"Dozens of observers, including many skilled amateurs, are monitoring these bright objects," says Eleanor Helin, the principal investigator for JPL's Near-Earth Asteroid Tracking (NEAT) program. "The close approaches of these asteroids offer a rare opportunity to learn about their physical characteristics, including what they're made of and their rotational periods," she added.

Above: NASA's 70-meter diameter Goldstone radar is the largest and most sensitive Deep Space Network antenna. Astronomers are using the Goldstone antenna to bounce radio signals off Near-Earth asteroids this month. [more information]

2000 QW7 and 2000 RD53 belong to a group known as "Potentially Hazardous Asteroids" or PHAs. There are 271 known PHAs, which by definition are asteroids larger than about 150 meters that can come closer to Earth than 0.05 AU (about 20 lunar distances). In spite of their menacing name, none of the PHAs we know of now are destined for a collision with our planet. But that could change.

"PHA orbits can be chaotic. Perturbations -- such as a gravitational nudge from Mars or Earth -- could change their orbits. We have to monitor them -- it's highly recommended," remarked Helin with a note of understatement.

A group of astronomers led by Jean-Luc Margot of the National Astronomy and Ionosphere Center detected radar echoes from 2000 QW7 and 2000 RD53 as they passed over the powerful Arecibo radar in Puerto Rico and NASA's Goldstone radar in the Mojave desert. By analyzing the echoes, researchers can construct three-dimensional maps of the asteroids and reduce the uncertainty of their orbital elements.

"Goldstone radar observations of 2000 QW7 and RD53 permitted velocity measurements accurate to better than 4 millimeters per second," says Jon Giorgini, a senior engineer in JPL's Solar System Dynamics Group "For 2000 RD53, it was possible to make direct range measurements to the asteroid, from both Arecibo and Goldstone planetary radars, accurate to at least 300-400 feet."

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With a precise orbit determined by radar data, Giorgini ran 2000 RD53's motion backwards and found that it had made an even closer approach at 9.3 lunar distances in 1933, but no one saw it. The next close encounter as near as this week's won't come until 2198.

According to Giorgini's calculations, 2000 QW7 will be back sooner than 2000 RD53. On Sept. 15, 2019, it will pass our planet 14 times farther away than the Moon -- about the same distance as this week's encounter.

"Evidence from the radar data suggests that 2000 QW7 is a slow rotator," added Jean-Luc Margot. "Its spin period is on the order of days, which is a puzzle for an object this size."

Collisions within the asteroid belt are expected to give space rocks plenty of spin, but 2000 QW7 joins at least two other NEAs (1999 JM8 and Toutatis) that rotate slowly.

"Various exotic possibilities have been proposed to explain how NEAs could lose their angular momentum," continued Margot. "These include close encounters with a planet, tidal despinning of a binary system, or disruption from a larger asteroid. Obtaining these radar measurements will help us understand ... their dynamical history."

Above: Astronomers at the Arecibo Observatory detected strong radar echoes from 2000 RD53 within 90 seconds of their first look at the asteroid. The frequency offset in this echo spectrum provides a highly accurate measurement of the line-of-sight velocity of the object. Image courtesy Jean-Luc Margot.

Slow rotation is just one of many puzzles attending Potentially Hazardous Asteroids, says Brian Marsden, director of the Minor Planet Center. Researchers still don't know how many PHAs inhabit the inner solar system, what they're made of, or exactly where they come from.

"There are two obvious possibilities," says Marsden. "PHAs could come from the asteroid belt or they might be inert comets. Undoubtedly it's a mixture of the two, but we don't know the fractions."

"We would expect some to be bona fide rocky asteroids," he continued. "After all, there are mechanisms that can bring main belt asteroids into Earth-crossing orbits. The principal one, involving what are called secular planetary perturbations, takes millions to hundreds of millions of years -- a short time compared to the age of the Solar System."

"As for the comets, it may be that they can masquerade as asteroids after their ices have been vaporized by solar heating. Is there enough particulate material for such a spent comet to remain coherent, or does it break up? Comet LINEAR broke up very nicely (when it passed by the Sun earlier this year)! An alternative is that a rocky crust might completely cover the comet's ice. We just don't know."

Left: This image captured by the Hubble Space Telescope shows Comet LINEAR disintegrating after it passed close to the Sun earlier this year. Could some comets hold together after all their ices vaporize? If so, they could be masquerading as Near-Earth asteroids.

Distinguishing between cometary and rocky NEAs is important in case we ever need to nudge one away from our planet. One of the most-often discussed scenarios for diverting a PHA involves launching a nuclear-armed rocket to intercept it. Exploding the warhead in the wrong spot could have unintended consequences. Scientists caution that a hailstorm of asteroidal fragments could be worse than one big piece -- like being hit by a shotgun instead of a rifle. Knowing what PHAs are made of and how they are put together is vital.

It's also vital to find such asteroids as early as possible. 2000 QW7 and 2000 RD53 didn't provide much advance warning. They were discovered 5 and 11 days, respectively, before their closest approaches to Earth.

"We can miss bright asteroids like 2000 QW7 for several reasons," explains Helin. "For instance, if an asteroid moves across the Milky Way during its closest approach, it might be hard to identify among the densely-packed background stars. Or if the asteroid is close to the Sun as it approaches, it could be lost in the Sun's glare. Many NEAs (like QW7 and RD53) are in highly elliptical orbits and spend most of their time as dim specks beyond the orbit of Mars. They brighten only as they come close to the Sun, then we can see them from Earth."

Right: This 1.5 MB MPEG video of 2000 QW7 was captured by John Rogers using a 0.30 meter Schmidt-Cassegrain telescope at the Camarillo Observatory, 55 miles north of Los Angeles. Amateur videos of RD53 are also available from spaceweather.com.

"The way to look at these objects is over a long period of time," noted Marsden. "Yes, some will come close to Earth and miss us -- as 2000 QW7 and RD53 have done -- but it's the subsequent passes that we have to worry about. Follow-up observations to establish precise orbits are very important if we wish to predict future encounters."

"Radar helps a very great deal (in refining asteroid orbits). We can also look for newly discovered asteroids in old images, and that helps, too. Nowadays we look for pre-discovery images as a matter of course."

Indeed, soon after NEAT identified 2000 QW7, a colleague of Marsden's found this asteroid in early-August data from MIT's LINEAR search program. Such prediscovery observations, and in particular confirmed "precovery" observations from an earlier year, can immediately refine an asteroid's orbit and make continued tracking easier.

"LINEAR did record 2000 QW7 almost a month before its closest approach," says Marsden, "but poor weather limited the observations to a few images on a single night, and it was moving too slowly to be picked out as unusual." It's another example of how search programs can miss PHAs. When they are faint, far away, and moving slowly against the background stars, PHAs can appear to be harmless main belt objects.

"We're accumulating asteroids at a furious rate," says Marsden. "At the turn of the century we knew of only 500 minor planets; now we've cataloged 17,349 with excellent orbit determinations. The rate of discovery is approximately doubling every two years."

The rate could increase further if a new British government initiative to identify hazardous asteroids bears fruit. With more telescopes on the lookout, astronomers will undoubtedly enjoy many more -- and perhaps uncomfortably numerous -- opportunities for close-up studies of these ever-scary space rocks.

The Anatomy of Asteroid Names Sometimes it seems that astronomers enjoy picking inscrutable names for the objects they study. What person on the street would guess that "2000 QW7" is a fascinating space rock? Nevertheless, there is a method to this naming madness. So many new asteroids are discovered each month that astronomers need an efficient way to catalog them. The first part of "2000 QW7" is simple -- it identifies the year of the asteroid's discovery (2000). Then comes "QW7." The first letter tells us that the object was identified during the second half of August. Each half-month is identified with a letter of the alphabet. January 1st-15th = "A"; January 16th-31st = "B"; August 16th-31st = "Q", etc. The letter "I" is omitted in this system. The second and third characters "W7" are a shorthand way of counting the number of asteroids found during the 2nd half of August 2000. The first asteroid discovered was "2000 QA"; the second was "2000 QB;" The second letter cycles through the alphabet until it reaches "Z" and then it goes back to the beginning with an extra number. So, the 26th asteroid discovered during the second half of August 2000 was "2000 QA1". Remember that "I" is omitted, so "A1" corresponds to the 26th asteroid, not the 27th. This means that 2000 QW7 was the 197th asteroid found in the second half of August 2000!

NEAT is managed by JPL for NASA's Office of Space Science, Washington, DC. The National Astronomy and Ionosphere Center is operated by Cornell University under a cooperative agreement with the National Science Foundation and with additional support from NASA.

Near Earth Objects - learn more about nearby space rocks

Asteroid Interactive orbits- You can view the 3D orbit of any Potentially Hazardous asteroid using this Java-based visualization tool from NASA's Jet Propulsion Laboratory.