Aug 31, 2000

A Close Encounter with a Space Rock

Today a half-km asteroid is passing by the Earth 12 times farther from our planet than the Moon. Scientists say it's an unusually good opportunity to study a near-Earth object.

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September 1, 2000 -- This morning a half-kilometer wide space rock is zooming past Earth barely 12 times farther from our planet than the Moon. In cosmic terms, it's a near miss. But don't bother grabbing your hard hats, scientists say, as there is absolutely no danger of a collision. Instead, the close encounter will afford astronomers a welcome opportunity to study a bright near-Earth asteroid from close range.

Today's hasty cosmic visitor -- known by researchers as 2000 QW7 -- was discovered just last weekend on August 26, 2000, with NASA/JPL's Near Earth Asteroid Tracking system (NEAT). QW7 caught the attention of NEAT project scientists because it was fast-moving and unusually bright. At 13th magnitude, amateur astronomers can easily spot the mountain-sized minor planet through 8-inch or larger telescopes.

Above: NEAT discovery images of asteroid 2000 QW7. The asteroid is not now a threat to Earth, but astronomers plan to continue monitoring the fast-moving space rock to learn more about future encounters. [more information]

According to NEAT principal investigator Eleanor Helin, QW7 offers an exceptional opportunity for Earthbound observers to study a near-Earth asteroid. "This is a very important object," she said. "It's so bright that amateur astronomers can track it now and through the end of this year. We should be able to obtain a precise orbit, as well as colors, a light curve and other physical properties during this discovery apparition."

sky map of asteroid location
A group of astronomers led by Jean-Luc Margot of the Arecibo Observatory has already made the first radar detection of the space rock using NASA's Goldstone antenna in the Mojave desert. "Radar measurements, in combination with optical data, can immediately shrink trajectory uncertainties by a factor of 1000 or more for a recently discovered object like 2000 QW7," says Jon Giorgini, a senior engineer in JPL's Solar Systems Dynamics Group.

"An improved orbit from the radar data will help us run the orbit backwards and search for pre-discovery images of the asteroid," added Helin. "It's a bit of a mystery why we haven't seen this one before."

Above: Amateur astronomers using 8-inch or larger telescopes can monitor 2000 QW7 for themselves this week as it races through the constellation Aquarius toward Pisces and Cetus. This graphic shows the southern sky as viewed from mid-Northern latitudes at midnight on September 2nd. Aquarius is about 40 degrees above the southern horizon. Orbital elements for the asteroid and an observing ephemeris are available from the Minor Planet Center.

Asteroid 2000 QW7 falls into a category of Near-Earth Objects (NEOs) called Potentially Hazardous Asteroids, or PHAs.


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"Technically an asteroid is a PHA if it can get within about 0.05 AU of Earth's orbit and if it's larger than a few hundred meters," explains Donald Yeomans, manager of NASA's Near Earth Object Program office at the Jet Propulsion Laboratory. "There are currently 266 known Potentially Hazardous Asteroids -- none of them pose an immediate threat to the Earth." (Editor's Note: 1 AU, or Astronomical Unit, is the average distance between the Sun and Earth. It equals 149 million km)

Although PHAs in their current orbits won't collide with Earth, astronomers monitor them because one day they might become dangerous. Gravitational nudges by Earth, Mars or Jupiter can potentially set such asteroids on a collision course with our planet, says Yeomans. At present, all known PHAs rank zero on the Torino Scale -- a numerical measure of asteroid collision hazards similar to the Richter scale for earthquakes.

Close encounters between the Earth and PHAs happen fairly often. Just last month, four such asteroids flew by Earth at distances ranging from 0.038 to 0.084 AU (or 15 to 33 times the distance between the Earth and Moon). The table below summarizes encounters for the months of August and September, 2000. More data are available from the PHA Earth Close Approach Table maintained at JPL's Near Earth Object web site.

 Earth-asteroid close approaches Aug.-Sept. 2000

Asteroid DATE
mmm-DD HH:MM
2000 CE59 Aug-06 17:33 0.0527 7.19 20.4 0.2-0.5
2000 PP9 Aug-08 11:51 0.0845 8.13 19.4 0.3-0.7
4486 Mithra Aug-14 08:14 0.0465 17.57 15.6 ~2
2000 QV7 Aug-15 13:44 0.0383 15.76 21.1 0.2-0.4
2000 QW7 Sep-01 12:54 0.0317 6.48 19.5 0.3-0.7
2000 ET70 Sep-04 10:39 0.1895 12.84 18.2 0.6-1.4
2000 DP107 Sep-19 13:20 0.0478 12.35 17.9 0.7-1.5
2000 QS7 Sep-20 04:54 0.0872 10.28 20.7 0.2-0.5

Legend: R is the asteroid's miss distance in AU (astronomical units) on the indicated DATE. For comparison, the distance between the Earth and the Moon is approximately 0.0026 AU. Vr is the relative velocity between the Earth and the asteroid at the time of the flyby. H is the asteroid's absolute magnitude (the visual magnitude an observer on Earth would record if the asteroid were placed 1 AU away). D is the size of the asteroid estimated from its absolute magnitude.

"Most PHAs originated in the asteroid belt between Mars and Jupiter," continued Yeomans. "One of the main mechanisms for transforming an asteroid's circular orbit in the asteroid belt into an elliptical one that can bring it close to Earth is a 3-to-1 orbital resonance with Jupiter. Main belt asteroids that go around the Sun three times for every orbital period of Jupiter will meet up with the giant planet at the same spot every third orbit. Jupiter's powerful gravity perturbs the asteroid's path, increasing its eccentricity with each encounter. Over time, as perturbations accumulate, the asteroid becomes a Mars crosser and then an Earth crosser.

Once an asteroid is in the 3:1 resonance region (at a heliocentric distance of 2.5 AU), it can evolve to a Mars crosser in about 100,000 years and then Mars' perturbations can evolve the asteroid to an Earth crosser in several tens of million years."  

Distribution of PHA Sizes
The most recent asteroid added to the list of potentially hazardous objects, 2000 QW7, seems fairly run of the mill as PHAs go, says Steven Pravdo, NEAT co-principal investigator and project manager. But the asteroid is attracting attention because it is bright and relatively near to Earth. Within the first day and a half of 2000 QW7's discovery, 23 different observatories had observed and reported measurements of its motion, and more are joining the monitoring effort every day.

"Other than its visual magnitude we don't know much about QW7," says Pravdo. "There are no images that resolve the asteroid, so we have to estimate its size from the observed brightness. Its absolute visual magnitude [the magnitude of the asteroid if it were placed 1 AU from Earth] is 19.5. If we assume a reflectivity between 5% and 25% -- a typical range for asteroids -- then QW7 must be between 330 and 740 meters across. That's about average for the size of a PHA."

Above: JPL's Steven Pravdo prepared this diagram showing the estimated size distribution of all known Potentially Hazardous Asteroids on August 29, 2000. More than half are 800 meters in diameter or smaller. The bars represent the number of objects in each size bin; the line is the cumulative fraction.

Potentially Hazardous Asteroids are a subset of a larger group known as "Near-Earth Objects" or NEOs. NEOs are comets or asteroids with perihelion distances (closest approaches to the Sun) less than 1.3 AU. Such objects can come within 0.3 AU of Earth's orbit.

Thanks to data from NEAT, scientists now estimate that there are between 500 and 1000 near-Earth asteroids larger than 1 km in diameter. That's less than half of the total expected before NEAT went online in 1995.

1.2-meter telescope used by NEAT
"Right now we know of 424 large, near-Earth asteroids," continued Pravdo. "That was a fairly small fraction of the 2,000 asteroids in our previous estimate. With our new calculations of between 500 and 1,000 such objects, this 424 figure represents a large chunk." NASA's goal is to find 90-percent of all large, near-Earth asteroids by 2010.

From December 1995 to 1999, NEAT operated with the Air Force GEODSS one meter telescope in Maui. Recently, NEAT researchers completed a major upgrade of the system. NEAT has been operating with

at the Maui Space Surveillance Site since March 2000. The 1.2-m telescope is the largest aperture used worldwide in a regular Near-Earth Object discovery program. Since March, NEAT has detected 30 near-Earth asteroids (including 5 new discoveries) and logged more than 10,000 other asteroid detections.

NEAT is managed by JPL for NASA's Office of Space Science, Washington, DC.

Web Links

Near Earth Asteroid Tracking - NEAT home page at JPL

Asteroid Population Count Slashed - Jan 12, 2000 press release

Near Earth Objects - learn more at this JPL web site

2000 QW7 Interactive orbit- You can view asteroid 2000 QW7's orbit from any angle thanks to this Java-based visualization tool from NASA's Jet Propulsion Laboratory.

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