A Close Encounter with a Space Rock
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."
"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.
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.
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."
"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.
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 withat 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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|For lesson plans and educational activities related to breaking science news, please visit Thursday's Classroom||Author: Dr. Tony Phillips
Production Editor: Dr. Tony Phillips
Curator: Bryan Walls
Media Relations: Steve Roy
Responsible NASA official: Ron Koczor