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 minor planet through 8-inch or larger telescopes.
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."
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."
Asteroid 2000 QW7 falls into a category of Near-Earth Objects (NEOs) called Potentially Hazardous Asteroids, or PHAs.
"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 R Vr H D
mmm-DD HH:MM (AU) (km/s) (Vm) (km)
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."
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."
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.
"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 a 1.2 m telescope 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.
August 28, 2000
About magnitude 13.6 when first detected, the asteroid will brighten to 12.7 within the next week as it makes a moderately close flyby, coming within 0.032 astronomical unit of Earth (about 12 Earth-Moon distances) in the first few days of September. Anyone with an 8-inch or larger telescope and clear, dark skies should have no trouble spotting the object visually during this period as it glides from Aquarius through Pisces into Cetus. Small CCD-equipped telescopes can also capture trailed images of 2000 QW7. (See the detailed ephemeris at the end of this AstroAlert.)
As soon as NEAT astronomers reported the object, the Minor Planet Center posted the position and direction of motion on the NEO Confirmation Page of its Web site. Within the next 30 hours, a total of 115 precise positions had arrived from 19 astrometric observatories around the world (including 7 positions obtained by the undersigned at the Drum Hill Station in northern Massachusetts). Williams then issued on Minor Planet Electronic Circular 2000-Q32, containing the following preliminary orbital elements (equinox 2000.0):
Epoch 2000 Aug. 24.0 TT Mean anomaly, M 353.92092 Semimajor axis, a 1.9510626 Eccentricity, e 0.4694395 Arg. of perihelion 190.39996 Long. of ascending node 158.81329 Inclination, i 4.16947
The elements show that this asteroid belongs to the Amor family and follows an orbit inclined 4.2 degrees to the ecliptic. It ranges as far out as the main belt of asteroids (roughly midway between Mars and Jupiter), but every 2.7 years it comes in to a point fairly close to the Earth's orbit. As far as is currently known, the object has not been detected by astronomers before. Its brightness suggests it may be about a half mile (0.8 kilometer) across.
The following ephemeris, calculated at Sky & Telescope from the above elements, gives the object's right ascension and declination at 6-hour intervals for the next two weeks. Also listed is its distance from the Earth (Delta) and the Sun (r) in a.u., expected visual magnitude, and the constellation through which it is passing.
When searching for the asteroid, keep in mind the parallax effect. Because the object is so close to the Earth in the next few days, it can appear displaced up to about 5 arcminutes from the geocentric positions tabulated below. (HINT: to make the numbers line up in straight, vertical columns, set your e-mail program to display this message in a font such as Courier that does not use proportionally spaced letters.)
Roger W. Sinnott
Associate Editor
Sky & Telescope
Ephemeris for Minor Planet 2000 QW7
Date UT R.A. (2000) Dec. Delta r Mag. Const.
2000 Aug 27 0h 21 33.1 -03 39 0.038 1.047 13.4 Aqr
2000 Aug 27 6h 21 37.9 -03 41 0.038 1.047 13.3 Aqr
2000 Aug 27 12h 21 42.7 -03 44 0.037 1.047 13.3 Aqr
2000 Aug 27 18h 21 47.8 -03 46 0.037 1.046 13.2 Aqr
2000 Aug 28 0h 21 52.9 -03 49 0.036 1.046 13.1 Aqr
2000 Aug 28 6h 21 58.2 -03 51 0.036 1.045 13.1 Aqr
2000 Aug 28 12h 22 03.6 -03 53 0.035 1.045 13.0 Aqr
2000 Aug 28 18h 22 09.2 -03 56 0.035 1.045 12.9 Aqr
2000 Aug 29 0h 22 14.9 -03 58 0.034 1.044 12.9 Aqr
2000 Aug 29 6h 22 20.7 -04 00 0.034 1.044 12.8 Aqr
2000 Aug 29 12h 22 26.6 -04 02 0.034 1.043 12.8 Aqr
2000 Aug 29 18h 22 32.7 -04 04 0.033 1.043 12.7 Aqr
2000 Aug 30 0h 22 38.8 -04 06 0.033 1.043 12.7 Aqr
2000 Aug 30 6h 22 45.1 -04 07 0.033 1.042 12.7 Aqr
2000 Aug 30 12h 22 51.5 -04 09 0.033 1.042 12.7 Aqr
2000 Aug 30 18h 22 58.0 -04 10 0.032 1.042 12.7 Aqr
2000 Aug 31 0h 23 04.5 -04 11 0.032 1.041 12.8 Aqr
2000 Aug 31 6h 23 11.1 -04 12 0.032 1.041 12.8 Aqr
2000 Aug 31 12h 23 17.8 -04 13 0.032 1.041 12.9 Aqr
2000 Aug 31 18h 23 24.5 -04 14 0.032 1.040 12.9 Aqr
2000 Sep 1 0h 23 31.2 -04 14 0.032 1.040 12.9 Aqr
2000 Sep 1 6h 23 38.0 -04 14 0.032 1.040 13.0 Aqr
2000 Sep 1 12h 23 44.8 -04 14 0.032 1.040 13.0 Aqr
2000 Sep 1 18h 23 51.6 -04 14 0.032 1.039 13.1 Aqr
2000 Sep 2 0h 23 58.4 -04 14 0.032 1.039 13.1 Psc
2000 Sep 2 6h 0 05.2 -04 13 0.032 1.039 13.2 Psc
2000 Sep 2 12h 0 11.9 -04 12 0.032 1.039 13.2 Psc
2000 Sep 2 18h 0 18.5 -04 11 0.032 1.038 13.3 Psc
2000 Sep 3 0h 0 25.1 -04 10 0.032 1.038 13.3 Psc
2000 Sep 3 6h 0 31.7 -04 09 0.032 1.038 13.4 Cet
2000 Sep 3 12h 0 38.1 -04 07 0.033 1.038 13.4 Cet
2000 Sep 3 18h 0 44.5 -04 05 0.033 1.038 13.5 Cet
2000 Sep 4 0h 0 50.7 -04 03 0.033 1.037 13.5 Cet
2000 Sep 4 6h 0 56.9 -04 01 0.033 1.037 13.6 Cet
2000 Sep 4 12h 1 02.9 -03 59 0.034 1.037 13.6 Cet
2000 Sep 4 18h 1 08.8 -03 57 0.034 1.037 13.7 Cet
2000 Sep 5 0h 1 14.6 -03 54 0.034 1.037 13.8 Cet
2000 Sep 5 6h 1 20.3 -03 52 0.035 1.037 13.8 Cet
2000 Sep 5 12h 1 25.8 -03 50 0.035 1.036 13.9 Cet
2000 Sep 5 18h 1 31.3 -03 47 0.035 1.036 13.9 Cet
2000 Sep 6 0h 1 36.5 -03 44 0.036 1.036 14.0 Cet
2000 Sep 6 6h 1 41.7 -03 42 0.036 1.036 14.0 Cet
2000 Sep 6 12h 1 46.7 -03 39 0.037 1.036 14.1 Cet
2000 Sep 6 18h 1 51.5 -03 36 0.037 1.036 14.1 Cet
2000 Sep 7 0h 1 56.3 -03 33 0.038 1.036 14.2 Cet
2000 Sep 7 6h 2 00.9 -03 30 0.038 1.036 14.2 Cet
2000 Sep 7 12h 2 05.3 -03 28 0.039 1.036 14.3 Cet
2000 Sep 7 18h 2 09.7 -03 25 0.039 1.035 14.4 Cet
2000 Sep 8 0h 2 13.9 -03 22 0.040 1.035 14.4 Cet
2000 Sep 8 6h 2 18.0 -03 19 0.041 1.035 14.5 Cet
2000 Sep 8 12h 2 21.9 -03 17 0.041 1.035 14.5 Cet
2000 Sep 8 18h 2 25.8 -03 14 0.042 1.035 14.6 Cet
2000 Sep 9 0h 2 29.5 -03 11 0.042 1.035 14.6 Cet
2000 Sep 9 6h 2 33.1 -03 08 0.043 1.035 14.7 Cet
2000 Sep 9 12h 2 36.6 -03 06 0.044 1.035 14.7 Cet
2000 Sep 9 18h 2 40.0 -03 03 0.044 1.035 14.8 Cet
2000 Sep 10 0h 2 43.3 -03 01 0.045 1.035 14.8 Cet
2000 Sep 10 6h 2 46.5 -02 58 0.046 1.035 14.9 Eri
2000 Sep 10 12h 2 49.6 -02 56 0.046 1.035 14.9 Eri
2000 Sep 10 18h 2 52.6 -02 54 0.047 1.035 14.9 Eri
2000 Sep 11 0h 2 55.5 -02 51 0.048 1.035 15.0 Eri
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