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2007 WD5 Mars Collision Effectively Ruled Out - Impact Odds now 1 in 10,000

Steve Chesley, Paul Chodas and Don Yeomans
NASA/JPL Near-Earth Object Program Office
January 9, 2008

Since our last update, we have received numerous tracking measurements of asteroid 2007 WD5 from four different observatories. These new data have led to a significant reduction in the position uncertainties during the asteroid's close approach to Mars on Jan. 30, 2008. As a result, the impact probability has dropped dramatically, to approximately 0.01% or 1 in 10,000 odds, effectively ruling out the possible collision with Mars.

Animation showing how the size of the uncertainty region of 2007 WD5 at its
encounter with Mars has decreased over the last 5 weeks
Animation showing how the size of the uncertainty region of 2007 WD5 at its encounter with Mars has decreased over the last 5 weeks as more and more data were added to the orbit solution. As shown in an earlier animation, the region sweeps past Mars from the lower right to upper left in this view. In all frames except the last of this animation, the uncertainty region intersects Mars as it moves past.

Our best estimate now is that 2007 WD5 will pass about 26,000 km from the planet's center (about 7 Mars radii from the surface) at around 12:00 UTC (4:00 am PST) on Jan. 30th. With 99.7% confidence, the pass should be no closer than 4000 km from the surface.

Updated Uncertainty region at closest approach to Mars
Updated Uncertainty Region for 2007 WD5 at encounter with Mars, shown as white dots. The thin white line is the orbit of Mars. The blue line traces the motion of the center of the uncertainty region, which is the most likely position of the asteroid.
Image of 2007 WD5 from the University of Hawaii 2.2-meter telescope on Mauna
Kea, Hawaii.
Image of 2007 WD5 from the University of Hawaii 2.2-meter telescope on Mauna Kea, Hawaii. The circled dot is the asteroid. Other dots are artifacts from cosmic rays. The stars are trailed because the telescope is tracking the asteroid as it moves among the stars. (Credit: Tholen, Bernardi, Micheli with support from the National Science Foundation).

The sequence of updates over the last few weeks has been typical of past potential impact scenarios, with the odds of impact initially surging and later plummeting towards zero. Early on, the uncertainty region is very large and the probability of impact is rather low. As the uncertainty narrows, but still includes the planet, the probability initially increases. But eventually, as in this case, the uncertainty region shrinks to the point that it no longer overlaps the planet, and the probability of impact begins a precipitous decline. This rise and fall of the computed hazard was most notably seen in Dec. 2004 when asteroid 99942 Apophis briefly reached a 2.7% chance of impact with Earth in April 2029. In every case, the height and the timing of the peak probability - and the subsequent decline - cannot be known until the uncertainty region has shrunk to the point where it no longer intersects the planet.

NASA's Spaceguard Survey continues searching for Near-Earth Asteroids such as 2007 WD5, endeavoring to discover 90% of those larger than 1 km in size, a goal that should be met within the next few years. Each discovered asteroid is continually monitored for the possibility of impact. For 2007 WD5, these analyses show there is no possibility of impact with either Mars or Earth in the next century.

This unfolding story and the present results have been made possible by the tracking efforts of many astronomers at several observatories around the world:

  • 2007 WD5 was discovered using the Mt. Lemmon 1.5-meter telescope by Andrea Boattini of the University of Arizona's Catalina Sky Survey, which is led by Steve Larson.
  • Follow-up from archival images taken by the 1.8-meter telescope on Kitt Peak in Arizona were provided by Terrence H. Bressi of the University of Arizona's Spacewatch Project, which is led by Robert McMillan.
  • Andy Puckett of the Univ. of Alaska obtained pre-discovery measurements from archival images of the Sloan Digital Sky Survey’s 2.5-meter telescope on Apache Point, NM.
  • Bill Ryan of New Mexico Tech's 2.4-meter Magdalena Ridge Observatory observed 2007 WD5 on several crucial nights, with critical support from university and observatory staff.
  • Observations from the 6.5-meter Multi-Mirror Telescope (MMT) Observatory in Arizona were provided by a team consisting of Holger Israel (Univ. Bonn), Matt Holman (Harvard/CfA), Steve Larson (Univ. Ariz.), Faith Vilas (MMTO), Cesar Fuentes (Harvard/CfA), David Trilling (Univ. Ariz.) and Maureen Conroy (Harvard/CfA).
  • The 3.5-meter telescope at the Calar Alto Observatory in Spain provided follow-up through a team consisting of Adriano Campo Bagatin (Univ. Alicante), Gilles Bergond (Calar Alto Obs.), Rene Duffard (Inst. de Astrofisica de Andalucia), Jose Luis Ortiz (Inst. de Astrofisica de Andalucia), Reiner Stoss (Obs. Astronomico de Mallorca and Astronomisches Rechen-Institut) and Javier Licandro (Inst. de Astrofisica de Canarias).
  • Fabrizio Bernardi, Marco Micheli and Dave Tholen of the Univ. of Hawaii Institute for Astronomy observed the asteroid at its faintest using the 2.2-meter UH telescope on Mauna Kea in Hawaii.


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