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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
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 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
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
- 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.