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Emily LakdawallaJanuary 10, 2008

Mars impact chance drops to 1 in 10,000, which may as well be zero

1 in 10,000 is still better than your chance of winning a lottery, but it's very, very unlikely. The newly discovered Earth-crossing asteroid 2007 WD5 has been generating a lot of excitement because at one point it had as high as a 1 in 25 chance of hitting Mars, but according to the Near Earth Object program the latest observations of the asteroid's path have "led to a significant reduction in the position uncertainties during the asteroid's close approach to Mars on January 30, 2008....Our best estimate now is that 2007 WD5 will pass about 26,000 kilometers from the planet's center (about seven Mars radii from the surface) at around 12:00 UTC (4:00 am PST) on January 30. With 99.7% confidence, the pass should be no closer than 4,000 kilometers from the surface."

They also posted an animation that neatly shows how the uncertainty in the position of WD5 has decreased over time:

Shrinking uncertainty surrounding 2007 WD5's close flyby of Mars

NASA / JPL

Shrinking uncertainty surrounding 2007 WD5's close flyby of Mars
In early December 2007, astronomers realized that newly discovered Earth-crossing asteroid 2007 WD5 would have an extremely close encounter with Mars on Jauary 30, 2008. Initially, the uncertainty in its position translated into a large uncertainty in where it would actually approach closest to Mars. In the diagrams in this animation, the stippled white band from lower left to upper right represents a long, skinny ellipse of possible positions for WD5 during its closest approach. Throughout December, as more observations of WD5 decreased the uncertainty in its position, the ellipse shrank, but it continued to intersect Mars, with the result that the likelihood of an impact steadily increased. By early January, however, the uncertainty (and the ellipse representing the possible position of WD5 during its closest approach) had decreased enough that there was no longer a significant chance of WD5 striking Mars.
The fact that the impact probability increased slowly over time before sharply decreasing is typical, the Near Earth Object program states, of past potential impact scenarios, including the Christmas 2004 scare of the close approach of asteroid Apophis to Earth. The problem is, you just can't know whether that sharp decrease is going to happen unless you keep performing observations. The observations of WD5 were performed by a long list of observers using pretty big telescopes ranging in size from 1.5 to 6.5 meters. I imagine that observing calendars had to be shuffled around quite a bit in order to perform the number of observations necessary to track WD5. I'll reproduce the list from the Near Earth Object program's article, because all of these people deserve credit:Now that the Mars impact probability has effectively gone to zero, WD5 remains an object of interest to scientists who study asteroids and who are concerned about the population of potential Earth impactors, but the Mars community is no longer interested. I sent out a couple of emails this week to chief scientists on the Mars Reconnaissance Orbiter and Mars Express missions to ask about whether they planned to observe WD5, and I received a response from Agustin Chicarro, Mars Express Project Scientist, this morning:
We just cancelled any potential observations of 2007 WD5, given the odds of impacting the surface of Mars. It is a pity, as Mars Express was the best positioned Mars orbiter to observe this asteroid. To study the trajectory is less exciting. I very much regret this turn of events, as it would have been very exciting to monitor an impact on Mars almost in real time and in particular to study its aftermath. Let's hope for another chance.
Some of you may say that Mars Express should study it anyway, to follow its position, but I would reply to you that you don't know (and neither do I) what kinds of observations Mars Express would have to cancel in order to make the attempt to study the trajectory of a tiny asteroid, one among tens of thousands in the solar system, whose paths are quite effectively studied -- as today's news has proven -- from observatories on Earth whose mission it is, in part, to study asteroids. Mars Express' mission is to study Mars.

I haven't heard back yet from the Mars Reconnaissance Orbiter folks but I'll let you know if I do.

(Here are my past posts on WD5: December 21, with chances at 1 in 75; December 27, at 1 in 25; January 2, at 1 in 28; January 8, at 1 in 40; January 10, at 1 in 10,000.)

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
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