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2007 WD5: More on the Mars-Crosser

The latest on the asteroid approaching Mars, with potentially Tunguska-like dangers, is that it will likely pass a safe 48,000 kilometers from the surface at about 1100 UTC on January 30. This news release describes the possibility of an impact as ‘unlikely,’ but goes on to say that if it does occur, the best view of the event will come from the Mars Reconnaissance Orbiter, whose High Resolution Imaging Experiment (HiRISE) would provide an unprecedented look at the crater.

While the size of asteroid 2007 WD5 approximates the object whose impact formed Meteor Crater (northern Arizona) some 50,000 years ago, the latter is thought to have been a metallic asteroid, while the one approaching Mars is probably stony. Current estimates of 2007 WD5 make it out to be 50 meters wide, traveling at some 13 kilometers per second. That’s enough to carry quite a punch, as the Tunguska impact proved in 1908, and as we may conceivably see at the end of January.

As we watch for updates, ponder the Catalina Sky Survey, which discovered the asteroid on November 20. The job of the CSS is to add to our inventory of near-Earth objects (NEOs), working under a congressional directive to NASA to identify objects one kilometer or larger to a confidence level of 90 percent or better. A later mandate brings that size down to 140 meters or larger. You can see the problem for objects in what we might call the ‘Tunguska-class.’ 2007 WD5 is almost three times smaller than our current programs are designed to track, though the CSS work on it proves that such identifications can be made.

2007 was a banner year for this particular survey. The Catalina team found 450 NEOs during the year, and that’s not the final count. Moreover, the number is growing: 400 were found in 2006, 310 in 2005. Even so, this is tricky work. The threat an object represents depends upon its impact energy, a quantity that demands knowledge of its size, density and velocity. That makes characterizing the objects that threaten our planet a major goal of observing programs, and an important objective for an early mission to an asteroid to further refine this information.

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  • andy December 22, 2007, 16:48

    You make the analogy between 2007 WD5 and the Tunguska event, but is the thin Martian atmosphere sufficient to trigger an airburst?

  • Administrator December 22, 2007, 17:00

    Good point, Andy, and I doubt the airburst possibility on Mars. A thin atmosphere indeed! Anyone with more information on this please jump in. Lack of airburst should mean different effects on Mars than on Earth, of course, but the larger point about the dangers of planet-crossing asteroids would surely be made by the creation of a sizeable crater before our eyes.

  • Ron S December 23, 2007, 0:50

    A quick check shows that surface atmospheric pressure on Mars is ~1% that at Earth’s surface, and about equal to that at an altitude of ~30 km above Earth’s surface. The Tunguska burst was below 30 km so I would expect that a similar object would reach Mars’ surface intact.

  • Mark Wakely December 23, 2007, 9:40

    Has anyone expressed any concern about the MRO either striking or being struck by ejecta from such a sizable impact? Given the thin Martian atmosphere and weaker gravity than Earth’s- and the fact that the MRO has the lowest orbit of any craft ever sent to Mars- I imagine some ejecta would reach that altitude and higher. Even micro meteorite-size particles could do serious damage, depending on speed and quantity. Since NASA hasn’t expressed any concerns for the MRO’s safety, perhaps it’s sufficiently hardened against such an event.

    At least, it would be nice to think so.

  • Lubo December 23, 2007, 14:15

    If it hits Mars, there must be a way to record the explosion, because such phenomenon is very rare :)

  • 42Cliff December 24, 2007, 14:23


    ScienceDaily (Dec. 19, 2007) — The stunning amount of forest devastation at Tunguska a century ago in Siberia may have been caused by an asteroid only a fraction as large as previously published estimates, Sandia National Laboratories supercomputer simulations suggest.

    “The asteroid that caused the extensive damage was much smaller than we had thought,” says Sandia principal investigator Mark Boslough of the impact that occurred June 30, 1908. “That such a small object can do this kind of destruction suggests that smaller asteroids are something to consider. Their smaller size indicates such collisions are not as improbable as we had believed.”

    Because smaller asteroids approach Earth statistically more frequently than larger ones, he says, “We should be making more efforts at detecting the smaller ones than we have till now.”

    The new simulation — which more closely matches the widely known facts of destruction than earlier models — shows that the center of mass of an asteroid exploding above the ground is transported downward at speeds faster than sound. It takes the form of a high-temperature jet of expanding gas called a fireball.

    This causes stronger blast waves and thermal radiation pulses at the surface than would be predicted by an explosion limited to the height at which the blast was initiated.

    “Our understanding was oversimplified,” says Boslough, “We no longer have to make the same simplifying assumptions, because present-day supercomputers allow us to do things with high resolution in 3-D.


  • Edg Duveyoung December 26, 2007, 12:20

    If asteroid 2007 WD5 hits Mars, what instrumentation will be able to capture/analyze the dust cloud etc.?

    I’m thinking Hubble or Earth-based scopes will be able to capture data better than the rovers or the satellites orbiting Mars right now.

    If we can “catch that cloud,” I’m betting we’d see water and organic signatures.

    ‘Course, afterwards, the crater and debris can be better analyzed from orbiters than Earth-based, er, right?

    And just to keep my reputation here “going,” let me say that if the God of War is going to be smacked in the puss, astrologically speaking, maybe George Bush should beware the Ides of January. I’m just sayin’!!!!! ;-)


  • ljk January 10, 2008, 11:53

    2007 WD5 Mars Collision Effectively Ruled Out As Impact
    Odds Widen To 1 In 10,000

    Pasadena CA (SPX) Jan 10, 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 …


  • Jay January 10, 2008, 13:26

    Maybe this was already mentioned so forgive me if you have already been asked this: Does anyone else find it alarming that scientists cannot accurately predict whether or not a space rock of this nature will actually hit or miss the planet Mars? Why was it so difficult to determine its exact path? It seems as if there were hundreds of possible impact locations which i found a little alarming. Again, I do not know or really understand the process of how exactly we track these things but just a thought. :)

  • Administrator January 10, 2008, 13:40

    Jay, it’s not too surprising that calculating a trajectory like this is tricky. It calls for observations over an extended period of time to really speak with any confidence about where such an object is going. As the data accumulate, the path of the asteroid can be refined, but you’re right, it’s a slow process.

  • ICE233 January 15, 2008, 11:28

    Is it true that recent data has shown that the rock has a very low probability of even hitting earth in 2029 and If it does what the hell can we do about it?
    I mean are there any technologies in place to prevent such an impact.

  • Administrator January 15, 2008, 11:40

    I haven’t seen anything at all on 2007 WD5 hitting the Earth in the coming century; you may be thinking of Apophis, which has been tracked for some time and which will come close to us in 2029 and again in 2036 (the latter course is harder to predict because of the gravitational effects of the 2029 pass). As for technologies to assist us in deflecting an incoming asteroid, we’d clearly be caught flat-footed right now. Such things need to be developed, and there are numerous ideas about how to do it, from big nukes to various forms of gravitational nudging. You can find more about these methods by looking into the asteroid and comet deflection section of our archives, available from the front page.