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Asteroid Deflection and the Odds

What would happen if asteroid 99942 Apophis ever hit the Earth? It’s about 1200 feet in diameter, and according to David Morrison (NASA Ames), that’s large enough to obliterate an area the size of England. The subject was under discussion at the recent American Association for the Advancement of Science meeting, and is reported capably in this Columbus Dispatch story, which quotes others on their own conclusions.

Jay Melosh, a geophysicist at the University of Arizona’s Lunar and Planetary Laboratory, said that if Apophis struck Earth, it would produce a 40-megaton blast, almost eight times larger than the most powerful nuclear bomb ever detonated. The explosion would create a crater more than 2 miles wide and obliterate buildings and bridges in a 4-mile radius. Melosh said everything around it would be buried beneath 20 inches of debris.

Nice to see a sober article discussing asteroid deflection in the popular press. Apophis probably isn’t going to make this kind of history, but the potential for getting blind-sided by an uncatalogued object is always there. Long-term thinking inevitably includes the premise of species survival, which is why a space-based infrastructure will eventually be forced upon us whether we want it in the short-term or not. Let’s hope it doesn’t take a Rendezvous with Rama-type survivable hit to make the case more emphatically.

Addendum: New Scientist covers Apophis scenarios and asteroid deflection in this recent article. Also see this post by Brian Wang on the Lifeboat Foundation weblog.

Comments on this entry are closed.

  • JD February 27, 2007, 16:33

    Hmm, actually the Soviets detonated a 48~52 megaton ( the Csar bomb). They had removed the third stage so it did not detonate at the theoretical 100 megatons it was designed for.

    Something isn’t quite right with the figures though. A 1200 ft bolloid should have a bit more oomf than 40 megatons, unless the impact volocity was very low. Also radius of destruction would be higher than stated in the quote even if it was a ground versus airial detonation.

  • Administrator February 27, 2007, 16:38

    JD, looks low to me, too, and I’m thinking the actual number quoted should have been 400 megatons, which would work out with the Csar bomb numbers, right?

  • Brian Wang February 27, 2007, 17:57


    NASA initially estimated the energy that Apophis would have released if it impacted Earth as the equivalent of 1480 megatons of TNT. A more refined later NASA estimate was 880 megatons.

  • Administrator February 27, 2007, 18:05

    That’s more like it — clearly the 40 megatons was a misprint in the original article unless there’s been some startling new thinking on the topic.

  • enzo February 27, 2007, 23:35

    An interesting online tool to calculate the consequences of an impact :

    Not sure how accurate, but there you go.


  • Eric James February 28, 2007, 1:45

    One can’t help but think of Chicken Little.

  • Drage February 28, 2007, 5:27

    It is good to see serious discussion about a possible bollide impact. My feeling is i am surprised we have taken this long to seriously consider how we would deal with a possible impact threat.

    Taking into account that we are seriously lucky as a species to be where we are today, it has always seemed strange that we would ignore a known danger, as if humanity is some helpless dumb animal. Especially a danger we can probably minimise. We have many spacecraft doing flybys and even making physical contact with Comets and Asteroids, so i dont see how slightly deflecting one would be too much of a problem for us…if the funding and urgency existed.

    Considering that so many scientists accept the anthropic prinicple at least in tis weakest form, it strikes me as odd we arent protecting our position and future in the cosmos.

    By the way, i know this is slightly off topic, but i remember reading a scifi book (i think it was Greg Bear) about how in the future species may try to destroy their stellar neighbors by mapping out all the big asteroids orbiting a home world, and then sneakily slightly pushing one into a trajectory which would impact the target planet.

    Because we would assume that a previously known bollide was not on an impact trajectory we might take our eye off it, and next thing we know….its direction has changed slightly and its too late to get a craft up to deflect it away.

    Sorry could not resist :-)

  • Administrator February 28, 2007, 9:21

    Let me know if you can recall the title of the Bear book — I like his work. Claudio Maccone has been doing a lot of work recently on asteroid deflection and possible strategies. I’ve got a new paper from him here that we’ll look at soon.

  • ljk February 28, 2007, 9:40

    Not sure about Bear, but Charles Pellegrino in his novel
    The Killing Star, postulated that ETI would send relativistic
    missiles to bombard the Sol system and wipe out most of
    humanity at one stroke.

    It was a pre-emptive strike designed to keep humanity
    from doing to them what they did to us. And there is virtually
    no defense against such an attack – except to do unto them
    before they do it unto us.

  • Administrator February 28, 2007, 9:54

    The Pellegrino scenario is grim indeed. What about Footfall? Wasn’t the premise there that an alien species was slinging massive objects at us?

  • Edg Duveyoung February 28, 2007, 10:32

    Footfall had these elephant-esque aliens who dropped rocks with the equivalent impact of nuclear bombs on every roadway intersection and thus stopped a lot of commerce etc. Then they invaded with foot soldiers — neat cover illustration of them with rifles in trunks — and the soldiers had one serious flaw — they were of the mindset that if an enemy allowed you to put your foot on his head, then that enemy would become your slave and be trustable — no longer an enemy. The humans allowed the foot thingy, and then just did what they wanted to do after that. The aliens had a hard time understanding that freedom on our parts. The aliens had this very large rock they were going to drop to really mess us up if things went south. I think it was called “the foot.” Would have killed our planet. Humans worked up several different efforts to defeat the aliens. I believe one thing was to plant thousands of atom bombs underground and to blast a huge hunk of earth into space with a continual blasting of the bombs. This “ship” was then crashed into the mother ship I believe, or it’s radiation from the bomb were a weapon. Not certain any more, but it was a very good read — developed a lot of characters. The alien psychology was made believable.


  • Drage February 28, 2007, 10:56

    Perhaps it was Footfall, I really cannot remember.

    But I think its interesting to wonder about just how altruistic or peaceful an advanced civilisation would be.

    Alot of people seem to assume that advanced Aliens will be friendly but im not so sure. Does being technologically advanced make a civilisation peaceful necessarily?

    I believe there is an interesting conversation going on at SETI and NASA about the viability of us sending big signals out in order to attract the attenton of any possible civilisations out there. Apparently some SETI hunters think we should do this because they are getting frustrated we have yet to hear anything.

    Personally I think maybe we should keep quiet for a while and stay under the radar until we have developed our space programs better.

    Someone who was against us making big noises made the analogy of “shouting out in the jungle”.

    Of course there may not be any civilisations for thousands of lightyears (if at all) so maybe we are all worrying for nothing :-)

  • Hiro February 28, 2007, 13:04

    I think Stephen Baxter also used the idea about an asteroid as a space weapon in the battle field. If I remember correctly, the title of the book is TITAN.

  • Drage February 28, 2007, 13:36


    You may be correct in that i was thinking of Baxter’s Titan. It sounds v familiar. problem is i read loads of scifi a few years ago and forgot most of the titles.

  • Administrator February 28, 2007, 14:35

    Edg, thanks, you bring Footfall back to me — it’s been years. And thanks for your tip on the Baxter, Hiro, which I haven’t gotten around to reading yet. Soon, I hope.

  • Brian Wang March 1, 2007, 1:16
  • Adam March 1, 2007, 5:37

    Asteroid deflection is pretty straightforward if we get a lot of warning. There’s decent plans for “gravity tug-boats” to give asteroids a little nudge to miss Earth at the right time. But how much accuracy in ranging of the asteroid is needed to get that advanced warning?

  • Administrator March 1, 2007, 8:56

    Adam, good point about accuracy, especially when we’re facing a problematic situation with radar sites like Arecibo…

  • Stephen March 1, 2007, 15:25

    In geologic time, does one eventually expect Apophis to strike Earth? Or is it more likely that it gets flung to the Sun or somewhere else?

    We might mine it to nothing first. The mighty Colorado River doesn’t make it to the ocean anymore. We continuously drink it dry.

  • Administrator March 1, 2007, 15:27

    Yes, there is a relatively close call by Apophis in 2036, followed by another a decade or so later. We have to keep an eye on this one.

  • Timothy J Mayes June 26, 2007, 2:45

    gravity tug boats are impractical . No object we can put in space
    will have enough gravitation to significantly effect the motion of an
    asteroid. Do the math and calculate with newtons law of gravitation ?