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Life in the Shooting Gallery
Jamais Cascio, 13 Mar 04

Last night, I attended the latest of the Long Now Foundation's monthly seminars about Long-Term Thinking: Apollo and Skylab astronaut Rusty Schweickart, talking about the threat to the Earth from asteroid impact over the next 100,000 years. The danger of asteroid or comet strikes on Earth is a topic we've mentioned a couple of times here at WorldChanging, but Schweickart's talk brought together quite a bit of information about the threat -- and what we can do about it.

The possibility of an extraterrestrial object being a threat to the planet is not something that many people concerned with more Earth-bound problems -- the environment, social justice, nuclear proliferation -- give much thought to. Regardless of the potential danger, asteroids seem pretty irrelevant. That presumption is, naturally, wrong. Schweickart gave one example which made clear just how broad the danger can be.

Once a year, on average, a single asteroid roughly 4-5 meters in diamter -- small enough to fit in a living room -- strikes the Earth. Moving at 47,000 miles per hour, however, when an asteroid of that relatively small size hits the atmosphere, it tends to explode -- and the energy released is roughly the same as a 10 kiloton nuclear bomb. Last year, during the height of tensions between India and Pakistan, one of these small space rocks hit the Earth and exploded over the Mediterranean Sea. If the rock had hit two hours earlier, it would have exploded high above Kashmir. In a war zone, with each side afraid of a preemptive nuclear attack from the other, how would that exploding meteor be interpreted?

Right now, we know of 1,100 large asteroids -- at least 1 kilometer, or about 10% of the size of the asteroid that probably killed off the dinosaurs -- that have orbits that come near the Earth. About 21% of these are considered PHOs ("Potentially Hazardous Objects"), as their orbits intersect Earth's orbit. Of those 1,100, 700 have had their orbits studied sufficiently to determine that they will not pose a danger to the Earth in the next century; 400 remain mysteries. But that's only the planet-killer size rocks. If you include the Near-Earth Asteroids of 150 meters or larger (which would still hit the Earth with hundreds of megatons of energy), there are over a million nearby. We live in a cosmic shooting gallery.

(More fun info about asteroids -- and what we can do about them -- in the Extended Entry.)

According to astronomers who specialize in watching asteroids, over the next hundred thousand years, Earth is likely to be struck by:

  • One 500 meter asteroid, delivering 6,000 megatons of energy, producing a 220 kilometer diameter region of total destruction.
  • Ten 200 meter asteroids, each delivering 400 megatons of energy, producing regions of total destruction about 2-3 times the area of Los Angeles, or about 90 kilometers in diameter.
  • One hundred 70 meter asteroids, each delivering 15 megatons of energy, producing regions of total destruction measuring 40 kilometers in diameter.

    We also have a 10% chance of being hit by a planet-killer of 1.2 kilometers in size -- which delivers 100,000 megatons of energy.

    Thinking in terms of the next hundred thousand years may seem a bit beyond reasonable long-term planning, so let's scale that down a bit. Over the next ten thousand years -- the length of time a nuclear waste repository needs to last -- Earth is likely to be hit by one 200 meter/400 megaton rock, and ten 70 meter/15 megaton strikes. These are avergages, based on lots of planetary research; we could be hit at any time. So what can we do about it?

    The first -- and most important -- thing we can do is crank up the search for Near-Earth Asteroids. The current Spaceguard program only has the resources to look for 1km+ sized NEAs; this means that smaller-but-still-dangerous rocks are simply not generally tracked with precision, and we'd have little warning before being hit.

    But what good is a warning if we can't do anything about it? If you're hoping that Bruce Willis and Ben Affleck are going to be able to save us by nuking the asteroid... sorry. Schweickart is strongly opposed to notions of using nuclear weapons to destroy dangerous incoming asteroids, for a number of reasons. Firstly, it probably won't work -- many asteroids are only about twice as dense as water, more like marshmallows than solid stone, and would be more likely to absorb the energy of a nuke than be torn apart by one. Secondly, it would lead to the proliferation of nuclear weapons in space -- and, from what Schweickart found at last week's Darpa-sponsored meeting in Anaheim, California, on the use of space, there are quite a few Pentagon officials eager to do just that.

    A somewhat better option would be ablation -- using massively powerful lasers or orbiting mirrors, focusing sunlight, to burn off layers of the asteroid over time, causing the ejected material to function like a rocket, pushing the rock slowly, changing its orbit so that it doesn't hit the Earth. This is a more elegant (and, depending on the asteroid, plausible) approach than trying to blow up a rock... but is likely to be very expensive. In addition, nobody's ever built lasers that powerful or mirrors that big.

    Best of all would be to push the asteroid directly, by attaching rockets to the rock itself. Traditional kerosene or liquid oxygen/hydrogen engines wouldn't do the trick -- a big, fast push would be more likely to break the asteroid up (only to see it gradually reform again) than to move it. We'd need slow, steady, long-lasting pushes. Fortunately, rockets that do that have already been invented and tested: electric ion engines. Given enough lead time -- a decade or more is best -- stick a good sized ion engine on an asteroid, push it in the direction it's already going for a couple hundred days, and you'll change its orbit just enough to cause it to miss the Earth entirely. There are two problems with this idea: if the engines give out before the push is finished, all you've done is change where on Earth the rock will hit; and sufficiently-powerful, long-lasting electric ion engines require nuclear reactors.

    The first is a political problem which will not be easy to resolve, but could have a technical prevention: make certain to have sufficient redundancy and extra capacity in the engines that even massive failures leave enough power to finish the job. The second is likely to be much more troubling for many WorldChanging readers. Given the possibility of launch vehicles exploding on their way up -- a problem that afflicts not just NASA, but pretty much every space program around -- would we be trading one risk (being smacked by an asteroid) for another (accidentally spewing plutonium into the atmosphere)?

    If the choice was between definitely being hit by a big rock and possibly having an accident at launch, most of us would choose to take the chance. But in order to make certain that an electric ion engine would in fact be able to do this, we'd need to test it first. Schweickart, though his B612 Foundation wants to do just that, and by 2015.

    It's a tricky issue. An asteroid strike sufficient to cause widespread destruction will happen again. It may happen soon. We have the technological capability to prevent it, with sufficient warning, but only by taking risks which could themselves lead to calamity. In time, we may be able to build non-rocket launch systems (such as Earth-to-orbit elevators) which would greatly reduce the risk of putting large nuclear reactors into space -- but the construction of such systems remains decades away, well past Schweickart's 2015 target.

    Which gamble do we take? That the next large asteroid impact is many decades or centuries away? Or that we can safely launch into orbit the nuclear power sources required for deflection? Given the political risks -- and financial costs -- of building the large electric ion engines, I suspect that we're going to choose the first gamble by default. I hope it's the right one.

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    Comments

    The ion engines that are being proposed are not particularly large -- the B612 site notes that " typical value for the force we will apply in reorientation and acceleration is about 10 Newtons, or about 2.25 pounds!"

    NASA's tests on its NEXT ion engine indicate that it produces nearly 1/4 Newton at its full power of about 7 KW. Thus a scale-up of 40X will be needed to move asteroids.

    Each solar power module on the International Space Station produces 60 KW of power. Ganging 5 or 6 of these together would not be hard, but building an equivalently powerful thermoelectric nuclear source is probably even less difficult.


    Posted by: Alton Naur on 13 Mar 04

    perhaps in one of those rare alignments of progressive and libertarian stars, er 'roids :D

    http://techcentralstation.com/031004C.html

    but really i think more just practical common sense, er enlightened self interest! like it's convinced me of the necessity of having a 'space force', altho how internationalised and overtly militarised it is is debateable...


    Posted by: reflexorset on 13 Mar 04

    Great post, Jamais! Wish I could've been there to hear Rusty's talk myself!


    Posted by: Alex on 14 Mar 04

    Alton, although Rusty didn't directly address the solar power alternative question, from my readings I don't think it would be a fully viable alternative because of the area required (the ISS solar modules are not small, so ganging together 5 or 6 would be a challenge). Moreover, since the B612 plan is to push the asteroid in the direction of its current travel vector, for half the trip the rocket would be facing away from the sun -- not an ideal situation for solar power.

    Thanks for the link, reflexorset.

    And I wish you'd been there, too, Alex!


    Posted by: Jamais Cascio on 15 Mar 04

    will a meter hit in semtember 2004


    Posted by: Brianna on 2 Apr 04



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