Defending the Earth

The 60-mile diameter Manicouagan impact feature in Canada

The 60-mile diameter Manicouagan impact feature in Canada

As astrophysicist Neil deGrasse Tyson has pointed out, we live in a cosmic shooting gallery. Less than a year ago a good-sized chunk of cosmic rock exploded over the Russian city of Chelyabinsk with a force of over 400 kilotons – over 30 times as powerful as the bomb that flattened Hiroshima. The impact was huge, blowing out windows and knocking people off their feet over hundreds of square miles – over 1500 people sought medical care for their injuries. And that was a fairly small rock – about the size of a school bus. There are much larger rocks out there with our name on them – like the 6-mile asteroid that dredged a hundred-mile crater (killing the dinosaurs in the process), or the even larger ones that excavated craters over 160 miles in diameter in Canada.

In fact, there are at least 4 craters on Earth that were formed by impacts large enough to cause mass extinctions – and these are only the ones we know about. Given that well over half the Earth’s surface is water-covered it stands to reason that there have been about twice as many huge water impacts as those on land. On top of that, we also have to wonder how many have eroded away, been covered by sediments, or destroyed by plate tectonics. Over the history of our planet it’s possible that we’ve had our bell rung by at least a dozen major impacts – every few hundred million years or so. Given that complex multicellular life has only been around for 500-600 million years most of these impacts would be invisible in the fossil record, but every one of them would have been catastrophic to life all over our planet – any of them would have been fatal to our civilization and would have pushed humanity to (maybe even past) the brink of extinction. And remember – it doesn’t take a dinosaur-killing strike to end our civilization – something far smaller is more than sufficient to put an end to our current technological civilization. Considering all of this, it might not be a bad idea to have some contingency plans.

Believe it or not there’s been a fair amount of work on this topic – watching the 1993 impact of Comet Shoemaker-Levy 9 leave Earth-sized bruises on the face of Jupiter convinced scientists that cosmic impacts can still play an important role in today’s Solar System. That led to Congress tasking NASA with locating all of the largest asteroids that have a chance of hitting Earth – to date the American programs have located over 2400 near-Earth asteroids, many of them large enough to pose a serious threat to our civilization.

Locating threats is a good first step but it would be nice to be able to do something other than passively watch an asteroid all the way to a collision – it would be nice to be able to deflect it somehow. Over the years there have been a number of suggestions, including gravitational tractors (parking a massive spacecraft nearby to let the gravity of the spacecraft tug the asteroid out of a collision course), using a giant mirror to heat one side of the asteroid to help divert it, and even coating half the asteroid with reflective materials to let the very slight pressure of reflected light push an asteroid out of our path. But the more dramatic methods – usually involving rocket motors or nuclear explosives – have pretty much been relegated to the realm of science fiction.

Part of the reason for this is that rockets and explosions are pretty dramatic and high-impact events – not only are they hard to get into position to use, but they are also just as likely to break an asteroid into pieces as to push it off course. This would seem to be OK – but in actuality, getting hit with three 2-mile diameter rocks is about as bad (maybe even worse) as being hit with a single 4-mile object. Unless whatever we were to do were to break the incoming object into pieces small enough to break up or burn up while passing through the atmosphere we might end up making things worse. Nevertheless, the concept of using nuclear weapons to help divert an incoming asteroid remains under consideration. In general, the further out we can predict a collision the more time we have to avoid trouble – and the gentler the methods we can use. But if we don’t see something until the last minute – a few years before collision – we might have to resort to more violent methods. This is where nuclear weapons might play a role, and according to a recent story in the Global Security Newswire, both Russian and American scientists are interested in using their skills to help develop weapons that might help to save our bacon.

So here’s the question – actually one of many – are nuclear weapons designers and the governments who employ them really interested in saving the planet, or are they just looking for a pretext to keep working on (and maybe testing) new and improved weapons? And a follow-on question – there’s a very real risk of a catastrophic collision in the next hundred million years, but a very small risk in the next century; do we face a greater risk from a possible asteroid collision or from developing and testing a new generation of nuclear explosives ostensibly aimed at averting such a collision?

I don’t have an answer to that one, but society needs to decide. If we, as a society, decides that the risk of a civilization-ending asteroid strike is sufficiently high that we need to have plans, backup plans, and an ultimate backup then we will need to not only design, but also to test new nuclear weapons that might someday save humanity – and we’ll also have to trust the governments and the scientists who design and test these devices that they will only be used for that purpose. If we don’t feel we can make this leap of faith then perhaps we ought to beef up our efforts to locate and track everything that poses a risk so that we don’t need to fall back on a last-ditch and last-minute effort to blow something out of our sky.

Personally, I think it makes sense to hedge our bets. There are only a few nations that have proven themselves capable of developing an asteroid-moving nuclear weapon and all of these nations have shown themselves able to resist the temptation to use these weapons in tense situations. I’d like to think that these nations will continue to show this level of restraint. And I also have to say that, to me, there is a certain symmetry in the thought that the weapons we thought might destroy civilization and launch a nuclear winter might one day be used to save the world.

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2 Responses to “Defending the Earth”

  1. Lance November 10, 2013 at 12:18 AM #

    Dr. Y:

    Your opinion about this subject is interesting and quite edifying but elicits the need to broaden the discussion somewhat. I don’t have any answers for you or your readership but I would like to point out a few issues to extend the thinking around this use of nuclear weapons. The issue of near-earth objects (NEOs) and the sometimes association with nuclear weapons presents some difficult problems to mull over. If I might be droll: This is a result of the world – indeed the cosmos – or at the very least, the solar system – being very complicated and dynamic systems.

    A legitimate movement has been afoot for some time that seeks to drive the number of nuclear weapons worldwide to zero. I am not speaking of the grass roots anti-war or anti-nuke movements but the more sober, scholarly studies that have studied the possibilities. I point you to Corey Hinderstein’s 2010 book Cultivating Confidence (produced by the Nuclear Threat Initiative) and to the study of the feasibility of reaching zero by Michael E. O’Hanlon entitled A Skeptic’s Case for Nuclear Disarmament (Brookings Institution, also 2010). In both cases, it can be ascertained that nuclear disarmament is a long way off but the world wide conditions that would make it possible are reviewed, outlined and not that all unrealistic. Although these conditions are at present (way) over the horizon, they speak to a reality that many nations aspire to. This is particularly true of the nations in the non-aligned movement and those non-nuclear weapons states that continually charge that the nuclear weapons states-parties to the Nuclear Non-Proliferation Treaty (NPT) – mainly the US and Russia – have not done enough to reduce their nuclear weapons caches. Witness the origin of the Global Zero movement in Paris by such notables as George Shultz, Bill Perry, Sam Nunn, and Henry Kissinger along with 95 or so others in 2008.

    Although I see the “poetry” in your final words about how weapons built to destroy us may save us, there is a parallel line of thought that believes that mankind must survive the nuclear period in which he currently resides in order to move into the next phase(s) of civilization – and that may include coincidentally, a manned return to solar system exploration. But I am jumping way too far ahead.

    Loss of nuclear weapons expertise is not hard to extrapolate under current conditions. No testing is allowed or is being done – thanks in part to the NPT and the listening devices that monitor for violations (more later on that). As engineers and scientists in the field retire, more effort will be needed to keep the corp charged with tending nuclear weapons up to speed. Its speculation on my part, but to broaden the customer base and keep them relevant, claims from nuclear weapons experts about the capability to destroy civilization-killing asteroids might be made.

    Can these weapons in fact do enough damage to a massive, perhaps iron based object? Perhaps, especially if the (right sized) target is still thousands of miles out in space – at least that has been the contention since the early 1990s. Others though disagree but not necessarily on the grounds of destructive capability. Asteroid hunter Tom Gehrels who operated the Spacewatch Program http://spacewatch.lpl.arizona.edu/index.html is skeptical because he does not want NEOs to be an excuse for new nuclear weapons research (at least that is how he felt in 1996 when interviewed in the July 28th New York Times Magazine). Everyone, even the asteroid hunters, have emotional responses to things nuclear. Again, the complicated universe reminds us how complicated it really is.

    There is also the little issue of the Outer SpaceTreaty (1967) which expressly forbids the stationing of nuclear weapons in earth orbit http://www.armscontrol.org/factsheets/outerspace.
    Although it is conceivable that the treaty could be amended, I imagine that as presently composed, it means asteroid-killing nuclear weapons would have to be launched from ground stations. And despite what an asteroid with just the right momentum could do to us, we all “know” somewhat viscerally, what a nuclear weapon gone astray that detonates in the atmosphere or on impact with the ground would do to us. Well, faced with a damned if we don’t and damned if we do (maybe) scenario, it’s perhaps a sure bet we would press the launch button – but not without a lot of sweating, protest and debate. Heck, we can’t even launch space probes carrying Pu-238 (albeit tens of lbs of it or more depending on the mission) without raising the hairs of those who expect the launch to fail catastrophically.

    Lastly, and somewhat in a contradictory vein but I think remaining consistent with dynamic, complicated cosmos backdrop to these musings, is the recent report that asteroid impacts might be more frequent than previously thought http://www.bbc.co.uk/news/science-environment-24839601 The listening devices used for detection of nuclear detonations apparently can detect a meteor impact of certain momentum as well. Again, how these two issues are married.

    There are at least 10 known impact craters scattered over the earth with diameters ranging from 25 to 110 miles. In the 20th century there were three significant impact events including Tunguska. So, a threat from larger impacts than the 20th century endured is there and may in fact be a little more probable. The question is, how probable? The parallel question is how probable is a nuclear detonation (or an exchange)? And please consider accidental launches due to questionable practice like Russian and American nuclear forces remaining on launch ready status in this day and age. One could argue that the probabilities are low for both. And one could be cold about it and calculate (more likely guesstimate) which has the lower probability: the civilization killing asteroid (meteor) impact or the nuclear detonation (by state or non-state actors)? Then decide if nuclear weapons should live or eventually die. After all, there may be other ways to deflect an asteroid (maybe).

    If only the equation were that simple in this complicated world – or should I say, complicated cosmos?

    • Dr. Y November 11, 2013 at 8:59 PM #

      You’re right – it’s a tough question, and there’s a lot more to it than just the science and engineering. In fact, I think that the science part is a lot easier than the political and social aspects. On the one hand, it would be embarrassing to be wiped out (or at least to have civilization collapse) because we had the right tool for the job but wouldn’t let ourselves develop it. On the other hand, while we know that there will be another impact, we don’t know if it’ll be tomorrow or next decade or in ten thousand years – having made significant progress in ramping down the risk of destroying ourselves with nuclear weapons it would be difficult to undo some of the treaties and other progress that have made the world genuinely safer.

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