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.

The post Defending the Earth appears on ScienceWonk, FAS’s blog for opinions from guest experts and leaders.