Death from the Skies! (4 page)

We’ve known about asteroids for two hundred years, and it’s taken us this long to recognize their danger. The dinosaurs never had a chance.

Of course, the big difference between us and the dinosaurs is that they didn’t have a space program.

You’ve seen the movie a hundred times: an asteroid miles across is discovered and its orbit puts it on a direct collision course with Earth. If we don’t do something, it’ll wipe us out. Enter the team of brave hero astronauts/oil riggers/military men. Heroically they launch into space, heroically face down the monstrous rock, and heroically blow it to smithereens, which then rain down harmlessly as gawkers look on.

That sounds, well,
heroic.
There’s only one problem: it won’t work.

Actually, there are lots of problems with this scenario. For one, there’s no guarantee that nuking an asteroid will destroy it. A lot of asteroids are almost solid iron, so throwing a nuclear bomb at one might only warm it up a little.

Even if an asteroid is made of rock, there’s no guarantee a nuke will disintegrate it. First, if it’s really big, a nuclear weapon may not do all that much damage to it. But it also depends on the asteroid’s consistency.

Some asteroids have been found to have very low density, which was initially puzzling. Rock has a density of about two to three grams per cubic centimeter (roughly an ounce per cubic inch, or two to three times the density of water). But some asteroids have lower density than that. An asteroid called 253 Mathilde, for example, which orbits the Sun between Mars and Jupiter, has a density of about 1.3 grams per cc. It must have a texture like Styrofoam. How can that be?

When asteroids were finally observed up close by space probes, they were seen to be heavily cratered. Obviously, asteroids eat their own: they hit each other, leaving giant pits across their surfaces. If an asteroid gets hit hard enough, it’ll explode, breaking apart completely. But if it’s hit just
softer
than that critical speed, it won’t blow apart: the shock from the impact will shatter it in place, like a hammer tapping a crystal egg. The asteroid’s own gravity will still hold it together, but it’ll be riddled with crevices and cracks. In essence, it’s a floating rubble pile.

What would happen if you tried to nuke something like that?

Asteroid expert Dan Durda from the Southwest Research Institute in Boulder, Colorado, wanted to find out. He discovered that the scientific literature on asteroids didn’t have much information involving experiments on actual asteroidal material, so he set about to correct that. He obtained some meteorites known to have come from asteroids. One was dense, solid, and rocky, and the other was more porous, more like 253 Mathilde than a chunk of, say, quartz.

He took his sample to NASA’s Ames Research Center in California, which boasts the ownership of an unusual gun: it uses compressed air to shoot projectiles at several kilometers per second.

Durda set up his solid specimen in the gun’s sights, and slammed a BB into it at five kilometers per second. As expected, the meteorite exploded, disintegrating into hundreds of pieces.

He then put a porous piece of rocky material in the crosshairs. When the projectile hit it, the meteorite
absorbed the projectile and didn’t shatter.

Durda asks, “What if an object like that were coming toward the Earth and you were trying to stop it? . . . How would it respond if we were to throw some sort of a projectile into it at a very high speed to try to break it up? What if you were to try to put a small nuclear device next to it to try to break it up? Would it actually respond in the way you normally think of a solid hunk of rock?

“You put a brick next to you, and take a hammer, and you slam the brick, and it goes flying into pieces . . . that’s what you think of when you talk about breaking up an asteroid.

“But you take a sandbag, and you whack it with that same hammer, and nothing happens. It just kind of goes
thud,
and that’s the end of it.”

That’s bad news for us. A rubble pile is pretty good at absorbing damage, and so a nuke won’t destroy it. If we see one headed our way, we can bomb the heck out of it, and it’ll laugh all the way down to impact.

Actually, many scientists are rethinking the idea of nuking an asteroid. A huge disadvantage of blowing up an incoming threat, even if we could, is that it would create thousands of small potential impactors out of one big one. That may sound better than the alternative (a giant one hitting intact), but even a rock a hundred yards across could easily take out a city. A dozen of those impacting at the same time would be disastrous no matter where they hit. While the explosions would be smaller, they’d be more spread out, scattering damage across the globe instead of confining it to one place.

Durda adds another danger of blowing up small asteroids. “If you take the cosmic composition of a typical asteroid of that size . . . there’s enough chlorine and bromine in that object to destroy the ozone layer. So it doesn’t matter whether or not that object hits all at once, in one big piece, or if all of that small debris left over from breaking it up into a million pieces still comes raining through and vaporizing into our atmosphere. You’re still depositing all of that very harmful stuff into our fragile atmosphere.”

It might be better, then, if there’s no way to stop it, to just let a smaller asteroid hit us.

That’s unsatisfactory, of course, especially if you’re sitting in the bull’s-eye.

But there may yet be another solution.

One idea is to drop a bomb not
on
the asteroid, but
near
it. Blowing up a bomb next to an asteroid, say, a few hundred yards away, would generate a huge amount of heat and vaporize part of its surface. The solid rock or metal would turn into a gas and expand rapidly, acting like a rocket. It would push a little bit on the asteroid, moving it.

It wouldn’t be much, but in space you don’t
need
much: every little push adds up. If you blow up several bombs, you can actually generate enough thrust to move the rock significantly. If you move it enough, it’ll miss the Earth entirely.

And a big advantage of this technique is that it would work on rubble piles too, though it’s not clear exactly how well.

There are some disadvantages to this method. You need a lot of lead time, for one thing. The farther away an asteroid is from impact, the less you have to change its orbit to make it miss us. Most experts think ten years’ warning is enough, though they’d be happier with twenty. A century would be just fine. This technique would work best on smaller asteroids since they’re easier to move, but a smaller asteroid is also fainter, and thus harder to find. Lead times would be shorter so there would be no room for mistakes. And getting one bomb to an asteroid is hard; getting twenty or more is a lot harder.

Another problem is that it’s nearly impossible to know how an explosion would affect the orbit of the asteroid. It might be enough to have the rock miss us, or it might nudge it into an orbit that will hit us on the
next
pass around the Sun.

For example, look at asteroid 99942 Apophis. It’s an Earth-crossing chunk of rock about 250 meters across, and is a potential impactor. At that size and mass, it would do considerable damage should it hit, exploding with the force of 900 megatons (more than a dozen times the yield of the largest nuclear weapon ever detonated). Apophis will pass by the Earth on April 13, 2029; there is no risk of impact at that time, but it will pass so close that it will actually be closer to the surface of the Earth than many weather and communication satellites.

The asteroid will approach so close to us, in fact, that its orbit will be seriously affected by Earth’s gravity, and just how much its orbit is changed depends on just how close it gets to Earth in 2029. In fact, there is a region of space called the
keyhole
such that if Apophis passes through it, the orbit will be changed precisely enough that on its next return in 2036, Apophis will impact the Earth.

This sweet spot is not terribly big, but our knowledge of Apophis’s exact trajectory isn’t good enough to completely preclude the asteroid’s passing through it. The odds are incredibly low, maybe less than 1 in 45,000, but it’s worth investigating.

And what if it turns out that Apophis will glide right through the keyhole? We’ll have just seven years to move it enough to miss us. A better idea is to prevent it from passing through the keyhole in the first place. If we get to Apophis before 2029, then we hardly have to nudge it at all; calculations show that changing its velocity by even a few thousandths of an inch per hour would work. So you might think that a well-placed nuclear weapon would do the trick.

Unfortunately, it won’t. That keyhole isn’t alone: there are dozens of keyholes, thousands. That first keyhole is just for a return of Apophis in seven years, but other keyholes will bring it back in ten years, twelve, twenty . . . instead of saving us, a detonation just buys a little bit of time, and there’s no guarantee that we can move it away from some other keyhole—or knock a chunk or ten of it into another keyhole.

Controlling
the resulting orbit is a key issue, and blowing up a nuclear weapon is not exactly subtle.
⁴
We need more fine-tuning on asteroid steering.

By now it may have occurred to you that maybe we don’t need a bomb. The impact of an asteroid on the Earth releases energy like a bomb, so why not try impacting the asteroid itself? If we hit it hard enough with some sort of impactor, we won’t need a nuke.

There is a very big advantage of this method: we’ve done it before. On, appropriately enough, July 4, 2005, NASA’s Deep Impact probe slammed into the comet Tempel 1, creating a flash seen by hundreds of scientific instruments across the world. The impactor was an 800 pound block of copper, which was steered into the comet at over six miles per second. The resulting explosion was the equivalent of about five tons of TNT detonating. The size of the resulting crater is unknown; the flash and debris hid the impact from the spacecraft’s camera.

Steering a probe into an object moving at several miles per second is an engineering triumph. There were no second chances, and even the exact shape of the comet nucleus was unknown until the probe got there.

On the other hand, the comet itself was three by five miles in size, which is pretty big. Had it been a small asteroid, it’s unclear whether the NASA engineers would have been able to hit it. Still, it was a first shot, and a successful one. Much was learned from the attempt that can be applied to ramming a potentially dangerous asteroid.

But it must be stressed that the impacting scenario suffers from most of the same issues as bombing an asteroid: it might shatter the asteroid, producing many smaller impactors; if the asteroid is porous it will simply absorb the impactor; and again we cannot control the resulting orbit, so we might just be pushing it into some other future impact event. While this might change the orbit enough to miss the Earth, it can’t be known in advance just how much, and in this game inches matter.

Still, there may be other ways to rid ourselves of a potential planet-buster. Perhaps, instead of blowing one up, we can instead gently
persuade
the asteroid to change its trajectory.

The B612 Foundation—named after the asteroid home of Antoine de Saint-Exupéry’s titular Little Prince—is, for lack of a better description, a kind of doomsday think tank, consisting of dozens of scientists, engineers, and astronauts whose express purpose is to figure out a way to save humanity from the threat of giant impacts. The foundation has held meetings, written papers, and had members (such as Apollo 9 astronaut Rusty Schweickart) testify before Congress about doomsday rocks.

Their Web site reads like a science-fiction novel, full of ways to stop an asteroid from hitting us. However, the emphasis is on the science. While many of the methods would be difficult to execute and are clearly only in the very early stages, others involve mature technology or adapting what we already have.

For example, one method is to physically land a rocket on an asteroid, secure it in place upside down, and then start firing it. Over time, the thrust will push the asteroid into a new orbit, making the rock miss us.

This may be the safest method, and it certainly makes sense, but in reality it would be pretty hard to do. For one thing, it’s not entirely clear how you would secure the rocket to the surface of the asteroid. What if the surface is powdery, or it’s a rubble pile, or it’s metal? For another, every asteroid spins, which means you can only fire the rocket for short periods of time when it’s pointed in the right direction. That means you need more lead time, and in many cases time is precious. Worse, some asteroids tumble chaotically, and for those a rocket would be nearly useless.

These problems kept the B612 Foundation members thinking . . . and they came up with an answer that is really quite surprising. What if you don’t land the rocket at all?

Asteroids are small compared to planets, but they still have mass. And any object with mass, said Isaac Newton, has gravity. The rocket itself has mass, and therefore gravity as well. So imagine this: a rocket is placed in a parking orbit
near
the asteroid, but not physically
in contact
with it. The asteroid’s gravity will pull on the rocket, making it fall toward the asteroid. In the same way, the rocket’s mass will pull on the asteroid. Now the rocket is fired, but very, very gently, just enough to counteract the fall toward the surface. The result is an asteroid tug. But unlike a barge or a tugboat on Earth that uses ropes to haul other boats, the rocket is virtually connected to the asteroid by gravity. Over time, the gravity from the rocket pulls the asteroid into a safe orbit.