Death from the Skies! (5 page)

 

But of course there are technical difficulties with this method too; there always are. The rocket cannot fire straight down, toward the asteroid, because it will push the asteroid back, negating the effect of the tug. So the rocket will have to be tilted outward, firing at an angle away from the asteroid. That means that pairs of rockets are needed to balance each other and keep the tug from spinning out of control.

The amazing thing about this method is that in some cases, the mass of the tug need not be all that much. For 99942 Apophis, for example, a tug massing only
a single ton
can be effective in moving the asteroid away from the keyhole even if it gets to the rock only two years in advance. To be fair, in general it will take longer to move an asteroid away from an impact trajectory; for Apophis we need only move it so it misses a small region of space, but for a direct-impact trajectory the asteroid needs to miss a whole planet. That means moving the orbit thousands of miles, which in turn means a longer lead time (or a more massive tug). One current thought, developed by Schweickart, is a hybrid solution: using a kinetic impactor (literally whacking it with another rock) or nuke to move the asteroid out of the immediate threat zone, then using the gravity tug to fine-tune the orbit so that we don’t get a surprise a few orbits later.

Still, as promising as these technologies sound, we need to be honest with ourselves. We currently don’t have the technology to implement
any
of these methods. We’re close—maybe only a few years from developing the gravity tug—but even lobbing a nuke at an asteroid is pretty difficult. A report written in 2007 from NASA to Congress suggests that sending an impactor to an asteroid is currently our only workable option.

But that is due to current knowledge and current technology. The B612 Foundation is hoping to incrementally test technology that will prevent an asteroid from hitting us. Even better, some of their ideas, like the gravity tug, allow us to manipulate the orbit of an asteroid any way we want. We might even be able to nudge one into a safe orbit around the Earth. It would be far too small for its gravitational pull to affect us, but close enough that we might be able to set up mining operations on it. That might sound far-fetched, but some estimates show that the metals in even a small asteroid could be worth trillions of dollars. That would make a mighty tempting target for industry.

Instead of its aiming at us, we would be well suited to aim for
it.

There is still another lurking problem about which we need to be aware. Asteroids tend to have nice, predictable orbits. They are dead hunks of rock and/or metal, so once we observe them for a while, we can predict their orbits for decades.

 

But asteroids aren’t the only threat. Comets are lovely, wondrous specters in the sky. Unlike asteroids, comets are like dirty snowballs: rock, gravel, and dust mixed in with ice holding it all together. When they get near the Sun, the ice melts.
⁵
Many comets have pockets of ice under the surface, and when those sublimate the gas vents out in a jet. This acts like a rocket, pushing the comet around. If the comet is spinning—and most are—this means the comet will get pushed around randomly. That makes it extremely hard to accurately predict their orbits, and that much harder to land a rocket in them, or to use a gravity tug.

And it gets worse. The solar system looks something like a DVD seen edge-on: the planets orbit the Sun in the same plane. Asteroids too tend to stick to that plane. That means looking for them is a lot easier; we only need to keep checking the same parts of the sky.

But comets are wild cards. They aren’t confined to the solar system plane, and can come literally from any part of the sky. This can significantly cut into the lead time we have to do something about a killer comet approaching Earth. While we might have decades of notice for an asteroid impact, we might only have a few years for a comet. Even comet Hale-Bopp, which was one of the brightest ever seen, and which delighted hundreds of millions of people, was only discovered about two years in advance of its passage of Earth. Had it been aimed at us, there wouldn’t have been a damn thing we could have done about it. Hale-Bopp’s nucleus—the solid part of the comet—was twenty-five miles across. Had it hit, it would have made the asteroid impact that wiped out the dinosaurs look like a wet firecracker.

But even a small comet could have a disastrous, well,
impact.
Assuming it wasn’t confused for a sneak attack of some kind, the direct consequence of a small impact or Tunguska-like airburst over a city could lead to thousands of deaths and billions of dollars of damage. If it happened over a major city or economic landmark—New York City, California’s Central Valley (where much of the nation’s fruits and vegetables are grown), Tokyo—the results could be far worse. The good news is that long-period comets like Hale-Bopp represent less than a few percent of the overall impact hazard, and most short-period comets are easy to spot.

So how big a danger are asteroid and comet impacts?

Statistically speaking, you’re not going to like the answer: the odds of getting hit are 100 percent. Yes, really. Given enough time, and if we do nothing about it, there
will
be impacts, and one
will
be big.

But the key part of that sentence is the “if we do nothing” part. The point is, we
can
do something. While the techniques described here sound like something out of a movie, they are all possible. Technically they’ll be tough, and they’ll be expensive. But the stakes are pretty high: global survival versus utter annihilation.

I think that given this, it’s about time we took these science-fiction ideas and made them science fact.

An occupational hazard of being an astronomer is getting free astronomy textbooks in the mail. Like e-mail spam (but tipping the scale at ten pounds), they come unannounced, and generally wind up in a used bookstore collecting dust (the real-world equivalent of the spam filter).

I can’t resist thumbing through them. I torture myself this way, knowing that I’ll find some odd chapter arrangement, some scientific error, some small turn of phrase that will irk me in some way. And always, without fail, I find it in the section about the Sun. Invariably, there will be some permutation of this sentence: “The Sun is an ordinary, average star.”

If you decide to read only this chapter and then close this book forever, then please walk away with just one thing:
the Sun is a star,
with all that this implies. The Sun is a mighty, vast, furiously seething cauldron of mass and energy. The fires in its core dwarf into microscopic insignificance all the nuclear weapons ever built by mankind. A million Earths would be needed to fill its volume, and the light it emits can be seen for trillions upon trillions of miles. Invisible forces writhe and wrestle for control on its surface, and when it loses its temper, the consequences can be dire and even lethal.

That
is what it means to be an “ordinary” star.

Let’s be clear—there
are
lots of stars like the Sun, and if you phrase it carefully, then sure, the Sun is average. The smallest stars have roughly one-tenth its mass, and the largest have a hundred times its mass, so the Sun is somewhere near the low end of the range. But this neglects the actual
population
of stars: low-mass stars are far, far more common than their hefty brethren. More than 80 percent of the stars in our galaxy are lower-mass than the Sun. Roughly 10 percent have the same mass as the Sun, and 10 percent have more. So really, in a standardized cosmic test, the Sun scores pretty well. Maybe a B+.