How to Defend Earth From Asteroids


The Earth sits in a cosmic shooting gallery.

Millions of asteroids orbit the Sun, some on paths that cross that of Earth. These range in size from grains of sand -- which create beautiful but harmless shooting stars when they burn up in our atmosphere -- to rocks the size of football stadia and even cities themselves. One such asteroid, 10 kilometers across, impacted the Earth 65 million years ago, and the resulting devastation wiped out the dinosaurs essentially overnight.

We know there are no rocks that big headed our way anytime soon, at least not for centuries. But it only takes one asteroid, 100 or so meters across, impacting the Earth to explode with the force of a dozen nuclear weapons. This is something we might wish to avoid!

So what do we do?

Find 'em

Step 1 is obvious enough: Find them. Smaller asteroids around 100 meters in diameter are tough to see because space is vast and distances large. We need big telescopes that can sweep across large areas of sky rapidly and find those moving targets. The good news is we have 'scopes like that, and we're building more.


Still, some of these cosmic bullets are on orbits that make them very tough to spot. They stay near the Sun, making them hard to observe from the ground. There are two solutions to that. One is to build a spacecraft that travels in a path closer to the Sun, so now it's looking away from the Sun rather than toward it to spot asteroids. Another is to launch a telescope that does the same thing but stays near Earth. Both have their advantages, and the good news again is that both of these methods are being actively pursued!

The B612 Foundation is a group of astronomers, astronauts and engineers who investigate the Asteroid Problem. They are seeking private donations to build Sentinel, a spacecraft that will travel on a Venus-like orbit, closer to the Sun, to look for hazardous asteroids.

NASA is looking into building the near-Earth mission, which is called NEOCam (for Near-Earth Object Camera). Both spacecraft would be tuned to look in the infrared, where warm asteroids give themselves away against the cold of space, and both expect to find as many as 90 percent of Earth-approaching asteroids bigger than 140 meters in size. Sentinel can look in regions of space NEOCam can't see, but NEOCam, being closer to Earth, can scan the skies faster and transmit the data more efficiently to Earth. Together, they will make a formidable team watching the skies.

Kill 'em

Suppose that NEOCam or Sentinel, or some ground-based survey, finds an asteroid with our number on it. What do we do?

One idea I'm fond of is the "kinetic impactor plus gravity tug," which I outline in the video. Basically, a space probe is sent to the killer asteroid at full speed, and slams into it. The force of the impact changes the velocity of the rock slightly, hopefully enough so that it will miss the Earth.

Just to make sure -- and also to make sure the asteroid doesn't simply circle the Sun a few more times and hit us at some later date -- a second probe is sent along, hitchhiking with the impactor. It can move into a station-keeping position near the rock, using its own gravity to very gently tug the rock into a safe orbit. This takes a lot of time, so the more lead-time we have, the better. In fact, it takes a lot of lead-time to build and launch the probes, as well as get to the asteroid. That's why finding all these asteroids as quickly as possible is so important.

Unfortunately, we don't have missions like these built and ready to go. We don't even have them designed! B612 is working on that, but NASA has not seriously investigated this kind of mission. Of course, NASA has a finite budget, so trying to find the money necessary to do this -- hundreds of millions if not billions of dollars -- is just not possible for them unless Congress and the president take it seriously and raise NASA's budget to do it.

What, me worry?

Right now, we don't know of any asteroids on an impact course with Earth that could do us serious, global damage. There is a handful we know of that are being watched carefully, but in all those cases the odds of impact are pretty low. If it helps, I'm not lying awake at night fretting about any of them.

I wrote a book, Death from the Skies!, and the first chapter was on asteroid impacts. I wound up spending some quality time thinking about the effects of the impact that wiped out the dinosaurs. I have a pretty vivid imagination, and had colorful, terrifying scenes of that day going through my mind as I wrote the grim details down. I've always been fascinated with big disasters -- who isn't? -- but that experience cemented my desire to be involved with all this. After all that, I suppose I would say I'm not worried, but I am concerned. Calculating the odds of a big impact helped, since they are so infrequent.

... but that's not to say no rock will ever hit us. Given enough time, and our inaction, an impact is inevitable. But we're clever animals, us humans, smarter than the dinosaurs ever were. We are just waking up to the dangers in the sky, and I think we're intelligent enough to recognize the threat and take it seriously. We've already begun taking steps in that direction. It will take time, and lots of expertise, and of course a lot of money. But that'll be a pittance compared to the cost of doing nothing.

In the case of astronomy, as is true with most of science, we don't spend money on it. We invest in it.
