10 Ways to Stop a Killer Asteroid

Nuclear weapons may not be original, but they're a known entity and, as a result, a logical choice if you need to blast a boulder to smithereens. This supermacho approach involves slamming a nuclear warhead into an approaching asteroid. There's only one problem: A direct hit on a large object might only break it into several smaller pieces (remember "Deep Impact"?). A better option might be to detonate a warhead near the asteroid, letting heat from the explosion sear one side of the rock. As material vaporizes from its surface, the asteroid would accelerate in the opposite direction -- just enough (fingers crossed) to steer it away from Earth.

If explosions aren't your thing, but you still want to hit something, then you'll appreciate another technique known as kinetic impactor deflection. The "kinetic" in this case refers to kinetic energy, which all moving objects have and the universe conserves. But we're getting ahead of ourselves. Turn the page to learn how the behavior of billiard balls just might save our planet.

If you've ever played pool, then you know about kinetic energy, which is the energy possessed by any moving object. The kinetic energy of a struck cue ball is what gets transferred to other balls on the table. Astronomers believe the same principle could deflect an earthbound asteroid. In this case, the cue ball is an unmanned spacecraft similar to the probe used in NASA's Deep Impact mission (not to be confused with the movie). The mass of the Deep Impact vessel was only 816 pounds (370 kilograms), but it was moving really, really fast -- 5 miles (10 kilometers) per second [source: NASA].

Kinetic energy depends on both the mass and speed of an object, so a small object moving fast still has a lot of energy. When mission engineers slammed the Deep Impact probe into the surface of the Tempel 1 comet in 2005, it was slated to deliver 19 gigajoules of kinetic energy. That's the equivalent of 4.8 tons of TNT, enough to shift the comet ever so slightly in its orbit [source: NASA].

Astronomers weren't looking to alter Tempel 1's trajectory, but they know now it could be done, should an asteroid or comet set its sights on Earth. Even with a success under their belt, scientists acknowledge the enormous challenge of such a mission. It's sort of like hitting a speeding cannonball with a speeding bullet. One wrong move, and you could miss your target completely or hit it off-center, causing it to tumble or crack into pieces. In 2005, the European Space Agency came up with the Don Quijote concept to improve the odds of a kinetic impactor mission (see sidebar).

You might classify nuclear weapons or kinetic impactors as instant-gratification solutions because their success (or failure) would be immediately apparent. Many astronomers, however, prefer to take the long view when it comes to asteroid deflection.

Electromagnetic energy produced by the sun applies pressure to any object in the solar system. Astronomers like to call it solar, or radiation, pressure and have long thought this stream of energy could be a source of propulsion for rockets. Just strap some sails onto a spacecraft, let them catch a few rays and the ingenious vessel will slowly, gradually, pick up speed as incoming photons transfer their momentum to the sail. Could something similar work on an asteroid? A couple of scientists think so. Assuming you had some time -- we're talking decades here -- you could fasten some solar sails on an asteroid, do a little tacking and steer the rock away from Earth.

Of course, even Bruce Willis might not be extreme enough to land on a hunk of rock and try to convert it into a cosmic sailboat. Another option would be to wrap the asteroid in foil or coat it with highly reflective paint. Either solution would have the same effect as a solar sail, harnessing the energy of incoming photons. Then again, who's going to try to wrap foil around a giant potato traveling, say, at 16 miles (25 kilometers) per second [source: Jessa]? Or carry a few million gallons of paint into space?

Luckily, there's another sun-centered solution that might not seem so wacky.

You're familiar with puffballs, right? They're the little round mushrooms we often see in fields and forests that reproduce by releasing spores through a topside exit hole. Poke a fresh puffball, and you'll see black smoke shoot out in a jet.

Strangely enough, astronomers think they can get an asteroid to do the same thing, though not by poking it. Instead, they envision parking an unmanned probe in orbit around an offending rock, then aiming a laser at the object's surface. As the laser heats up the rocky substrate, steam and other gases will erupt in fast-moving jets. According to Newton's laws of motion, each burst of gas applies a tiny force in the opposite direction. Heat the asteroid long enough, and you'll have it hissing like a teakettle and moving, centimeter by centimeter, off its original course.

Some see the laser as the limiting factor in this scenario. What if it can't draw enough power to sustain long-term heating? You could arm the probe with an array of mirrors. Once you get the spacecraft in orbit around the asteroid, you simply unfurl the mirrors and orient them so that they direct a beam of concentrated sunlight toward the object's surface. This provides the necessary heating without the need for a high-powered laser.

Then again, why not use the orbiting spacecraft without all of the tricks and gimmicks? Doesn't it have mass and, as a result, gravity? And doesn't gravity pull on nearby objects? Why, yes, Sir Isaac, it does.

Every object in the universe, even something as small as a pebble, has gravity. You can't feel a pebble's gravity because its mass is so small, but it's still there, tugging away on anything that comes close. The close part is important because gravity is also related to the distance separating two objects. The closer they are, the greater the gravitational attraction.

A spacecraft zipping through the solar system obeys the same principles, exerting a gravitational pull directly proportional to its mass and inversely proportional to the distance between it and another object. Now, compared to an asteroid, which might have the mass of Mount Everest, a spacecraft is pretty puny, but its gravity can still make things happen. In fact, if you place an unmanned probe in a close orbit around an asteroid, it will pull ever so slightly on the rock. Over a period of 15 years or more, this almost infinitesimal tug could deflect the asteroid's orbit just enough to protect Earth from a nasty blow [source: BBC News].

Astronomers refer to this as a gravitational tractor and think it's a viable solution -- as long as they know about a potential collision years in advance. Early detection is just as critical to the next idea on the list.

If the gravitational tractor concept seems too delicate and prissy, you're in luck. A few scientists are proposing another way to make use of a spacecraft that doesn't require slamming it into an asteroid or entering a passive orbit. They studied busy harbors here on Earth and observed how tugboats nudge large ships up to the wharf. Then they developed an asteroid-deflection scenario using a similar technique.

Here's how it works: First, you build a special ship with powerful plasma engines and an array of radiator panels to dissipate heat from the onboard nuclear reactors. After you're alerted of a threat, you launch the vessel and fly it to the offending asteroid. Then you ease the space tug close to the rocky surface and attach the vessel using several segmented arms. Finally, you go easy on the throttle and start a slow, gentle push. If all goes well, 15 to 20 years of pushing in the direction of the asteroid's orbital motion will deflect it just enough to avoid a catastrophe [source: Schweickart].

Still not convinced? Then grab your mitt and keep moving to the next page.

Remember those baseball pitching machines you faced when you were a kid? They had a feeder tube and a wheel assembly to shoot the balls out at 50 to 60 miles (80 to 97 kilometers) an hour. Wouldn't it be great if you could set up a pitching machine on an asteroid? Not to take batting practice, but to save the world?

As crazy as it sounds, astronomers have an idea to do just that. They call their machine a mass driver, but it works the same way. It scoops up rocks from the surface of an asteroid and hurls them out into space. With each throw, the machine applies a force to the rock, but the rock, thanks to Newton's action-reaction law, applies a force back to the machine -- and to the asteroid. Throw a few hundred thousand rocks, and you'll actually shift the asteroid's orbit.

Of course, the concept has invited some criticism. How do you get the mass driver on the asteroid? And how do you keep it powered? A pitching machine plugs into an electrical supply, but extension cords are tough to manage out in space. And what if the darn thing breaks down? A relief pitcher may not be available to finish the game.

Maybe baseball is the wrong sport. Maybe another backyard favorite offers a better solution.

In 2009, a doctoral candidate at North Carolina State University proposed a novel asteroid-deflection technique in his dissertation. This was the idea: Attach one end of a tether to an asteroid and the other end to a massive weight known as a ballast. The ballast acts like an anchor, changing the asteroid's center of gravity and diverting its trajectory over the course of 20 to 50 years, depending on the size of the rock being moved and the weight of the ballast.

The student didn't work out every detail, but he estimated that the tether would need to be somewhere between 621 miles and 62,137 miles (1,000 and 100,000 kilometers) long. He also suggested a crescent-shaped attachment bar similar to those found on globes. This would allow the asteroid to rotate without tangling the tether (no one likes a tangled tether).

Now, if you think this sounds just too wacky to work, you should know that astronomers have embraced space tethers for years. In fact, NASA has used them successfully on several missions to move payloads in Earth's orbit. Future missions call for delivering material to the moon by handing off payloads across a series of tethers.

Still, a tether and ballast system, like most solutions in our countdown, requires time. And time requires early detection. As we'll see next, asteroid detection may be far more important than deflection.

When it comes to asteroids, you want to be like the Rolling Stones and put time on your side (yes, you do). Luckily, steps are being taken to survey and detect near-Earth objects, or NEOs.

NASA addresses NEO detection through two surveys mandated by U.S. Congress. The first, known as the Spaceguard Survey, seeks to detect 90 percent of NEOs 1 kilometer (0.621 miles) in diameter. Congress had set the original deadline as 2008, but the work continues as astronomers keep discovering and learning more about these enigmatic rocks. The second survey, the George E. Brown Jr., Near-Earth Object Survey, seeks to detect 90 percent of near-Earth objects 459 feet (140 meters) in diameter or greater by 2020. Both surveys rely on powerful telescopes to repeatedly scan large areas of the sky.

As of March 2012, those telescopes had discovered 8,818 near-Earth objects. Almost 850 of those NEOs were asteroids with a diameter of approximately 1 kilometer or larger. Nearly 1,300 were labeled as potentially hazardous asteroids, or PHAs. PHAs must be at least 492 feet (150 meters) wide and must come within 4.65 million miles (7.48 million kilometers) of Earth [source: NASA]

Now, if you're prone to panic, remember that the key word is "potentially." Not every space rock that makes a close approach to Earth will make an impact. Still, it's a sobering number, especially when you realize that the solar system likely contains hundreds of thousands, or even millions, of asteroids. How many have we just not seen? And how many will go unnoticed until it's too late?

As we grapple with that final question, we must face a harsh reality: Despite our best efforts, a catastrophic impact could be in Earth's future. Next, we'll consider a few civil defense strategies that might be necessary if an asteroid comes knocking.

So, the tether on your tether-and-ballast system got tangled. The gravity tractor wasn't built Ford-tough. What do you do now about that killer asteroid barreling toward Earth? Well, if you tried one of the mitigation strategies just mentioned, the asteroid is most likely (a) big and (b) far away. That gives you some time to prepare for impact, although you won't have any historical precedent to provide best practices.

In fact, many astronomers point to fictional accounts -- "On the Beach" by Nevil Shute, for example -- as the best source material about what we might do and how we might fare in a true global cataclysm. Clearly, astronomers would try to pinpoint where the asteroid would hit so ground-zero areas could be evacuated, and governments would try to build underground bunkers, store food and water, collect animal and plant species, and shore up the global financial, electronic, social and law-enforcement infrastructures. The impact of a smaller asteroid -- say, one about 984 feet (300 meters) wide -- could devastate a region the size of small nation.But a rock bigger than 0.621 miles (1 kilometer) wide would affect the whole world. A rock larger than 1.86 miles (3 kilometers) would end civilization [source: Chapman].

Tsunamis, firestorms and earthquakes might cause additional damage. Either way -- impact in the ocean or land -- public officials might only have days or hours to evacuate heavily populated areas. Millions of lives would likely be lost.

Given these scenarios, you can see why governments around the world are so interested in keeping asteroids far from our biosphere. You can also see why dollars don't always drive decisions -- because the cost of failure far exceeds the cost of even the most elaborate deflection concept.