The Doomsday Asteroid is coming. An immense boulder with our name on it is cruising through the Solar System, and we all know what will happen when it arrives.

Four and a half billion years ago, our moon was formed when a planetoid the size of Mars hit the early Earth, ripping a chunk out of it. For the first billion years of the Earth’s existence, comet and asteroid impacts kept its surface temperature too high to sustain life, but may have seeded the planet with the raw materials necessary for life to evolve soon after. Sixty-five million years ago, a giant asteroid impact near Chicxulub in the Yucatan was responsible for the mass extinction of all non-avian dinosaurs, along with many other animals and plants. In 1908, a much smaller meteor burst in the air a few miles above Tunguska, Siberia with a force a thousand times greater than the Hiroshima bomb, causing widespread devastation over hundreds of square miles. Impactors from space have already substantially affected Earth’s geology, biology and history, and there are plenty more rocks where those came from. It’s only a matter of time before another one comes to visit us. Catastrophically.

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How Big Is Big?

With an asteroid impact, size and location are everything. An asteroid ten to fifteen kilometers across, like the one that caused the Cretaceous-Tertiary Extinction, would set off global firestorms and earthquakes, and the dust it would kick up into in the stratosphere would cause a severe “impact winter” that could last many years. On land, these effects would lead to worldwide catastrophic extinctions. Sulfate aerosols, ozone destruction and smoke would raise the acidity of the oceans, so even creatures that swim or crawl underwater would not be immune. And if the asteroid fell into the sea in the first place, destructive tsunamis would flood hundreds of kilometers inland, drowning low-lying areas around the world. If this happens, it’s curtains for the human race, and for most of the other species we know and love. (If it’s any comfort, some mammals will probably survive. At least, they did last time, along with crocodiles and cockroaches.)

A smaller rock one to two kilometers across could still annihilate civilization. Direct earthquakes and fires would be limited to the area a few hundred miles from ground zero, but the dust would decrease sunlight to the level of a cloudy day almost globally, blighting world agriculture with summertime freezes. The sulfates, smoke and water vapor injected into the air would magnify these effects, and the ozone layer would probably be destroyed. Again, if our rock goes into the ocean, its shorelines would be flooded tens of kilometers inland. We’d probably all freeze, drown, or starve.

Naturally, the smaller the rock, the better our chances. An asteroid 300 meters across shakes the ground considerably but ignites only localized fires, and the dust is well below catastrophic levels. However, it still creates a crater five to ten kilometers across, which could cause extensive devastation to a heavily populated area, and with a sea impact the tsunamis would cause unprecedented flooding.

Did I mention that the Tunguska meteor was only tens of meters across?

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Identifying the Threat

How will we find out about the next deadly space invader? With blind luck, we’d get a few weeks’ warning.

We can do better than that. An international effort called the Spaceguard Survey has been underway since the 1990s to find and characterize all Near Earth Objects (NEOs) that threaten us. NASA, in collaboration with the U.S. Air Force, is the main supporter of NEO research in the U.S., and in 1998 Congress directed NASA to detect 90% of all Near Earth Asteroids over a kilometer across by 2008—a target which has recently been achieved. NASA’s Wide-field Infrared Survey Explorer (WISE) satellite has just completed an accurate census of Near Earth Asteroids, and soon wide-field surveys conducted by the Pan-STARRS Project and Large Synoptic Survey Telescope will increase our knowledge still further.

NASA’s Jet Propulsion Laboratory maintains an online list of local rocky space junk. As I write—October 2011—there are over 8,000 NEOs catalogued. Of these, 1,254 are described as potentially hazardous, meaning they exceed 150 meters and will swing within 7.5 million miles of Earth—less than 20 times the Earth-Moon distance, a close approach in cosmic terms. Most NEOs are in complicated but predictable orbits around the Solar System. In all likelihood, when the Big One is identified we’ll have decades, maybe even centuries to prepare.

All right. Let’s assume Spaceguard has detected the Doomsday Asteroid. We know its size and composition from its brightness and spectrum, and its ETA from its orbital elements. We face a global catastrophe. What now?

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Blow It Up

Obvious, right? A few well-placed nuclear bombs should do the trick. However, the obvious thing to do may also be the dumbest. Vaporizing a rock a mile across takes an immense amount of energy. Even if we have the power and skill to disrupt our earth-threatening asteroid, many of those chunks would still head straight towards us, potentially causing widespread devastation. Same problem with the “kinetic kill” approach, where we throw a heavy spacecraft or other large object into the asteroid’s path hoping that the high-velocity impact will break it up or knock it off target.

As it turns out, the math shows that, given a year’s advance warning, it takes roughly a thousand times less energy to deflect our doomsday rock than to disrupt it. Trying to blow up an incoming asteroid would definitely be a desperation measure, a hail-Mary play with a strong chance of (literally) blowing up in our faces.

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Push It Aside

This was the approach adopted by one of the earliest studies: Project Icarus, initiated at MIT in 1967 by Professor Paul Sandorff as a class assignment for 21 advanced system engineers. Their mission was to assume that the mile-wide Icarus asteroid was heading for Earth, and devise a means of stopping it using existing technologies. The final—rather ingenious—solution involved the launch of six Saturn V rockets within a six-week period, each armed with a 100 megaton nuclear bomb, accompanied by five Atlas-Agena rockets carrying probes to send feedback to the asteroid-killer team. Detonating the six bombs in succession, within a hundred feet of the Icarus asteroid, would vaporize its surface and knock the rest of it off course.

Forty-five years later, it’s still a better bet to use our nuclear missiles to shove the asteroid aside instead of blowing it apart. However, our lack of control is a serious concern. The risks of failure are still daunting.

A steady shove is greatly preferable to a short, sharp shock. We don’t need to tear the asteroid apart, merely delay it. The Earth is 12,700 kilometers across, and orbits the Sun at thirty kilometers per second, so it travels a distance equivalent to its diameter in about seven minutes. That’s how much leeway we need, and the more notice we have, the gentler the push needed to buy us that time. Just as an example: Given thirty years (and enough fuel), the Space Shuttle’s main engine, attached to a 1 kilometer asteroid and fired from its surface, could deflect it sufficiently to miss the Earth.

With enough lead time, another gentle alternative is to put a heavy spacecraft—a so-called “gravity tractor”—into orbit around the asteroid, powered by an ion thruster. Over a few years their mutual gravitational attraction will tug the asteroid gently off course. This method works just as well on a floating rubble pile; even a loosely-bound collection of rocks will be deflected by gravity. In a similar approach, an ion thruster can be aimed directly at the asteroid from a rather lighter spacecraft, providing a steady force to deflect it, while another thruster firing in the opposite direction keeps our spacecraft on course.

We might install a mass driver on the asteroid’s surface. A mass driver is a kind of electromagnetic catapult; a series of electromagnets fired in sequence along a long tube can accelerate a projectile in a magnetizable casing to high speed. As every action has an equal and opposite reaction, shooting a continuous stream of heavy asteroid-rock projectiles would exert a steady force in the opposite direction, shepherding the asteroid away from its disastrous date with our homeworld.

A hundred years from now, even grander options might be available. We might attach an immense solar sail to the asteroid and use the solar wind to “blow” it off-course, or use high-powered lasers to ablate the asteroid’s surface directly.

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What Are The Odds?

At last, some good news! According to our best estimates and the geological record, Tunguska-sized events happen only once every two to three thousand years. Roughly speaking, kilometer-sized asteroids hit Earth only once every million years or so.

Even better: In September 2011, NASA’s WISE mission revealed there are far fewer nearby asteroids with planet-destroying power than previously thought. The NEOWISE survey shows there are probably fewer than a thousand near-Earth asteroids larger than a kilometer; 911 of these have already have been found and tracked, and none pose any threat to Earth in the next few centuries. As for the even weightier dinosaur-killer class of asteroids, those over ten kilometers across, it’s believed that we have now identified all of them.

Plenty of smaller rocks remain. The expected number of midsize asteroids a hundred meters to a kilometer across has shrunk from 35,000 to 19,500, but the Spaceguard projects have only identified around five thousand of these.

Nonetheless, this is encouraging. It appears that the odds of Death by Meteor are miniscule, and that we should get lots of warning of such a calamity. But let’s keep watching the skies anyway, okay?