A mountain of rock and iron is hurtling towards us from space. Apophis -- a 300-meter diameter asteroid -- is still millions of kilometers distant. But in 2029, it will make a spectacularly close passage by our planet. When it does, its orbit around the Sun will be affected. A shift of just a few hundred kilometers, and Apophis could return in 2036 to slam into Earth, creating widespread devastation.

Will Apophis pass through the "keyhole," the small area on its 2029 path that would cause it to hit Earth on its next orbit in 2036? We have to find out, because if an impact is likely to occur, we're going to need all the time possible to plan and implement space missions to deflect it away from Earth.

The most accurate way to track and determine the orbit of a potentially dangerous asteroid is to send a space probe there and "tag" it. But that is something that, right now, no one knows exactly how to do.

The Planetary Society, not content to wait for governments to come to the rescue, has come up with a plan to help advance our efforts to prepare for the inevitable -- whether it happens with Apophis in a few years, or another object a few decades from now.

With our members' support, The Planetary Society will challenge the most innovative and brilliant minds on the planet today to design a space mission to visit Apophis and "tag" it for tracking.

How Close is Close?

Our best information indicates that, on April 13, 2029, the asteroid will pass over London at less than one tenth the distance to the Moon and 4,000 miles inside the geostationary satellite orbit. In the fading twilight on April 13, Londoners will actually be able to see Apophis in the sky. They will have to look just to the west of due south, about 45 degrees above the horizon, to catch this magnitude 3 object (about the same as a medium-brightness star) as it passes behind Earth, headed toward the just-set Sun. It will dim slightly over the next 40 minutes as it moves almost horizontally to the west, passing closest to Earth in the west-southwest at 21:40 local time. The asteroid will pass over London at less than one tenth the distance to the Moon and 4,000 miles inside the geostationary satellite orbit.

What will be invisible to all of us on that evening is the 28-degree turn that Apophis will take as it whizzes past us. Apophis will end up in quite a different orbit on April 14 from what it had on April 12, shifting from an orbit 323 days long to one of about 428 days. Exactly what its new orbital period will be depends on precisely how far behind Earth it passes on April 13, and the result could be disastrous.

If, by chance, Apophis passes by Earth so that its new orbit has a period of about 426.125 days, the asteroid and Earth will come back to the identical orbital positions in exactly seven years. The big "if" in all this is the very low probability that the orbit of Apophis will end up not at 426.1250 days but, in fact, about 30 seconds shorter than that, or 426.1246 days. In that very specific and improbable instance, Earth and Apophis will come together on April 13, 2036 in a cosmic collision the likes of which happen here on Earth about once every 50,000 years.

This narrow "window" through which Apophis could pass to bring about such a collision is called a keyhole. The likelihood that Apophis will pass through this keyhole is extremely low, but it could happen, and the reason we have a program to discover and track near-Earth asteroids is to convert such statistical possibilities into measured certainties.

When Will We Know if We're Safe?

So will Apophis pass through this keyhole, or won't it? The answer is that we don't know yet. Although we have been tracking this asteroid since early 2004 and we have more data on it than on most near-Earth asteroids we've discovered, the data are not accurate enough yet to answer this question.

In order to know prior to 2029 that the asteroid will in fact collide with Earth in 2036, we have to have more accurate knowledge of its orbit than we normally would have -- in fact, thousands of times greater accuracy.

Using the best telescopes existing now, and allowing for inevitable uncertainties, we will be able to predict the probability of impact with MN4 to be no higher than about 1 in 150 by 2014, even if we're headed for a direct impact! Radar data that we hope to get in 2012 may help, but probably not enough to allow a clear choice.

In the current situation with Apophis, there are critical decisions to be made, options to be evaluated, and actions to be taken. One of those choices is to gather much better information about where the asteroid is going soon enough to do something about it, if necessary. By launching a scientific mission to Apophis, we can do excellent and valuable science, and in addition, we can know whether or not we'll have to deflect the asteroid.

It turns out that if you want to know the orbit (trajectory) of something in space, the most accurate way to do so is to install a radio transponder on it. That's what we do with our spacecraft; it's what enables us to fly cheaply out to Saturn (or wherever) by doing the same orbit-altering trick that Apophis will do using Earth in 2029. We can make these very clever orbit-changing maneuvers, swinging by Venus and/or Earth on the way out to deep space, because we know precisely where the spacecraft is. The trick, then, is to tag Apophis -- maybe by affixing a lander there, or orbiting the asteroid with some kind of beacon -- in order to know with certainty, by 2014, whether the asteroid is going to be a pest in 2036.

A mission to tag an asteroid has never been studied. The Planetary Society's Apophis Mission Design Competition will inspire people to come up with practical, do-able plans -- not only for the ideas people have already bandied about, but for those yet to be thought of.

We're going to back this contest with a $50,000 cash reward, along with the alluring possibility that NASA or another space agency will actually transform the design into a real mission.

Scientific and engineering competitions like this, as you probably know, have long been used to attract and inspire brilliant minds to solve tough problems. On first glance, asteroid tagging may seem simple enough. But right now, the space community isn't sure how to best do this.

We've already determined most of the criteria for the competition. Contestants must design a mission that can approach Apophis, place the necessary gear, and provide astronomers the information they'll need to determine if the asteroid is a real threat or not. The mission plan will need to meet realistic mission constraints -- from spacecraft mass, to overall cost -- and will need to be done quickly to give humanity time to mount a deflection mission, if it's needed.

At the conclusion of the Apophis Mission Competition, we'll debut the winning design at a conference attended by the space community's science and engineering professionals. And after that: it'll be on to government space centers to turn the top idea into an actual mission.

We already have in interest of NASA and ESA, and we're working with space agencies in Japan and Russia. We also have the support of the Association of Space Explorers -- the renowned group of Earth's astronauts and cosmonauts -- and the leading professional organization of aerospace engineers, the American Institute of Aeronautics and Astronautics.

In the end, Apophis may not threaten us at all. Once we've captured its trajectory precisely, we may see it as just another harmless denizen of our solar system. But Apophis is not alone: millions of asteroids are out there, tens of thousands of them periodically approaching Earth. Among them -- history is our witness -- a few will, sooner or later, pose a lethal challenge to humanity.

The rational thing to do is to take this opportunity to prepare, to get our plans in order, our responses ready. And that's what the Apophis Mission Competition is all about: "getting ready."