Look out below!

In 1908, the skies over Siberia lit up in a sudden and massive explosion: an asteroid, 40 m wide, had entered earth’s atmosphere and was breaking up in a multi-megaton burst. Although the asteroid itself didn’t make it to the ground, the shock wave and massive fireball that resulted destroyed 2,000 sq. km of forest, laying waste to the ground below. The Tunguska Event, as it’s called, took place in a remote area, so no human lives were lost. If the blast happened over Toronto, London or Shanghai, it would be another story.

Thousands of asteroids, most of them untracked, swarm around our planet; some are over 10 km wide. “Right now, the most probable amount of warning we’ll have for an asteroid impact is zero, because we don’t know where most of them are,” says Robert Jedicke, 46, a University of Hawaii astronomer originally from Niagara Falls, Ont. Jedicke is part of a team at UH’s Institute for Astronomy that’s working to change that. A new program, called Pan-STARRS, will combine the world’s most powerful asteroid-tracking telescope with the largest digital camera ever built. The first of four planned telescopes is set to begin its full scientific mission any day now. “In the past 200 years, we’ve discovered half a million asteroids,” he says. The first telescope alone “should find a comparable number in a single year.”

Asteroids, which are leftovers from the creation of our solar system, pepper our planet more often than most realize. “Basketball-sized objects come in every day, and Volkswagen-sized objects come in once or twice a year,” says Don Yeomans, who manages the Near-earth Object Program Office at NASA’s Jet Propulsion Laboratory. Those under 25 m in diameter cause little damage. (Sand-sized particles burning up in the atmosphere can be seen from the ground as “shooting stars.”) Events like Tunguska happen two or three times every 1,000 years. Asteroids one kilometre across or wider strike our planet only about once or twice every million years, but their impact is devastating on a global scale: 65 million years ago, a 12-km asteroid crashed into earth, killing off 70 per cent of all species, including the dinosaurs.

In 1998, NASA set about discovering and tracking asteroids one kilometre in diameter or greater. Scientists now say they’ve found about 83 per cent of them, and that none threaten us within the next century; but when it comes to those under one kilometre—including potential city killers, like the one at Tunguska—it’s another story. A 140-m asteroid “packs about 250 megatons of equivalent energy,” Yeomans says. “Even five megatons is a substantial nuclear weapon.” The U.S. Congress is now pushing NASA to find asteroids that measure 140 m and up. It’s these objects that Pan-STARRS will be hunting for.

Tracking asteroids, which are only visible by the reflected light of the sun, requires a telescope with a wide mirror to concentrate light: the fainter the object, the larger the mirror that’s needed. Instead of building one giant telescope, which can be prohibitively expensive, Pan-STARRS will combine images from four smaller ones watching the same patches of sky. (PS1, the first of the four, is ramping up to its full-time mission now. All four telescopes, called PS4, will be in use within the next few years.) At a cost of roughly $10 million apiece, they are as powerful as a single 3.5-m telescope, at half the price.

Within each telescope will be a 1.8-m mirror and the biggest digital camera ever made, with 1.4 billion pixels over an area of 40 sq. cm. (Your average digital camera has about five million pixels on a chip just a few millimetres across.) “The sheer amount of information we’ll be able to generate is amazing,” says Jedicke, who notes that each telescope will gather about six gigabytes of image data per minute—enough to fill up a typical laptop in under an hour.

Asteroid detection isn’t the program’s only goal. Within the next 10 years, Pan-STARRS should be able to catalogue up to a billion stars that have never been seen before within our galaxy, the Milky Way, and a billion new galaxies, too, says project manager William Burgett. It will be searching for comets, brown dwarfs (celestial bodies that are smaller than stars), and unknown planets: Burgett expects to find up to 70 new planets within the next few years. Pan-STARRS will also search for supernovas in distant galaxies. Because these exploding stars have a constant brightness, and thus seem dimmer when they’re at a greater distance, supernovas help scientists track the universe’s expansion. The project, he says, will provide “huge amounts of information about how our universe works.”

And what if Pan-STARRS finds an asteroid on a collision course with earth? As long as we have enough warning, it shouldn’t be a problem, Jedicke says. In 2005, for example, NASA scientists successfully collided a spacecraft called Deep Impact with a comet. “If we can do that, we can slam a bomb into it, and blow it up,” Jedicke says, although he admits that shooting nuclear weapons into space isn’t the most popular choice.

Scientists could also position a large spacecraft near an earth-threatening asteroid, creating a slight gravitational tug that would pull it off-course. They may have a chance to test this method soon: a 270-m asteroid, called Apophis, is set to make a close approach to earth 20 years from now. Computer models show that, if it passes through a specific 600-m gap, Apophis could swing back to strike the earth in 2036. “If we can move it out of that keyhole, just a little bit, it’ll miss us,” Yeomans says, noting that a gravity tractor might be a good way to do it.

How many other Apophis-size rocks are on a potential collision course with earth? Nobody knows for sure. “One of my colleagues observed that there are more people working in a single McDonald’s than there are trying to save civilization from an asteroid,” Jedicke says. Pan-STARRS will help us find those asteroids, one would hope, before they find us.