Sixty-five million years ago, a Manhattan-size meteorite traveling through space at about 11 kilometers per second punched through the sky before hitting the ground near what is now Mexico’s Yucatán Peninsula. The energy released by the impact poured into the atmosphere, heating Earth’s surface. Then the dust lofted by this impact blocked out the sun, bringing years of wintry conditions everywhere, wiping out many terrestrial species, including the nonfeathered dinosaurs. Birds and mammals thus owe their ascendancy to the intersection of two orbits: that of Earth and that of a devastating visitor from deep space.
We humans need not wait, like dinosaurs, for the next big rock to drop. We have an advanced understanding of the heavens and a spacefaring technology that could soon enable us to alter the orbits of any celestial object on a collision path with us. That capability just might come in handy.
We got a taste of the challenge in December 2004, when scientists at NASA and the Jet Propulsion Laboratory (JPL), in Pasadena, Calif., estimated there was a nearly 3 percent chance that a 30-billion-kilogram rock called 99942 Apophis would slam into Earth in 2029, releasing the energy equivalent of 500 million tons of TNT. That’s enough to level small countries or raise tsunamis [PDF] that could wash away coastal cities on several continents. More recent calculations have lowered the odds of a 2029 impact to about 1 in 250 000. This time around, Apophis will probably miss us—but only by 30 000 km, less than one-tenth of the distance to the moon.
But let’s not rejoice too quickly. We know next to nothing about that asteroid’s porosity, composition, and tensile strength. It’s possible that tidal stresses during its 2029 approach could cause it to break apart, adding to the odds of an Earth impact during another rendezvous further down the line.
There is some disagreement about the best course of action. In the United States, experts tend to want to experiment with various deflection techniques by first sending robots or even astronauts to asteroids that do not threaten Earth. But in Russia, many asteroid watchers believe the risk of a collision between Apophis and Earth has been underestimated. These analysts contend that we should therefore concentrate our experiments on this particular asteroid.
To be sure of diverting any interplanetary intruder, we would need several strings to our bow. A method that could swiftly deflect a hunk of iron might blow an icy rock into several parts, each of which could then become a danger. And the gentler method now being discussed—to vaporize part of the surface of the asteroid, creating an outpouring of gas that would generate a propulsive force—would do no more than warm a meteorite made of iron. So we’ll doubtless need to devise several strategies for dealing with threatening asteroids.
So I have proposed a new tool, one that would use the pressure of light to nudge threatening objects into safe trajectories. That I’ve been asked to explain it at all in a magazine article shows that there’s indeed one thing we can rejoice in: the enhanced awareness of the problem. The mention of killer asteroids no longer raises jeering comparisons to the cries of Chicken Little, now that we know celestial impacts are far more common than once thought.
The largest and most famous Earth impact in historical times occurred in Tunguska, Siberia, in 1908, when an object perhaps 30 meters wide entered the atmosphere and exploded aboveground, with the strength of several megatons of high explosive. It leveled forests and dispersed reindeer herds, and the dust it kicked up produced colorful sunrises and sunsets throughout Europe. Fortunately, the devastated area was sparsely populated, so few people were hurt.
Astronomers now know a good deal about the nature and location of objects posing threats of both the Yucatán and Tunguska kinds. Some of these objects are comets—celestial icebergs that spend most of their time in the depths of space far from the planets. At intervals of 100 000 years or more, stars may approach our solar system closely enough to disrupt the solar orbits of some of these comets, pushing them sunward. They would then swoop through the inner solar system at great speed. It is not impossible that such a comet is what destroyed the dinosaurs.
The main threat comes from what are known as near-Earth objects. They usually reside between the orbits of Venus and Mars, although their orbits aren’t very stable. Most are eventually flung out of the solar system, but replacements wandering in from the main asteroid belt maintain their population. Some 7000 near-Earth objects have been identified so far. As many as 100 000 more, all larger than the Tunguska object, may await discovery. This guesstimate, by analysts at JPL, is based on the assumption that astronomers are far better at spotting mountains than molehills.