By Dan Durda, Southwest Research Institute, Boulder, Colorado
Reprinted from Astronomy’s 60 Greatest Mysteries; Sky & Telescope Media, LLC; Cambridge, MA; 2013, page 12-13.
OUR HOME WORLD orbits the Sun amidst a swarm of projectiles. Most of these diminutive denizens of near-Earth space are emissaries from the main belt of asteroids between Mars and
Jupiter, a reservoir of millions of rocky and metallic minor planets. Others may be the burnt-out nuclei of comets that wandered in from the outer solar system. These near-Earth objects (NEOs) interest astronomers as fossil building blocks of planet formation, but they concern everyone because they might impact our planet and cause widespread destruction.
Ongoing telescopic searches have revealed that there are about 1,000 Earth threatening NEOs larger than a kilometer (0.6 mile) across. Several tens of thousands of smaller, football-field-size objects approach or cross Earth’s orbit, although at present we have catalogued only a few percent of them.
These searches, known as the Spaceguard Survey, were prompted by a growing realization in the 1980s and ’90s that asteroid and comet impacts have affected life on Earth and pose a credible natural hazard. The impact of a Mount Everest size asteroid 65 million years ago wiped out most of the dinosaurs and some three-fourths of all species of plants and animals, clearing the way for mammals’ rise to prominence and our own evolution. In 1908, a football field-size asteroid exploded in the lower atmosphere over a remote area of Siberia, blasting flat 1,000 square miles of forest.
Fortunately, it’s highly unlikely that we’ll ever face the impending doom of an extinction-level impact. But it’s not out of the question that someday soon we’ll discover a smaller asteroid on a threatening path, dangerous enough to merit preventative action. How could we divert it from its collision course and prevent local or regional devastation?
Scientists and engineers have proposed a variety of ideas for deflecting a threatening asteroid onto a non-threatening trajectory. It’s a good idea to have a lot of options because we don’t know how much warning time we’ll have, or what type of object we’ll face. A technique or technology that might work well for one scenario might not work for another.
Hollywood’s favorite solution — nuking it to smithereens — might actually work, depending on the asteroid’s size and internal structure. But this big-hammer approach is inherently unpredictable and uncontrollable and the resulting shotgun blast of small debris will still be on a collision course for Earth. These and other problems make this our option of last resort.
We could slam a spacecraft into the asteroid to nudge it off course. We actually have some experience with this. On July 4, 2005, NASA’s Deep Impact mission flew an 815-pound (370-kg) copper impactor into a comet nucleus in order to study the comet’s structure and composition.
The impact changed the comet’s velocity by a minuscule amount. Like the nuclear option, though, we would not have much control over what happens to the asteroid after the impact. We could end up with several fragments, making the impact problem even worse, or end up accidentally pushing the asteroid onto a different impact trajectory and simply delaying the inevitable.
If — as is likely to be the case — we have significant lead time before the impact, slower, gentler deflection options would be a better bet and could even be used in concert with more powerful but less controllable techniques.
The gravity-tractor concept, proposed in 2005 by NASA astronauts Edward Lu and Stanley Love, is a particularly creative solution. Their idea is to park a spacecraft near the asteroid, angle its highly efficient solar- or nuclear-powered ion thrusters out to the side so that the plumes do not hit the asteroid, and hover beside the object for several months or more with long, continuous thrusting. The spacecraft’s gravitational pull on the asteroid will allow the vehicle to effectively tow the asteroid along with it and off the collision course, using gravity as
Since we don’t have to make contact with the asteroid, the gravity tractor provides an effective and highly controllable deflection technique. It can change the asteroid’s orbital speed in any direction required to most efficiently prevent an impact, and scientists can monitor the mission’s progress continuously and take corrective action if necessary.
Big rocks occasionally fall out of the sky, and the dinosaurs went extinct because they didn’t have a space program. But we have the option to use our knowledge of NEOs and develop appropriate space technologies to make sure we don’t suffer that same fate. The asteroid impact hazard is one natural disaster we can predict and prevent.