The Earth is no stranger to asteroids and comets. Early in the Earth’s history, they may have helped deliver water and organic materials, sparking the development of life. But once life got its footing, the impacts did not stop—nor were they as beneficial. Asteroid impacts likely played a role in mass extinctions millions of years ago, and today they pose a small but significant threat to the future of our civilization.
Modern observing techniques have revealed thousands of near-Earth objects (NEOs) over the past 20 years, but many more are likely hiding in plain sight—if only we had the ability to see them. Still, even our incomplete knowledge of NEO numbers suggests that a severe impact—like the Tunguska event —could occur once every several hundred years. But even smaller objects can cause considerable damage on Earth. The object that exploded over the Russian city of Chelyabinsk on February 15 was only 18 m in diameter, yet it produced a blast wave that damaged buildings and injured some 1,500 people.
Currently, we think catastrophic impacts—those from objects 1 km across or more—occur around every 600,000 years. Fortunately, no one place on Earth is more prone to impacts than another. That means the probability of an object striking a populated area is simply the ratio of urban areas to the total surface area of the planet. As you might guess, this ratio is increasing as our population grows and becomes more urban, but it is still very small.
That said, we shouldn’t be indifferent, either. Preventing serious disruptions to our way of life will depend on developing ways of dealing with the impact hazard. To safeguard Earth from NEO strikes, many spacefaring nations have been considering programs to detect and deflect asteroids and comets. One of those isNEOShield , an international collaboration established in January 2012, a project that I coordinate from my position at the German Aerospace’s (DLR) Institute of Planetary Research in Berlin, Germany. In total, 13 partners from research institutions and industry are investigating means of preventing impacts of NEOs. The European Union is supporting the project with 4 million euros, while our partners, which include organizations in the US and Russia, are contributing another 1.8 million euros.
Before we can successfully deflect an asteroid or comet, we have to know more about “the enemy”—its size, shape, rotation rate, density, composition, and so on. To start, we can look at existing data from the perspective of asteroid defense. With this, we can devise new observation techniques and mitigation strategies.
Here at NEOShield, we’re investigating a variety of possible mitigation techniques, including the “kinetic impactor,” in which we would ram a spacecraft into an asteroid, using the transfer of momentum to change the asteroid’s orbit slightly, causing it to miss the Earth. It’s not without risk, though. If we were to ram a small, loosely packed asteroid, it could break into many smaller, but still potentially hazardous, fragments. That’s one reason why NEOShield partners are carrying out laboratory experiments in which projectiles are fired at materials thought to be analogous to those in asteroids. The results of such experiments, together with computer modelling and simulations, will provide insight into how asteroids would respond to a kinetic impactor.
Another method is to use a “gravity tractor.” This technique would require us to steer a spacecraft close to a NEO, at which point the small but significant attraction between the spacecraft and the asteroid would work like a tow rope. With adequate forewarning, changes in the orbital velocity of the NEO of just a few centimetres per second or less would be enough to avoid a catastrophic impact on the Earth. The gravity tractor method would be relatively slow, but it would have one big advantage: we wouldn’t have to know anything about the interior structure or surface properties of the object because the surface would remain untouched.
The two approaches could be combined, too. For example, a gravity tractor could be deployed after a kinetic impactor has struck, allowing small corrections to be made to the NEO’s new orbit. A major goal of NEOShield is to develop detailed designs of demonstration missions to test mitigation techniques such as the kinetic impactor and gravity tractor.
When to Act
Not all asteroids on a collision course with Earth will require mitigation. The first NEO to trigger a space-borne mitigation attempt will most likely be 75–300 meters in diameter. Impacts of objects larger than this occur very infrequently—less than every 100,000 years on average—and smaller objects probably wouldn’t cause much damage. Plus, people in the affected area could be evacuated in time.
In the unlikely case of an extremely large, threatening NEO or very short warning time, we would have to resort to more desperate measures. In this case, a kinetic impactor or gravity tractor probably wouldn’t be sufficient to solve the problem. In those cases, we would need the greatest force we could apply in a short amount of time—a nuclear explosion. Obviously we have no actual plans to test that kind of mission, but we’re studying the possibility as part of the NEOShield project.
Though the probability of Earth being struck by a medium to large NEO is very small, the consequences could be catastrophic. Currently, there are heated debates over how much money should be spent on setting up a defense system. One solution is to treat it as an insurance question. What is the probable cost of ignoring the problem over a long period of time in terms of loss of life and property? We know from research that we should be spending much more than we currently do. NEO impacts are potentially catastrophic but easily preventable, unlike other natural disasters.
Perhaps that alone is reason enough for us to act. The Chelyabinsk fireball and the close flyby of asteroid 2012 DA14—both on February 15—are stark reminders of our Solar System’s violent history. There is no “if” regarding the next big impact on the Earth. It’s just a matter of time.