Movies love to depict the end of the world. Some, such as “”http://www.imdb.com/title/tt1190080/“>2012”, present rather unrealistic scenarios while others, such as “”http://www.imdb.com/title/tt0120591/“>Armageddon”, although with scientific inconsistencies galore, describe a more likely picture. But worry not, the world is not going to end any time soon — at least not because of an asteroid impact.
NASA astronomers are keeping a sharp eye on the sky to map the orbits of nearby asteroids to reassure us that none of them is going to hit the earth in the next few decades. In the event that astronomers do find a potentially destructive asteroid coming our way, a few deflection mechanisms have even been considered. What is needed is to go from the paper to space to make sure these mechanisms work. But even that seems to be on its way.
Asteroids are small rocky bodies that orbit the Sun much like the earth and other planets do. Professional and amateur telescopes map the skies every night in search of these space rocks. Because astronomers understand exactly what physical laws govern the motion of these bodies, once they detect one, they need only a few observations to determine where and when it will be in the next few decades. Determining longer-term orbital paths is more complicated because, over time, the gravitational pull of nearby planets may alter the asteroids’ orbits. But that is why NASA closely tracks the objects it finds.
If there is a large enough body at risk of colliding with the earth in the near future and causing serious damage, alarm bells will ring. The current research program is designed to find the objects that pass within 50 million kilometers of our planet, a third of the distance from the earth to the sun. Of these, an estimated 1,050 have diameters of a kilometer or more; an impact by such a large asteroid could have worldwide effects. Fortunately, of the 903 large objects found so far, none is expected to hit the earth any time soon.
But most of us are aware that large asteroids do sometimes hit our planet. The most famous example is the impact thought to have caused the extinction of the dinosaurs about 65 million years ago. The 15-kilometers-wide asteroid is believed to have released a billion times more energy than the Hiroshima atomic bomb upon impacting the earth at Chicxulub in Mexico.
Small asteroids may also cause damage, even if only at a local or regional level. In 1908, an asteroid a few tens of meters across hit central Siberia near the Tunguska river. The resulting explosion, roughly 1,000 times more powerful than the Hiroshima bomb, flattened around 2,000 square kilometers of forest. It is not believed to have claimed victims; humans were lucky enough that the collision occurred in a deserted area as opposed to a densely populated city where millions could have been wiped out.
Fortunately, these events are rare. On average, Tunguska-sized asteroids collide with the earth every 200 to 300 years, while catastrophic impacts similar to the Chicxulub event are much more infrequent. Still, the earth is not free from danger. While worldwide damage is unlikely, an impact capable of causing local or even regional damage is much less so.
That is when the “deflection team” would come to save us. Given a decade or so warning, current technology offers a few solutions to turn the hit into a miss. One of them consists in changing the asteroid’s velocity by crashing large amounts of copper or lead into it. The closest astronomers got to testing this solution was when they crashed a spacecraft into a comet a few years ago. In that case the craft was too light and only negligible changes to the comet’s orbit have been detected so far.
The nuclear-weapons option is also viable, but not quite in the “Armageddon” way. It consists in setting off a few nuclear weapons above the surface of the asteroid to slightly change its orbit while making sure the object is not fractured. The shell of material resultant from the explosion would hit the surface of the asteroid facing it. By expanding and blowing off, the shell would be expected to produce recoil on the asteroid, changing its speed ever so slightly. If this change were to act for several years, the orbital path of the asteroid could be altered enough to avoid collision with the earth. The more spectacular alternative of blowing up the asteroid, popular in the movies, wouldn’t be as useful since the pieces from the explosion could still pose a threat.
While these options may work in theory, it is advisable to test them before they are actually needed to prevent a collision. But there is good news for those worried about future asteroid impacts. According to Russell Schweickart, a former astronaut now focusing on planetary defense, writing for the NYTimes a few days ago, “The White House Office of Science and Technology Policy has just recommended to Congress that NASA begin preparing a deflection capacity”. So it seems NASA may soon be testing these deflection technologies in space to make sure they will work if a collision needs to be avoided.
Some things are just like in the movies: the Americans are in the lead when it comes to saving the world from a catastrophe.