Near-Earth asteroids, generally defined as asteroids with orbits that bring them within just tens of millions of miles of planet Earth, are one of the most important subjects of space research today. Charting the orbits of near-Earth asteroids is important for pure scientific curiosity, as well as the possibility that some of them could eventually be mined for precious metals. Much more importantly, however, it is vital to know the orbits of near-Earth asteroids in order to forecast whether any are likely to impact the planet in the foreseeable future.
The threat posed by near-Earth asteroids is very real. David King, of the Lunar and Planetary Institute in Houston, Texas, explains that although “impact events” caused by large near-Earth asteroids are quite rare, when they do occur, the consequences can be devastating. Many geologists and paleontologists believe that the Chicxulub impact in Mexico, by a 6-mile-wide asteroid, likely contributed to the extinction of the dinosaurs 65 million years ago. More recently, in 1908, a much smaller asteroid burst over the remote Tunguska region of Siberia with the force of a nuclear weapon. That asteroid was just a few hundred feet wide. Fortunately, it did not enter the atmosphere over a more densely populated region in North America or Europe.
Will another impact like Tunguska, or Chicxulub, happen soon? Without charting the orbits of near-Earth asteroids, it is impossible to know. Thousands of tiny rock objects enter the Earth’s atmosphere every year, causing nothing more harmful than shooting stars – flashes of light as they burn up in the atmosphere. Larger objects would not burn up harmlessly, however, and by the time they entered the atmosphere, it would be much too late to take any measures to protect humanity from harm.
NASA and other space organizations around the world have acknowledged the importance of charting the orbits of near-Earth asteroids. NASA now has a program, the Near Earth Object Program, devoted to tracking near-Earth asteroids and calculating the possibility of a future impact event. Over the past several years, the program has identified about 4700 potentially hazardous asteroids – ones that are large enough to cause regional or global damage, and which also pass within five million miles of Earth. (This may seem like a very wide envelope, but, to put it in perspective, it’s only 25 times as far from Earth as the Moon.) Extra effort is then put into charting the future orbits of these asteroids to rule out the possibility of an impact.
When this charting process identifies an asteroid which may pass very close to Earth in the near future, it is assigned a grade on a risk assessment scale, like the 10-point Torino Scale. Right now, NASA is tracking only two objects with a Torino Scale score of greater than zero: asteroids 2011 AG5 and VK184. Both of these have a score of just 1 point, however, meaning that an impact is very unlikely and that astronomers expect that they will rule out any chance of impact once they have a chance to refine their calculations. The highest level ever reached on the Torino Scale was asteroid 99942 Apophis, which was briefly given a score of 4 in 2004. If Apophis had struck the Earth, it would have released an amount of energy equivalent to hundreds if not thousands of nuclear bombs. Astronomers briefly thought there was a chance of an impact occurring in 2029, but subsequent calculations have all but ruled out any impact by Apophis.
One important argument against spending increasingly scarce taxpayers’ money on charting the orbits of near-Earth asteroids is that impact events are not only extremely rare, but that we currently lack the technology – at least outside of the movies – to do anything to prevent an impact anyway. The relevant technologies are in their infancy. However, Micah Shembri writes that there are several designs on the drawing board that could be used in an emergency, including kinetic energy impactors and gravity tractors.