There are billions of small asteroids and comets in the solar system. Of these, only a small fraction actually cross Earth's orbital path, and only a small fraction of those are large enough to survive a descent through the Earth's atmosphere. Nevertheless, it is possible - and, in the past, has happened every few tens of millions of years - that a large enough object could collide with the Earth, unleashing a catastrophic wave of destruction. The Torino Scale is currently used by astronomers to measure the likelihood that an object will collide with Earth and, if the likelihood is high, what should be done about it.
- About Asteroid Impacts -
Every year, enormous numbers of small space objects enter the Earth's atmosphere; many of these, small meteoroids, burn up in dazzling displays called meteor showers. However, sufficiently large objects would not burn up during entry. Indeed, especially large objects - those several miles wide or more - could, if they collided with Earth, unleash a force equivalent to millions or billions of nuclear weapons, cause devastating megatsunamis, and scatter enough dust and debris into the atmosphere to cause rapid and dangerous global cooling.
Fortunately, there are fewer such large objects in the inner solar system than there used to be early in our solar system's development, and in the vast majority of cases our atmosphere protects us. (One need only look at the scarred surface of the Moon to see how important the atmospheric shield is.) Nevertheless, once in a while a dangerously large object passes by the Earth. Studying Earth's history, we can begin to guess how devastating a major collision would be. The 110-mile-wide Chicxulub crater in Mexico, for instance, is believed to have been caused by an asteroid about 6 miles wide, and was probably the cause of the extinction of the dinosaurs.
According to NASA's Ames Research Center, there are 1100 near-Earth objects more than one kilometre in diameter (enough to pose a serious threat to us), and more than a million larger than 40 metres wide (enough to get through the atmosphere and potentially unleash a force equivalent to a large nuclear explosion, as occurred at Tunguska, Russia, a century ago). NASA estimates that a collision with a 2-kilometre object will occur as much as twice every million years, and release about a million megatons of destructive force. NASA maintains a growing catalogue of such objects as part of its Spaceguard Survey, though there is still a real chance that an asteroid on course for Earth would actually strike without being detected.
- About the Torino Scale -
The dinosaurs, of course, never saw the asteroid coming. However, with current telescopes and our knowledge of orbital dynamics, there is at least a reasonable chance that professional and amateur astronomers would be able to spot such a large object well in advance, and predict with some likelihood whether it will strike the Earth. Over the past twenty years, NASA has used one particular tool, the Torino Scale, to measure the likelihood of an asteroid impact, as well as its potential severity.
The Torino Scale is a ten-point scale, between one and ten. (Objects also receive a "zero" rating on the Torino Scale if they have been identified as possible threats, but then ruled out by subsequent calculations or estimated to be so small that they would be harmless.) Objects are rated a "1" on the scale if there is a very small likelihood that they will pass near or even collide with the Earth, but scientists are confident that updated calculations will rule out any danger. Currently, NASA's Sentry Risk Table list one object, 2007 VK184, at risk level 1. This 130-metre wide object has a very low chance of striking Earth in 2048, and, if it did, would cause enormous but not global devastation.
Levels 2 through 4 on the Torino Scale indicate objects which have an unlikely but not minimal chance of striking Earth - generally, calculated as 1% or greater. According to NASA, there is no reason for public concern about such objects unless the impact might occur within the next decade. Currently, NASA lists no objects at this level of the Torino Scale. However, the European organization NeoDYS currently lists asteroid 1950 DA as a 2 on the Torino Scale, because there is a chance the object could strike Earth in 2880. In 2004, the asteroid Apophis was assigned a Torino Scale rating of 4 because of a 3% chance it would collide with Earth in 2029, with a force equivalent to 500-1500 megatons of TNT (in comparison, the last serious impact, at Tunguska, was about 10 megatons). However, subsequent observations have ruled out any chance of Apophis hitting Earth, so it has been redesignated at level 0 of the scale.
The next two sections of the Torino Scale, levels 5 through 7 and 8 through 10, are indications that a very large object is likely to strike the Earth. Levels 8 through 10 indicate that scientists have calculated that the collision will definitely happen, causing localized destruction or tsunamis (level 8), destruction of an entire region (level 9), or destruction of human civilization (level 10). As of June 2010, no object has ever been given a score above level 4 (Apophis) on the Torino Scale. However, NASA recommends that any object given a Torino rating of 5 or above should result in emergency government planning to deal with a potential impact.
- What Would Be Done? -
The Torino Scale, and a more technical version known as the Palermo Scale, are a basic tool for risk assessment. However, this does not help much if an incoming asteroid or other space object is detected, but nothing is done to deflect it or to mitigate the risk to life on Earth from a major collision. For better or worse, the lack of international governance on space issues means that it could be very difficult to decide which countries would do what in the event of an emergency.
However, the American government has devoted very small amounts of funds to developing ideas of what might be done, technically, to save the Earth from a major collision if an incoming asteroid was detected far enough in advance. These so-called asteroid defense schemes generally involve using unmanned spacecraft to knock the object slightly off course, missing the Earth - not quite so exciting as Deep Impact or Armageddon, but hopefully safer and more effective.