What are the chances of an asteroid hitting the Earth?

Bill Read looks at the latest research into the potential threat from Near Earth Objects

Mention the words "meteor" or "asteroid" and everyone probably thinks of Hollywood disaster movies such as Armageddon or Deep Impact. But how much of these films are based on fact? Is there a real danger of an asteroid hitting the Earth? The answer is yes - there is a risk. In fact, the Earth is being bombarded with pieces of rock and metal every day. Most pieces are very small and burn up in the atmosphere as shooting stars, while larger pieces occasionally reach the ground as meteorites. The real danger comes from larger pieces of debris, known as Near Earth Objects (NEOs), which measure between 10 and 300 metres across and are believed to have originated from an asteroid belt between Mars and Jupiter.

When an NEO hits the Earth, it brings about widespread devastation, loss of life and possible climate change. It is believed that an NEO hitting the Earth 65 million years ago may have brought about the extinction of the dinosaurs. The last collision occurred in 1908, when a 100 metre-wide NEO crashed into an uninhabited region of Siberia called Tunguska, causing an explosion that burned an area of 2,000sq km. It is inevitable that, sooner or later, another impact will occur.

It is not known how many NEOs are currently in existence but, thanks to the introduction of new Earth- and space-based telescopes, more are being discovered all the time. In 1975 a total of 70 had been discovered but, in 2007, the latest statistics from NASA revised this figure to a much larger 4,559.

Once a new NEO is detected, the next stage is to determine where it is going and what orbit it is following. From this, a probability can be calculated of the likelihood of an NEO hitting the Earth; in most cases it is extremely low.

If an NEO is detected heading for Earth, the good news is that there are feasible methods to avert a collision. However, fans of Armageddon may be disappointed to learn that none of them involve sending Bruce Willis into space to try to imbed a nuclear bomb in an approaching meteorite. Trying to explode an NEO is not a wise countermeasure, as it will only result in the Earth being hit by several objects rather than just one.

The effective way to deal with an approaching NEO, according to scientists, is not to destroy it but to deflect it away from a collision course. This can be achieved either by speeding it up, so that it does not collide with the Earth but instead passes in front of it, or to slow it down so that it passes behind. The deflection does not have to be large, as a small course change will build into a large one over time.

A simple way to change the speed of an NEO is either to ram a spacecraft into it or hit it with an explosive device (a method known as kinetic energy deflection). However, for the best effect, the explosive device would have to be nuclear and runs the risk of breaking up the asteroid. Also, politicians and many other people and organisations are not keen on the idea of anyone deploying nuclear weapons in space.

A second, more complicated approach is that of low-thrust deflection, which consists of methods that continue to affect the speed of an NEO over time. These include attaching a solar electric propulsion unit to the NEO to push it or using "gravitational coupling", in which a spaceship flies close to an NEO and diverts it by using its own gravitational pull. Other ideas include altering the orbit of an NEO by reducing the amount of sunlight reflected off its surface, by either flying a reflector close to it or even (in the case of smaller NEOs) by painting its surface black. Alternatively, the speed of the NEO could be adjusted by firing lasers at it or even by attaching solar sails to pull it faster.

The practicality of sending unmanned missions to asteroids has already been proved. In 2005, the Japanese Space Agency's Hayabusa mission collected samples from the asteroid Itokowa. On 4 July 2005, NASA crashed a craft into the comet Tempel 1 on its Deep Impact Mission. However, space agencies are suffering from budgetary constraints for future missions. To succeed, it is vital that potentially dangerous NEOs are detected in plenty of time so that a deflection can be carried out well in advance - ideally 20 years - before a potential collision. In March 2007, NASA admitted that it had insufficient funds to find all the NEOs that might pose a threat to Earth, let alone prepare any countermeasures.

Of all the NEOs currently detected, one in particular is causing scientists concern. The object in question is the 280 metre-wide asteroid MN4 (now renamed Aphophis after the Greek name for the Ancient Egyptian god of darkness and chaos), which will have a very close approach (within 30,000km) to Earth on 13 April 2029. The probability of a collision varies according to different calculations but could be between 1 in 3,600 and 1 in 5,500. Additional radar observations in 2013 may alter this figure. However, if Aphophis flies through a particular "keyhole" in space in 2029, it greatly increases the likelihood (to 1 in 2,000) of the NEO hitting the Earth on the next encounter in 2036. If this were to happen, Aphophis would hit the Earth at a speed of 20km per second, releasing around 1,000 megatons of energy and causing huge loss of life and long-term effects on the environment. Deflecting Aphophis is feasible but action needs to be taken earlier rather than later.

One opportunity to intercept Aphophis using the kinetic energy deflection approach is the 2011 Don Quijote mission organised by the European Space Agency (ESA). The mission will comprise two unmanned spacecraft: Sancho, which could reach the asteroid in 2015 and go into orbit around it to study its composition, and an impactor, Hidalgo, which would leave Earth in 2015 and could be used to crash into the asteroid in 2017.

Whether Don Quijote goes to Aphopsis has yet to be decided; if it does, then ESA might save the world.

Bill Read is features editor of 'Aerospace International' magazine