Why are we asking this now?
Scientists at the US National Aeronautics and Space Administration (Nasa) have taken the first radar images of Asteroid 2007 TU24 which yesterday passed within 538,000km (334,000 miles) of Earth – about one and a half times the distance between the Earth and the Moon. The asteroid is roughly 250m (800 feet) in diameter, which is big enough for it to have caused a global environmental catastrophe, had it collided with our planet.
Many asteroids that pass close to Earth have orbital trajectories that make it possible for them at some point in the future to hit the planet, with potentially devastating consequences. Calculations, however, suggest that this was the closest approach to Earth that this particular asteroid will make in the next 2,000 years. So we are safe for the moment – at least from Asteroid 2007 TU24.
Are there many of these objects out there?
Yes, but the risk they pose is comparatively small, although not negligible. So far, scientists have identified more than a hundred asteroids that are large enough to pose a threat if they collided with Earth, and, crucially, which also orbit within a certain distance to the orbital trajectory of the Earth around the Sun. Nasa scientists have begun a project to identify, catalogue and track 90 per cent of all potentially hazardous objects greater than 140m in diameter by the end of 2020.
For these objects to be deemed potentially hazardous, their orbits around the Sun must pass within a certain distance to the Earth. This distance has been set at 0.05 astronomical units from the Earth – in other words, within 5 per cent of the distance between the Earth and the Sun.
How do scientists assess the risks?
They use a network of telescopes and radar installations. It takes a number of observations over a period of time to assess the orbit of the object around the Sun, and hence the risk that it may at some time make a close approach to Earth. Sometimes, when scientists discover a new asteroid, they can look at archived images of the sky to assess where the object had been before it was detected. This "pre-recovery" process is useful when determining the risk of a future collision.
The first thing scientists want to know is roughly when the asteroid will next cross the Earth's orbit around the Sun.
At this early point in assessing the risk of a direct hit, the scientists draw up an oval "target" known as the "error ellipse", which is the area of space adjacent to the Earth that the asteroid will pass through. If the Earth falls within this oval target, then there is a risk of a collision. As more observations are made, this error ellipse progressively decreases in size and the probability of a collision increases. Eventually, the Earth falls out of this error ellipse and the scientists can then confidently predict that there is zero risk of a collision.
This is why an initial observation of a new asteroid is sometimes followed by assessments of an increasing probability of a collision, followed by a sudden drop to zero when scientists refine their calculations and find that the Earth no longer falls inside the error ellipse.
What are asteroids?
Asteroids are essentially large chunks of rock, or space debris. Some scientists have likened them to "builders' rubble" left over from when the planets of the solar system were created more than 5bn years ago. The word asteroid means "star-like" because they can shine in the sky like points of light – in fact, they reflect sunlight, as they are cold, inert objects. They can range in size from specks of dust to giant boulders several miles in diameter. The largest known asteroid, Ceres, is 913km across.
Anything else we should worry about?
Comets are also space objects that pose a risk to Earth. These "dirty snowballs" are made of rock, ice and organic compounds. They may, in the past, have deposited water on Earth and even the building blocks of life in the form of simple organic substances. Comets are less likely to hit the Earth than asteroids, but when they do collide they can cause even greater damage because of their size.
What would happen if an asteroid struck the Earth?
It depends on how big it is. Objects less than a metre in diameter are quickly destroyed by the Earth's atmosphere, which burns them up as flashes of light. We call these meteor showers or "shooting stars". Larger objects can explode in the atmosphere, releasing enough energy to cause damage on the ground. Perhaps the most famous example of this was the explosion over Tunguska in Siberia in 1908 which flattened some 80m trees over an area of 2,100sq km. Luckily, the explosion occurred over a relatively uninhabited region. If it had occurred over London or Moscow, these cities would have been totally destroyed.
When an asteroid is larger than about 50m in diameter, there is a strong possibility of it reaching the ground to form a crater. This would result in millions of tons of soil and rock being vaporised or sent as debris into the atmosphere. In addition to the huge explosion, equivalent to several hundred nuclear bombs going off, the atmospheric debris would encircle the globe, cut out sunlight and cause an extended winter that would kill off many plants and animals. This is thought to have happened about 65m years ago, when a giant asteroid caused a mass extinction, including the demise of the dinosaurs.
Does anyone take the asteroid risk seriously?
The US Government is certainly taking it seriously by ordering Nasa to complete its survey of near-Earth objects as soon as it can – with 90 per cent of the survey completed by 2020. The Near-Earth Object Survey Act was signed into law by George Bush at the end of 2005. The British Government established its own Task Force on Potentially Hazardous near-Earth Objects in 1999, and the resulting report made 14 recommendations on how to track objects and assess the risks.
What can we do about it?
Scientists can only speculate on what we could do if a large object was ever identified as being on a collision course with Earth. Some have suggested the use of nuclear missiles to shatter an asteroid into pieces, but this could make matters worse, with tens or hundreds of sizeable fragments heading our way.
Another possibility is to nudge it off course with non-nuclear, "kinetic" missiles. A further possibility is to use a "slow push" method of deflection, such as the deployment of a space sail that uses the force of the solar wind, but this would require many years or even decades of planning and preparation.
So should we be worried?
* It has happened in the past, with devastating consequences for life on Earth
* The probability is higher than most people think, roughly equivalent to the risk of death in an air crash
* It is certain to happen at some point in the future; the only question is when
* In climate change and terrorism, the world faces much more pressing problems than asteroids
* There is nothing much we can do about the asteroid threat anyway, so why worry?
* The chances of the Earth being struck by an asteroid are extremely small