Finding the killer asteroid
NASA have just published a report on how the should look for asteroids that may be on killer orbits and collide with the Earth, like in the Hollywood blockbuster Deep Impact.
Space is unfortunately a big place and asteroids are pretty tiny by comparison, being only metres or kilometres across. This makes them very hard to spot. Despite this NASA estimates that there are around 20,000 asteroids bigger than 140 metres across that would wipe out a whole city if they hit the Earth, and we don't know where most of them are. Such an asteroid would leave a crater around 3 miles in diameter and destroy everything for miles around. We believe that a much smaller object, about 10 metres across, hit Siberia in the early 1900s and laid waste to 2,000 square km. We estimate that an object on this scale hits the Earth roughly once every 250 years. There may even be some huge asteroids bigger than 10 km on collision course with Earth. These are much rarer, hitting about ever 25 million years, but one of these would cause Tsunamis, climate change, earthquakes and volcanic eruptions, devastating whole continents.
The solution is to put a telescope in space to look specifically for them. But where? It seems that the best place is near Venus. This is because sometimes asteroids come from the direction of the sun, which makes them hard to spot, like a fighter pilot attacking from the sun. Because Venus is closer to the Sun than Earth we can spot any asteroids coming from that direction. Unfortunately this mission would cost about 700 million pounds (1 billion dollars) and NASA don't have the budget to do it. Until they do we'll just have to keep our fingers crossed that the next killer asteroid hasn't already got us in its sights.