Those of us that study one aspect or another of the Earth impact hazard are sometimes asked, "If one of these dangerous space rocks really is on course for Earth, how much warning will we have before it collides with our planet?" The answer has been - and still is - that the most probable situation is a warning time of just a few seconds. That is, despite the excellent work of telescopic "Spaceguard surveys", most objects capable of causing a serious amount of damage have not yet been discovered. So the first the inhabitants of Earth would know is when the intruder from space starts to vaporise on its high speed passage through the atmosphere, producing an immense fireball. Admire the brief, spectacular show, and hope it lands or explodes far enough away that you escape the worst effects.
Astronomers' progress in finding the larger near-Earth objects in interplanetary space, although incomplete, has been substantial. As these objects, on impact, could produce devastation on a global scale, it seems fair to regard this part of the task as the most important. But what of lesser impactors whose effects on impact would still be destructive, albeit on a localised - up to continental - scale? These objects have lesser potential consequences, but they are much greater in number than the larger bodies. Not globally damaging, but more likely to happen.
The terminal explosion of the "Tunguska bolide", an event whose centenary is this year, flattened trees covering an area of two thousand square kilometres in the middle of Siberia. Back in 1908, the Tunguska object, which would have been the size of a large building, was not of course seen out in space before it hit the atmosphere creating a fireball as bright as the Sun. A hundred years later, a handful of Tunguska-sized asteroids have been spotted by telescopes, but most remain unseen.
The asteroid designated 2008 TC3, which made the news on October 6th before colliding with the Earth on October 7th was too small to constitute a part of the "Earth impact hazard". Such asteroids, around the size of a car, are generally prevented from reaching the ground intact by the atmosphere, which acts as a shield. 2008 TC3, for example, finally exploded in the atmosphere above Sudan. But this asteroid represents a world first in a way that has exciting implications for studies of the near-Earth environment.
Before 2008 TC3, there were two sorts of observation which were always distinct. Astronomers can use telescopes to see asteroids and comets out in space and visible by reflected sunlight. On the other hand, when grains or boulders in space run into Earth, the meteor phenomenon results. Even a solid grain millimetres in size can produce a meteor or "shooting star" visible to the naked eye. Larger boulders or even asteroids give correspondingly brighter meteors or fireballs.
However, every single meteor or fireball seen had been derived from a solid object that was essentially too small, and therefore too faint, to be picked up by telescopes while still out in space. Tunguska-sized asteroids, and also smaller objects, down to five or ten metres, have occasionally flown by our planet and been recorded by telescopes. More often (and the smaller the asteroids, the more often it is) they fly past unobserved. The much larger near-Earth objects reflect more light and so are easier to see, but fortunately they are lesser in number and do not impact so often. They have not done so in modern times. The dinosaurs had no Spaceguard telescopes to spot what was heading their way.
The smallest asteroids imaged in space have been no larger than the objects giving rise to the brightest, occasionally seen, fireballs. But never was the same asteroid seen both in space and on impact. The available observational techniques do not tend to allow it. After 2008 TC3 was found by the Mount Lemmon Survey in Arizona, and observed during the following several hours from Arizona and Australia, Steve Chesley of NASA's Jet Propulsion Laboratory was able to calculate that it would collide with Earth twenty hours after discovery. Finally an impacting asteroid has been tracked on its final approach to the planet. The eventual idea, for near-Earth objects in the destructive size range (Tunguska and above), will be to catalogue their orbits well enough to provide up to decades of warning time of a potential impact. Although astronomers still have much work ahead to attain that ideal situation, 2008 TC3 is a sign that we are on the way. This asteroid itself was not destructive, but as surveys improve we reach a stage where the identification of colliding objects in space before impact can sometimes (once, so far) happen.