When we talk about impact craters, we conveniently refer to the obvious physical size of the crater that is formed. But the damage extends far further. As with a terrorist bomb, where a small crater in the road marks the site of the outrage, but damage extends across a whole shopping street, so it is with impacts. We should be careful not to let the relatively small size of the crater itself limit our perception of the risk from the explosion that creates that crater.

A recent THEMIS release on Gratteri Crater, Mars illustrates this dramatically. The full image is a 26 Mb download but the linked page allows you optional zooms through a flash interface and does not force the full image on you. It's worth a look, for many reasons.

This is one of the handful of Mars' craters that are recent enough and large enough to show visible rays. It must have been formed in the last few million years, although the exact date cannot be known.

The rays are the traces of jets of ejecta that have fallen back to the surface and scoured trails across it. Gratteri crater is 6.9km in diameter, the result of an impact with an asteroid (or cometary) fragment approx 350m in diameter, similar to many of the "small objects" detected in NEO surveys.

We might summon-up a mental map of our home country, or some other well-known area and imagine the horrific but limited damage due to that 6.9 km "hole". But the damage zone is far wider:-

For a distance of about 10 crater radii (i.e. some 70 km), the entire surface is scoured clean. pre-existing craters and surface fetaures are heavily modified. Relief of tens to hundreds of metres has been expunged, either blasted clean, or buried under the thick ejecta layer. No man-made structure short of a nuclear bomb shelter or a major dam embankment could survive at all in this zone. (But even if you were IN a nuclear shelter, you and your air pipes would be buried by tens of metres of rubble and red-hot ejecta.

In the next 70-100 km zone the rays begin to develop - they are lanes of more total destruction in a fabric of desolation. Again, almost all man-made structures would be reduced to their foundations and buried in rubble and ejecta in this zone. Beyond this, a blast wave would sweep the surface - the classic base-surge of a Nuclear detonation or a pyroclastic column collapse, and the rays extend out to at around 600 km from the impact one. Each ray is a swath of severe damage. With distance your odds of survival increase, as the rays become weaker and more separated. Probably, a rain of gravel and cinders would affect everything out to hundreds of km, starting fires in buildings, forests, and croplands on a regional basis.

The ensuing firestorm would probably be more lethal than the initial impact, for sites in the few-to-several hundred km zone.

Although the odds of such an impact are extremely low - recurrence rates are in the 1-10 million year range for one of this size on Earth, the consequences would be devastating to the region of impact and economic shock waves would engulf the entire Earth. it is important, then, to get impacts in perspective. The sky is not falling, so we should not act like Chicken Little, but the risk is not negligible either. It should be a strategic long-term goal of our space program to work towards identifying bodies that pose risks and developing strategies to mitigate that risk.