Day 2

As expected, Kathy has a plan. We’re going to measure the diameter of the crater the easy way – by trigonometry from one part of the crater, sighting onto an opposing part. On one part of the crater wall we mark out a rough 100 m baseline and then using giant protractors (knocked together Blue Peter style with a few bits of wood), we measure angles between the baseline and our target point at the two end points. The aerial view showed us how the crater was far from a perfect circle, so we know we should do the same measurements in as many different places as possible to get a decent estimate. Before hand we figured between five and ten measurements would be ideal; now on the ground we change our plan - we do one more for luck. I know, not very good but you should see the size of this thing!

Kate’s also having trouble taking this all in. It’s clear that a misunderstanding is creeping in. The challenge is what is the size of the object that hit, but me and Kathy keep going on about the energy, rather than the size, of the impact. That’s because it is the amount of the energy released by the impact that causes the crater, not simply its size. Two similar sized objects will create very different craters if they’re travelling at different velocities, and the combined effect of velocity and size is what we are meaning by energy. In the heat of Arizona, our explanations are getting frazzled. Kate can’t work out how we’re going to get at energy; to be honest, neither can we. We need to get back to the workshop and think of some way of getting at the explosive energy of the impact and then a simple way of extracting from that the size of the impactor body. So it’s back to the helicopter and off to see what Mike has been doing back at the ranch with his bucket of sand.

Mike, as it happens, has been doing some brilliant experiments dropping marbles and iron balls into flour and then sand at gradually increasing heights. This is great stuff- and really easy to try at home. If you do try it, you’ll find the same amazing thing that he did. That there is a nice linear relationship between crater size and drop velocity – his data points plot on a wonderfully convincing straight line. So, it should just be a simple case of scaling up from his small-scale results to our large-scale crater (once Kathy calculates the size). But I’m really not convinced that this ‘up-scaling’ will work. For me, the small-scale impacts are fundamentally different in terms of process to those that evacuate out the kilometre-sized craters. For one thing, Mike’s marbles don’t vapourise.

Enter a cowboy with a gun. We’ve got a hired hand to fire a bullet into sand to try to get a better simulation of a high-velocity impact and explosion. It’s still not perfect, since our bullet isn’t vapourising either, but we are certainly getting craters that are much larger than we would expect from the small bullets being fired. We plot it on Mike’s graph – it’s way off the line. The higher velocities are still not giving us the size of crater we need.

We’re at a bit of a loss, but while we’re stumped as to how best to proceed, we take time out to admire Jonathan and Ellen’s fantastic telescope and its incredible view of the Moon. Even then we can’t get away from our problem. All of the craters on the Moon appear perfectly round, but you might expect some to be elliptical since not all impacts would be direct hits - some would come in low angle. In fact, many of them probably do, but the explosion and the vaporisation of the meteorite body throws material out in all directions, producing a nice round crater. Back to that flipping problem. Perhaps a good night’s sleep will give me some inspiration.