Friday, January 12, 2007

Asteroid impact effects

A while ago, I wrote an (unpublished) science-fiction novel. Here is an extract where the two main characters are discussing what a grazing asteroid impact on the earth would look and feel like.
They both turn round on the log, and, with the sun on their backs, look towards London. Right on the horizon, Stephen can just make out tiny skyscrapers, a distant toytown.

“That’s right,” says Emma, “you can just see the towers at Canary Wharf, next to the Thames in East London. Do you know how far away they are? About 30 kilometres from here.”

Emma is warming to her theme.

“You’re trying to divert the asteroid to just miss the earth - what we call a grazing trajectory. You want to leave it as late as possible to minimise the chances of countermeasures, so you’re going for a very, very close encounter, right?”

Steven nods. That’s exactly the plan.

“Suppose you get things a little wrong. Suppose the asteroid were to come in over London, for example, with the ultimate grazing trajectory. It just comes really close to the ground and then flies out of the atmosphere again. Do you know whether that would be good or bad? What the effects would be?”

Steven hasn’t thought about it in detail. He considers: it’s two kilometres wide, weighs 33 billion tons and is travelling at 53 kilometres per second.

“It’s going to create one massive fireball and shockwave, and cause huge localised damage.”

“It’s twelve seconds to closest approach, Steven, and this is your first real sight of the asteroid. Look there, out over London.” she points. “ It’s currently 640 kilometres out, right there on the horizon, and it’s just hit the top of the atmosphere, 30 kilometres up. It looks like a brilliant point of light. That’s the plasma shock in front of it.

“Nine seconds to go now. It’s 18 kilometres up and 480 kilometres out. The asteroid now sits behind a 6 kilometres wide plasma disk at 6,000 degrees centigrade - the bow-shock. It looks like the sun, but it’s 50% wider, and it already feels twice as hot. It’s so hot you can barely stand it .

“Two seconds to closest approach now, Stephen and although it’s still 75 kilometres away and 440 metres up, it looks nine times wider than the sun and it’s beginning to loom. Another half a second and you burst into flames ... it’s eighty times the heat of the sun and all around you, the countryside is on fire.

“Do you know when it reaches London, right there on your far horizon? Just 570 milliseconds before it gets to you. At that point, the bottom of the plasma shock disk is only 70 metres above London’s streets and the disk is 23 times the size of the sun. It’s dominating your northern horizon. You’re already toast, of course, but steel structures around you are melting under 500 times normal solar intensity.

“In the next few tenths of a second, your remains will have the privilege of encountering the heart of a thermonuclear fireball, as the plasma shock fills the entire sky.

“It goes without saying that London would be completely destroyed, by the thermal effects, and then the blast wave. Most of southern England would be set ablaze and flattened in fact. And that’s an asteroid miss. Although the plasma shock front itself touches down, the asteroid never got closer than two kilometres up.”

“I should say, though, for completeness that the asteroid semi-disintegrates in this scenario. The combination of searing heat and heavy aerodynamic loading as it hits the denser atmosphere causes the surface to ablate and tear off. The asteroid tumbles and fragments, and pieces impact the earth at 50 kilometres per second despite the main bulk missing us. There are going to be a lot of Meteor Craters in Southern England and Northern France. "