Asteroid impacts and EMPs

[Willowmound]

I'm trying to work out the range of the electromagnetic pulse created by an asteroid impact of a given size. So far I've been struggling. I know that a comet eight miles across, arriving at 135,000 mph, would send an EMP right around the world. But how far would the EMP of a comet or asteroid of, say, four miles across reach?

Any ideas?

[GeorgeK]

In the science fiction section there is a sub-forum called Science Fact. There are physicists who stroll by there on a regular basis.

[Mac H.]

I'm trying to work out the range of the electromagnetic pulse created by an asteroid impact of a given size. So far I've been struggling. I know that a comet eight miles across, arriving at 135,000 mph, would send an EMP right around the world. But how far would the EMP of a comet or asteroid of, say, four miles across reach?

Any ideas?At first read I can't see why an asteroid impact would have an EMP at all.

An asteroid impact is basically a kinetic weapon ... a lump of rock.

Even if it was electrically charged and generated a field as it moved through a magnetic field, it wouldn't have the sharp transients of an EMP - so it wouldn't damage power grids, etc.

There seems to be some arguments that ionization of the air will produce an EMP .. but that doesn't seem to make sense. I still can't see it have the sharp transients.

The chatter on the web that argues that EMP occur seem to be a little wrong.

eg:


In January 2000, a meteor only 15 ft (5 m) across entered the atmosphere and exploded over the town of Whitehorse in the Canadian Yukon. The blast created an electromagnetic pulse (EMP) similar to that of a high-altitude nuclear detonation and disabled a third of the region's electrical power grid.
- answer by Justine Whitman (http://www.aerospaceweb.org/about/bios/justinewhitman.shtml), 4 March 2007
Compare this with eye-witness accounts, however, and the 'a third of Canada's electricity grid was disabled' seems a little hollow:


http://www.ufobc.ca/yukon/ykfireballjan2000.html
The light was indeed very bright; one observer saw the meteor streaking by and a daylight sensor on a nearby streetlight caused the street light to turn off. Many described the color of the light as that from an arc welder. The intensity of the light varied probably due to the meteor coming apart. Each time a large piece would break off, new material was exposed presenting greater surface area for burn-up. Some witnesses observed smaller fragments within the tail of the main object.

Witnesses in Whitehorse and other communities heard a sonic boom a few minutes after the light show. Seismic information from Geological Survey of Canada indicated that a possible impact or a ground concussion from the sonic boom might have been registered in the Carcross Area. Indeed, witnesses in that community heard a very load explosion that caused snow to slide off some roofs.

Nope - no EMP which shut down a major part of the electricity grid!

I think people are just getting confused .. although I've been wrong before, and will be wrong about at least 3 things today. Maybe this is one of them.

Mac

[Willowmound]

Thanks for that answer. The notion of the EMP I have from a program called Super Comet: After the Impact (http://www.youtube.com/watch?v=zc86dSl7IDc), which I happened to find in its entirety on YouTube. It seems pretty well researched to me, featuring interviews with a number of scientists.

The comet in the program is eight miles across, though. That's a lot bigger than the 5 metre pebble that hit Canada. The lump of death I have in mind for my story should be about 6 miles across. When this thing hits Kansas, windows in London are blown in.