

The 'shot' – that’s what Ambassador James Jeffrey called the decision to use an an Aegis SM-3 to try to shoot down satellite USA 193 in the next 3-12 days.

Holding the aside the politics of this — which are terrible — the briefing on debris risk left me cold. I have to say that I am very, very uneasy about this decision — our missile defense tests have been heavily scripted to minimize debris creation and modeling of debris creation isn’t an exact science.

The burden of proof really should be on these guys to demonstrate that the risks to the ISS and other objects in space are minimal.

General Carwright, to his credit, provided enough technical information to model the intercept. David Wright is working on that right now — for those of you who can’t wait, the important numbers are:

1. The intercept will occur at 240 kilometers (130 nautical miles)

2. The mass of the satellite is 2,300 kg (5,000 pounds)

3. The mass of the interceptor is 20 kg. (From CBO)

4. The closing velocity will be 9.8 km/s (22,000 mph), suggesting a virtually head-on collision.

Other pertinent observations. At 240 km, the satellite should be traveling 7.8 km/s; the SM-3 has a burnout velocity of 3 km/s.

I am very worried about the debris creation — particularly the debris that the light-weight interceptor will kick into higher orbits when it hits the massive (bus-sized) satellite. Think, as Geoff Forden suggested, of a ping pong ball hitting a superball.

Virtually all the debris should come down quickly. Cartwright said 50 percent would come down within two orbits, with the rest coming down in weeks and months. That seems plausible, at first blush.

But those two orbits could be hairy and some of the debris will remain in orbit. Michael Griffin, NASA Administrator, said there are “good times and bad times” to conduct the intercept, based on the position of the ISS but that “bad times are not all that bad” comparing the risk to an order of magnitude lower than flying the shuttle.

Last I checked, the Space Shutle's PRAN (probability risk assessment number, of complete loss of life) was 1 in 100. Extrapolating, there would be only a 1 in 1000 chance of wiping out the ISS.

Great.

Anyway, we should be able to get some real numbers in the next 24 hours.

– Jeffrey Lewis, cross-posted at ArmsControlWonk.com

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