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U.S., Russian sattellites crash

Soyuz-Apollo, collider edition



It's been the first time when full-fledged satellites collided. Both were communication satellites - one ton (Kozmos 2251) against half a ton (Iridium LLC, both above Siberia). The Russian vehicle won, turning Iridium into smoke, but its own fate wasn't much better. ;-)

The debris may be a threat to ISS and other objects. When they determine the coordinates of the dangerous pieces, ISS may get ready for a slalom!

See also: The New York Times
So far, the density of objects in the orbit has been low enough that no one has bothered to write down any traffic rules for the satellites. I am afraid that this will become necessary in the future.

An order-of-magnitude estimate follows. Feel free to fix it if it is unrealistic.




If you allow their average velocity differences to be 15,000 km/h (the actual relative speed in this case was 40,000 km/h), it takes one tenth of a day for a satellite to probe all longitudes. If you assume the cross section to be 3 squared meters and divide it by the total cross section of the space for orbits - say 10,000 km (latitude) times 1,000 km (vertical) which is 10^{13} squared meters, you deal with 3 x 10^{-13} of the space every tenth of a day.

Because there are 3,000 satellites or so, we deal with 5 million pairs or so. Multiply it by 3 x 10^{-13} to get 1.5 x 10^{-6} per tenth of a day or 50% per century (and I could have added the factor of two or four somewhere, too). So it shouldn't be too shocking that we have already seen a collision. ;-) The space people should celebrate that no one has been killed so far and work on the traffic rules and synchronization.

What I have in mind is a simple NASA program that will predict the trajectories of those 3,000 objects plus a few thousand of annoying pieces for a few orbits in advance and will recommend satellites to change the velocity so that they will be a few meters away from the other object at the time of the expected collision. ;-)

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