[X] My Assistant

[Guest_Bobby_*]

Just read at MSNBC that a large U.S. spy satellite will hit Earth by the end of February or early March.
Better keep our hard hats ready???

Here is the article:

http://www.msnbc.msn.com/id/22857051/

[ElkGroveDan]

http://www.space.com/news/080219-satellite-shootdown.html

The collision between the fired missile and the satellite would not only break the massive hunk of metal into pieces but would also speed up its tumble through Earth's atmosphere.

"If you want to bring something down, you slow it down. You apply a force on it which results in it being slowed down and decrease in its orbit," Carrico told SPACE.com. "Right at that point where they want to engage [the satellite] is at the edge of the atmosphere, so you're bringing it down faster."


I believe that was the point I was trying to make above.

[Jim from NSF.com]

http://www.space.com/news/080219-satellite-shootdown.html

[i]The collision between the fired missile and the satellite would not only break the massive hunk of metal into pieces but would also speed up its tumble through Earth's atmosphere.


I believe that was the point I was trying to make above.

The force that is going to bring down the fragments is drag, not the impact. The fragments have a lower ballistic coefficent

[ElkGroveDan]

The force that is going to bring down the fragments is drag, not the impact. The fragments have a lower ballistic coefficent

You left out this part:

"If you want to bring something down, you slow it down. You apply a force on it which results in it being slowed down and decrease in its orbit,"

This is what I have been saying, and they weren't referring to atmospheric drag. The impactor will slow it down and it will begin to drop.

[helvick]

If I'm understanding ugordan's explanation of the dynamics correctly what will actually happen is that the ASAT will slow it all down a little. That situation is unstable so the debris cloud will fall lower into the atmosphere and speed up in the process as it swaps gravitational potential energy for kinetic energy.

The satellite is (believed to be) about 2600kg and is currently orbiting at around 7.8km/sec. I've no idea how the ASAT intercept is supposed to work but lets assume that they put it into a similar orbit moving in the opposite direction. The impact head of the ASAT is not likely to mass much more than 100kg and probably will mass less than 25kg if it's similar to the earlier US ASAT tests. For a perfect collision where a 100kg ASAT warhead transferred all it's momentum into the satellite the resulting object would be moving at 7.22km/sec. That's now unstable at that altitude and so it will fall until its rising velocity matches the orbital velocity at the altitude it finds itself. My [highly unreliable and full of gross simplifications] back of the envelope calculations put that at about 18 km lower down.

That alone will significantly increase the drag but the really important factor is that the end result of a successfull intercept will be a debris cloud that has a massively increased effective cross section compared to the original satellite and that is what will really bring it down fast.

[ugordan]

My [highly unreliable and full of gross simplifications] back of the envelope calculations put that at about 18 km lower down.

According to my fooling around with Orbiter, a 7.22 km horizontal velocity at 250 km altitude brings you 700 km below the surface at perigee.

Lithobraking is the word.

My back of the envelope calculation says a head-on collision with a static 100 kg impactor should give a resulting velocity of 7.5 km/s, not 7.22 km/s. If the impactor is 10 kg, that comes down to 7.77 km/s (a perfect inelastic collision). I haven't done a check for 7.77, but my gut feeling say that, too, guarantees reentry in less than half an orbit. Depending on how solid the impactor and the satellite are, the impactor might partially rip-through the satellite implying an even smaller deceleration.