Spy Satellite to Hit Earth by late February to March Options

[Tman]

Time and location of this first shot down attempt is well chosen. There's a graphic here (text in German) that show roughly the orbit(s) (in orange) after the possible hit. The most part of the following two orbits are over the sea and the rest good enough for military secrets. The large green colored circle shows the area where the satellite (or debris) can be seen from the ground.

[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

[ugordan]

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

I believe the space.com story doesn't mean the impact will bring down the fragments, it will lower the perigee so it runs into the denser atmosphere much sooner. Hence the "speed up its tumble through Earth's atmosphere". It's inevitable the fragments will be robbed of some energy by the impact in addition to increased drag due to fragmentation you mention.

[helvick]

I know this is a stupid question and I'm sure I should know the answer myself but I'm having trouble getting my head around it. Lower orbits have a higher orbital velocity than higher orbits so if this exercise is (as seems likely) going to be a head on collision then it will slow down the whole satellite and probably seriously slow down quite a lot of the debris. Ignoring drag which I fully accept will act much more effectively on the resulting impact compromised debris why doesn't this deceleration cause the whole thing to end up in a higher orbit?

[ugordan]

why doesn't this deceleration cause the whole thing to end up in a higher orbit?

Because this event will take away from the total energy of the satellite. Higher orbits have higher total angular momentum, but a lower kinetic energy. If you very gradually take away energy from an orbiting satellite (say in a circular orbit via air drag), it will actually be speeding up a bit because the orbit will effectively remain circular and just the radius will decrease. Overall, it still loses energy because the gravitational potential energy drops off more rapidly than the kinetic energy is increasing.

It's a bit counterintuitive, but true. A lower orbit is a higher speed one, yet one with a lower angular momentum. In our solar system, the majority of the energy is carried by the outer planets, not inner ones.

[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.

[stevesliva]

I heard somewhere that the KV was only 10kg... can't find the discussion now. Some more info here:
http://blog.wired.com/defense/2008/02/the-weapon-that.html

Ahh, here it is, in the FAS anti-ASAT take:
http://www.fas.org/blog/ssp/2008/02/us_pla...ti-satellit.php

Says the interceptor is 20lb.

[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.

[rlorenz]

A couple of things crack me up about this whole business

1. the hydrazine cover story..... as if stuff like this doesnt re-enter all the time. If anything gets decomposed
or dispersed during the entry, it'll be the hydrazine. This has to be a total non-issue that the spin doctors have
figured would be a good angle for public consumption (NB hypergolics are a problem for launch failures
where their toxicity etc sticks around, but for hypervelocity entry, I think not....)
2. the way all the TV coverage seems to use Magellan and Cassini footage or models.... I guess to a lot of
people a spaceship is a spaceship...
3. And now they are talking about delaying the shot because of weather... let's hope the Iranians/Koreans/whoever
don't play nasty and try to attack us on a cloudy day........ :-)

(I know, I know, it's for the observations to see how it all went, not for the intercept itself...)

[Jim from NSF.com]

A couple of things crack me up about this whole business

1. the hydrazine cover story..... as if stuff like this doesnt re-enter all the time. If anything gets decomposed
or dispersed during the entry, it'll be the hydrazine. This has to be a total non-issue that the spin doctors have
figured would be a good angle for public consumption (NB hypergolics are a problem for launch failures
where their toxicity etc sticks around, but for hypervelocity entry, I think not....)

You haven't been keeping up with things, it is solid hydrazine. Many propellant tanks (Delta II, Columbia, etc) have survived hypervelocity entry

[helvick]

ugordan - yeah I got a better quality envelope and it looks like the orbiter perigee is probably correct. It surprises me that even a small 10kg impactor could actually bring down a 2600kg satellite from a low earth orbit, even if there was no atmosphere involved. But it does appear to be true. Quite cool actually. Now let's see if it actually works.

[Jim from NSF.com]

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.

But the hit isn't head on. It is around 90 degrees. the missile and warhead fly straight up into the path of the satellite. Both vehicle are destroyed by the kinetic energy into small fragments.

[Pavel]

http://www.spaceref.com/news/viewpr.html?pid=24802

So we're pretty comfortable right now that we'll have windows available to us through about the 29th or 30th.

I'm not sure they'll have a good chance to hit it on February 30.

[centsworth_II]

MSNBC.com and CNN.com reporting a hit.
No story, just the headline.

edit:
AP story now out: "The operation is so extraordinary that Defense Secretary
Robert Gates, not a military commander, made the final decision to pull the trigger."

This post has been edited by centsworth_II: Feb 21 2008, 03:59 AM