Comments on: SpySat update

By: Buzz Parsec

Buzz Parsec — Sun, 02 Mar 2008 23:14:26 +0000

Saying the hydrazine dispersed in the upper atmosphere isn’t really a very good description of the situation. What happened (most likely) when the tank was ruptured or destroyed by the collision is the hydrazine evaporated into a large cloud of gas which is still in orbit around the earth. As individual molecules in the cloud strike molecules of air in the upper atmosphere, they are moving at a relative velocity of 5 miles per second, so the collisions will rip the molecules apart, leaving an ionized cloud of hydrogen, nitrogen, and whatever air molecules it happens to hit (mostly nitrogen and oxygen, with a little argon, carbon (from CO2) and hydrogen (from water.) The ions, after repeated collisions, will slow down enough to merge with the air, and recombine to form N2, H2, H2O, etc. in the upper atmosphere. Small amounts would probably form various nitrogen oxides, ammonia, nitric acid, etc., but much less in quantity (and much less toxic overall) than the original hydrazine. It would take a while for the hydrazine cloud to reenter (or merge, depending on your point of view), but only a single collision between a hydrazine molecule and an air molecule would be enough to remove that hydrazine molecule from orbit, as opposed to the billions (trillions?) of collisions needed to cause a macroscopic chunk of the spacecraft to reenter.

By: Reed

Reed — Fri, 29 Feb 2008 05:59:41 +0000

Jeffersonian
b) The tank is believed to have been destroyed, as intended, dispersing the hydrazine in the upper atmosphere. See http://www.washingtonpost.com/wp-dyn/content/article/2008/02/25/AR2008022501053.html?hpid=sec-nation for example.

Given the energy involved, any successful impact would be expected to thoroughly disrupt the structure (i.e., “blow it up” even though the warhead isn’t explosive.)

Some parts of the satellite might survive re-entry, but less than would be expected to survive if it re-entered intact. Where they might re-enter is unpredictable.

A comment on c) the large/dense (technically those with the highest ballistic coefficient) objects would actually be the last to re-enter, because they are least affected by drag, and so their orbits decay more slowly.

By: John Phillips, FCD

John Phillips, FCD — Fri, 29 Feb 2008 05:36:23 +0000

Jeffersonian. Hydrazine is toxic, though you can argue how much, and the problem was that they suspected the full titanium tank with a tonne or so of fuel IIRC, was frozen solid making it much more likely to survive reentry and being released on hitting the ground. By the way, they weren’t aiming specifically for the tank, their targeting isn’t that good plus as I understand it the tank is pretty well protected, but just at the centre of the satellite as at the velocities they would meet the kinetic energy involved would very likely smash the whole thing apart. Which appears to be what has happened.

If the satellite or any substantial pice that remained after reentry hit the ground or water you would have little but debris of various sizes to recover and possibly an area of contaminated ground. They didn’t know where the satellite would fall as, unlike when previous satellites have come down, they have never had any control over this one and by know was in an unstable orbit where they could only make a rough estimate when it came down. I.e. if you don’t know when it is coming down you don’t know where it is coming down until relatively close to the event. Most previous large satellite downings have usually been under controlled conditions in the sense that the remaining fuel on board has been used to bring it down in safe areas such as an area of the Pacific. Which is probably where this one would have been aimed if they had had enough control over the satellite. Though not all have gone to plan and ended up where intended, such as Skylab.

By: Jeffersonian

Jeffersonian — Fri, 29 Feb 2008 04:53:09 +0000

I still don’t quite get the issue re: the fuel tank(s). The toxicity of concentrated Hydrazine is one of the reason for interception, right? But I’ve come across conflicting info; so maybe somebody here can help me. I’ve heard several versions:
a)The tanks were to be punctured so that the Hydrazine could leak and spread in the uppermost atmosphere
b)The tanks were blown and a cloud of Hydrazine was released and would then slowly disperse in the upper atmosphere
c)because the tanks were the largest/heaviest units on the satellite, they would likely remain relatively intact and therefore would be tracked for eventual retrieval on land or water
d)The tanks were supposed to drop in the Pacific
Anybody know?

By: James Reynolds

James Reynolds — Fri, 29 Feb 2008 04:44:12 +0000

I just looked at AstroProf’s page; a good explanation, but I have a sort-of-unrelated side question. Does AstroProf know that there was an ad for ‘Expelled’ on the sidebar? It isn’t the sort of ad that I’d expect to see on that sort of page, and the sight of Stein’s knees (in schoolboy shorts) will haunt my dreams!

BA, you know him, so maybe you could tell AstroProf about it? Oh, and I just referred the note about your lecture to my Niece Maggie, the Chemistry grad student at UC-Boulder.

By: Reed

Reed — Fri, 29 Feb 2008 01:02:42 +0000

Sorting out the fragments takes time, they may only have definitive orbital elements for the 17 most obvious ones. Bits have been reentering since shortly after the impact, so the population is quickly diminishing too.

@mangus
All of the fragments are so low that they well re-enter quite soon. That’s the fundamental difference between this and the Chinese ASAT test. Even though the impact could have added a lot of energy to some particles, they would still end up in an elliptical orbit that dips down to the the height they were at the time of impact. Orbital mechanics doesn’t let you raise your entire orbit with one short impulse. The cube/square relationship of mass vs. surface area means that smaller particles generally reenter more quickly than large ones would.

By: magnus

magnus — Fri, 29 Feb 2008 00:10:39 +0000

How do they know that there are still not dangerous fragments left after this shot? Did they only manage to track 17 out of thousands? I don’t like littering in general, and littering of space is the worst kind (i.m.o). Actually, at orbital velocities, a tiny fragment of paint is a lethal projectile for a space walker.