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On-orbit Satellite Collision
ngunn
post Feb 13 2009, 11:58 AM
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I don't get this (from spaceweather):

"This injection of debris substantially increases the population of space junk at altitudes near 800 km. Collisions are now more likely than ever. Fortunately, the International Space Station orbits Earth at a much lower altitude, 350 km, so it is in no immediate danger. The Hubble Space Telescope is not so safe at 610 km. In the days ahead, researchers will carefully study the make-up and dynamics of the debris cloud to estimate when bits will begin to drift down to lower altitudes."

If you collide two (equal) objects at 90 degrees you'd kill one-over-root-2 of the velocity right there. Far from expecting the debris to spread out at the same altitude and only drift down slowly I'd expect a lot of it to fall down pretty quickly.

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ugordan
post Feb 13 2009, 12:03 PM
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QUOTE (ngunn @ Feb 13 2009, 12:58 PM) *
If you collide two (equal) objects at 90 degrees you'd kill one-over-root-2 of the velocity right there. Far from expecting the debris to spread out at the same altitude and only drift down slowly I'd expect a lot of it to fall down pretty quickly.

You're assuming a totally inelastic collision. What appears to have happened is that two distinct debris clouds were created that roughly follow original objects' orbits.


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tedstryk
post Feb 13 2009, 12:53 PM
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QUOTE (AndyG @ Feb 12 2009, 02:52 PM) *
There's a trillion cubic kilometres of space in LEO, up to 2000km. Less than ten thousand objects above 10cm across.

A miss is as good as a mile in vacuum: I'm utterly stunned that this could occur. Paint flakes and the like, impacting in lower orbits, yes - but to quote Harry Hill...

"What are the chances of that happening?"


That math isn't really fitting, since satellites are not distributed randomly. Based on the launch site and purpose, certain types of orbits are used far more than others.


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ngunn
post Feb 13 2009, 02:34 PM
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QUOTE (ugordan @ Feb 13 2009, 12:03 PM) *
You're assuming a totally inelastic collision.


I know - a VERY crude approximation just to make the general point.

Kinetic energy was lost (a lot of it since both satellites were completely destroyed) so the mean velocity of the fragments must be less than it was before the collision. Most will therefore be moving too slowly to remain in circular orbit and so must drop down to a lower perigee at the antipodal point.
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remcook
post Feb 13 2009, 03:54 PM
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And that's the reason why they expect most bits to burn in the atmosphere relatively soon. Yet the apogee stays at the same altitude in that case. And some bits will probably gain a bit of momentum (for instance, if stuff like fuel explodes or just being ejected in the right direction) and move a bit in one direction and/or their orbital plane will change a bit and their orbital periods will be slightly different, so things will spread out eventually. The amount of energy lost during the collision is probably not so high as you imagine relative to the enormous amount of energy they had, since the material of the structures must not have given much resistance to such an impact.
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dvandorn
post Feb 13 2009, 05:38 PM
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The dynamics of the impact also affect the kinetic energy transfer. Remember, you don't just "lose" energy, it's translated into other forms of energy, other vectors, etc.

If the two bodies hit pretty much directly, then much of the "lost" orbital energy would be used up in melting, shattering and vaporizing the structures of the bodies. If the collision path would have only a half or a quarter of one of the bodies intersecting the other, less kinetic energy is used up in vaporization and the resulting debris' orbits are altered less than in a direct impact. If only a very small physical interaction occurs (i.e., if an antenna on one vehicle snags an antenna on the other vehicle), the two bodies will still break up (especially if the "just barely" contact comes at 11 km/sec!), but they'll break up due to the extreme rotational rates imparted by the "brush-by". This last scenario changes the orbits of the resulting debris the least.

I don't know if our "security assets" are able to image satellites at 800km range well enough to characterize the resulting debris from the Cosmos-Iridium collision, but the extent of debris and its orbital characteristics (as tracked by radar) ought to give us some idea as to how direct a hit they endured.

-the other Doug


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mcaplinger
post Feb 13 2009, 05:41 PM
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As noted by kwan3217, there is already a very nice service to look at collision probabilities: http://celestrak.com/SOCRATES

If you look at today's top 10, the highest probability is 2.785E-03 (1 in 359). This is certainly a lot higher than I would have expected. I wonder if Iridium was using this service, and if so how, and if they were concerned at all before the collision.


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Disclaimer: This post is based on public information only. Any opinions are my own.
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ugordan
post Feb 13 2009, 10:53 PM
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Via NSF.com - a couple of simulations showing debris evolution: http://www.agi.com/corporate/mediaCenter/n...iridium-cosmos/


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Geert
post Feb 14 2009, 02:42 AM
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QUOTE (mcaplinger @ Feb 14 2009, 12:41 AM) *
As noted by kwan3217, there is already a very nice service to look at collision probabilities: http://celestrak.com/SOCRATES


That is indeed a extremely interesting service, and I'm equally amazed at the results, didn't know also there were regularly so many encounters at such close ranges!

If the Iridium collision didn't even make it into the top 10 I'm not surprised that no prior avoidance action was undertaken, as has been mentioned already if you start maneuvering for each of these close encounters you will run out of fuel quite soon, and indeed it causes problems with your commercial service as satellites will be drifting out of their slots, etc, etc.

Still, now the question opens, if the Iridium collision did not make it into the top 10, then how accurate where/are the details of the orbits? Did either of the satellites recently manoeuvre ( I know the Cosmos satellite was dead, but it could have been losing fuel or some pressurized gas, causing a slight propulsion), how accurate is the tracking really and how often are those parameters updated?

At such speeds and with almost 90 degree intersection, you need only a very, very, minor tracking error and the situation completely changes...
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Vultur
post Feb 14 2009, 02:47 AM
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It will be interesting to see how (or if) regulatory bodies will respond. (For that matter, what regulatory body would have jurisdiction? The US government could make laws requiring the US companies to take precautions, but they have no authority over non-US companies, or the space agencies of other nations. It seems unlikely that the UN will respond, but you never know...)
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nprev
post Feb 14 2009, 04:22 AM
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Well, the right answer IMHO would be at this point to add it to the duties of the International Civil Aviation Organization (ICAO); they're sort of like an international version of the FAA. Any sort of regulation's gotta be multinational, obviously.


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ngunn
post Feb 14 2009, 01:41 PM
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From SpaceWeather:

"LISTEN UP: The US Air Force Space Surveillance Radar is monitoring the skies above Texas for echoes from satellite fragments. Try listening on Saturday, Feb. 14th between 10:45 and 10:55 am CST (1645 - 1655 UT). That's when Iridium 33 would have passed over the radar intact had it not been shattered."

On the spaceweather site 'Try listening' is an active link (I think).
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Leither
post Feb 14 2009, 08:23 PM
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QUOTE (Vultur @ Feb 14 2009, 03:47 AM) *
It will be interesting to see how (or if) regulatory bodies will respond. ...... It seems unlikely that the UN will respond, but you never know...)


Well we might not have to wait too long……in July 08 the UN General Assembly recommendation that the Scientific and Technical Subcommittee of the Committee on the Peaceful Uses of Outer Space should consider space debris as an agenda item at it next session. The next session (the 46th) started on 9th Feb 09 in Vienna and according to the agenda (published in Dec 08) it’s due to discuss space debris this Monday/Tuesday (16 -17 Feb 09). What timing…no guesses as to what they will be talking about! It will probably be a closed session but the daily report might be interesting.

The GA also ”.. noted with appreciation that some States were already implementing space debris mitigation measures on a voluntary basis, through national mechanisms and consistent with the Inter-Agency Space Debris Coordination Committee Guidelines and the Space Debris Mitigation Guidelines …..(and) invited other Member States to implement, through relevant national mechanisms, the Space Debris Mitigation Guidelines.” Space Debris Mitigation Guidelines cover both Prevention of on-orbit collisions and Post-mission disposal. huh.gif
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leper
post Feb 15 2009, 12:30 AM
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I'm surprised this happened in my lifetime! The future is now...
Does anyone know if de-orbit tethers have been / will be added to any existing/future payloads?

I wonder how many more collisions like this there can be before the debris situation becomes unmanageable....
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Juramike
post Feb 16 2009, 02:11 AM
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CNN reporting that a fireball was imaged and sonic booms were heard over Texas: http://www.cnn.com/2009/US/02/15/texas.sky.debris/index.html


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