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On-orbit Satellite Collision
stevesliva
post Feb 12 2009, 05:40 PM
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QUOTE (AndyG @ Feb 12 2009, 09:52 AM) *
There's a trillion cubic kilometres of space in LEO, up to 2000km. Less than ten thousand objects above 10cm across.

"What are the chances of that happening?"


I'm curious, too. But if you expand your timescale to infinity, perhaps the chance is pretty close to one. What are the chances in any given year? Any given decade? And given the choice of orbits, is the space being considered really so large?
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stevesliva
post Feb 12 2009, 05:43 PM
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QUOTE (algorimancer @ Feb 12 2009, 11:19 AM) *
Yes, considering that they (apparently) routinely track fragments the size of bolts and coins (at least in LEO), I find this a tad suspicious. Just how "dead" was that Russian satellite? Not to be going off on a conspiracy rant or anything, just finding my belief a little strained here -- particularly in light of the recent Chinese and US ASAT operations.


If it was less than dead, you'd hear about its divergence from its expected orbit.

While it's Iridium's fault, I'm surprised there was no prior warning. I would have thought the US trackers would anticipate any two objects--dead or alive--occupying the same space.
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Geert
post Feb 12 2009, 05:43 PM
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QUOTE (ugordan @ Feb 12 2009, 08:39 PM) *
They hit at almost a 90 degree angle and that comes out to something like 11 km/s relative velocity. Ouch.

http://spaceweather.com/swpod2009/12feb09/deak1.gif


That picture says it all, the more you think about it the more perplexed you are that this could happen, such a tremendously small chance...

Two satellites, each over 1 ton in weight, hitting at 11 km/sec, must have been quite some fireworks in the Siberian sky, some of the fragments might even have made it into solar orbit... Shape and progression of this debris cloud will indeed be an interesting mathematical experiment...

Regards,

Geert.
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ugordan
post Feb 12 2009, 05:48 PM
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QUOTE (stevesliva @ Feb 12 2009, 06:43 PM) *
While it's Iridium's fault, I'm surprised there was no prior warning. I would have thought the US trackers would anticipate any two objects--dead or alive--occupying the same space.

How exactly is this Iridium's fault?

As for advanced warning, I'm puzzled as well.


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djellison
post Feb 12 2009, 05:50 PM
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At 7.5km/sec - it takes about 0.00013s for a spacecraft to cover the, say, 1 metre width of an Iridium sat.

I wonder if anyone was watching it do an iridium flare at the time smile.gif


Doug
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stevesliva
post Feb 12 2009, 05:50 PM
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QUOTE (ugordan @ Feb 12 2009, 01:48 PM) *
How exactly is this Iridium's fault?

As for advanced warning, I'm puzzled as well.


Same way it's the skipper's fault if a boat runs aground on a charted shoal.
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ugordan
post Feb 12 2009, 05:56 PM
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QUOTE (stevesliva @ Feb 12 2009, 06:50 PM) *
Same way it's the skipper's fault if a boat runs aground on a charted shoal.

Bad analogy. Iridium doesn't have assets to track every conceivable, potentially harmful object in the sky. Other organizations specialized in that do. It's very likely these other organizations already know Iridium 33 is/was a functioning satellite and as such would benefit from collision predictions. Or are Iridium guys expected to go to those same organizations every day and ask "I know we asked this yesterday, but could you see if there's any possibility of a collision today?"

The skipper is blind and a third person needs to inform him if/when he gets into very shallow waters.


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dvandorn
post Feb 12 2009, 06:06 PM
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The only way in which I think you could say that the collision was Iridium's fault is in the fact that, of the two satellites, Iridium was the only one that was still "live" and had any capacity for collision avoidance.

However, just because Iridium was capable of maneuvering doesn't mean that its controllers were aware of the collision threat. I think if there is any "blame" to lay here, it's with the agencies that track the satellites, who could have sounded a warning and given Iridium's controllers the opportunity to make a collision avoidance maneuver.

-the other Doug


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mcaplinger
post Feb 12 2009, 06:38 PM
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QUOTE (dvandorn @ Feb 12 2009, 10:06 AM) *
However, just because Iridium was capable of maneuvering doesn't mean that its controllers were aware of the collision threat.

Wasn't the Cosmos stage in the published NORAD two-line element sets? I'd say that Iridium should have been looking at those.

EDIT: sure it was: COSMOS 2251, NORAD ID 22675.


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djellison
post Feb 12 2009, 06:45 PM
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QUOTE (dvandorn @ Feb 12 2009, 06:06 PM) *
doesn't mean that its controllers were aware of the collision threat.


They should have been - as MC cites, the details of the defunct satellite were available, along with most other chunks up there.

Not keeping track of your satellites and comparing them to the published NOARD elements, is like sailing along a coast without charts.
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ngunn
post Feb 12 2009, 06:59 PM
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A question - could a defunct satellite suddenly spring a leak and thereby start to deviate from it's predicted path?
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stevesliva
post Feb 12 2009, 07:29 PM
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QUOTE (ngunn @ Feb 12 2009, 02:59 PM) *
A question - could a defunct satellite suddenly spring a leak and thereby start to deviate from it's predicted path?


Or bump into something smaller first...
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mcaplinger
post Feb 12 2009, 08:20 PM
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One has to keep in mind that satellite paths are not predictable for more than a couple of weeks under any circumstance due to non-gravitational effects like drag, light pressure, etc, especially for LEO orbits. That's why NORAD regularly updates their element sets.

Unless there's reason to think that Cosmos 2251 suddenly changed course or that the data in the NORAD sets was wrong, I'd place the blame for this totally on Iridium. If they weren't looking for collisions, it was basically a hope that they would stay lucky. With 90+ satellites in LEO, they were really in a position to pay more attention.


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nprev
post Feb 12 2009, 08:33 PM
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Mike, you're right; even the GPS constellation (which is in MEO) downloads ephemeris updates to ground receivers fairly often.

Seems like the question is who's responsible for predicting such collisions? From the discussion thus far, I suspect that the US & Russia watch the ISS like a hawk, of course, and probably also all their government-owned assets (at least those that are active). Could it be that Iridium & other private spacecraft are expected to do this on their own with nothing provided to them but updated ephemerides? If so, this seems unwise & definitely not in the best interests of all stakeholders for the sole reason that nobody wants to deal with a debris cloud.


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AndyG
post Feb 12 2009, 08:48 PM
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QUOTE (djellison @ Feb 12 2009, 05:50 PM) *
At 7.5km/sec - it takes about 0.00013s for a spacecraft to cover the, say, 1 metre width of an Iridium sat.


I need to delve into the statistical probabilities, but given the apogee/perigee variation for the satellites was ~ 26km for Cosmos 2251 and ~ 15km for Iridium 33, that there was complete overlap, and that the inclinations were similar (74 versus 86.4 degrees) that's still a huge amount of sky to exactly meet one-on-one in that tenth of a microsecond that counted.

It's beyond lottery-winning "unlikely".

...But maybe it'll raise a few eyebrows and result in positive decisions for the future.

Andy
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