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On-orbit Satellite Collision
AndyG
post Feb 12 2009, 08:51 PM
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QUOTE (stevesliva @ Feb 12 2009, 05:40 PM) *
...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?


rolleyes.gif

A decade is infinitesimally smaller than infinity. Which would suggest the chance is 0.

As to "who's to blame", the answer, as Mr Newton would have to say, is "both of them". smile.gif

Andy
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mcaplinger
post Feb 12 2009, 08:53 PM
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QUOTE (nprev @ Feb 12 2009, 12:33 PM) *
Could it be that Iridium & other private spacecraft are expected to do this on their own with nothing provided to them but updated ephemerides?

That is certainly the case now, and frankly, it wouldn't have been that big a deal for Iridium to have done this. The argument could be made that it shouldn't be up to them, but absolving satellite operators of any responsibility for collision avoidance also seems like the wrong way to go.


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nprev
post Feb 12 2009, 09:02 PM
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Thanks, Mike. Wow. Think we might have found the disconnect.

Liability issues aside, spacecraft collisions present hazards to everything operating in orbit; it's a lot more than a two-party issue. IMHO, it looks like there really does need to be an orbital 'ATC' of some sort, and this doesn't have to be more than a continuously updated model; maybe 3 or 4 software engineers keeping it happy, plus a few more people to broadcast alerts.

Worth doing, and maybe a good small business idea for any of you computationally gifted types out there. The root data's provided for free... wink.gif


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imipak
post Feb 12 2009, 09:04 PM
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QUOTE (algorimancer @ Feb 12 2009, 03:19 PM) *
I find this a tad suspicious.

What are the chances that a deliberate attempt to set up a collision between orbits with those orbital characteristics would succeed? Statistics and frequency probability are deep and frequently counter-intuitive waters.



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stevesliva
post Feb 12 2009, 09:30 PM
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QUOTE (AndyG @ Feb 12 2009, 04:48 PM) *
that's still a huge amount of sky to exactly meet one-on-one in that tenth of a microsecond that counted.


Tenth of a millisecond wink.gif 5000 tries per year... but I getcha. Not only do the orbits have to exactly intersect, but they have to be at the point of intersection at the same time.

Making the wrong assumption that two orbits will always intersect at exactly one place, with a 100 minute orbit, the chances of being at that particular place are 6000/.00013= 1 in 46 million per orbit? And then say it does 50000 orbits/decade-- a 0.108% chance of it happening in a decade. (Times 70 satellites)

No idea what the chances are of two orbits intersecting, though. And I know they probably don't, for long. Brings the chances way down. So if they intersect once per day, divide by 3650, for a .0000296% chance, about 1 in 3.4 million.

That does get you up into winning-the-lottery range.
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OWW
post Feb 12 2009, 09:31 PM
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Speculation: Maybe Cosmos 2251 very recently suffered an (undetected) fragmentation and the debris cloud intersected with Iridium's orbit.
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AndyG
post Feb 12 2009, 10:02 PM
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QUOTE (OWW @ Feb 12 2009, 09:31 PM) *
Speculation: Maybe Cosmos 2251 very recently suffered an (undetected) fragmentation and the debris cloud intersected with Iridium's orbit.

It helps the odds immensely. And a tiny lumplet, at 11km/s, would do the job.

Andy
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nprev
post Feb 12 2009, 10:19 PM
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Thing is, all the press reports seem to imply a head-on collision. Were there any hints that the Cosmos had a pre-existing associated debris cloud?

Lots we don't know; a very odd event.


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ngunn
post Feb 12 2009, 11:08 PM
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QUOTE (stevesliva @ Feb 12 2009, 07:29 PM) *
Or bump into something smaller first...


A very good point. One would expect the first event to lead to others. Would we necessarily detect the first one, or would we catch on a little later in the process?

I'm not so sanguine as some of the the statistical assessments above. I think we have a serious problem here.
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djellison
post Feb 12 2009, 11:12 PM
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QUOTE (AndyG @ Feb 12 2009, 08:48 PM) *
It's beyond lottery-winning "unlikely".


For any one satellite on any one orbit - yes.

But 3000 satellites, orbiting 14 times a day, 365 days a year, for, say, 10 years - and infact, it's not beyond the realms of possible in any way shape or form.

Doug
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dvandorn
post Feb 13 2009, 02:10 AM
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Statistics are what people play around with while awaiting actual empirical data.

We now have empirical data. Put enough satellites and associated pieces of crap in orbit and, eventually, things start colliding. That's not a statistical analysis -- it's the empirical description of an observed phenomenon.

Also remember that, for everything that actually occurs, the statistical likelihood of it having happened is exactly 1 in 1. wink.gif And as for the "lottery-winning" odds, please keep in mind that, at least in the United States, lotteries with odds of a single given person winning that work out to something like 100 million to one are won by *someone* every few weeks. So, while the odds that one given satellite may impact another given satellite may be very, very low, the odds that *some* satellite out there will collide with some *other* satellite are obviously a lot higher.

-the other Doug


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kwan3217
post Feb 13 2009, 04:58 AM
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From the http://www.satobs.org/seesat/Feb-2009/index.html SeeSat-L list: Closest predicted approach of the two objects was 800m plus or minus whatever the error in a two-line element is. There are six predicted close approaches within 100m in the next five days. Closest one is something like 52m. http://celestrak.com/SOCRATES/top10minrange.asp . Probably none of them will collide. One of the guys on SeeSat says that these two didn't even make the top 10 hazard list.

You pays your money and you takes your chance. Just because something bad happened doesn't mean that someone is at fault. If you are running Iridium, maybe you have a conjunction like this once every couple of days with one of your satellites. You have to maneuver to dodge, and maneuver again to get back into the proper slot. Do that enough and you will run out of gas a lot faster than just doing nothing.

And, I highly recommend SeeSat. It is for Earth orbiters what UMSF is for the rovers.
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ilbasso
post Feb 13 2009, 04:59 AM
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They have moved the ISS orbit - what, 8 times? - because of a satellite coming within a few miles of the predicted location. That's a pretty large area of uncertainty. I'm trying to imagine a scenario where they require every active satellite that might pass within a few miles of another piece of debris change its orbit. How many satellites would be changing their orbits every year, and how badly would that complicate the job of those who are trying to keep track of them all? Is it worth the hassle to do all that moving for an incredibly low probability that any one of those "near misses" might be a collision? Yes, for a manned vehicle. It depends, for an unmanned one - depends on criticality of function, etc etc.


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PFK
post Feb 13 2009, 07:52 AM
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QUOTE (ugordan @ Feb 12 2009, 01:39 PM) *
They hit at almost a 90 degree angle and that comes out to something like 11 km/s relative velocity. Ouch.

Large Hadron Collider, who needs it?! smile.gif
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nprev
post Feb 13 2009, 11:40 AM
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The only thing to be sure of is that this event is gonna increase costs for private LEO flights. It's a pretty safe bet that the insurers are going to factor this in for all future policies.


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