On-orbit Satellite Collision Options

[ElkGroveDan]

Two satellites collide in orbit
BY WILLIAM HARWOOD

Posted: February 11, 2009

In an unprecedented space collision, a commercial Iridium communications satellite and a presumably defunct Russian Cosmos satellite ran into each other Tuesday above northern Siberia, creating a cloud of wreckage, officials said today.

Iridium satellite
An artist's concept of an Iridium satellite orbiting the Earth. Photo: Iridium

The international space station does not appear to be threatened by the debris, they said, but it's not yet clear whether it poses a risk to any other military or civilian satellites.

"They collided at an altitude of 790 kilometers (491 miles) over northern Siberia Tuesday about noon Washington time," said Nicholas Johnson, NASA's chief scientist for orbital debris at the Johnson Space Center in Houston. "The U.S. space surveillance network detected a large number of debris from both objects."

MORE.....


http://spaceflightnow.com/news/n0902/11iridium/

[mcaplinger]

http://www.thespacereview.com/article/1314/1 is a very good summary of this event.

[stevesliva]

http://www.thespacereview.com/article/1314/1 is a very good summary of this event.

In this vein, I think it can be argued that the US military committed a sin of omission in the case of the Iridium-Cosmos collision. The US military maintains the best and most complete satellite catalog in the world and had the data to provide collision warning to Iridium. But as was discussed earlier in this article, the US military only looks at a limited list of satellites for collisions. It also appears that at some point they stopped providing collision warning for the Iridium constellation.

Given the complexities of the conjunction assessment process, it is understandable that the US military does not have the resources or capability to screen all of the estimated 900 payloads in Earth orbit. But both the US military and Iridium knew that there were many close approaches with the Iridium constellation and that eventually one could collide. Perhaps both thought that if they stopped looking at the problem it would go away.

My bold. Ha. Given that supercomputers and salaries are an order of magnitude cheaper than satellites, perhaps that will change. (Although the Iridium constellation is odd in that is was essentially gotten for free after bankruptcy.) The conclusions in the article are a little more nuanced. Good article!

[helvick]

Nice post on the collision by Diandra over at CocktailPArtyPhysics.

She mentions two things that I hadn't come across before - firstly that the predicted closest approach of the two satellites prior to the impact had been around 600m and secondly that the increased atmospheric drag caused by the recent Sunspot minimum is a prime suspect in explaining why the predicted orbits were sufficiently incorrect to turn that 600m into 0.

600m would still seem like far too close for comfort to me but does anyone know what the normal error in such things would have been expected to be?

[Tom Womack]

Nice post on the collision by Diandra over at CocktailPArtyPhysics.

She mentions two things that I hadn't come across before - firstly that the predicted closest approach of the two satellites prior to the impact had been around 600m and secondly that the increased atmospheric drag caused by the recent Sunspot minimum is a prime suspect in explaining why the predicted orbits were sufficiently incorrect to turn that 600m into 0.

600m would still seem like far too close for comfort to me but does anyone know what the normal error in such things would have been expected to be?

http://celestrak.com/SOCRATES/top10maxprob.asp almost always shows at least one expected pass at 0.1km or less, with an estimated probability of collision more than one in a thousand; since we don't see a collision every few years, I suspect their model is not perfect.

From a 500km circular orbit, a change in velocity by 1 centimetre per second changes the radius of the orbit by 18 metres, so you have to know the velocity very accurately indeed.