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ISS Dodges Space Junk For First Time In Five Years
Kligat writes "For the first time since 2003, the International Space Station has utilized the rockets on the European Space Agency's Automated Transfer Vehicle to dodge leftover remnants of a defunct satellite. The Russian Cosmos-2421 was launched in June 2006 to track Western Navy vessels and is believed by NASA to have exploded — 'likely due to a self-destruct command issued by Russian officials' according to the article — leaving 500 pieces of space debris. Ordinarily, the rockets on the ATV are used to take the ISS away from Earth's atmosphere and reduce drag. In this case, the 5-minute firing caused the ISS to move downward because it was already near the top of its acceptable range. Estimated probability of impact was 1 in 72, and an avoidance maneuver is called for if the probability is greater than 1 in 10,000. The space junk was predicted to pass the ISS within just a mile."
I would guess for the same reason the people at the use probability for theirs. Interestingly, there's one object on there that meets the "more probable than 1 in 10,000" threshold for Earth (two decades after Apophis and with half the explosive energy, though).
Depends on the error ellipse of the orbit determination for the junk, and it sounds like the uncertainty is a good fraction of a mile in size. But in any case, the miss distance is a mile after the course adjustment, not before.
According to that list, there are 12 objects with a probability >1/10,000, and 2 with a probability > 1/1000.
Note that the uncertainty on these orbits is frequently many 1000's of km; the orbits of things in LEO are much better determined.
The orbital trajectory of every piece of debris from a spy satellite that was intentionally blown up isn't so well known, especially when the nation controlling the satellite wants it to be a secret.
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At the risk of being redundant, it's roughly a 1 in 72 chance that their calculations of a "miss" are off. Calculations of this sort involve a margin of error, from not precisely knowing locations of these objects to not being able to do forecasting accurately enough. Debris A gets hit by debris B (which somehow evaded your radar), sending off two new chunks of metal which weren't even IN your original calculations. I'm actually impressed that they can put solid numbers on these things, but I guess that's what supercomputers are for.
Yay for safety margins.
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That's a heck of a close call, considering the ISS is traveling at 4.8 miles per second. That's little like a car at highway speed running a red light and missing another car by less than one car length.
Patrick Doyle
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Here's a graph of ISS altitude for the last year, if anyone is interested in hard data. (The steady downward slope is due to atmospheric drag, and the sharp increases are from firing maneuvering thrusters to maintain altitude. Presumably, the recent abrupt drop was the maneuver described in the article.)
The calculated miss distance was about a mile, but there was uncertainty in that miss distance such that there was a 1 in 72 chance it wouldn't miss the ISS, but instead hit it directly.
To answer your question (at a high level), the sensors and models that are used to track and predict the debris locations have associated mathematical models that can put a number on the uncertainty of where that debris is. The uncertainty takes into account things like how many radar obs were made, the inherent accuracy of the radar, uncertainties in atmospheric drag, etc. You can never know exactly where an object is, only an approximation of the current and future location and a mathematical confidence in that estimate. In this case that confidence was sufficiently low (and the risk of impact high enough) that a collision avoidance maneuver was executed.
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The central ISS modules (Zarya, Zvezda) are Russian. Actually, the docking port the ATV is using is also Russian, using the "probe and drogue" technique. I would call their contribution quite remarkable.
You wish. There are over 140 US objects tracked by us amateur satellite trackers which are classified - i.e. they officially do not exist and the only public data on their orbits comes from us amateur trackers. Not NASA, the DoD or any other US government agency.
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