MKremer - 12/9/2006 10:47 AM
yinzer - 12/9/2006 11:44 AM
If something like a nut or a washer drifts away (as opposed to being heaved), isn't its ballistic coefficient going to be low enough that it will appear to continually move away from the station, forward and down?
Not especially - it would be about the same as, say, undocking one of the Soyuz or Progress ships and immediately allowing it to free-drift. Its 'new' orbit would bring it back around near the station again.
The only hope with small parts and pieces that drift off is that their lighter mass allows quicker atmospheric friction decay, or that they can be tracked fairly quickly so just one small thruster "burp" is all that's needed to avoid it from then on.
I'm unconvinced.
There's the behavior coming from the initial separation impulse, which is determined by the Clohessy-Wiltshire equations and gives the receding-climbing-closing-diving loops described in the Oberg article. If the separation impulse is directly up or down, the thing will come back half an orbit later and hit the space station with exactly the same velocity with which it left.
There's also the behavior due to differential atmospheric drag; this will make the more draggy object move continually forward and down relative to the less draggy object. Assuming an aluminum nut 2 cm in diameter and 1 cm tall, and using the ISS data from
here, it looks like the nut will be about 10 times more draggy than the ISS.
Given that ISS loses between 50 and 200 m of altitude per day, the nut (the densest of the 3 things) should lose altitude at nearly a kilometer per day. I'd think this would totally dominate the relative motion, and after even a couple of hours the thing should be out of the vicinity of the ISS never to return.
Atlantis is officially cleared for re-entry. All inspections by the ground teams were completed in half the estimated time of 5 days. Shannon says that Discovery will be in great shape for a night launch in December and that they feel very good about launching at night again due to the lighting from the boosters and the available inspection techniques.
yinzer said -
I'm unconvinced.
There's the behavior coming from the initial separation impulse, which is determined by the Clohessy-Wiltshire equations and gives the receding-climbing-closing-diving loops described in the Oberg article. If the separation impulse is directly up or down, the thing will come back half an orbit later and hit the space station with exactly the same velocity with which it left.
There's also the behavior due to differential atmospheric drag; this will make the more draggy object move continually forward and down relative to the less draggy object. Assuming an aluminum nut 2 cm in diameter and 1 cm tall, and using the ISS data from here, it looks like the nut will be about 10 times more draggy than the ISS.
I 'm not quibbling with your figures (or your reasoning), but how about a SS bolt 3cm long by 0.5cm (with a 1cm flange), departing zenith (or within 10° forward) @ 20cm/sec?
Being somewhat ignorant about mass -vs- cross section -vs- atmospheric drag, would a more-massive object be susceptable to the same or less atmospheric drag forces than a less-massive object of the same size/shape?
Not to say I'm very worried about it - I would think even if it were even possible to get near the station within 1 or 2 orbits, the most it could do would be to bounce off, which would cause it to lose energy and enter a lower, faster orbit and decay to re-entry. (assuming it didn't bounce off a sensitive/easily damaged area)
That bolt has a ballistic coefficient about 50% higher than the aluminum nut, but it will still decelerate much faster than the ISS. 20 cm/sec will boost it into an orbit with a mean altitude 300 m higher, and receding 100 km per orbit. Assuming that the altitude loss due to drag figure of 500 m / day is reasonable, this orbit will have decayed to completely within the ISS orbit long before it the ISS catches back up to it.
If Jorge sees this, he can verify or correct my math.
As for collision, well, 20 cm/sec is about what you'd get if you dropped something from a foot up on Earth. Vast areas of the ISS will be completely immune to this, but there might be areas that could be damaged. Chips out of windows or lodging in a moving part would be my main concerns, albeit not big ones.
OK, thx, sort of what I assumed, if all the details are correct for that kind of piece escaping during an EVA.
It stands to reason the longer the ISS is there and unfinished, the more 'stuff' has a chance of going off on its own during EVAs (of which they may or may not have figured to happen over the years), and the more potential danger the ISS is in during its planned construction.
After Core Complete it's pretty much a wash, assuming a somewhat higher orbit, and less obvious opportunities for debris generation.