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9 Ideas For Coping With Space Junk
An anonymous reader writes "The space age has filled Earth's orbit with all manner of space junk, from spent rocket stages to frozen bags of astronaut urine, and the problem keeps getting worse. NASA's orbital debris experts estimate that there are currently about 19,000 pieces of space junk that are larger than 10 centimeters, and about 500,000 slightly smaller objects. Researchers and space companies are plotting ways to clean up the mess, and a new photo gallery from Discover Magazine highlights some of the proposals. They range from the cool & doable, like equipping every satellite with a high-tech kite tail for deployment once the satellite is defunct, to the cool & unlikely, like lasers in space."
You need to get through all the orbits of uncontrolled junk, which takes a lot of calculation and you can only move so fast.
You have to be able to get out of the way of new junk that's moving so fast you can't accelerate quickly enough to wait and see if it might miss you.
Calculation of junk trajectory is only so precise, so you have to leave a 'safety buffer' of sorts.
There's more junk up there than we have cataloged. There will always be new junk, and collisions alter orbits of existing junk such that our known trajectories become inaccurate and we have to relocate and recalculate all the time.
So finding a safe zone which requires the least fuel usage to stay alive is becoming more challenging.
Tiny fragments that wouldn't harm anyone if you threw it at them are deadly, equipment-wrecking projectiles at high velocity. Think about a small piece of metal, like a penny. Not a problem if you drop it on your foot. Not going to destroy a vehicle if you drop if off a towering skyscraper, even. But, in space where there's no[t enough] atmosphere to slow it down or burn it up, it can theoretically approach any speed... and a 1,000 MPH penny is a fearsome entity to a fragile laboratory measurement device. We might not even be able to track that very accurately, but if you guess wrong... you transfer that momentum into multiple new shards of former expensive equipment!
So getting things into space is really getting more complicated and keeping things alive up there takes a lot more calculation and fuel as the probability of stray objects increases. Does that cut down on the exaggeration factor?
Consider that each object (in low Earth orbit) is in a separate orbit. Each pair of orbits crosses twice on opposite sides of the Earth. The eccentricity of each orbit causes the object to traverse a range of altitudes, defining the subset of all the LEO objects that are possible collision risks at any given time. The risk for two particular objects colliding is low, but each object has many other opportunities as it crosses thousands of other orbital tracks each time it circles the Earth. Then integrate over all the objects. The probability is a nested summation - integrated over time.
For example, assume there are about one hundred spacecraft (active and defunct) occupying a particular semimajor axis "zone". Each satellite orbits once every 90 minutes, ie, 16 orbits/day. Each satellite crosses the orbit of another about 200 times in that 90 minutes. Usually the other spacecraft is somewhere else entirely, but there are a lot of opportunities.
Establish a "comfort radius" - say, one kilometer. If Le Petit Prince is sitting on a satellite, he will get very nervous if another spacecraft zooms through this keyhole at 10 km/s. A typical low Earth orbit is about 42,000 of these comfort units long. So the odds (ignoring altitude for the moment) of finding a spacecraft within the same part of the orbit - during each passage - is 1/42,000. Multiply by the 200 opportunities makes this 1/210 (0.5%) per orbit or about 7.5%/day/spacecraft. There are 100 spacecraft in this zone, so that amounts to about 4 close encounters per day (divide in half since it takes two to tango) in which some spacecraft passes directly above or below another by a few kilometers.
Accounting for altitude requires a bit more physics (inverse square law and all that), but basically amounts to a similar argument of dividing the altitude range traversed by each satellite into comfort zones. The odds of passing through the keyhole drop, but not dramatically - and the orbit crossings keep piling up about a hundred thousand per day per altitude range. With each close encounter, the odds of an impact are basically very simple. What is the volume of a typical spacecraft divided by the 1 km^3 volume? (The second spacecraft either will or won't be occupying the same volume at the moment of closest approach.) Satellites can be surprisingly large - Hubble is about the size of a schoolbus - but figure a Volkswagen van or at least a Beetle.
Bear in mind that this is just one particular altitude range, the same thing is happening at different altitudes. Some spacecraft are in highly elliptical orbits and cross through several such zones. In short, what seems to be a three dimensional problem is really one dimensional. After the spacecraft collision a couple of years ago some of us were scribbling on a blackboard. A physical model would be needed to get the precise answers, but a ball park figure is that we can expect the apparently astronomically rare event of two LEO spacecraft colliding to happen about once per decade (in the absence of active station keeping). Then account for all the debris, not just complete spacecraft.