A simple question with a simple answer that I don’t know of.
If there is so much space junk floating around the earth, why hasn’t it shown up on the hundreds of photos taken of Earth? Or has it?
Earth is a very large object that reflects a lot of light. Space junk is “small” and does not refelct much light. If you set the camera for the amount of light reflected by earth, the amount of light reflected by the junk will be too dim to show up.
Then there is the subject of focus. If the earth, which is very far from the camera, is in focus, the space junk, which is much closer to the camera will be out of focus.
Then there is size. If you are taking a picture of something the size of the earth, little things don’t show up. When you take picture of a parade, football game, etc, your picture does not show flys, mosquitoes, etc [unless they happen to be very close to the lens nad then they are still out of focus.]
There there is the amount of space. Space junk is spread out over a huge area. There is not much out there, still mostly empty space. The chance of a piece of space junk being in the right place is low. A picture is a cone. Small near the camera, larger as you move away. So the field of view is small near the camera.
Then there is velocity. Unless it happens to be orbiting with you so the velocities are the same it will just be a streak across your pictureof the earth.
You can compensate for all of these effects and take a picture of the space junk, but it would just show up in a picture being taken of the earth.
I agree with starfish’s analysis, but I’d add that the thing to appreciate is that most space debris is very small and very spread out. That it’s a problem is almost entirely due to its velocity. A piece can pack a huge punch when it hits and so the smallness doesn’t protect you and it can sweep out very large spaces in a given time and so the sparseness of the stuff doesn’t protect you either.
Oh, OK. Thanks! 
From the way someone was talking about it today, in one of my classes, we were practically in a cage of debris. I would have figured you could at least see it if it were that bad. I don’t think it’s that bad—yet.
Does NASA know the location of all the space junk in orbit? They must have to take this in consideration when launching, right?
They do track as much as possible, for the reasons noted earlier. As more missions are made into orbit, this becomes ever more important. The ESA has an excellent image showing where th junk ends up:
http://www.esoc.esa.de/external/mso/debris.html
The US has a satellite whose primary purpose was to track missiles. The coolant supply for that instrument package ran out less than a year after launch, but other abilities still work, including a method to track previously-lost space junk:
http://www.space.com/businesstechnology/technology/space_trafficcontrol_001102.html
If I ever get the chance to achieve orbit (long shot (no pun intended), but a guy can hope), I’d like to know that as much deadly stuff as possible has been accounted for.
Direct tracking is really only a way of following the relatively large pieces, where (from memory) relatively large counts as something bigger than a nut. The range of stuff that’s involved includes such diverse items as paint flecks and drops of fuel, some of which don’t easily show up on radar.
A lot of effort in the field is more towards flux estimates. These are statistical, telling you what the population of stuff in different orbits is rather identifying every last item. For the small pieces, there’s really no foreseeable alternative and even the estimates are pretty scanty. Radars can give a handle on what’s there without actually tracking it. But the really hard numbers are from in situ measurements. For instance, when the solar panels were replaced on Hubble, the old ones were pored over to look for impacts. The resulting study (sorry, no cite, but I think it was by the University of Sussex - I’m recalling a presentation) was able to distinguish debris from micrometeorites by differencies in the little craters they make and so get seperate numbers for these two fluxes. Now, of course, most of these objects weren’t the dangerous ones - none disabled the HST - but the significance is that this sort of number greatly constrains modelling. And it’s the models that are our best understanding of the problem. People have constructed models of what the debris cloud looks like using things like known launch patterns. It “helps” that much of the problem has historically derived from a few instances (there was a particular failed Russian booster that leaked, for instance). Such modellers are desperate for hard data to use in refining their models. Tracking really only helps in one regime and studies like the Hubble one help by giving numbers elsewhere. But measurements are very few and far between. Anybody who can come up with a neat way of measuring the fluxes will find themselves with a lot of friends in the space debris community.
Of course, the other importance of direct tracking is that it allows you to take evasive action as appropriate. When you’ve only got flux estimates to go on, the best you can do is avoid certain regions.
Debris also tends to be concentrated in a few specific areas. Geostationary orbit, for instance, is quite cluttered just from the stuff we want up there, much less the junk, and most other satellites are launched into an orbit as close to the equator as possible, to save fuel costs, so that’s another problem area for space junk. Over Alaska, by contrast, you’ll hardly ever have to worry about space debris, because almost nothing passes over Alaska in orbit.