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What is space junk?
There are more than 20,000 prices of space junk orbiting the earth, that are of size larger than 3.5 inches. And 5,00,000 pieces the size of marble or larger.
Here is an image of the Earth with space junk as seen from space.
Although in an image the space junk looks few and inconsequential, what must not be forgotten is that the threat it poses is real and huge. If you have a 720p HD image from space, and Earth fills it vertically, each pixel is 17 kilometers across (12,742/720). Seeing a piece of space junk would be impossible as the average size of space junk is 10 cm. A satellite or space debris would only look like another star unless it was a movie and you could see it moving.
To be far enough away to see the Earth in more or less its entirety, you need to be a few thousand kilometers away - if it is a genuine photo. So even if you block out the Earth and Sun (easy to do in space) and look at a dark sky, the satellites and space debris would be just about impossible to see unless you are pretty close to it. From geostationary orbit, even the International Space Station would be a faint star of magnitude 4 or so at its brightest (−5.9), hard to spot though visible to naked eye.
There are around 3,600 satellites in orbit. A big satellite is about the size of a bus. Imagine 3,600 busses parked around the earth. Would you be able to see them? That would be one bus for every 55,000 square miles.
The reason these particles are invisible in the photographs is that the high majority of them are less than half an inch in size. The only non-illuminated things visible from space are large clouds and continents. That's the size things have to be to be visible from space. So, it is only logical that space junks won't be visible in an image of earth from the space.
And once you're in orbit, there is even more area. Satellites orbit from about 200 miles up to over 22,000 miles up. (22,236 miles up are geosynchronous orbit. At this orbit, a satellite goes around the Earth once every 24 hours, so it follows the rotation of the Earth, and is always above the same spot on the ground.) At 22,000 miles, up, if we take our same 3,600 busses, each bus would have nearly 1,700,000 square miles to itself. There is lots of room in space.
There are 500,000 pieces of space junk in space. Those are spread out over the twenty million cubic kilometers of space that is within earth orbit. That's one piece of junk per 400 cubic kilometers. That's not very dense.
You are mistaken because there are over 170 million pieces of debris smaller than 1 cm (0.39 in) as of July 2013. There are approximately 670,000 pieces from one to ten cm. The current count of large debris (defined as 10 cm across or larger) is 29,000. Over 98 percent of the 1,900 tons of debris in low Earth orbit (as of 2002) was accounted for by about 1,500 objects, each over 100 kg.
You can now imagine why astronauts don't see rubbish floating and orbiting over their heads.
Scale is something we tend to visualize incorrectly. Relatively small and nearly invisible doesn't mean something is not a threat. Even though space junk is relatively minuscule
it poses real danger to the operational space devices and vehicles.
Even tiny paint flecks can damage a spacecraft when traveling at high velocities. In fact a number of space shuttle windows have been replaced because of damage caused by material that was analyzed and shown to be paint flecks.
Greatest danger is posed by non-trackable debris. "The greatest risk to space missions comes from non-trackable debris," said Nicholas Johnson, NASA chief scientist for orbital debris.
On Feb. 10, 2009, a defunct Russian satellite collided with and destroyed a functioning US Iridium commercial satellite. The collision added more than 2,000 pieces of trackable debris to the inventory of space junk. The amount of space junk has increased drastically over the years, which is a cause of concern of the space agencies.