Empty space. In discussions of things astronomical, the term pops up so often it's gone beyond cliche.

"A voyage into empty space." "Across the vastness of empty space." Yet in point of fact, space is never empty, neither between the most distant galaxies nor in the space above our heads here on earth.

Outer space is often described as a perfect vacuum -- which would certainly meet the definition of empty -- but it's not; there are always a few hydrogen atoms floating around.

Closer to home, it's not atoms that float around the Earth -- well, they're there, of course -- it's matter much more mundane, and man-made: space debris, or to put it more plainly, space junk.

There's a veritable floating junkyard of used-up rocket stages, de-commissioned satellites, and a mass of orbital debris in sizes down to fractions of an inch, the result of orbiting objects colliding and disintegrating.

NASA has tracked nearly 21,000 pieces of orbital debris larger than 4 inches, and estimates there are 500,000 pieces in sizes of at least a half-inch and 100 million pieces of smaller size.

The principle sources of the larger pieces are explosions and collisions of satellites, the space agency says.

This was highlighted last week when NASA released a report describing how its Fermi Gamma-Ray Space Telescope dodged an orbiting bullet in the form of a disused Cold War-era spy satellite.

A NASA system that tracks space debris predicted the Fermi telescope would come dangerously close to Cosmos 1805, a Russian spy satellite launched in 1986 that, although long de-commissioned, was still speeding around the Earth at 15,000 mph.

In a desperate move, the Fermi team used the telescope's on-board thrusters -- intended only to guide it downward into the atmosphere to burn up at the end of its operational life -- to move out of the way of the Russian satellite's path.

Controllers fired the thrusters for just a second, moving Fermi sufficiently for it and the Russian satellite to miss each other by 6 miles.

But this was just one incident involving just one piece of space junk; there are bound to be more in the future.

That was borne out in another report last week of an object punching a small hole in a solar panel of the International Space Station.

Canadian astronaut and Expedition 35 Commander Chris Hadfield photographed a small but very visible "bullet" hole, which he put down to a small meteoroid.

"Bullet hole -- a small stone from the universe went through our solar array. Glad it missed the hull," Hadfield wrote in his Twitter blog.

But some experts said they believed the hole was probably caused by a random piece of space junk.

"It's unlikely this was caused by a meteor, more likely a piece of man-made space debris in low Earth orbit," said Jim Scotti, a planetary scientist at the University of Arizona's Lunar and Planetary Laboratory.

A near-miss by a satellite and a small hole in a solar panel might seem minor affairs, but larger, more destructive events are bound to happen -- and in fact already have.

In 2009 two communications satellites, the U.S. Iridium 33 and the Russian Kosmos-2251, collided over Siberia at a speed of more than 26,000 mph.

The collision destroyed both satellites and added yet more orbiting debris to Earth's halo of space junk.

As manned missions move forward in the plans of space agencies of a number of countries, there should be considerable concern over the possible consequences of our leaving so much litter in our own astronomical back yard.

Empty space, indeed. Time to set up garbage collection.