In the movie Gravity, a group of astronauts finds out first-hand what it might be like for space explorers to run into an orbiting bit of space debris. The results are not good, although at least one astronaut makes it through safely. Although the movie has generated a lot of discussion among space experts about its accuracy in some places, it does highlight a growing problem that us folks on Earth are facing.
It's particularly important for people in the space launch business because there are nearly 20,000 pieces of space debris up there. Most of it ranges from such small objects as gloves and cameras to pieces of rockets and artificial satellites. There's enough "stuff" up there to pose a real danger to such objects as Hubble Space Telescope and weather and communications satellites, as well as those of us on Earth. That's the bad news. The good news, for us on Earth at least, the chances of something hitting us on land are fairly small. It's far more likely that a piece of space debris will fall into the oceans, or at least into an unhabitated part of a continent.
To keep launch vehicles and orbiting satellites from running into these bits of space junk, organizations such as the North American Aerospace Defense Command (NORAD) have tracked the junk as it orbits the planet. Before every launch (and as satellites orbit the globe), the positions of all known debris must be known so that the launches and orbits can proceed without damage.
The safety of our satellites, astronauts, space stations, and other necessary objects "up there" is important. Thus, knowing where all the "stuff" is lets operators steer clear of it and avoid harm.
However, the problem of space junk isn't just limited to rocket scientists. As mentioned above, it's also an issue for us Earth-dwellers. The old adage "what goes up must come down" is an important part of the problem.
In essence, pieces of junk in orbit can and do get caught up in our planet's atmosphere. That slows them down, in a process called "atmospheric drag". If we're lucky, and a piece of orbital debris is small enough, it will likely vaporize as it falls to Earth under the tug of our planet's gravity. (This is exactly what happens to meteoroids when they encounter our atmosphere and the resulting flare of light we see as they vaporize is called a meteor. Earth regularly encounters streams of meteoroids, and when it does, we often see meteor showers.) But, larger pieces of space junk can pose a threat to folks on Earth as well as get in the way or orbiting stations and satellites.
Earth's atmosphere is not the same "size" all the time. For example, scientists need to know how the density of the atmosphere changes over time in the low-Earth orbit (LEO) zone. That's an area several hundred miles above the surface of our planet where most orbiting materials (including satellites and the International Space Station) exist.
Heating by the Sun helps "swell" our atmosphere, and waves propagating from lower in the atmosphere can also have an effect. But, there are other events that affect our atmosphere and could have the effect of catapulting larger objects toward Earth's surface. Occasional solar storms cause the upper atmosphere to expand. These erratic solar storms (caused by coronal mass ejections) can zip from the Sun toward Earth in less than two days, and they produce rapid changes in air density.
Again, most space "junk" falling to Earth can and does vaporize on the way down. But, larger pieces can land and cause damage on our planet. Imagine being in the neighborhood if a large piece of a defunct satellite fell on your house? Or, imagine what would happen if a large solar storm resulted in enough atmospheric drag to pull a working satellite (or a space station) into a lower and more potentially dangerous orbit? It would not be good news for the satellite operators or the astronauts working aboard the International Space Station.
The U.S. Air Force (which is involved with NORAD), and the U.S. National Center for Atmospheric Research (NCAR), the University of Colorado at Boulder, and the U.S. National Oceanic and Atmospheric Administration Space Weather Prediction Center work together to forecast space weather events and the effects they have on our atmosphere. Understanding those events will help us all in the long run by understanding the same effects on the orbits of space junk. Ultimately, the junk trackers will be able to forecast more accurate orbits and trajectories of space debris in near-Earth space.