Student Features

What Goes Up Doesn't Always Come Down
12.02.04
Debris falling from the sky
People have been launching objects into space for almost 50 years. That adds up to a lot of stuff! Most of it has fallen back to Earth. These objects have either landed, or burned up in the atmosphere. A few of them have been launched beyond Earth's gravity. These objects travel to other worlds or explore space. But, many of the objects that have been sent into space are still in orbit. They are endlessly circling the Earth.

Image to right: Orbital debris falls through the sky. Credit: NASA

This "junk" that is circling the Earth is called orbital debris. On one extreme, debris can be as small as tiny flecks of paint that have come off spacecraft. On the other, large debris could be satellites that are no longer working. The most common source of orbital debris larger than 1 centimeter [cm] (0.39 inches) is the explosion of objects orbiting the Earth. These are often rocket upper stages. These can contain fuel or high pressure fluids.

To keep astronauts safe, scientists keep track of all the debris in orbit. They sort them by their size. The diameter of the piece is how scientists classify it. There are about 11,000 known objects that are bigger than 10 centimeters. Scientists believe that there are more than 100,000 pieces of orbital debris between 1 cm and 10 cm. And, there are tens of millions of pieces smaller than 1 cm! All pieces of debris larger than 10 cm are carefully tracked. That information is used to estimate the number of small pieces of debris. Even though this system cannot detect every piece, it can give scientists an idea of the debris that is out there.

Image of a meteorite shield
To determine how many pieces of small debris (smaller than 1mm) are in orbit, scientists study the Space Shuttle when it returns to Earth. To do this, they look for damage from debris impacts. When the Space Shuttle returns from a mission, scientists count the number of impacts it suffered. They then compare the number of hits to the amount of space the Space Shuttle traveled through. This helps them estimate how many of the tiny objects are in orbit around the Earth.

Image to left: A meteorite shield is tested. Credit: NASA

In low Earth orbit, most "space junk" is moving super fast. It can reach speeds of 4.3 to 5 miles per second! And if a spacecraft is moving toward the debris, this speed can seem even faster! The average impact speed of a piece of orbital debris running into another object is 22,370 miles per hour [mph]! Since it is moving so quickly, a tiny piece of orbital debris can cause a lot of damage. In fact, an 8.8 pound piece of debris could create the same impact as a car moving at 60 mph!

International Space Station
Since we keep track of larger debris, manned spacecraft are able to dodge them. When an object is expected to come within a few miles of the Space Shuttle, it changes its path to avoid the object. The International Space Station (ISS) can also move away from debris in its path. Plus, the ISS is also the most heavily shielded spacecraft ever. It can survive impact with smaller pieces of debris.

Image to right: The International Space Station floats above the Earth. Credit: NASA

Since the smallest pieces of debris cannot be tracked, collisions with them are bound to happen. The Space Shuttle often returns to Earth with tiny impact craters. And, impacts can even create small cracks in the front windows! Even though the spacecraft run into this debris quite often, the debris rarely runs into other debris. If fact, we only know of one time where this actually happened!

You may be wondering about the creation of more space debris. As we launch more and more objects, will Earth orbit turn into a dangerous, crowded junkyard? Space agencies around the world are working to make sure that does not happen. Since 1988, the United States has had an official policy to keep the creation of new orbital debris to a minimum. NASA even has an Orbital Debris Program Office at Johnson Space Center. This office looks for ways to stop us from creating more orbital debris. They are also looking for ways to get rid of the debris that is already there. Many U.S. aerospace firms also follow guidelines to reduce the creation of debris. We're not in this fight alone, though. The Russian, Japanese, French, and European space agencies are keeping the creation of new debris low, too.

Many things are being done to reduce the problem of orbital debris. The upper stages of launch vehicles, and some satellites, are being placed in lower orbits. This will cause them to re-enter the atmosphere and burn up sooner. Debris in orbits below 373 miles usually falls back to Earth within a few years. Objects at heights of more than 621 miles can stay in orbit for more than a century.

For years, humans have understood how important it is to protect our environment on Earth. Now, as we explore space, we are learning how important it is to protect the environment off the Earth, too.


Glossary

collision -- an accident resulting from violent impact of a moving object; a crash

debris -- the remains of something that has been destroyed or broken up

stage -- one of two or more propulsion units of a rocket vehicle; the second stage of the rocket fires after the first one has run out of fuel and dropped off from the rocket

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Adapted from NASAexplores