Johnson and his team have devised a computer model capable of simulating past and future amounts of space junk.
The model predicts that even without future rocket or satellite launches, the amount of debris in low orbit around Earth will remain steady through 2055, after which it will increase.
While current efforts have focused on limiting future space junk, the scientists say removing large pieces of old space junk will soon be necessary.
Researchers present an overview of the space junk problem in tomorrow's issue of the journal Science.
Ripping Holes
Since the launch of the Soviet Union's Sputnik I satellite in 1957, humans have been generating space junk.
The U.S. Space Surveillance Network is currently tracking over 13,000 human-made objects larger than four inches (ten centimeters) in diameter orbiting the Earth. These include both operational spacecraft and debris such as derelict rocket bodies.
"Of the 13,000 objects, over 40 percent came from breakups of both spacecraft and rocket bodies," Johnson said.
In addition, there are hundreds of thousands of smaller objects in space. These include everything from pieces of plastic to flecks of paint.
Much of this smaller junk has come from exploding rocket stages. Stages are sections of a rocket that have their own fuel or engines.
These objects travel at speeds over 22,000 miles an hour (35,000 kilometers an hour). At such high velocity, even small junk can rip holes in a spacecraft or disable a satellite by causing electrical shorts that result from clouds of superheated gas.
Three accidental collisions between catalogued space-junk objects larger than four inches (ten centimeters) have been documented from late 1991 to early 2005.
The most recent collision occurred a year ago. A 31-year-old U.S. rocket body hit a fragment from the third stage of a Chinese launch vehicle that exploded in March 2000.
"We've been fortunate that in all three cases only a few [new] debris [fragments] have been created," Johnson said.
Best-Case Scenario
Previous space junk projections have assumed that new satellites and rockets would launch in the future.
The new study, in contrast, looks at what would happen to the amount of space junk if no rocket bodies or spacecraft were launched in the next 200 years.
"This is kind of a best-case scenario," said lead study author Jer-Chyi Liou, principal scientist and project manager for orbital debris with the Engineering Science Contract Group at NASA's Johnson Space Center in Houston.
The results suggest that new fragments from collisions will replace the amount of objects falling out of orbit and back to Earth. Beyond 2055, however, fragments from new collisions will exceed the amount of decaying debris.
"The debris population will continue to grow," Liou said. "We know it will only get worse."
Removing Junk
Johnson, the program manager for orbital debris, says space-faring nations agree that the space junk problem needs to be addressed. There is even a special organization called the Inter-Agency Space Debris Coordination Committee, made up of space agencies from ten countries and the European Space Agency.
So far, efforts have concentrated on preventing new debris. Johnson believes it may be time to think about how to remove junk from space.
But that is a difficult proposition.
Previous proposals have ranged from sending up spacecraft to grab junk and bring it down to using lasers to slow an object's orbit to cause it to fall back to Earth more quickly.
Given current technology, those proposals appear neither technically feasible nor economically viable, Johnson admits.
But, he says, the space-junk problem needs more attention.
"It's like any environmental problem," he said. "It's growing. If you don't tackle it now, it will only become worse, and the remedies in the future are going to be even more costly than if you tackle it today."
