The Pentagon's research and development division is studying concepts to remove dangerous space debris from orbit, an endeavor long dismissed as too costly but potentially feasible with technology advancements.

Artist's concept of space debris around Earth. Image not to scale. Credit: ESA

The study is called Catcher's Mitt and will explore technically and economically feasible ways to rid Earth orbit of space junk that threatens active space missions, including the International Space Station and valuable satellites.

The Defense Advanced Research Projects Agency, the Department of Defense's technology development arm, is leading the Catcher's Mitt study.

The analysis was the subject of a conference sponsored by DARPA and NASA this week in Washington. Nearly 300 participants and approximately 60 speakers attended the event.

Representatives from NASA, the European Space Agency, Russia, Japan and industry were at the first-of-a-kind conference, according to DARPA.

Presenters discussed exotic concepts including lasers, tethers, solar sails and methods of capturing debris to move objects out of often-used orbits. Some of these techniques, if practical, promise to be less costly than older concepts.

Removing space debris from orbit is not a new idea. There have been evaluations of techniques for such an effort since the 1970s, but they have all fallen flat because of insurmountable technical and cost hurdles.

Any debris removal system will also have to contend with legal and policy issues. Nations and companies can retain ownership of hardware in orbit after it is retired, so an international agreement or legislation may be required.

"Cost is often the principal factor impeding orbital debris removal concepts," said Nicholas Johnson. "Several concepts appear to be technically feasible, but the expense of removing a single large object or numerous small ones has not yet proven cost-effective. However, technology advances and perhaps future reductions in space transportation costs could alter this situation."

Despite the expense and challenges, experts believe the only way to solve the space debris problem is to remove objects from orbit.

"NASA studies have shown that the current Earth satellite population is unstable and will lead to the creation of new debris via accidental collisions at a rate faster than the debris will be removed by natural forces," Johnson said. "The only means of curtailing this debris growth is to remove existing resident space objects, preferably the larger and more massive ones in highly congestion regions first."

International space agencies have agreed to guidelines to improve the space debris environment. The guidelines aim to reduce the odds of orbital collisions, limit debris release events, minimize the potential for on-orbit breakups, and set standards for disposal of old rockets and satellites.

A depiction of a rocket's upper stage exploding, creating more space debris. Credit: ESA

But the population of space debris is predicted to grow at faster rates in the future. Two significant events fundamentally changed the debris environment in the last three years, creater an even greater sense of urgency.

In January 2007, China tested an anti-satellite weapon and destroyed a weather satellite, creating a cloud of debris comprising nearly 3,000 objects.

An accidental collision between a Russian military satellite and a commercial communications spacecraft in February added nearly 2,000 more objects to the crowded low-altitude orbital catalog.

Roughly 15,000 objects larger than 4 inches circle Earth, and about 95 percent of those are fragements or unusable satellites and rocket stages. Hundreds of thousands more pieces of debris, smaller than the palm of a hand, also pose a serious threat to operational missions, but military radars can't track those objects.

Scientists predict a cascading number of orbital collisions in the coming years, as more objects are launched and liberated during impacts.

The likelihood of a debris collision with an individual spacecraft is relatively low, but the consequences are catastrophic and the odds will only increase in the coming decades, according to analysts.

"Current analysis indicates collisions between orbital objects could potentially lead to a sustained growth in the debris population. Debris mitigation alone will not be sufficient to prevent a continual increase in the number of debris objects," says a DARPA fact sheet on the Catcher's Mitt study.

Johnson said he expects about one accidental collision every five years for the next few decades, each producing more debris.

"Later in this century the collision rate will increase," Johnson said. "Therefore, the sooner an effective debris removal technique can be employed, the better."

DARPA is collecting information for the study from this week's conference, independent utility analyses by the military and NASA, and a formal request for information released to industry in September.

"DARPA intends to use the results of these three approaches to determine whether DARPA investment in a new program is warranted and where and how to be most effective. If justified, potential follow-on efforts might include a new DARPA led program, or DARPA support for an effort led by another U.S. government organization," said Wade Pulliam, a program manager with the DARPA Tactical Technology Office.