HOUSTON – The world’s space-faring nations must forge a cooperative strategy for the costly and technically challenging elimination of orbital debris that poses a growing threat to strategically important robotic as well as manned spacecraft, according to Nicholas L. Johnson, NASA’s chief scientist for orbital debris and the top U.S. representative to the United Nation on the issue.

Efforts over the past two decades to mitigate the growth through changes in the design and operation of launch systems and satellites have given way to a new concern – the increasing risk of collision between existing debris in Earth orbit, adding to the fragment population faster than the junk can make a destructive descent into the atmosphere naturally. “It took a lot of effort, energy and money to get these things into space, and by golly, it will take a lot of that to get it out,” Johnson told a Nov. 29 gathering of the Houston section of the American Institute of Aeronautics and Astronautics. “That is our challenge right now.”

A failure to methodically address the issue could place future generations of vital communications, navigation and weather satellites in jeopardy, he said.

Johnson’s concerns echo those of a Sept. 1 report issued by the National Research Council, Limiting Future Collision Risk to Spacecraft: An Assessment of NASA’s Meteoroid and Orbital Debris Programs. The prospect of new debris from collisions between existing space junk has reached a “tipping point,” according to the NRC, which urged a re-examination of internationally recognized restrictions that prevent any one nation from sweeping away the debris from another country’s spacecraft.

Last year, the Obama administration’s national space policy recognized the threat as well and assigned NASA and the Department of Defense with the responsibility of developing remediation technologies and techniques.

NASA’s Johnson Space Center, where Johnson leads NASA’s orbital debris assessment efforts, was recently charged by the agency’s chief technologist with forging a response that includes contributions from experts throughout the agency.

“It’s not a sense of urgency, but is a sense of responsibility,” said Johnson, who consults regularly on the issue with the Inter Agency Space Debris Coordination Committee and the International Academy of Astronautics as well as the U.N. “We know we can’t do it today, surely not economically and in some cases the technology is not quite there yet either,” he said. “But we know in the the long term we have to do something. So we need to start thinking about the problem seriously.”

The Pentagon’s U.S. Surveillance Network currently tracks 22,000 large objects in Earth orbit, with a combined mass exceeding 13.4 million lb. Only 1,000 of the total represent operational satellites. Experts place the population of debris greater than a half inch in size at 500,000, any one of which could prove lethal to the six-person International Space Station.

The debris threat has leaped in the past four years as a result of two major events. In 2009, a defunct Russian spacecraft collided with an active U.S. communications satellite. Two years earlier, China blasted one of its own polar orbiting weather satellites in an anti-satellite demonstration that prompted worldwide alarm.

Twice this year, the space station has been forced to make an evasive maneuver to avoid a debris collision threat – doubling the previous annual average for evasive action. Last week, a collision threat to the station appeared, then disappeared so quickly that the crew was nearly forced to take refuge in their Soyuz transport capsule.

Last year, NASA carried out six debris avoidance maneuvers among its 60 various robotic spacecraft, Johnson said.

The debris population grows by a “couple of hundred” fragments annually, mostly in response to an average half dozen “fragmentation” events, typically the result of a propulsion system explosion or weakened pressure tank.

About one piece of debris falls harmlessly back to Earth daily.