Space debris orbiting the Earth is building up to a critical point, and it's threatening the environment and spacecrafts alike, say scientists.

And objects colliding in space are leading to fragmentation, thus creating more debris.

Unless active debris removal is enforced, the situation will only worsen, and might pose the biggest engineering challenge of the 21st century, says J.C. Liou of NASA's Orbital Debris Program Office.

Despite full compliance with mitigation measures by nations to control space junk already in place, Liou says "It would not be enough. ... We have reached that point. We need to take more aggressive measures."

Orbital debris is an environmental problem, says Liou. "And it's everybody's problem, not just the United States' or a NASA problem. All the spacefaring nations share the same environment."

In order to achieve this goal, Liou says an international strategic plan of "four C's" is needed: consensus (on ADR), cooperation (removal targets may belong to a different country), collaboration, and contributions (cost-sharing).

"As the international community gradually reaches a consensus on the need for ADR, the focus will shift from environment modeling to completely different challenges: technology development, systems engineering, and operations," Liou wrote in the July edition of NASA's Orbital Debris Quarterly News.