The proliferation of garbage in low Earth orbit has reached a point where it will increase in the coming decades even if all rocket launches were canceled starting now, according to research by NASA's Johnson Space Center.

The satellites and rocket stages designed and launched before the seriousness of the problem was recognized are like time bombs, waiting to break apart in the coming years on combustion of their remaining fuel thereby multiplying the pieces of space garbage.

At some point, the growing population of orbiting debris increases the likelihood that pieces will collide into each other, spawning still more space junk.

The problem is especially acute at altitudes of between 900 and 1,000 kilometers.

Research by NASA's Nicholas L. Johnson and J.C. Liou conclude that even if all launches were stopped immediately, the debris population would remain constant for about 50 years, then increasing noticeably after that as a result of in-orbit collisions.

No less an expert than Arthur C. Clarke, who first discovered the virtues of geostationary orbit 36,000 kilometers above the equator for communications satellites, warned that space exploration is more likely to be shut down by low-orbit debris than by anything else.

In response to Space News inquiries, Johnson said July 26 that while removal of large pieces of debris might be the only answer to the problem, "no cost-effective means to remove derelict large objects from Earth orbit has yet been identified. This remains an area of research."

The breakup in May and June of two Russian rocket stages illustrates the problem.

In May, the third stage of a Russian Tsyklon rocket spontaneously exploded, creating more than 50 pieces of debris large enough to be tracked by the U.S. Space Surveillance Network of ground-based sensors. The network can identify objects in low Earth orbit as small as 10 centimeters. For objects in geostationary orbit, objects of 1-meter-diameter and larger can be tracked.

In June, a motor from a Russian Proton rocket engine launched in the late 1980s broke up. The engine was in a highly elliptical orbit with a perigee of 655 kilometers and an apogee of 18,410 kilometers, according to a NASA study of Space Surveillance Network data.

Tsyklon and Proton rocket stages and motors are well-known among those tracking orbital debris. The May breakup was the fifth of its type since 1988 for a Tsyklon stage. Simulations of the trajectory of the 50-plus pieces of junk created in the May event suggest that nearly half of them will re-enter Earth's atmosphere and burn up this summer.

The June Proton motor breakup was the 34th of its kind and created more than 70 detectable pieces of debris.

Tracking relatively small pieces of debris needs to be done regularly because their trajectory can be predicted only for a few days, Johnson said. "Hence, it is not possible to determine if any Tsyklon or Proton breakup debris might come close to any operational spacecraft for other than very short periods," he said.

Tsyklon and Proton once both employed a design that did not permit their upper-stage engines to be rendered passive in orbit, meaning they would not explode from the combustion of their remaining fuel, or following a collision with another piece of debris.

Johnson said Tsyklon vehicles, built by the Yuzhnoye Design Bureau of Ukraine, continue to feature the kind of upper stage that makes them dangerous as debris generators. The Proton vehicle upper stage has been redesigned, he said, and now features engines that are rendered passive in orbit.

Johnson said there remain slightly more than 100 Tsyklon stages and more than 50 Proton stages orbiting the Earth with the same debris-generating design.

The Inter-Agency Space Debris Coordination Committee, whose members include most of the active space-faring nations, has developed guidelines for satellites and rockets to reduce the chance that they will contribute to the debris problem once in orbit.

The U.S. Federal Communications Commission, which licenses U.S. satellites, also has developed mandatory rules for debris mitigation.

NASA keeps what it calls a "box score" of nations responsible for the current debris population as measured by ground-based radars and telescopes. As of July 4, the U.S. Space Surveillance Network was tracking 9,680 pieces of debris.

Not surprisingly, the former Soviet Union and the United States are running a close race for first and second place among the nations responsible for debris, with 4,151 and 4,058 pieces, respectively.