Shuttle flights and other space activity could become too risky within 20 to 30 years if nations continue to litter the most traveled paths, the Office of Technology Assessment warned yesterday.

For instance, the congressional agency said, the Hubble Space Telescope, which has enough problems because of faulty optics, has been given a 1-in-100 chance of being severely damaged by space debris during its 17-year lifetime.

"Even small debris particles can produce high levels of damage," said Ray Williamson, who directed the study. "The average collision velocity in low Earth orbit is about 10 kilometers {6.2 miles} a second."

At such speed, an object having 1/35th the weight of an aspirin tablet would have the impact of a .30-caliber bullet, he said.

The National Aeronautics and Space Administration is including shielding for critical elements as it designs the space station and is looking into possible collision avoidance maneuvers for the shuttle, according to the OTA report.

In the only documented collision, a tiny paint chip damaged the windshield of space shuttle Challenger in 1983 to the extent that it had to be replaced.

As of last week, the U.S. Space Command was tracking 6,645 artificial objects orbiting Earth, each larger than a softball and weighing a total of about 4.5 million pounds. Only 6 percent were operating satellites.

Don Kessler, NASA's debris expert at the Johnson Space Center in Houston, said the agency is warned about once every shuttle mission by Air Force trackers that objects are in the path. About once every five missions, he said -- when the object is in an approaching "box" a mile high, a mile wide and three miles long -- NASA is advised to change course slightly.

Such a maneuver has never been made, he said, because even within that box the chances of a collision are estimated at one in 100,000.