WASHINGTON D.C. - A U.S. Defense Department report says tests of space-based missile interceptors that could take place beginning in 2012 will create debris that could threaten the international space station, space shuttles and satellites in low Earth orbit.

However, to the study, conducted by the Missile Defense Agency (MDA), the threat is not severe because the debris likely would re-enter the atmosphere before colliding with anything, and NASA could move the international space station to avoid debris if necessary.

The MDA posted the "Draft Programmatic Environment Impact Statement" for national missile defense systems on its Internet site Aug. 9.

Most activities associated with land-based missile interceptors have no significant impact on the environment, or could be handled relatively easily by following safety procedures for handling hazardous waste and reducing harmful emissions in the air, the review concluded.

The MDA plans to begin testing space-based missile interceptors that rely on kinetic energy to destroy their targets around 2012. Debris from these tests is unlikely to cause problems on Earth, as it would probably burn up upon re-entering the atmosphere or land in an ocean or otherwise unpopulated area, the report said.

However, even tiny particles generated by collisions in space could pose a threat to people and spacecraft in orbit, the report said. Many of these particles would be too small for the Pentagon's space surveillance systems to track, the report said, noting that those surveillance systems generally cannot track objects less than 10 centimeters across.

Particles less one-tenth of a millimeter in diameter could damage a spacecraft, and would be especially troublesome if there were many collisions, the report said. Debris from one-tenth of a millimeter to 1 millimeter across could cause significant damage, and anything larger could penetrate a critical spacecraft component, like a flight computer or propellant tank, and lead to loss of the spacecraft, the report said.

The report said a chunk of debris 10 centimeters in diameter, the smallest sort of debris ground-based radars can track, might be as destructive to an orbiting spacecraft as 25 sticks of dynamite.

Spacewalking astronauts could have their space suits penetrated by debris even 1 millimeter in diameter, the report said.

The report states that in most cases, debris created by a missile defense test would re-enter the atmosphere before completing a full orbit, and therefore would put satellites at risk only briefly. In other cases, spacecraft like the international space station could be moved to avoid collisions with debris, the report said.

A Defense Department official said missile defense tests today are designed to avoid creating debris that could collide with satellites, and that the locations of manned and unmanned spacecraft will continue to be taken into consideration if the Pentagon conducts space-based missile defense tests.

Nicholas Johnson, chief scientist and program manager of NASA's Orbital Debris Program Office at Johnson Space Center in Houston, said missile intercepts are capable of producing debris large enough to damage the international space station, but the probability of impact "might indeed be small depending upon the nature and number of intercepts."

Johnson said he meets periodically with his counterparts at the MDA, and their discussions to date have focused on the potential for orbital debris hazards resulting from ground-based interceptor testing. He said the guiding principle has always been to alter any planned tests to eliminate any risk to the space station, as opposed to moving the station.

"Any debris, orbital or sub-orbital, created during future MDA tests which pose a risk to NASA space assets, including the [space station], are of concern to NASA," Johnson said. "Consequently, NASA has been engaged with MDA and other [Defense Department] organizations since early this year to address this issue."

Johnson said he has not engaged his MDA counterparts in detailed discussions of orbital debris hazards posed by a space-based missile defense system, primarily because such a system does not yet exist. Johnson said even ground-based interceptor tests have the potential to create at least temporary debris hazards in low Earth orbit.

Should missile defense tests produce debris large enough to be tracked by U.S. space surveillance assets, there is a collision-avoidance process that calls for the space station to maneuver if the risk from known orbital debris exceeds a certain threshold, Johnson said.

He said that the space station maneuvers once or twice a year to avoid known debris but that "to date none of these maneuvers have been associated with MDA tests."

While opponents of space-based interceptors have often focused on the political implications of using such systems, the debris threat discussed in the MDA report presents another "serious argument" against their development, said Theresa Hitchens, vice president of the Center for Defense Information, a think tank here.

The debris could pose a significant threat of impact with a spacecraft even in the course of one orbital pass, and that threat is multiplied if there is repeated testing, said Hitchens, who has publicly opposed the use of space-based weapons.

The debris threat also raises the issue of how much technical information about space-based intercepts the MDA can provide to other countries to protect their own satellites, and what capabilities those countries may have to avoid collisions, she said.

Frank Gaffney, president of the Center for Security Policy, a defense advocacy group based here and proponent of space-based missile defense systems, said that MDA likely could conduct the intercept tests at a low enough altitude in space that satellites would not be threatened.

However, the loss of spacecraft may be an acceptable risk in the case of a real attack if it means stopping a nuclear warhead from landing in the United States, he said.

The Pentagon also should take the threat of debris to its satellites as an incentive to push forward more aggressively with the development of rockets and payloads that can be launched on short notice to replenish its constellations, Gaffney said.