An artist's concept of ARGOS orbiting the Earth. Photo: Boeing

The University of Chicago's space dust instrument (SPADUS) has detected a cloud of tiny debris particles that was scattered into space when the upper stage of a Chinese Long March 4 rocket unexpectedly exploded March 11 after five months in orbit.

The U.S. Space Command detected the explosion immediately, but the Chicago finding marked the first time that scientists have been able to link ultra-small particles to the break-up of a particular satellite, said Nicholas Johnson, chief scientist for orbital debris at NASAšs Johnson Space Center in Houston.

The largest of the particles would barely be large enough to see with the naked eye. But they race around Earth 19 times faster than a speeding bullet, fast enough to scratch a space shuttle window on impact, or cause potential problems during a space walk.

"They are the kind of particles that hit spacecraft all the time," Johnson said. "We typically replace one space shuttle window after every mission because of impacts from orbital debris and natural meteoroids."

NASA routinely assesses the probability that a space shuttle or the International Space Station will pass through a known stream of orbital debris. The shuttle and the space station can maneuver around the larger pieces, but NASA wants to know the whereabouts of the smaller ones as well, Johnson said.

"For example, if we think there's a chance of the space station going through one of these very small particle clouds, we may decide to postpone a spacewalk," he said.

The smaller particles also pose a minor but continuous threat to orbiting satellites, said Anthony Tuzzolino, the University of Chicago scientist who heads the SPADUS project.

"It's just like sandblasting something," Tuzzolino said. There is damage. It's not catastrophic damage, but slowly evolving, erosive damage."

SPADUS was launched aboard the Air Force's unclassified Advanced Research and Global Observation Satellite (ARGOS) on Feb. 23, 1999, from Vandenberg Air Force Base in southern California. ARGOS follows a polar orbit at an altitude of 516 miles. The $2 million SPADUS instrument is funded by NASA, the Office of Naval Research, the Naval Research Laboratory and Lockheed Martin.

The Boeing Delta 2 rocket awaits liftoff from Vandenberg with ARGOS onboard. Photo: Boeing

During its first year in orbit, SPADUS recorded 195 impacts. Some impacts were produced by natural cosmic dust or cometary debris flying in from interstellar space, but most were the result of man-made orbital debris, Tuzzolino said. SPADUS normally detects one orbital debris impact every two days. When Tuzzolino found three spikes in his data earlier this year, he ran an analysis and found that the increased impact rate was due to three debris clouds.

The most pronounced of the three debris clouds is the only one that Tuzzolino and Johnson have firmly linked to the exploding Chinese rocket. That cloud produced approximately 40 impacts from March 25 to April 1. After Tuzzolino reported his finding to Johnson, the latter immediately suspected the Chinese rocket as the source.

Comparing the impact data with the trajectories of the ARGOS satellite and the Chinese rocket, they found a strong correlation. SPADUS generally recorded more impacts when ARGOS crossed or neared the orbital plane of the Chinese rocket.

"The time association is more than coincidental," Johnson said. The Chinese rocket exploded at an altitude 60 miles lower than ARGOS, but it is well-known that such events throw debris into a higher orbit, Johnson said.

The U.S. Space Command routinely tracks all orbital objects larger than four inches in diameter. NASA attempts to monitor the smaller objects, which sometimes can be detected with ground-based radar and optical sensors.

After each explosion in space -- approximately 160 have been identified -- NASA runs computer simulations to estimate the number and size of pieces that might be generated. NASA's model simulates the orbital behavior of particles ranging from a centimeter (less than half an inch), to a millimeter (four-hundredths of an inch).

"The particles that Tony is picking up are much, much smaller than a millimeter, so this will be the first time we've been able to make some estimate as to how many of those there might be," Johnson said.

The most recent explosion of the 2,200-pound Long March 4 rocket produced more than 300 fragments large enough to be tracked by the U.S. Space Command. The Long March 4 has flown four times: once in 1988 and 1990, and twice in 1999. Its upper stage was designed to remain in orbit after delivering its satellite payload, as with most such rockets, Johnson said. But one month after the 1990 launch, the upper stage broke into nearly 100 detectable fragments.

"We believe the cause of the explosion was residual propellant left on board the upper stage," Johnson said. It's a problem that most of the spacefaring nations have experienced, including the United States. The Chinese reported that they changed their operational procedures to address the residual propellant problem, but one of the 1999 attempts apparently failed.

"We discovered these types of break-ups almost 20 years ago. We have pretty much fixed our vehicles and we've helped everybody else who's had a problem fix theirs, as well. It doesn't happen nearly as often as it used to," Johnson said.