An October explosion in orbit of the upper stage of a Russian Proton rocket has left hundreds of pieces of debris circling the globe at altitudes that pose a danger to operational satellites but apparently not to the international space station, according to a Jan. 15 assessment by NASA.

The U.S. space agency said the Proton Breeze-M upper stage, which weighs 2,600 kilograms, exploded because of a 10,000-kilogram fuel tank that was still attached to it after the stage shut down prematurely following liftoff Aug. 6.

The failure left the Breeze-M stage and its fuel tank in an orbit of 250 kilometers by 5,015 kilometers, according to NASA’s Orbital Debris Office at the Johnson Space Center in Houston.

The explosion occurred at 290 kilometers. NASA said the U.S. Space Surveillance Network detected 700 pieces of debris and had catalogued 111 of them by late December.

To get a better fix on the extent of the debris field, NASA solicited assistance from the Massachusetts Institute of Technology Lincoln Laboratory’s Haystack radar and the Goldstone radars operated by NASA’s Jet Propulsion Laboratory in California.

These radars are able to detect centimeter- and millimeter-size objects in low Earth orbit. The results confirmed that the total debris population was “much larger … than could be tracked by the Space Surveillance Network.”

The debris field crosses the path of the international space station, which flies at an altitude of about 415 kilometers, twice each orbit. But an assessment of the threat to the station has concluded that it is minimal, NASA said.