This image provided by NASA shows an artist concept of an Iridium satellite in earth orbit. Two big communications satellites collided Tuesday in the first-ever crash of its kind in orbit over Siberia.

U.S. and Russian officials traded shots Thursday over who was to blame for a huge satellite collision this week that spewed speeding clouds of debris into space, threatening other unmanned spacecraft in nearby orbits.

The smashup 500 miles (800 kilometers) over Siberia on Tuesday involved a derelict Russian spacecraft designed for military communications and a working satellite owned by U.S.-based Iridium, which served commercial customers as well as the U.S. Department of Defense.

A prominent Russian space expert suggested NASA fell down on the job by not warning of the collision. But U.S. space experts said the Russian has the wrong agency.

The U.S. military tracks the 18,000 objects in orbit, monitoring only certain threats because it lacks the resources to do everything, said Maj. Regina Winchester, spokeswoman for U.S. Strategic Command, which oversees the military’s Space Surveillance Network.

Iridium spokeswoman Elizabeth Mailander said the company can move any of its 65 satellites out of the way if it gets a precise warning ahead of a crash. Such a warning was not made Tuesday, Mailander said.

But the company has never redirected a satellite before because the warnings they get aren’t precise enough and there are just too many satellites to be constantly rejiggering their orbit, she said.

“Ours was where it was supposed to be and it was functioning,” Mailander said. She said Iridium hasn’t talked with Russian space officials.

No one has any idea yet how many pieces of space junk were generated by the collision or how big they might be. But the crash scattered space junk in orbits 300 to 800 miles (500 to 1,300 kilometers) above Earth, according to Maj. Gen. Alexander Yakushin, chief of staff for the Russian military’s Space Forces.

Experts in space debris will meet next week in Vienna at a U.N. seminar to come up with better ways to prevent future crashes, said NASA orbital debris program manager Nicholas Johnson.

Igor Lisov, a prominent Russian space expert, said Thursday he did not understand why NASA’s debris experts and Iridium had failed to prevent the collision, since the Iridium satellite was active and its orbit could be adjusted.

“It could have been a computer failure or a human error,” he said. “It also could be that they only were paying attention to smaller debris and ignoring the defunct satellites.”

But that job belongs to the U.S. Department of Defense’s Space Surveillance Network, which was created with NASA’s help.

The network’s top priority is protecting astronauts — warning if there is a threat to the international space station or manned spacecraft. And it gives NASA precise warnings for about a dozen satellites that could be maneuvered out of the way, something that happens once in a while, Johnson said.

There are 800 to 1,000 active satellites in orbit and about 17,000 pieces of debris and dead satellites, like the Russian one, that can’t be controlled, he said. The U.S. space tracking network doesn’t have the resources to warn all satellite operators of every possible close call, Johnson and Winchester said.

“It’s unfortunate that we cannot predict all of the collisions all of the time,” Winchester said.