MOSCOW (AP) — 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," said Winchester.

A private Web site, named Socrates, does give daily risk of crash warnings for satellites and Iridium, with 65 satellites, frequently is in the top 10 daily risks, Johnson said. However, the Iridium satellite wasn't on Tuesday's warning list, he said.

Lisov said the debris may threaten a large number of earth-tracking and weather satellites in similar orbits.

"There is a quite a lot of satellites in nearby orbits," he told The Associated Press. "The other 65 Iridium satellites in similar orbits will face the most serious risk, and there numerous earth-tracking and weather satellites in nearby orbits. Fragments may trigger a chain of collisions."

Both the U.S. surveillance network and Russian Space Forces are tracking the debris, believed to be traveling at speeds of around 200 meters — or about 660 feet — per second.

NASA said it would take weeks to know the full magnitude of the crash, but both NASA and Russia's Roscosmos agencies said there was little risk to the international space station and its three crew members.

Russian Mission Control spokesman Valery Lyndin noted the station's orbit has been adjusted in the past to dodge space debris.

The space junk also is unlikely to pose a threat to the space shuttle set to launch Feb. 22 with seven astronauts, U.S. officials said, although that issue will be reviewed.

The Iridium orbiter weighed 1,235 pounds (560 kilograms), and the decommissioned Kosmos-2251 military communications craft weighed nearly a ton. The Kosmos was launched in 1993 and went out of service two years later in 1995, Yakushin said.

Some Soviet-built, nuclear-powered satellites long out of action in higher orbits may also be vulnerable to collisions, Lisov said. If one of them collides with the debris, the radioactive fallout would pose no threat to Earth, he said, but its speeding wreckage could multiply the hazard to other satellites.

Iridium said the loss of the satellite was causing brief, occasional outages in its service and it expected to fix the problem by Friday. The Bethesda, Maryland-based company said it expected to replace the lost satellite with one of its eight in-orbit spares within 30 days.

The replacement cost for an Iridium satellite is between $50 million and $100 million, including the launch, said John Higginbotham, chief executive of Integral Systems Inc., which runs ground support systems for satellites.

AP Writer Vladimir Isachenkov in Moscow and AP Technology Writer Peter Svensson in New York contributed to this report. AP Science Writer Seth Borenstein contributed from Washington.

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