(Adds Russia nuclear, Pentagon, European space officials' quotes)
By Guy Faulconbridge
MOSCOW, Feb 12 (Reuters) - Showers of debris from the collision between a U.S. satellite and a defunct Russian military satellite forced scientists on Thursday into an urgent calculation of the chances of another crash in space.
Concern centres around the safety of the manned International Space Station while a Russian expert talked of the dangers posed by a crash involving one of the Soviet-era satellites powered by nuclear reactors.
Tuesday's crash, which Russian officials said took place at about 1700 GMT above northern Siberia, is the first collision between satellites that has been made public.
It happened in an orbit heavily used by satellites and other spacecraft and the U.S. Strategic Command, the arm of the Pentagon that handles space, said countries might have to manoeuvre their craft to avoid the debris.
Pentagon spokesman Bryan Whitman said it was not possible for the U.S. military to track and predict the movements of all 18,000 man-made objects in space all the time.
Asked if the military had identified any danger to its satellites from the debris, he said: "No. They're tracking those pieces now but it will take some time to get a full picture of the orbits of those objects."
Russian Space Forces said it was monitoring debris that was spread over altitudes between 500 km (310 miles) and 1300 km (807 miles) above earth.
"The collision of these two space apparatuses happened by chance and these two apparatuses have been destroyed," Major-General Alexander Yakushin, the forces' first deputy commander, told Reuters.
Debris could hit Soviet-era satellites with nuclear reactors, Interfax news agency reported, quoting an unnamed Russian space expert.
"The debris got scattered in all directions, including upwards, where old Soviet satellites are 'buried', those which were intended for surveillance over the navy of a potential enemy ... (and are) equipped with nuclear reactors," Interfax quoted the expert as saying.
"There is a threat of collision with these spacecraft and therefore of the appearance of radioactive debris in orbit," the expert said.
The collision between the Iridium Satellite LLC-operated satellite and the Russian Cosmos-2251 military satellite occurred at about 485 miles (780 km) above the Russian Arctic.
That is an altitude used by satellites that monitor weather, relay communications and perform scientific observations.
European space scientists were aware the Russian and U.S. satellites were going to pass close to each other. But the difficulty of predicting orbits and "noise" from thousands of pieces of debris had made a definitive prediction of a collision impossible, European officials said.
"The 'catalogue' of objects and debris showed a possible approach between the paths of the two satellites but an approach doesn't necessarily mean a collision, and you would need more information to be certain," said Philippe Goudy, deputy director of the French space operations control centre at Toulouse.
The International Space Station orbits at 220 miles (350 km), substantially below the collision altitude. One Russian and two U.S. astronauts are currently aboard the station.
The orbit of the ISS can be changed by controllers from Earth but even a tiny piece of debris can cause significant damage to the station as it travels at eight km (five miles) per second.
The crash has underlined concerns about how crowded the orbit paths around earth have become in recent decades.
The collision happened not far from the orbit of a defunct weather satellite blown to pieces by a ground-based missile in a Chinese weapons test in 2007. European and U.S. officials argue the resulting debris made it harder to identify crash risks.
Scientists say predicting collisions is difficult because of the unpredictable behaviour of other objects, solar radiation and the gravitational effect of the moon and earth, while at 800 km molecular wisps of atmosphere can gently skew orbits.
But experts said the chances of a collision are extremely low. "The orbital altitude where the collision took place is among the most crowded in low Earth orbit," Texas-based security consultancy Stratfor.com said in a research note.
"But statistically speaking, the enormous scale of space makes the chance that this kind of direct collision would occur completely by accident infinitesimal."
The European Union said on Thursday leading nations should adopt a code of conduct for civil and military activities in space. [ID:nLC240693] (Additional reporting by Conor Sweeney, Tatiana Ustinova in Moscow, Tim Hepher in Paris and Jim Wolf, Andrew Gray in Washington; editing by Christian Lowe)