By Paul Rincon
Science reporter, BBC News
The explosion of a Russian rocket stage in space may have created over 1,000 pieces of orbiting debris which could threaten other spacecraft.
Arabsat 4A was launched on a Proton rocket
The rocket section exploded on 19 February, generating as much debris as the destruction of a satellite by China last month, if not more.
This space wreckage could remain in orbit for years, experts say.
The anti-satellite test conducted by China on 11 January generated 817 confirmed fragments.
After the deliberate destruction of the Feng Yun 1C polar orbit weather satellite in January, the risk of a mission critical collision with the space station was raised by 60%, according to European Space Agency (Esa) estimates.
But this figure is likely to fall over the coming months as some fragments burn up in the atmosphere.
The most recent explosion was caused by a Breeze-M upper stage which was part of a Proton rocket launched from Baikonur Cosmodrome in Kazakhstan on 28 February 2006. It was intended to deliver the Arabsat 4A satellite into a geosynchronous orbit around Earth.
However, the booster rocket on the upper stage malfunctioned, putting the satellite in the wrong orbit. The satellite itself was intentionally de-orbited.
But on 19 February this year, almost a year after the failed launch, the upper stage - which still contained some of its fuel, exploded. The US Space Surveillance Network has so far picked up 1,111 pieces of debris.
After rockets deliver their payloads into orbit, it is standard practice to release remnant fuel, in order to avoid explosions.
This operation could not be performed by the failed Breeze stage, leaving a sizeable amount of propellant inside the stage, and triggering a high-energy explosion.
Esa officials estimate that the risk of a mission critical collision between debris from the Breeze-M stage and the ISS has been raised by 10%.
But unlike the orbiting detritus from the Chinese test, the debris from this event is likely to remain in orbit for years.
Nasa says there is little threat to the space station (Image: Nasa)
"The closest point to Earth is maintained [by the debris] at a fairly constant altitude for a long, long time," Heiner Klinkrad, head of Esa's orbital debris office told BBC News.
"These risk elevations, in contrast to the Chinese event, are longer lasting."
The Breeze-M upper stage is in an eccentric orbit which ranges from 15,000km-500km from Earth.
Its closest approach to Earth brings it into proximity with scientific, military and commercial satellites as well as the International Space Station (ISS).
The two events, which occurred within weeks of each other, are the worst for space debris in the history of space launches.
Ruediger Jehn, an orbital debris analyst at Esa, said the Chinese A-Sat test alone had caused roughly a 10% increase in collision warnings for the European Envisat and ERS remote sensing satellites.
About 10-30% of objects curently orbiting close to the satellites are debris from the Chinese A-Sat test.
Nasa officials said the debris was not expected to pose a threat to the International Space Station or the upcoming launch of space shuttle Atlantis in March.
Only particles of about 10cm or above can be tracked using radar. But both explosions would have generated millions of smaller fragments which are difficult to detect.
The space station has shields to protect it against objects under 1cm in size. Anything larger than 1cm would be perfectly capable of penetrating these shields.
If a threat arises from objects of 5-10cm and above, the space station can adjust its position to avoid the debris. But that leaves a gap, objects intermediate between these two size ranges are difficult to avoid.
No one is sure what caused the 19 February 2007 explosion of the Breeze-M stage. It could have been due to the corrosive hypergolic fuel it carried. Alternatively, it could have been caused by the impact of another piece of space debris, or a micrometeor.