A piece of space junk orbiting Earth has forced NASA to tweak upcoming plans for the Friday departure of Japan's first cargo ship to the International Space Station.
The debris, a wayward chunk of an old Russian satellite, poses no risk to the space station, but could be a threat to the H-2 Transfer Vehicle, an unmanned Japanese freighter making its maiden flight, after it departs Friday.
Japan's space agency launched the cargo ship last month and planned to undock it tomorrow at about 12:05 p.m. EDT (1605 GMT). But the Russian piece of space debris is expected to pass through the station's neighborhood around that time, so NASA opted to keep the Japanese spaceship attached to the orbiting lab for one extra orbit. It will now undock at around 1:30 p.m. EDT (1730 GMT).
The space station will not have to fire its thrusters to dodge the space junk. Delaying the HTV-1's departure will ensure that the unmanned spacecraft will be safe after it leaves the station and enters a different orbit, NASA officials said.
NASA typically moves the space station if the odds of a space debris impact are within a 1-in-10,000 chance. The agency also likes to keep a box-like buffer around the station free of any debris. That safety zone extends about 15 miles (25 km) around the space station, as well as about a half-mile (0.75 km) above and below it. The station flies in an orbit about 220 miles (354 km) above Earth.
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The Russian satellite debris will not encroach within that perimeter, NASA spokesperson Kelly Humphries told SPACE.com.
"They tracked it for a few days and determined it was going to be outside what we call the pizza box, and that the station will be safe," he said. "They just wanted to give HTV the same courtesy."
Space debris has been a growing threat for satellites and crewed spacecraft in orbit. Earlier this year, two communications satellites collided to create two vast clouds of space junk orbiting Earth. China also added to the debris mix in 2007 by intentionally destroying a satellite in orbit during an anti-satellite test.
Some experts have said the more than 20,000 objects being tracked by various agencies today already represent a serious threat to satellites and other spacecraft. They have called for a renewed push to limit the amount of junk from new satellites and develop clean-up measures for the orbits near Earth.
Japan's HTV-1 cargo ship launched to the space station on Sept. 10 and arrived on Sept. 17 after a smooth shakedown cruise. The vessel is the latest in a series of disposable unmanned international spacecraft to deliver supplies to the space station.
The spacecraft is due to undock Friday with nearly 1,600 pounds (728 kg) of trash and then plunge into the Earth's atmosphere on Sunday to be intentionally destroyed over the Pacific Ocean.
