Boston, March 30 (AP): Zooming through low-Earth orbit at 17,500 mph, Sunita Williams completes the standard marathon distance every 5.4 seconds.

But for next month's Boston Marathon, the US Navy commander will run the equivalent distance on a treadmill - 338 kilometres above Earth in the international space station, and tethered to her track by bungee cords so she does not float away.

"She thought it would be cool if she gave it a try," said Williams' sister, Dina Pandya, who will run the race the traditional way with almost 24,000 other runners. "She said, 'I'll call you on Heartbreak Hill.'"

Although the world's oldest annual marathon starts at 1930 IST on Earth, Williams might not be able to run at that time because her sleep schedule - a fairly arbitrary matter in space - is set for the arrival of a Soyuz mission.

"I'm not sure the timing will be that she'll be awake," Pandya said. "They're going to be on Russian time, so they're kind of sleep-shifting."

Williams, 41, qualified for the Boston race by finishing last January's Houston Marathon in 3 hours, 29 minutes, 57 seconds. On December 9, Williams took off on the space shuttle Discovery and it became clear she was not going to make it to the starting line.

"I considered it a huge honour to qualify, and I didn't want my qualification to expire without giving it a shot," Williams told the Boston Athletic Association, which organises the race.