Indian-American astronaut Sunita Williams is keeping the company of the rain-drenched Boston marathoners up in the space by running on her treadmill in the International space station, where she will circle the Earth at least twice.

Sunita started the race on time at 10 a.m. EDT (1930 IST) with race No. 14,000 taped to the front of the treadmill. She has placed two laptop computers on either side of the treadmill just to watch a live feed of the race from Boston and keep track of where the ISS was flying.

Sunita, 41, an accomplished marathoner, will be running as fast as eight mph during the 42.16 kilometres simulated run, restrained to the treadmill in a harness so she wouldn't float away.

She is wearing her bib number 14,000 that has been issued to her by the Boston Athletic Association.

"Just want to let you know the team here is cheering for her and by our calculations she already has completed her 26 miles," Mission Control told her crew mate, Michael Lopez-Alegria, a few minutes into the race.

While racers in Boston bundled up on a raw day, the space station was balmy enough that Sunita was just wearing her navy shirt and shorts. Lopez-Alegria and Sunita woke up several hours earlier because of the marathon.

About 45 minutes into the start of the race, or a trip halfway around the world in the space station, Lopez-Alegria reported to Mission Control: "Suni's still smiling. That's a good sign."