Washington: Indian American astronaut Sunita Williams has already voted by absentee ballot, but if she wanted she could have voted in the US presidential election from her home in space on election day itself.

Williams, a captain in the US Navy, who is the current commander of the International Space Station, voted from Russia using the same absentee ballot that troops abroad use before she went on her second space sojourn in July.

So did flight engineer Kevin Ford, the second American on the six-member crew currently at the space lab. He voted with a Texas absentee ballot while stationed in Russia before the launch.

But if they wanted they could vote from space as NASA astronauts aboard the space station have the option of voting from space, thanks to savvy technology and a 1997 bill passed by Texas lawmakers.

Astronauts working on the space station receive a digital version of their ballots in a secure email, which are beamed up to them from Mission Control at the Johnson Space Centre in Houston, and then sent back to Earth, ABC News reported citing a NASA official.

"They send a secure ballot to Mission Control," NASA spokesman Jay Bolden was quoted as saying. "They'll send it up to our astronauts onboard, and they fill it out and send it back as a private email. Astronauts do have privacy in the matter."

The ballot is then sent directly to voting authorities. "The election folks count it and tabulate it just like any other vote," Bolden said.

Astronauts can vote this way in any kind of US election. Six astronauts have used the method to vote in presidential elections since it became law in 1997.

NASA astronauts Leroy Chiao, Edward Michael Fincke and Greg Chamitoff have all voted while aboard the space station.