Two American astronauts orbiting the Earth at 17,500 miles an hour on the International Space Station have voted in the US presidential election.

Using an elaborate system to ensure their ballots remain confidential, International Space Station Commander Edward Michael Fincke and ISS Flight Engineer and Science Officer Greg Chamitoff cast their votes by secure digital transmission.

Mission Control at Johnson Space Center in Houston, Texas shipped digital ballots to the space voters at an altitude of some 354 kilometers (220 miles) above the planet, and local election officials sent passwords directly to them.

The ballots were returned, encrypted, via Mission Control. Only the two Texas county clerks involved -- one astronaut lives in Harris County in Texas and another lives in Brazoria County -- and the astronauts had the keys to break the encryption.

"Nobody can get into the ballot," said Jennifer Ballard, election supervisor in Harris County, Texas. "I know because I've tested it. I tested it extensively."

The astronauts urged their Earth-bound compatriots to vote in a message broadcast by NASA. "Voting is the most important statement Americans can make. It's a cherished right," Station Commander Fincke said in the pre-recorded video. "So this election day, take time to go to the polls and vote. If we can do it, so can you."

"We do a lot of things to try to support their stay on the space station, which can last six months or more," said Johnson Space Center spokeswoman Nicole Cloutier.

An American astronaut voted from space for the first time in 1997, from the Russian Space Station Mir, and since then a total of four Americans have voted in space. A Texas law establishing special procedures for space voting was passed in 1997.

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