Kvinner i rommet - Peggy Whitson med amerikansk rekord - Curt Lewis

US astronaut Peggy Whitson breaks American spaceflight record

• Commander of international space station set to break a big record

• U.S. astronaut Peggy Whitson is smashing records left and right.

Whitson, 57, broke the record for the most cumulative time in space by an American astronaut early Monday, streaking past the 534-day record held by Jeff Williams. The 879-day global record, held by Russian cosmonaut Gennady Padalka, still stands.

By the time she returns to Earth in September, Whitson will have spent 666 days in floating above the planet. She hopes she won't hold the title for long.

"I'm not here because of the record," Whitson told ABC News' David Kerley via video teleconference from aboard the ISS earlier this month. "I'm definitely here for conducting the science."

The research she's doing is "a really important stepping stone" to sending astronauts on even longer missions to Mars -- "the sooner the better," Whitson hopes.

However, "we still have some critical questions to answer," including the medical complications that come with living in zero gravity, like bone density and muscle constriction, she told Kerley.

"I think the biggest hurdle probably for the human body is going to be the radiation ... and probably the easiest solution is to get there faster so that you take less risk along the way." she said.

Whitson, an Iowa native, is no stranger to shattering records. In 2008, she became the first woman to command the ISS, and just last month -- during her eighth spacewalk -- Whitson surpassed NASA's Sunita Williams for the woman with the most cumulative "extra-vehicular activity" time.

Her journey hasn't always been smooth sailing.

During re-entry following her second mission in 2008, her Soyuez capsule experienced a technical glitch, sending it hurtling into a violent dive and exposing the crew to forces eight times more powerful than the earth's gravity for more than a minute.

Nevertheless, her time in space is "one of those rides you hope never ends," Whitson tweeted Sunday. "I am so grateful for all those who helped me on each of my missions."