US astronaut Peggy Whitson breaks American spaceflight 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 previously held by Jeff Williams. The 879-day global record, held by Russian cosmonaut Gennady Padalka, still stands.

By the time Whitson returns to Earth in September, she will have spent 666 days in space. 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 International Space Station earlier this month. "I'm definitely here for conducting the science."

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

"We still have some critical questions to answer," she told Kerley, including around the medical complications that come with living in zero gravity, like effects on 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."

President Trump and first daughter Ivanka Trump called into the ISS from the Oval Office to congratulate Whitson on her achievement Monday morning.

"This is a very special day in the glorious history of American space flight...That's an incredible record to break," President Trump said. "And on behalf of our nation, and frankly, on behalf of the world, I'd like to congratulate you. That is really something."

Whitson said it was an huge honor to break this record and to represent everyone at NASA "who make this space flight possible and who make me setting this record feasible."

She also said the International Space Station is providing "a key bridge from us living on earth to going somewhere in deep space," and it is crucial to the Mars mission.

President Trump asked about a timeline for sending Americans to Mars, to which Whitson responded, "It will approximately be in the 2030s."

Trump then said he hopes to make that happen in his first or second term, "So we'll have to speed that up a little bit, OK?" Trump said.

"We'll do our best," said Whitson, laughing.

ABC News' Gina Sunseri and Jeffrey Cook contributed to this report.