Veteran Fyodor Yurchikhin and first-timer Jack Fischer set off in bright conditions at 8.13am United Kingdom time.
NASA astronaut Jack Fischer and Russian cosmonaut Fyodor Yurchikhin are on the docket for this flight, scheduled for take off at 3:13AM ET from Baikonur, Kazakhstan. About nine minutes into their voyage, the stuffed dog leaped into the air, then began to drift at the end of the string around its neck.
This is the first two-person launch to the ISS in more than a decade, with Russian Federation scaling back its space station staffing until a much-delayed science laboratory is flown to the $100bn (£78bn) space station next year.
Just six hours after launch, the pair docked at the space station.
Whitson, who flew to the station in November along with Russian cosmonaut Oleg Novitskiy and French astronaut Thomas Pesquet, will remain aboard with Fischer and Yurchikhin until September.
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Fischer's wife thanked him for what she said was "the most unexpected bouquet of flowers" that she received as he was launching into space.
Yurchikhin, 58, is a veteran cosmonaut making his fifth spaceflight with the Expedition 51 mission. "Fyodor told me I should be doing that", he said.
She is expected to receive a congratulatory phone call on Monday from U.S. President Donald Trump, NASA said on Wednesday. Commander Whitson, 57, in the midst of her third long-duration mission, is due on Monday to beat the 534-day record for cumulative time spent in space by a US astronaut.
Now on her third long-duration mission, she already holds the record for the most spacewalks conducted by a female astronaut.
Yurchikhin and Fischer are set to complete a five-month mission at the station.