Two astronauts were on Friday ready for a 12-month stay at the International Space Station (ISS) in a longest human expedition to space so far and testing the limits of human research, space exploration and the human spirit.
NASA's Scott Kelly and Russian cosmonaut Mikhail Kornienko will be part of research to help the US space agency and the international partners reduce risks and better understand how to ensure that astronauts will thrive on longer missions.
The launch was set for 3.42 p.m. EDT on Friday (1.12 a.m. IST on Saturday).
Most expeditions to the space station last four to six months.
By doubling the length of this mission, researchers hope to better understand how the human body reacts and adapts to long-duration spaceflight, the US space agency said in a statement.
This knowledge is critical as NASA looks toward human journeys deeper into the solar system, including to and from Mars, which could last 500 days or longer.
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It also carries potential benefits for humans here on the Earth, from helping patients recover from long periods of bed rest to improving monitoring for people whose bodies are unable to fight infections.
Long exposure to a zero-gravity environment can affect the human body in multiple ways.
Some physical symptoms can include changes to the eyes, muscle atrophy and bone loss.
Human psychology is also an important area of study, as the effects of living in isolated and small spaces will be important to understand ahead of future human missions to Mars.
There are seven key elements of research on the one-year mission.
Functional studies will examine crew member performance during and after the 12-month span.
"Behavioural studies will monitor sleep patterns and exercise routines. Visual impairment will be studied by measuring changes in pressure inside the human skull," the statement read.
Metabolic investigations will examine the immune system and effects of stress.
Physical performance will be monitored through exercise examinations.
Researchers will also monitor microbial changes in the crew, as well as the human factors associated with how the crew interacts aboard the station.
A number of spaceflight endurance records will be broken during the one-year mission, including the most cumulative time in space for any US astronaut.
Kelly will spend 342 days off the planet resulting in a total of 522 days in space, allowing him to surpass current US record holder Mike Fincke's mark of 382 days.