A Russian Soyuz-21 spacecraft is going through final pre-flight checks at the Baikonur space centre in Kazakhstan.
The crew, which includes an American, is due to blast-off later and should dock with the International Space Station on Thursday.
The rocket launches just ahead of the 50th anniversary of Yuri Gagarin’s first ever manned space flight.
At the same time NASA delayed the launch of space shuttle Endeavour to April 29 from April 19 because of a scheduling conflict at the International Space Station.
Endeavour’s mission will be the second-to-last for NASA’s shuttle programme and will deliver a particle detector called the Alpha Magnetic Spectrometer to the orbital outpost.
NASA pushed back the launch date to avoid having Endeavour’s mission overlap with the arrival of a Russian cargo ship at the station.
Soyuz prepares for ISS space flight