Three spacefarers are set to blast off for a six-month mission to the International Space Station (ISS) from the Russian cosmodrome in the Kazakh city of Baikonur.

The crew - Alexander Gerst of Germany, Serena Aunon-Chancellor of the US and Sergei Prokopyev of Russia - are scheduled to lift off on Wednesday for an "Horizons" mission to the ISS to conduct research 400km above the Earth.

Gerst recently showed himself relaxed about the trip to the ISS in the service of the European Space Agency (ESA) thanks to his experiences on the Blue Dot mission in 2014.

As Soyuz co-pilot, he has greater responsibility than last time, when he was a simple on-board engineer. He will also be taking command of the ISS from October for three months.

Ahead of blast-off, a Russian Orthodox priest blessed a rocket on Tuesday.

The blessing of the Soyuz rocket is one of several traditions of Russian space travel. Others include the planting of a tree and the watching of a classic Soviet-era film from the 1970s.

Some of the luck-bringing customs date back to space pioneer Yuri Gagarin, but the blessing was first introduced in the 1990s.

The priest, who introduced himself at Father Sergei, told dpa on Tuesday he had blessed every rocket and crew for 20 years. "All of them have come back to Earth," he said.