Live coverage: 400th Proton rocket takes off with Russian satellite

Russian officials have delayed until Saturday the launch of a Proton rocket from Kazakhstan with the Spektr-RG X-ray astronomy observatory, an ambitious mission jointly developed with Germany to probe dark energy.

A Russian Soyuz booster arrived at its launch pad Sunday in Kazakhstan for two days of final countdown preparations ahead of liftoff with a Russian cosmonaut and two U.S. astronauts Tuesday on the way to the International Space Station.

A Proton rocket blasted off from the Baikonur Cosmodrome in Kazakhstan at 2040 GMT (4:40 p.m. EDT) Friday with Turkey’s Turksat 4B communications satellite.