A Russian Soyuz spacecraft arrived at the International Space Station Friday afternoon, bringing the number of astronauts at the ISS to six.

Expedition 26 crew members Dmitry Kondratyev, Catherine Coleman and Paolo Nespoli docked to the Rassvet mini-research module at 3:11pm Eastern time, NASA said. They launched from the Baikonur Cosmodrome in Kazakhstan at 2pm on Wednesday and will remain at the ISS until May 2011. They join Commander Scott Kelly and Flight Engineers Alexander Kaleri and Oleg Skripochka, who have been there since October 9 and will leave in March.

Kelly, Kaleri, and Skripochka kept busy while waiting for the new crew members to arrive. NASA said that Kelly on Thursday was monitoring bowling ball-sized satellites programmed by U.S. high school students to perform virtual spacecraft tasks as part of a student competition.

Kaleri and Skripochka also participated in a few experiments, including one designed to detect pressure leaks to improve spacecraft safety and to observe how crew members physically and mentally adapt to long duration missions in microgravity. They were also charged with the more mundane task of checking filters and windows in the Russian segment of the station.

The last three crew members to leave the ISS - American astronauts Doug Wheelock and Shannon Walker, as well as Russian cosmonaut Fyodor Yurchikhin - landed in Kazakhstan on November 26.

NASA has been trying to send its own shuttle, Discovery, to the ISS since early November, but weather, electrical issues, and cracks have pushed it back until at least February 2011. On Friday, NASA actually conducted a fueling test on Discovery since it was during fueling that NASA discovered those cracks.

Earlier today, NASA said "Discovery's instrumented external tank has been filled with 535,000 gallons of liquid oxygen & hydrogen. Data will take time to review."