Two Russian cosmonauts on Thursday completed a marathon spacewalk in which they launched a student-made satellite honouring the first spaceman Yuri Gagarin after initially aborting the delicate task.

The Russian space agency announced that the six hour and 22 minute mission from the International Space Station (ISS) was "successfully completed" and the Gagarin mini satellite was in orbit and busy sending signals to Earth.

The spacewalk was the 35th conducted by Russian cosmonauts since construction of the international orbiter began in 1998.

But the latest excursion by Sergei Volkov and Alexander Samokutyaev came as the world's gaze focused on Russia's ability to pick up the mantle from the retired US shuttle programme.

Updated versions of Soviet rockets will now provide the world's only link to the ISS and space officials in Moscow wanted this mission not only to highlight their achievements but also the legacy of the world's first man in space.

The 50th anniversary of Gagarin's April 12, 1961 space shot was to have been crowned by the cosmonauts' launch of a simple mini-satellite called Kedr (Cedar) -- the call sign used during the historic mission.

The toaster-like device was designed to transmit Gagarin's message of global harmony by airing greetings in more than a dozen languages that could be received by amateur radio enthusiasts.

But television pictures from space showed Volkov and Samokutyaev open the hatch 20 minutes behind schedule and then have trouble untangling themselves from the numerous cords that linked their bulky Russian suits to the station.

The two men then spent about 30 minutes tethering themselves into place before taking their first tentative steps into space with the 30-kilogram (66-pounds) satellite in hand.

They ended up aborting their initial attempt an hour later after suddenly realising that the Kedr had only one of its two antennas in place.

Ground control outside Moscow insisted that the Kedr came equipped with two. But the cosmonauts said they knew nothing about it.

"I came here three months before Sergei (Volkov) and it was already just the one antenna," Samokutyaev was quoted as saying by Interfax.

The satellite's developer later told Russian reporters that the missing antenna was actually folded inside the Kedr for safekeeping during its transport to space.

"There is no one to blame here," Kedr developer Sergei Samburov told the RIA Novosti news agency.

"The cosmonauts will try to catch the (folded) antenna by the pinkies of their gloves and pull it out," the satellite developer said.

But the Russian space agency contradicted the developer by appearing to blame the cosmonauts themselves for somehow mangling the antenna as they lumbered their way out of the hatch.

"One of the two antennas about seven centimetres long (about three inches) was damaged while the cosmonauts were conducting their spacewalk," Roskosmos said in a statement.

It added in a later statement that Kedr had still managed to send 19 clear messages to Earth in a sign that "all of its systems were functioning normally."

The Kedr design team's website also posted brief messages from a space fan in Tokyo and another in Africa who claimed to have received radio and photo images from the craft.

Volkov for one could hardly conceal his joy once he managed to set the little craft into orbit.

"It's going!" he exclaimed as the Kedr spun off into orbit. "It's going great."