KOROLEV, Russia (AFP) – A group of volunteers isolated from the outside world for eight months stepped out on a mock-up of Mars on Monday, reaching the half-way point of their experimental "voyage" to the Red Planet.

Outfitted in heavy white space suits and clunky boots, the Russian and Italian volunteers left their capsule and took their first careful steps on the red sands of Mars -- all without ever leaving a Moscow research centre.

"We have made great progress today," Vitaly Davydov, the deputy head of the Russian Federal Space Agency, pronounced as live footage of the virtual space walk beamed into the country's real-life space control centre.

"The crew feel fine," he added as journalists crowded around the surreal video feed in Korolev outside Moscow.

The "space walk" marks a key moment in an unprecedented experiment in which six volunteers are spending 520 days in isolation to test how humans would respond to the pressures of the there-and-back voyage to Mars.

They planted the flags of Russia, the European Space Agency (ESA) and China and took samples of the dusty soil in a 92-minute experiment that Russia and Europe hope to repeat for real by 2040.

"It's a very exciting experience," said Christer Fuglesang, head of the science and application division at the ESA, which is organising the experiment together with Moscow's institute of biomedical problems.

"They have been very creative," added Fuglesang, who was the first man from Sweden to reach space.

Officially called Mars-500, the project joins six men from Europe, Russia and China in an experiment that focuses on the physical and psychological strains of extended space travel.

Although not weightless, the crew of 20- and 30-year-olds is spending its time in tight quarters that is preventing their bodies from getting their normal doses of exercise.

The virtual space craft's living quarters measure just 20 metres (yards) long and less than four metres across, with special armchairs set up on planet's surface to help the men deal with the 32-kilogramme (71-pound) suits.

"We have to experience the impact (of extended travel) on the crew," said ESA representative Martin Zell.

"I believe in this mission. Until now, it has been very successful. I am fairly sure that they will succeed."

The first of the three space walks included the Russian volunteer Alexander Smoleyevsky and Italy's Diego Urbina, who will be later joined by China's Wang Yue.

Three other men, Romain Charles from France and Sukhrob Kamolov and Alexei Sitev from Russia, will remain "in orbit" in the main module.

In experiments, the volunteers will control a wheeled robot to explore the mock-up of the surface of Mars, which is covered with sand and a scattering of rocks.

For around a month, the crew will carry out scientific experiments in the environment designed to mimic that of Mars.

Besides taking samples, they will also measure the planet's magnetic poles and conduct other fieldwork using tools designed specifically for Russia's mooted manned voyage to the Moon.

They will then rejoin the three colleagues who stayed "in orbit" around the Red Planet on February 27 for the long return "trip" back to earth, with the experiment due to finish with the mock landing in November.

Russia plans to send a real flight to Mars in 20-30 years, possibly in a joint effort with US space agency NASA.