As humanity's continuous presence in space begins its fifth year aboard the International Space Station, the next inhabitants rocket into Earth orbit late Wednesday for a six-month expedition.

The Expedition 10 crew. Credit: NASA

Mission commander Leroy Chiao and flight engineer Salizhan Sharipov blast off at 0306:26 GMT (11:06:26 p.m. EDT) aboard Russia's Soyuz TMA-5 spacecraft from the same Baikonur Cosmodrome pad where Yuri Gagarin started manned spaceflight in 1961.

A two-day pursuit of the space station culminates with docking to the Russian Pirs module early Saturday at 0425 GMT (12:25 a.m. EDT) to begin a week-long transition between the incoming Expedition 10 and outgoing Expedition 9 crews.

Joining the two-man Expedition 10 crew during launch is rookie cosmonaut Yuri Shargin. The Russian Space Forces engineer will visit the International Space Station before returning to Earth on October 24 with the Expedition 9 crew of commander Gennady Padalka and flight engineer Michael Fincke.

The station has been permanently occupied since the arrival of Expedition 1 on November 2, 2000. Two dozen crew members have lived on the complex during tours-of-duty ranging between four and six months.

Expedition 10 will mark the fourth mission to launch since the Columbia accident 20 months ago, which forced the station program to reduce the crew size from three to two people due to limited supplies.

"Since the Columbia tragedy, we have been flying two-person crews to the station, and the primary goals of those crews has been to keep the station going by doing repairs, on-orbit maintenance, and also in the meantime performing different experiments as well as performing some assembly tasks as well. So we'll be continuing in that vein of work," Expedition 10 commander Leroy Chiao said.

The prime objectives of Expedition 10's planned 193-day voyage include tending to station systems, conducting science experiments, receiving two unmanned resupply ships, performing a pair of Russian-based spacewalks and preparing the complex for the first post-Columbia space shuttle visit expected next spring.

The shuttle return-to-flight mission, called STS-114, will deliver much-needed supplies to the station, replace a failed control-orienting unit and ferry a large amount of equipment back to Earth.

Shuttle Discovery had been targeted for launch in March 2005 while Expedition 10 was still aboard the station, but NASA recently postponed the shuttle return until mid-May. Expedition 10 returns home April 25 after being replaced by Expedition 11.

Nevertheless, Chiao and Sharipov have jobs readying for the shuttle arrival.

"A lot of the preparations that they will be doing include pre-packing cargoes that will be returned on the shuttle flight (and) reconfiguring stowage on the ISS," said Susan Brand, NASA's Expedition 10 increment manager.

The station's Pressurized Mating Adapter-2 shuttle docking port, the U.S. airlock module Quest and Node connecting module have been used for storage in the absence of shuttle flights. Expedition 10 must clear those areas for Discovery's docking, spacewalks and mounting of an Italian-made cargo module during STS-114.

Throughout the Expedition 10 mission, science research will be conducted with U.S. and Russian experiments. Despite the shuttle grounding that has halted the launch of large new science equipment, the station residents have continued running experiments.

"We can still do medical studies and biological studies using ourselves as test subjects," Chiao said. "So we'll be continuing, in our way, the science that will be needed to continue with the vision of a voyage towards Mars."

A key aspect of station science is gauging spaceflight and the extended exposure to weightlessness on humans.

"We understand that humans will be on the moon, on Mars and other planets, and the beginning stages of humankind in space is the study of space impact to the human body, and we are test subjects in this respect in microgravity," Sharipov said.

"Medical/biological experiments comprise the largest part of the Russian science program. For example, the research is being done on how the muscles are operating in space, how the bones are functioning in space, how various medications are affecting the human body. All of that will help for research in preparation for long - very long, a many-years'-duration - flight in space.

"We are not forgetting that when humans stay a long (time) in space it will be necessary to have food the same as we have on Earth; that's why we have an experiment which is called Plants that is a study how to grow plants in space."

The science agenda includes a range of other research too.

"We'll perform various kinds of experiments," Sharipov continued. "One very interesting experiment is called SRS, which is a low-temperature synthesis in space; high technologies that would allow us to develop new materials. In biotechnology, for example, we are using stem cells for possible development of a vaccination for AIDS treatment. We are studying ecology of the Earth from 400 kilometers above; geophysical experiments, we are researching as well, study nature of various occurrences, physical occurrences. We are performing such experiments that would help humanity to live better and progress."

To keep the station stocked with food, water, supplies and rocket propellant, the Russian Progress 16P cargo freighter is scheduled for launch December 23. It will make an automated docking to the station on Christmas Day.

The station program keeps a 45-day reserve of onboard supplies. The crunch caused by the lack of shuttle flights and delays launching the next Progress means Expedition 10 literally will be eating into the onboard food reserve starting December 19.

"Because it is a very small intrusion into the (reserve) and because the Progress is very reliable, we have no issues with that and we're ready to go," said Mark Geyer, NASA's station manager for integration and operations.

Another Progress is scheduled for launch February 28 with docking expected March 2, Brand said.

Chiao and Sharipov will don Russian Orlan spacesuits and venture outside the station for spacewalks in January and March.

"The first spacewalk will be for installation of the universal work space on the Service Module, and installation of Rokviss experiment, which is a joint experiment for the European Space Agency and Russia. We will have to install an antenna and pick up some equipment that is already outside the station," Sharipov explained.

The second excursion will be dedicated to outfitting the station for the European Space Agency's Automated Transfer Vehicle (ATV) cargo ships that begin launching atop Ariane 5 rockets in October 2005.

"During the EVA, they will install GPS navigation antennas, a space-to-space radio system for ATV proximity operations, a television camera and all of the cabling to support the equipment," Brand said.

"They'll also be installing internal ATV equipment and cabling, and testing all of the individual components in the end-to-end system."

Other highlights of Expedition 10 include upgrading four major U.S. software systems in December and a fifth in March. The advances will allow ground controllers to operate the station's Canadian-built robot arm, Expedition 10 lead flight director Annette Hasbrook said.

"On the Russian side, we will be continuing to install acoustic reduction hardware on fans and ducts that are part of the Service Module ventilation system," Brand continued.

"And throughout the increment we will be conducting medical operations, maintenance, public affairs, onboard training and housekeeping activities."

The Expedition 11 crew launches April 15 to replace Chiao and Sharipov, who return to Earth on April 25 aboard the same Soyuz capsule in which they fly this week.

Neither Expedition 10 crew member has flown on Soyuz or experienced long-duration spaceflight. Their previous missions have occurred aboard U.S. space shuttle flights.

Chiao, 44, has flown three shuttle missions to log 36 days in orbit and conducted four spacewalks totaling over 26 hours of EVA time.

His rookie flight, STS-65, was aboard Columbia for a Spacelab science mission in the summer of 1994. He followed that with the STS-72 mission on Endeavour in 1996, which retrieved a Japanese science satellite and conducted a pair of spacewalks to test tools and techniques for station assembly. Chiao put the EVA experience to use in 2000 by performing two International Space Station construction spacewalks during the STS-92 mission on Discovery.

Sharipov, 40, a colonel in the Russian Air Force, has one spaceflight to his credit. He flew aboard shuttle Endeavour in early 1998 during the nine-day STS-89 mission to the Russian space station Mir.

"The only part we haven't experienced is how do you do an entire long-duration flight, but given our experience I don't anticipate that that'll be a problem," Chiao said. "We work very well together; our personalities complement each other, and we both kind of have the same views on how things ought to be done. And so I think that everything will be just fine."