Duties of this two-man crew will differ greatly from those of its three-member predecessors: as a result of the grounding of the American shuttle fleet after the loss of the Columbia on Feb. 1, space station building has come to a halt and normal supply lines are choked. The new crew will have less time for research and will have to pay extra attention to food, water and other consumables, which are in increasingly short supply without regular shuttle visits.

Dr. Edward T. Lu, the American half of the team, said the conservation efforts would make the mission more interesting and encourage the crew to be more imaginative. "We're going to probably have to improvise, with help from the ground, quite a lot more than other crews have done," he said in a preflight interview. "And I think that's going to make it much more challenging."

Hatches between the Soyuz and the station opened at 3:27 a.m., followed by laughter and hugs of the new arrivals by the three-man station crew, anxious to go home after more than five months aboard. The three — two Americans, Kenneth D. Bowersox, a Navy captain who is station commander, and Dr. Donald R. Pettit, the science officer; and a Russian, Nikolai M. Budarin, the flight engineer — are to return to Earth on Saturday in another Soyuz capsule attached to the station.

The crews moved to the Russian Zvezda service module, the station's main living area, for congratulatory phone calls from American and Russian space officials. NASA's deputy administrator, Frederick Gregory, thanked the astronauts for their current and future efforts to sustain the station, particularly in light of the Columbia disaster.

"The International Space Station partnership has been tested by a great challenge," Mr. Gregory said. "But the partnership has risen to the challenge."

Mr. Malenchenko, 41, and Dr. Lu, 39, are called the Expedition Seven crew because they are the seventh set of astronauts to live on the station in more than two years of continuous occupancy.

They will spend most of their time just keeping the station running, taking care of routine operations and maintenance. Unlike previous crews, Mr. Malenchenko, the station commander, who previously served on the Russian Mir space station, and Dr. Lu will have no assembly tasks to perform. No spacewalks are planned.

Their time for research has also been curtailed, but NASA said the crew would devote more than 200 hours to life science, physics, chemistry and material science studies. Dr. Lu, a physicist and engineer designated the mission science officer, will continue the popular science education activities Mr. Pettit pioneered with his "Saturday Morning Science" demonstrations. In addition, a science team on the ground will continue to operate some experiments remotely and gather data from others that function autonomously.

Other tasks assigned the crew include overseeing two major upgrades in computer software aboard the station in preparation for future expansion and operating the outpost's robotic arm to use its cameras to monitor the exterior.

The 16 nations building the station came up with the plan to maintain it with a two-person caretaker crew. The plan calls for Russian Soyuz spacecraft to bring up and return a pair of astronauts every six months and for unmanned Progress cargo vessels, which carry far less than shuttles, to haul up essential supplies each three months or so until shuttle flights resume.

The 198-ton station is less than half finished, with construction halted at the beginning of what was planned to be one of its busiest building periods. Five shuttle flights, five Soyuz and Progress missions, and two dozen spacewalks were scheduled this year to finish the outer framework of the $60 billion station.

The new crewmen are somewhat familiar with the space station because they visited it together at an earlier stage. Dr. Lu and Mr. Malenchenko were aboard the shuttle Atlantis when it delivered supplies to the station in September 2000 to set it up for the first permanent crew. The men performed a spacewalk together to connect cables between two Russian modules, and left the outpost crammed with so many supplies that they had to be stuffed into almost every unused space.

Such an abundance of provisions, including large plastic vessels of water produced by space shuttle fuel cells, no longer exists on the station. The new team has been instructed to conserve resources whenever possible, including wearing clothing longer and using electric batteries sparingly.

Water, the most critical resource, will be more difficult to conserve, Mr. Pettit of the current crew said earlier. "There's not a whole lot we can do to conserve because we don't use water wantonly on the space station," he said. "For example, I probably use three or four ounces of water every time I take a shower. It really doesn't take much to take a sponge bath, and that's what we do up here."

The biggest water usage is in food and drink, he said, and it would not be wise to scrimp on water consumption and risk dehydration and other health consequences.

The crew is to be resupplied by two Progress cargo ships, one arriving in June and the other in late summer. Each robot craft is to bring 4,000 pounds of water, food, clothing, fuel, station equipment and personal items.

There has been some discussion about how a two-person crew will fare in the station during a long stay, although pairs of astronauts were occasionally posted aboard Mir. Mr. Malenchenko and Dr. Lu are friends who have flown and trained together for years, and say they foresee no problem. While both men plan to bunk together in the Zvezda module, there is a third sleep compartment elsewhere in the station in case one wants more solitude. Unlike most crews left on the station, this one will have no visitors during its stay.

Captain Bowersox, of the current team, said a two-person crew would have a different social dynamic. "Having two people on station would change things quite a bit, it would change the dynamic of how you support each other emotionally," he said. "It's really nice where if two people are getting a little bit irritated, one of them can go and talk to the third or the third can act as a referee."

A shuttle was supposed to bring the current crew back home in mid-March; instead, the astronauts will have to return in the Soyuz that has been docked at the station since October as a rescue vehicle. The crew, already trained to fly the craft, took refresher courses in recent weeks, and Mr. Budarin is to pilot the craft during its three-and-a-half-hour return to Earth, landing on the flat steppes of Kazakhstan in central Asia.

Although American astronauts have flown into space before on Soyuz craft, this will be the first time they have landed in a Soyuz capsule and in a foreign country. NASA has arranged for the crew to be met at the landing site by officials carrying entry visas and customs claims documents, as required by Kazakhstan for anyone entering its territory, including those parachuting down in a space capsule.

The trip home is of added interest because it will be the first return of a modernized TMA-model Soyuz, which has new computers, instruments and digital display screens, as well as new braking rockets that fire shortly before touchdown to cushion impact with the ground.