Ready for immediate occupancy, move-in condition: Four-room mobile home with no bath, no kitchen, no beds, and the world's most spectacular view.

In fact, the world is the view.

Last week, when the space shuttle Discovery pulled away from the $60 billion International Space Station, it left behind the three astronauts who will inaugurate the scientific mission of mankind's most sophisticated home in space. Though still far from finished, the ISS is now officially the largest structure ever placed into orbit, with computers and lab equipment that would have been the envy of the crews on any of the world's previous space stations.

And yet, in its present form, the ISS has limitations that may leave its multinational crews hankering for the good old days of earlier space stations like Mir, the 15-year-old Russian station that met its fiery end last week.

There is no real kitchen yet, so the astronauts will eat canned or freeze-dried food. (There is a freezer, but it's being used for blood and urine samples). There is no shower, so they'll have to rely on sponge baths. And while the total amount of enclosed space is greater than previous stations, there is no single area as roomy as the huge interior of Skylab, the US station of the 1970s whose round main cabin was big enough for astronauts to mimic ''jogging'' around the outside wall.

And no part of the ISS is a refuge from politics. Many of the station's current limitations, intended as temporary, could end up being permanent if cutbacks proposed in the Bush administration's new budget are implemented. Among other things, that would eliminate the planned ''habitation'' module, with its kitchen, sleeping quarters and shower. And some research facilities also could face the chopping block.

But now, at least, there is enough equipment in place so that the ISS can begin its job as a laboratory. The scientific mission of the station - much of which concerns how the human body reacts to weightlessness - is a prerequisite for eventual human missions to Mars and beyond. But for now, most of the work is considerably less glamorous: Ensuring survival of the craft and its crew.

The high life

The new mission began two weeks ago, when the crew of Captain Yury Usachev and Americans James Voss and Susan Helms pulled themselves through a 3-foot-wide hatch and entered an interior that counts as relatively roomy, by space standards.

To its occupants, the station resembles a series of hallways meeting at right angles, like four big panel trucks parked close together with hatchways connecting them. There are few windows, a constant hum of mechanical equipment, and almost every surface is covered with racks of electronic equipment - not just the walls, but the ''floors'' and ''ceilings'' as well.

The structure is a series of large modules connected through ''nodes,'' smaller cylinders with six hatches. The first node, called Unity, was attached last fall, and the US laboratory module was added to one end of Unity last month. In the coming years, new modules and nodes will be attached one at a time, adding labs and living space.

Some aspects of space-station architecture have changed little since the former Soviet Union launched the first space station in 1971. The ISS core module, launched in 1999, is a modified version of the core of Mir, which itself was adapted from the seven Salyut stations that preceded it.

The ISS computers, thanks to the long testing period before a new system can be approved for launch, are actually a few years behind what you can buy at Staples. Most of the station's functions can be controlled from IBM laptops running Windows 95. Its laboratory equipment is mostly standard devices that can be shared by a variety of different research projects, unlike the custom-built equipment needed for earlier experiments. And its huge solar panels will ultimately provide more than twice the power of those on Mir, where astronauts frequently had to turn off some equipment to conserve power.

It will take more than 30 space flights over the next five years to finish building the station. When - and if - the station is fully complete as planned, in 2006, the ISS will have as much cabin space as two 747 jumbo jets, and weigh a million pounds.

Spin city

For all the extra room afforded by the ISS, even the most basic functions of daily life are made complex in space. Lights and the drone of pumps and fans are constant, so astronauts need earplugs and eyeshades to catch any sleep. And without gravity, astronauts can strap themselves down, but the floating sensation never ceases.

Dizziness, disorientation and nausea are the most obvious biological effects of living in weightlessness, and the first ones that astronauts experience when they arrive. Sometimes they can even cause further hazards - for example, as the ISS was being constructed last year, one astronaut assigned to mount an emergency exit sign became disoriented in the symmetrical central node and put the sign on the wrong hatch. (The mistake was quickly corrected.)

But disorientation is far from the most serious long-term effect of living in space. Other problems are immune-system suppression, whose cause is not yet understood, and possible radiation damage. But many of the most crippling effects are caused by the simple absence of gravity.

On Earth, the constant pull of gravity means we're essentially getting constant exercise - everything we move, including our bodies, is affected by gravity's tug. This helps keep muscles strong and bone mass high.

In space, without that resistance, muscle and bone mass start to deteriorate. Norman Thagard, an astronaut who is also a medical doctor, experienced the effects firsthand as the first US astronaut to visit Mir, where he spent 41/2 months. In spite of twice-daily exercise on the station's stationary bicycle and treadmill, ''I lost 12 percent of my bone mass,'' he said. ''That's a problem - we don't know when the bone loss stops.''

On a future mission to Mars, which according to most proposed mission plans could involve at least two years in space, astronauts could emerge with muscles so weak they would be unable to stand up in a planet's gravity - or with bones that fracture on impact.

Increasingly, specialists in space medicine wonder whether the ''artificial gravity'' produced by a spinning space station may be the most effective way of maintaining the crew's strength. Perhaps the most famous space station of all was the huge wheel in ''2001: A Space Odyssey.'' Based on real designs by rocket pioneer Werner von Braun, the huge spinning station with its hotel and roomy lounges made it possible for people to walk around just as if they were on Earth, held down by centrifugal force that exactly mimicked Earth's gravity.

But a spinning spacecraft could pose its own problems. Experiments at Brandeis University, which has a 20-foot-diameter spinning room at its Graybiel Laboratory, suggest centrifugal ''gravity'' could have debilitating effects. If someone in the spinning room attempts to touch his fingertip to his nose, for instance, the effect is comically similar to someone failing a sobriety test. A ball thrown straight ahead swerves dramatically to the side. Any head movement is an invitation to instant sickness.

So far, however, centrifuges have been tested only on Earth. The idea of a space-station centrifuge big enough to house animals has ''been under discussion since the '70s,'' said Lawrence Young, Apollo professor of astronautics at the Massachusetts Institute of Technology and director of the Space Biomedical Research Institute. ''It's been the top recommendation'' of every group that has ever evaluated proposed life-sciences research equipment for space missions, he said.

One such centrifuge, about a yard in diameter, is part of the plan for a Japanese research module to be added to the space station in about two years. Another, 8 feet in diameter, is being designed in California for launch in 2006, but is potentially threatened by proposed budget cuts.

Even before the latest budget proposal, the smaller centrifuge was threatened by escalating costs, Young said. ''There's been a lot of talk about scaling back, which really cuts the legs out of basic biology experiments,'' Young said.

Most space policy analysts expect that, in the end, such crucial cuts to the space station are unlikely to prevail. For one thing, it calls for reducing the crew from seven to three - the minimum number required to run the basic functions of the station. Space policy analyst John Pike calls this shortsighted: ''Like having a school with janitors but no teachers.''

For the next few months, NASA estimates, the crew will be spending most of its time on maintenance and construction, with only about 10 hours per week for research. But, as more lab equipment is installed - and if the cutbacks don't decimate the plans - the role of research will gradually increase.

If the budget cuts really are implemented, MIT astronautics professor Charles Oman said, the station could end up becoming ''a Potemkin village for science'' - all show and little substance. But if enough of the scientific capability is retained, he said, its power as a research facility will be unmatched. For the first time in the Space Age, he said, ''it's a station built for science.''

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