SPACE CENTER, Houston (AP) _ Endeavour's astronauts crossed a new threshold into the international space station today, turning on the lights and preparing the outpost for future assembly crews.

Shuttle commander Robert Cabana and Russian cosmonaut Sergei Krikalev floated side by side into Unity, the American-made piece of the seven-story station, after opening a series of hatches leading from the shuttle Endeavour. They carried lanterns to see inside; the 36-foot cylindrical chamber was pitch-black, so their first official act was to flip on the lights.

The six crew members shared the moment with the world: They took in cameras to provide everyone back on Earth with the first look inside the 250-mile-high space station.

``This is a very significant and almost momentous event,'' said NASA's lead flight director, Bob Castle.

It was slow going to get through the three hatches leading from Endeavour into Unity. The astronauts, wielding checklists and wrenches, had to make sure the air pressure was constant from one area to the next before moving on.

``This is our goal _ building a space station and setting the pace for the future,'' Cabana said earlier in the day. ``When you get to look out the window and see Zarya and Unity joined together, and knowing that you get to go inside ... it's pretty awesome.''

The five Americans and one Russian had plenty of work to do inside, beginning with the routing of air ducts from Endeavour into Unity.

Cabana and astronaut Jerry Ross had to wire up a communications system inside Unity. The others were to continue on into the Russian-built Zarya control module, where Krikalev was to replace a charging component on a faulty battery.

Other duties include transferring tools from the shuttle to the space station for later use. The astronauts will even leave behind clothes for the first permanent station crew, scheduled to arrive in January 2000.

The work will continue through Friday, when the astronauts will exit the space station and close the hatches to get ready for Tuesday's return to Earth.

On Wednesday, in preparation for their journey inside the station, Ross and astronaut James Newman ventured out on a seven-hour spacewalk and attached two 100-pound antennas to the outside of Unity.

The antennas are part of the communications system to be wired up today. Once activated, the system will provide a direct, virtually uninterrupted communication link between Unity and Mission Control. Otherwise, U.S. flight controllers would have to rely on the sporadic coverage provided by Russian ground stations.

In a tense and meticulously planned operation, Newman also pried open a stuck antenna on Zarya. After several pokes with a 10-foot pole, the 4-foot strip antenna shot out.

``There it goes! It's gone!'' Ross shouted.

``You got it deployed. All right,'' Mission Control radioed up.

Other spacewalking chores completed by Ross and Newman included erecting a sunshade over a computer mounted to the outside of Unity and covering three of four protruding pins on the chamber with thermal blankets.

Unfortunately for Ross, the thermal cover for the fourth pin got loose and floated away; he lost two tools during Monday's spacewalk. ``I don't believe this,'' he groaned.

It was the second time this week that Ross and Newman floated out to work on the 35-ton station taking shape in the shuttle's cargo bay. During a 7 1/2-hour spacewalk on Monday, they connected electrical and data cables to Unity, enabling it to be powered up.

One more spacewalk is planned for Endeavour's 12-day mission. On Saturday, Ross and Newman will conduct a photo survey and take out a sack of tools that will be used by future station visitors. Newman also may try to unjam the other Zarya antenna.