The astronauts aboard the linked shuttle and space station rested up Wednesday for the fourth spacewalk of their mission, a caulking gun and goo test.

"Three down, two to go," Mission Control told the astronauts in a wake-up message.

Two of the crew members will float outside Thursday night to squirt salmon-colored goo into the crevices of extra space shuttle thermal tiles that were deliberately damaged for the test. NASA wants to see how well the caulking gun and patching material work, in case they're ever needed for a real repair.

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The tools were developed in the wake of the 2003 Columbia disaster. The shuttle was destroyed and all seven astronauts were killed during re-entry because of a hole in the wing.

The shuttle astronauts spent the first half of their mission putting together the space station's new Canadian robot, Dextre, and installing a Japanese storage compartment that will be followed by Japan's enormous Kibo lab in May.

During Thursday's spacewalk, astronauts Michael Foreman and Robert Behnken will work on sample tiles that were carried up in Endeavour's payload bay.

The experiment was supposed to be conducted during a shuttle flight last fall but was scrapped because of urgent repairs needed for a ripped solar wing at the international space station.

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NASA would like the test results before Atlantis blasts off at the end of August on one last repair mission to the Hubble Space Telescope. The astronauts on that mission will not be able to use the space station as a refuge if their shuttle is damaged during launch; they won't be in the same orbit.

Another space shuttle will be on the launch pad ready to fly to the rescue if necessary. Nonetheless, NASA wants the Hubble crew to have as many shuttle repair methods available as possible.

"Having this in our bag of tricks is really going to be helpful," Behnken said in a series of broadcast interviews Wednesday night.

As Endeavour's 16-day flight hit the halfway point, the seven shuttle astronauts and three station residents got several hours off to enjoy the views of Earth and to call or write home.

"We're having a great time taking a break, just relaxing a little bit. I think we really needed it," said the space station's commander, Peggy Whitson. She and her crewmates had to drastically change their bedtime to sync up with Endeavour's night-shift schedule, "so it was nice being able to get eight hours of sleep."

Two more spacewalks are planned before Endeavour departs the orbiting complex early next week.

Late Wednesday, Japanese astronaut Takao Doi got a congratulatory call from his country's prime minister, Yasuo Fukuda. Doi showed off the space station's new Japanese compartment — adorned with the Japanese flag — as well as the boomerang he took into orbit. He promised to show the prime minister the video he took of his boomerang-throwing experiment, once he's back on Earth, but offered no hint as to whether it returned to him in space.