Old NASA satellite tumbling to Earth Friday or Saturday

The Upper Atmosphere Research Satellite during its deployment in September 1991.

A dead 6-ton satellite baffled NASA experts Friday by slowing its descent toward Earth and delaying its ultimate crash until the early part of the weekend.

The space agency is now predicting the satellite will crash down to Earth late Friday or early Saturday, Eastern Time. Increased solar activity had been causing the atmosphere to expand and the satellite to fall more quickly, but that's no longer such a major factor, experts said. What's more, the orientation of the satellite apparently has changed in orbit, and that's slowing its fatal plunge.

Friday morning, NASA cautioned there is now a slim chance any surviving debris will land in the United States. Earlier this week, NASA said North America would be in the clear and that the satellite would strike somewhere Friday afternoon.

STORY: Shuttle accident cut plans to retrieve now-falling satellite

"It is still too early to predict the time and location of re-entry with any certainty," NASA said in a statement.

The Aerospace Corp., based in California, is estimating the strike sometime between about 6 p.m. and 4 a.m. EDT, which would make a huge difference in where the debris might wind up. Those late-night, early-morning passes show the satellite flying over parts of the United States.

Any surviving wreckage is expected to be limited to a 500-mile swath.

The Upper Atmosphere Research Satellite, or UARS, will be the biggest NASA spacecraft to crash back to Earth, uncontrolled, since the post-Apollo 75-ton Skylab space station and the more than 10-ton Pegasus 2 satellite, both in 1979.

Russia's 135-ton Mir space station slammed through the atmosphere in 2001, but it was a controlled dive into the Pacific.

Most of the UARS satellite will disintegrate, but 26 pieces — representing 1,200 pounds of heavy metal — are expected to rain down somewhere. The biggest surviving chunk should be no more than 300 pounds.

Chances are the wreckage will slam into the ocean; nearly three-quarter of the Earth is covered in water.

In any event, no one has ever been hurt by falling space junk to anyone's knowledge, and there has been no serious property damage. NASA put the chances that somebody somewhere on Earth will get hurt at 1-in-3,200. But any one person's odds of being struck have been estimated at 1-in-22 trillion, given there are 7 billion people on the planet.

UARS was launched in 1991 from space shuttle Discovery to study the atmosphere and the ozone layer. NASA shut it down in 2005 after lowering its orbit to hurry its demise. With a satellite-retrieval mission ruled out following the 2003 Columbia disaster, NASA did not want the satellite hanging around orbit posing a debris hazard.

Space junk is a growing problem in low-Earth orbit. More than 20,000 pieces of debris, at least 4 inches in diameter, are being tracked on a daily basis. These objects pose a serious threat to the International Space Station.

Station astronauts enjoy Labor Day weekend

Other than some routine maintenance and housecleaning, space station crew will have a chance to relax

The six astronauts aboard the International Space Station will enjoy a long Labor Day weekend — the last extended break for three of them before they head back to Earth on Sept. 15.

The space station will do a bit of routine maintenance and housecleaning work on Saturday (Sept. 3), NASA spokesman Kelly Humphries said. But other than that, the astronauts will have a chance to put their feet up for a spell, or catch up on some science work if need be.

"If they're aching to do something they haven't had time to do, they can go off and do that experiment," Humphries told SPACE.com. "They pretty much manage that on their own when they have these days off." [ Photos: Building the International Space Station ]

Saying goodbye to the station
Three astronauts — American Ron Garan and Russians Andrey Borisenko and Alexander Samokutyaev — are slated to leave the space station on Sept. 15. The three days off may be especially sweet for them, giving the spaceflyers a chance to say goodbye to the orbiting lab.

"Ron and his colleagues are going to get a three-day weekend to enjoy being in space one last time, before they do the final preparations and pack up to come home," Humphries said.

Garan, Borisenko and Samokutyaev arrived at the station on April 6. They were originally slated to leave on Sept. 8, but last week's crash of Russia's unmanned Progress 44 cargo ship delayed their departure.

The Progress was doomed by a problem with its Soyuz rocket, which is very similar to the rocket the Russians use to launch crews to the space station. No astronauts will launch on a Soyuz until the issue is identified and fixed, NASA officials have said.

Russian engineers are making progress in their investigation. They've traced the source of the failure to a malfunction in the gas generator in the Soyuz’s third-stage engine, Russian officials announced earlier this week.

An unmanned space station?
The station's other three astronauts — American Mike Fossum, Russian Sergei Volkov and Japan's Satoshi Furukawa — are scheduled to come home in mid-November.

After the retirement of NASA's space shuttle fleet in July, the Soyuz is the only way to take astronauts to orbit. If it's not back up and running by mid-November, the space station could be unmanned for the first time in more than 10 years.

And that's part of the reason NASA decided to delay the departure of Garan, Borisenko and Samokutyaev.

Keeping them around for even one additional week will allow the crew to get more science work done, which could be invaluable if the station goes to a skeleton crew of three or is unmanned altogether for a spell, NASA officials have said.

The space station could survive just fine without a crew for a while, officials have said. Controllers on the ground can move it out of the way of potentially dangerous space debris, and its propellant can be topped up by robotic spacecraft.

However, having no crew aboard means any problems that crop up aboard the station would likely not get fixed, making flying a more risky proposition. And having anything less than a full crew of six astronauts would cut back on the scientific research that can be done on the orbiting lab.