This STS-48 onboard photo is of the Upper Atmosphere Research Satellite (UARS) in the grasp of the RMS (Remote Manipulator System) during deployment, September 1991. (NASA Marshall Space Flight Center (NASA-MSFC))

Sometime late Friday — possibly during happy hour, to use a conveniently ambiguous time period, but maybe deep into the evening — a tumbling NASA satellite is expected to enter the upper atmosphere, partially melt, disintegrate and spray the planet with charred debris.

NASA, faced with the inherent uncertainties of space-junk dynamics, continued late Thursday to offer a fuzzy prediction about when and where the Upper Atmosphere Research Satellite (UARS) will crash.

Thursday night, the space agency said that the 35-foot-long satellite would probably reenter Friday afternoon or early evening (Eastern time) and that it wouldn’t be over North America at that time.

But this has proved to be a squishy situation with enormous, globe-spanning margins of error.

UARS appeared to be on a trajectory to splash into the desolate South Pacific sometime Friday night, according to a map published by the Aerospace Corp., which uses Air Force tracking data. The map indicated that if the satellite crashed just 20 to 25 minutes later, it would be over North America.

This was a significant change from a previous projection by the same organization, which showed UARS coming in several hours earlier and reentering the atmosphere just off the west coast of South America.

UARS circles the planet in a little more than 11 / 2 hours. A small change in the reentry time will result in thousands of miles of difference in the location.

On Thursday, the satellite was barely more than 100 miles up, steadily losing speed and altitude. When it’s about 42 miles above the surface, it will probably break up as the aluminum frame melts away, said Bill Ailor, an Aerospace Corp. engineer who has been monitoring the satellite.

Ailor said fuel tanks often explode during reentry. NASA expects UARS to break into about 100 pieces that will fireball across the sky, visible for hundreds of miles. About 26 of those pieces should survive reentry and crash to the surface, with the largest chunk weighing more than 300 pounds.

But most of the planet’s surface is open ocean, and it’s unlikely that anyone will be hurt, according to NASA, which puts the odds of even one human being, somewhere, being injured at 1 in 3,200.