NASA hedges predictions about time, place of satellite’s crash

This screen grab image provided by NASA shows UARS attached to the robotic arm of the space shuttle Discovery during a mission in 1991, when UARS was deployed.

WASHINGTON — 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 Atmospheric Research Satellite (UARS) will crash.

On Wednesday, the space agency said that the 35-foot-long satellite would probably re-enter Friday afternoon 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.

Thursday evening, UARS appeared to be on a trajectory to splash into the desolate South Pacific sometime Friday night (Eastern time), 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 re-entering the atmosphere just off the west coast of South America.

UARS circles the planet in a little more than 1 1 / 2 hours. A small change in the re-entry 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 re-entry. 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 re-entry 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.