ROBERT SIEGEL, HOST:

Last night, a bright fireball with a long tail streaked across the sky in the western part of the U.S. You could see it from San Diego, Las Vegas, Salt Lake City and places in between.

UNIDENTIFIED MAN #1: Whoa.

UNIDENTIFIED MAN #2: (Unintelligible).

UNIDENTIFIED PERSON: Whoa.

UNIDENTIFIED MAN #2: Dude, (unintelligible).

SIEGEL: That's a video from Utah, one of many that were posted online. Everyone was asking - what was that - a meteor or falling space junk? Well, NPR's Nell Greenfieldboyce has the answer.

NELL GREENFIELDBOYCE, BYLINE: Jonathan McDowell heard about the fireball a few minutes after it happened. He's an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. And he immediately checked to see if any low-orbit satellites were expected to re-enter the earth's atmosphere about that time.

JONATHAN MCDOWELL: Very quickly, I spotted that this large Chinese rocket stage was on the verge of re-entry.

GREENFIELDBOYCE: And, yep, the fireball was this returning Chinese rocket. That was confirmed today by the U.S. Strategic Command. It had been tracking the rocket since it launched last month. Now, hunks of falling space junk burn up all the time, but this one was pretty big - more than 30 feet long.

MCDOWELL: The only bigger event this year was a Russian rocket stage that re-entered over Vietnam on New Year's Day.

GREENFIELDBOYCE: McDowell says it's just luck that this one came down when and where so many people could see it. And while it's possible a few small pieces of metal might have survived to hit the ground somewhere, he says there was no real danger to people. It was just a pretty show. Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.