"I shot an arrow into the air, It fell to earth, I knew not where ...," goes an old verse.

That arrow didn't cause any harm. But on Sept. 12, 1991, NASA shot a 12,500-pound satellite into the sky -- and its fall to Earth may be more troublesome.

The satellite -- the size of a school bus -- is about to tumble out of orbit. And we "know not where" -- or "whom" or "what" it may hit when it comes down. The 35-foot-long satellite is expected to crash somewhere between northern Canada and the southern tip of South America, in the ocean or on land.

We don't know exactly "when," either. It could be anytime from today through Saturday.

We can't do anything about it, though fortunately it will partially burn up on re-entry and break into dozens of pieces. Of the chunks that survive, the biggest may weigh about 300 pounds.

Heads up!