It might seem as if the sky is falling, but it's just space junk

Not to sound too Chicken Little-like, but a flaming spy satellite bigger than a Hummer is about to come crashing back to Earth in a month -- potentially falling in the United States, possibly on a presidential candidate, perhaps smashing a truckload of puppies, or even squeezing onto your couch while you are watching an episode of "The Wire."

And it has company.

"There are oodles and oodles, thousands of these things up there," says a remarkably calm Larry Ciupik, a veteran astronomer at Chicago's Adler Planetarium & Astronomy Museum.

Hundreds of tons of massive metal objects traveling faster than anything on Earth are going to return in violent re-entries eventually.

What's more, "there's no real good way of predicting" exactly where and when they will fall, Ciupik says.

And yet, in the half-century since Sputnik, 17,000 manmade objects have re-entered the Earth's atmosphere, and the closest they've come to wreaking death and destruction is one suspect report from Fidel Castro that a small U.S. rocket part killed a Cuban cow in the 1960s.

"Obviously, the atmosphere is a good filter," says Ciupik, noting that many rocket and satellite parts simply burn up during re-entry. Most parts that make it to Earth fall in the ocean or unpopulated areas such as deserts or Rudy Giuliani presidential victory parties.

Of the most famous potentially damaging returns, a few chunks of the U.S. Skylab came down in a remote part of Australia in 1979, and the Soviets' Mir space station harmlessly broke apart above the South Pacific in 2001.

"There's a challenge to lifting heavy stuff, so we typically don't build them too substantially. They are relatively flimsy," Ciupik says of satellites and other spacecraft. "It's only the really, really big stuff that is worrisome."

Despite all this debris raining down, a sighting is rare even among people who spend a lot of time looking at the sky. Ciupik remembers seeing one about 25 years ago while he was driving with a co-worker on the Calumet Expressway.

"We actually saw a rocket body streak across the sky. It actually looked like a Flash Gordon craft, with little sparkles coming out the back," Ciupik remembers. Only a few small pieces of the Russian rocket were left by the time it crashed without causing a fuss in Ohio.

Our government publishes a monthly "Spacewarn Bulletin" (nssdc.gsfc.nasa.gov/spacewarn) that lists all the objects with decaying orbits that should re-enter the atmosphere in the next 60 days.

More predictable (and less worrisome) is finding objects still rotating around the Earth, Ciupik says.

The Web site www.heavens-above.com shows the orbits of everything from satellites to comets to the International Space Station. Plug in your town, and the Web site tells you when, where and what you can spot flying overhead.

With 61 nations having put at least one object into space, the heavens are home to thousands of spacecraft and debris from those spacecraft.

There are so many things moving across the sky, a night photo Ciupik took last November of Venus and the moon inadvertently caught the bright streak of the International Space Station to become NASA's Astronomy Picture of the Day.

So enjoy the show. And don't worry. That falling spy satellite has the same chance of hitting you as it does of landing on Osama bin Laden.