Left unchecked, a growing cloud of space debris, made up of bits and pieces of old rockets and satellites hurtling around Earth at orbital velocity, threatens the safety of future space flights, NASA said.

The danger is clear: a piece of debris just .12 inches in diameter traveling at speeds of about 6.2 miles per second packs the punch of a bowling ball traveling 60 mph, according to a report released Friday by NASA and the Department of Defense."The equivalent energy of running into a centimeter-sized particle (.4 inches wide) is like getting hit by a 400-pound safe going 60 miles an hour," said Donald Kessler, an expert on space debris at NASA's Johnson Space Center in Houston.

U.S. Space Command currently tracks about 7,100 objects larger than 10 centimeters - about 4 inches - in diameter. While smaller objects are not directly tracked, estimates that include objects down to one centimeter across - .12 inches - push the total to at least 17,500 pieces of space junk and possibly much higher.

And that does not include billions of paint flakes and clouds of burned solid-rocket propellant.

"My own opinion is it's not out of hand yet but it's certainly to the point that it requires consideration of spacecraft design," Kessler said in a telephone interview. "It's urgent that we adhere to the policies in the report."

The study released by the National Aeronautics and Space Administration and the Department of Defense concluded that too little is known about the threat of orbital debris, although it is clear efforts need to be made to check its growth.

While U.S. Space Command routinely tracks objects the size of a softball and up, there is a large uncertainty about how many smaller objects are whizzing through space undetected.

"The current tracked population is about 7,100 objects, 10 centimeters or larger," Kessler said. "An educated guess, just based on modeling of (rocket) breakups and so forth, you would guess there are something of the order of maybe 100,000 centimeter-sized objects."

It is the centimeter-sized objects in low-Earth orbit that pack the wallop of a 400-pound safe moving at 60 mph.

Because of the uncertainty about the actual debris population, the study concluded, it is difficult to assess the true risk faced by manned and unmanned operational spacecraft.

But the report said the principal source of orbital debris is the breakup of rocket boosters and satellites. Other debris is made up of inactive satellites and objects that are discarded during routine space activity, such as lens caps, empty propellant tanks and so on.