NASA satellite soon to return to Earth... in pieces

Almost 20 years to the day after it was launched into space to collect data on Earth's atmosphere and interactions with the sun, NASA's Upper Atmosphere Research Satellite is coming back home--in pieces--and there's a higher than normal chance one of them will hit someone.

But before you run to grab your diamond and titanium alloy umbrella that I know you have somewhere in the back of the hall closet for just such an occasion, it's important to note that there's only a 1 in 21 trillion chance that a piece of the space junk will hit you specifically, according to an AP report. A NASA scientist apparently told the AP that there is a 1 in 3,200 chance that a piece of the satellite will hit someone on Earth, which is much higher than the 1 in 10,000 threshold NASA has adopted as an acceptable risk. That rule was put in place after the UARS satellite was launched in 1991.

CNET attempted to confirm the figure, but NASA's East Coast media headquarters is closed for the day. We've reached out through other channels, but did not immediately receive a response. I called U.S. Strategic Command at Vandenberg Air Force Base in California, which also houses the Joint Space Operations Center that "works around the clock detecting, identifying, and tracking all manmade objects in Earth orbit, including space junk," according to a NASA release.

A communications officer there told me that she believed NASA had worked with another agency to come up with a risk model for the UARS re-entry. NASA often uses software called--in typical NASA naming style--ORSAT, for Object Re-entry Survival Analysis Tool, to figure these sorts of things out.

Point is, NASA says in a posting on its Web site, there's a very small risk of a piece of UARS damaging anything or anyone.

Since the beginning of the Space Age in the late-1950s, there have been no confirmed reports of an injury resulting from re-entering space objects. Nor is there a record of significant property damage resulting from a satellite re-entry.

UARS is a 6.5 ton piece of space trash right now, having ceased operations in 2005. It's fitting that it would go out by causing a potential minor freak-out. This is the satellite that helped us confirm the existence of a hole in the ozone layer, after all.

Far bigger crafts have re-entered the atmosphere, including Skylab and the Russian Space Station--both landed in the ocean.

Normally, NASA can steer a dead satellite to a watery ocean grave, but UARS reportedly doesn't have enough fuel for that approach to work this time, meaning it will be consigned to blazing its way into the atmosphere, most of it burning up as it goes.

NASA says it's too early to know exactly when UARS will begin its descent, but it's expected to re-enter sometime in the next few months. Updates from the Joint Space Operations Center will come weekly until about a week before anticipated re-entry, when they'll become more frequent. NASA says that owing to the satellite's particular orbit, it will re-enter somewhere between 57 degrees north and 57 degrees south of the equator. That means that most of Alaska will be spared the possibility of raining space debris, but the rest of North America is fair game. NASA estimates that wherever it lands, the debris footprint will be about 500 miles wide.

NASA will be holding a teleconference on the satellite's re-entry Friday morning at 11 a.m. EDT.