A 6.3-ton NASA science satellite, decommissioned in 2005, is expected to plunge into Earth's atmosphere later this month, breaking up in a shower of debris. Experts said today they expect 26 components, the largest weighing more than 330 pounds, to survive re-entry heating to hit the surface somewhere between 57 degrees north and south latitude.
But a NASA space debris expert said the threat to the public is minimal and that statistically, the odds are good the debris will land in an ocean or some other sparsely populated area.
"We looked at those 26 pieces and how big they are and we've looked at the fact they can hit anywhere in the world between 57 north and 57 south and we looked at what the population density of the world is," said Nick Johnson, chief scientist with NASA's Orbital Debris Program at the Johnson Space Center in Houston.
"Numerically, it comes out to a chance of 1 in 3,200 that one person anywhere in the world might be struck by a piece of debris. Those are obviously very, very low odds that anybody's going to be impacted by this debris."
The centerpiece of a $750 million mission, the Upper Atmosphere Research Satellite, or UARS, was launched from the shuttle Discovery in September 1991. The solar-powered satellite was designed to address a wide variety of atmospheric questions, including the depletion of Earth's ozone layer 15 to 30 miles up, a trend blamed on the release of man-made chemicals called chlorofluorocarbons, or CFCs, used in refrigerants, foam products, and various solvents.
The long-lived satellite was decommissioned in 2005 and one side of its orbit was lowered using the last of its fuel to hasten re-entry. No more fuel is available for maneuvering and the satellite's re-entry will be "uncontrolled."
Air Force Maj. Michael W. Duncan, deputy chief of space situational awareness with U.S. Strategic Command at Vandenberg Air Force Base, Calif., said it is too early to predict exactly when--or where--UARS might fall.
"Late September is the best estimate we can give right now," he said. "There are so many factors that will affect it between now and that point in time. The atmosphere changes on a daily basis, it's impossible to say how that's going to impact this re-entry.
"We do know with 99.9 percent accuracy that it will re-enter the atmosphere somewhere between 57 degrees north and 57 degrees south, which means it will be anywhere from northern Canada to southern South America. That is truly the best estimation we can give you at this point in time."
UARS was released from the shuttle Discovery into a 357-mile-high orbit inclined 57 degrees to the equator. Steadily losing altitude because of increasing atmospheric drag, the satellite's current orbit measures 152 miles by 171 miles.
Duncan said STRATCOM will track UARS with powerful radar systems and provide increasingly frequent updates as the spacecraft's altitude continues to drop and its trajectory becomes easier to predict. But even two hours before entry, the impact zone will only be known to within about 6,000 miles.
UARS is the largest U.S. satellite to fall back to Earth since the 17.5-ton Compton Gamma Ray Observatory was guided to a safe, controlled re-entry in June 2000. A 2.5-ton National Reconnaissance Office spy satellite was shot down in 2008 to prevent toxic hydrazine fuel from reaching the ground.
Johnson said no such toxic materials were part of UARS.
"Satellites re-entering is actually very commonplace," he said. "Last year, for example, we averaged over one object per day falling back uncontrolled into the atmosphere. The majority of these, of course, were very, very small, they burn up completely and no parts reach the surface of the Earth.
"On the order of one moderate-sized object, on average, falls back to Earth every week, an old intact spacecraft of intact rocket body. Some of these vehicles do have components that reach the surface of the Earth. When they do, they typically fall into an ocean area or some desolate region. Typically, we find one piece a year from one of these re-entries."
Old rocket bodies or defunct satellites as large as UARS re-enter about once per year on average.
"Last year, there were 75 metric tons of spacecraft and rocket bodies falling back to Earth in an uncontrolled manner," Johnson said. "In perspective, UARS is less than six metric tons. So it's a very small percentage of the annual re-entry of satellites.
"Finally, just to remind everyone, throughout the entire 54 years of the space age, there have been no reports of anybody in the world being injured or severely impacted by any re-entering debris."