A defunct NASA satellite is expected to re-enter Earth's atmosphere by early October, sparking concerns that some debris might shower down on populated areas.
Most of the 6.5-ton Upper Atmosphere Research Satellite's parts, known by its acronym UARS, will burn up in Earth's atmosphere when it finally falls from orbit later this month or early October.
Although NASA has said the chances of the wreckages of the defunct satellite falling to the Earth is very remote, yet the space agency officials cautioned that the uncontrolled falling of the debris may cause risk to billions of people.
The agency says there is little chance that the falling debris will create risk to public safety or property. 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, NASA said in a report.
The report further stated that it is difficult to predict the date of re-entry, as well as what geographic area may be affected, because it depends on the spacecraft's orientation as its orbit decays. As re-entry draws closer, predictions will become more reliable.
According to Nicholas Johnson, NASA's chief scientist for orbital debris (yes, there is such a job), the odds of any specific person getting hit are about 1 in 21 trillion, MSNBC reported. That's a chance of about 0.0000000001 percent. For perspective, the odds of any given person winning the lottery are 1 in hundreds of billions, depending on the lottery design -- but nowhere near the trillions.
There is a much greater, but still minimal, chance that a piece of the satellite will hit someone on Earth: 1 in 3,200, or about 0.03 percent.
Once the satellite re-enters Earth's atmosphere, NASA will be able to project with more accuracy the general area within which it will land, but it will be a wide area, not specific, and even those projections could be wrong. One of the problems with predicting the landing spot is that the satellite will break up once it re-enters the atmosphere.
Pieces of various sizes -- the largest probably 300 pounds, or the weight of a refrigerator -- could land across an area of 500 miles. That means pieces could fall over most of the world's six inhabited continents as well as its three largest oceans.
As of Sept. 8, 2011, UARS' orbit was 152 miles by 171 miles (245 km by 275 km) with an inclination of 57 degrees, which means that any surviving components of the spacecraft will land within a zone between 57 degrees north latitude and 57 degrees south latitude.
Launched from the space shuttle Discovery on Sept. 15, 1991, to collect data on Earth's atmosphere and its interactions with the sun, the $750 million spacecraft orbited the planet more than 78,000 times, before ceasing operations on Dec. 14, 2005. It used 10 onboard scientific instruments to collect data on a variety of atmospheric chemicals, including carbon dioxide, ozone, chlorine, methane, nitrogen oxides and chlorofluorocarbons.
In its first two weeks of operation, UARS data confirmed the polar ozone depletion by providing three-dimensional maps of ozone and chlorine monoxide near the South Pole during development of the 1991 ozone hole. In addition, UARS collected data on the chemistry, dynamics and radiative inputs to the upper atmosphere far beyond its designed lifetime.
The United Kingdom and Canada also provided instruments for this mission, the first spacecraft launched as part of NASA's systematic, comprehensive study of the Earth system. Although it was designed to last 18 months, upgrades extended UARS' life for years beyond its expected lifespan.
If you find something you think may be a piece of UARS, do not touch it. Contact a local law enforcement official for assistance, NASA said.