A defunct NASA satellite is expected to plunge back to earth on Friday, raising concerns that blazing hot debris may shower down on the unsuspecting terrestrial population.
NASA said on Thursday to expect pieces of the satellite to start falling in the afternoon, Easter Daylight Time, but the projection isn't 100 percent accurate.
It is still too early to predict the time and location of re-entry with any more certainty, but predictions will become more refined in the next 24 hours, NASA said late Thursday.
Variations in solar activity -- which influence to orbit of the satellite -- have made pinpointing the precise trajectory difficult. In the days leading up to Friday officials have changed their estimates a number of times.
We're really never confident, said Nick Johnson, Chief Scientist of NASA's Orbital Debris Program on Saturday. Even at the last message, T minus 2 hours, there will be a lot of uncertainty, plus or minus 10,0000 kilometers.
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The 6.5 ton satellite, known as the Upper Atmosphere Research Satellite (UARS) was originally expected to re-enter the Earth's atmosphere either late September or early October, NASA said.
It's a little bit unpredictable, and as a result, it's coming in a little faster than we originally anticipated. said Dr. Mark Mateny of the Orbital Debris Program at Johnson Space Center in Houston, Texas.
Most of the vessel is expected to burn up in the atmosphere, but the agency said there was still some potential for problems.
The risk to public safety or property is extremely small, and safety is NASA's top priority, the space agency wrote in an advisory.
NASA estimates a 1-in-3,200 chance that a satellite part could hit someone, though 1,200 pounds should survive the re-entry.
Things have been re-entering ever since the dawn of the Space Age; to date nobody has been injured by anything that's re-entered, said NASA orbital debris chief Gene Stansbery. That doesn't mean we're not concerned.
The satellite ran out of fuel in 2005. As of Sept. 8, 2011, the orbit of UARS was 152 miles by 171 miles (245 km by 275 km) with an inclination of 57 degrees.
Because the satellite's orbit is inclined 57 degrees to the equator, any surviving components of UARS will land within a zone between 57 degrees north latitude and 57 degrees south latitude . This translates to anywhere in the six inhabited continents in a worldwide swath from south of Juneau, Alaska, to just north of the tip of South America.
UARS was a $750 million mission deployed from the shuttle Discovery in 1991 to study the Earth's atmosphere and its interactions with the sun.
It measure important ozone depletion related to climate change. According to NASA reading from UARS gave evidence that Chorine in the atmosphere is at the root of the polar ozone hole.
In August researchers recommended that NASA and the U.S. State Department coordinate with other nations to work on plans to remove space junk from Earth's orbit, warning debris was reaching a tipping point.