NASA says crashing science satellite is low risk to public
BY STEPHEN CLARK
Posted: September 9, 2011
NASA expects an abandoned research satellite to crash back to Earth later this month, scattering more than 1,000 pounds of debris somewhere on the surface as the rest of the craft vaporizes in the upper atmosphere.
"We just won't know precisely where it's going to come down until it's come down," said Nick Johnson, chief scientist for NASA's orbital debris program at the Johnson Space Center in Houston.
Johnson said 26 components from the Upper Atmospheric Research Satellite, or UARS, will survive re-entry and reach Earth's surface. The total mass of the objects expected to make it to the surface is more than 1,100 pounds.
But officials said the satellite will most likely come down over the ocean or an unpopulated land mass. There is a 1-in-3,200 chance a piece of debris could injure or kill a person, according to an assessment by NASA.
"I hope [people] don't get too concerned because this is something with a very low probability of anyone being hurt or anyone's property being damaged," Johnson said. "Unless we build these things out of paper mache, we can't reduce the risk to zero," he later added.
UARS was built before NASA and international standards were employed to limit human casualty risks from re-entering spacecraft to less than 1-in-10,000.
Launched by the space shuttle Discovery in 1991, UARS operated for 14 years measuring changes in the upper atmosphere. The $750 million mission's suite of sensors was tasked with detecting the erosion of ozone, a molecule that blocks ultraviolet light that can cause skin cancer and damage food crops.
UARS is now on the cusp of being captured by the atmosphere as it circles Earth at altitudes between 152 miles and 171 miles. When NASA decommissioned the 12,500-pound satellite in 2005, controllers used leftover propellant to lower its orbit from 340 miles to expedite its re-entry.
Atmospheric drag gradually reduces an object's velocity in lower orbits, bringing satellites closer to Earth and quickening their demise.
UARS overflies Earth between 57 degrees north and south latitude. The satellite could fall on any region within those latitudes, but most of the territory is oceans and unpopulated land masses.
Johnson said an object with the size and risk of UARS typically falls to Earth about once per year. The last NASA satellite as large as UARS to re-enter the atmosphere was the Compton Gamma Ray Observatory, which was guided to a controlled return over remote Pacific Ocean.
A 5,000-pound National Reconnaissance Office satellite was shot down in 2008 because its fuel tanks were full of toxic hydrazine propellant. Defense officials said they wished to avoid showering the caustic fuel in the atmosphere.
But UARS has no fuel left for a controlled re-entry, also dispelling concerns the satellite would litter the ground with toxic materials.
"Throughout the entire 54 years of the space age, there's been no report of anybody on the world being injured or severely impacted by any re-entering debris," Johnson said.
"We continue to say that late September is the best estimate we can give right now," Duncan 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."
For now, the Air Force's only prediction is bounded by the satellite's orbit.
"We do know with 99.9 percent accuracy that it will enter between 57 degrees north and 57 degrees south, which means it will be anywhere from northern Canada to southern South America," Duncan said. "That is truly the best estimation we can give you at this point in time."
The bulk of the Earth's population lives between 57 degrees north and south latitudes.
"Once you get up to 57 degrees, plus or minus, you've pretty much encompassed all 7 billion people on the planet," Johnson said.