The department announced Thursday that it will "engage" the decaying satellite, which it earlier deemed to be low risk. The department said this week that the chances that the "uncontrollable U.S. experimental satellite" will hit a populated area are small, but "the potential consequences would be of enough concern to consider mitigating actions."
The NROL-21 USA-193 satellite was launched for the Defense Department in December 2006 and failed within hours. The solar arrays never deployed, and in January 2007, U.S. officials reported that they were unable to communicate with the spy satellite, said GlobalSecurity.org, a military information Web site.
The satellite, estimated to weigh 7,000 pounds, is expected to continue disintegrating upon re-entry into the Earth's atmosphere. Government experts and GlobalSecurity employees said that the odds of debris landing on solid ground, instead of in the ocean, are about one in four. That's because about 75% of the Earth's surface is covered by water.
Now government experts believe about 2,500 pounds of it could survive disintegration during re-entry. That includes 1,000 pounds of propellant fuel (hydrazine), a hazardous material, the Defense Department said. Amateur satellite watchers have said they believe the satellite is headed toward North America.
"The president has decided to take action to mitigate the risk to human lives by engaging the nonfunctioning satellite," the Defense Department explained in a news announcement. "Because our missile defense system is not designed to engage satellites, extraordinary measures have been taken to temporarily modify three sea-based tactical missiles and three ships to carry out the engagement."
The Defense Department said it had to make a decision to shoot the satellite down before choosing the best time and place. The department will use modeling and analysis to do that, and military experts said they are confident their mission will succeed.
"Contact with hydrazine is hazardous," the Defense Department explained. "Direct contact with skin or eyes, ingestion or inhalations from hydrazine released from the tank upon impact, could result in immediate danger. If this operation is successful, the hydrazine will then no longer pose a risk to humans."
U.S. Strategic Command, or STRATCOM, has acknowledged that re-entry of space debris is an inexact science because of limitations on tracking equipment and environmental factors, like variations in gravitational fields, solar radiation pressure, and atmospheric drag.
Several government agencies are involved in monitoring and planning for re-entry of the satellite.
"In the event the engagement is not successful, all appropriate elements of the U.S. government are working together to explore options to mitigate the danger to humans and to ensure that all parties are properly prepared to respond," the Defense Department said. "In the unlikely event satellite pieces land in a populated area, people are strongly advised to avoid the impact area until trained hazardous materials teams are able to properly dispose of any remaining hydrazine."