Low-flying helicopters heard and seen over the Washington metro area recently are part of a radiation assessment program by the National Nuclear Security Administration, or NNSA, the federal agency that maintains this country's nuclear weapons stockpile.
The helicopters began flying over the area in late December, measuring naturally occurring radiation, according to the agency's website.
The flights will continue until through Friday during daylight hours.
Pilots fly at about 80 mph at 150 feet or higher over a 70-square mile radiation assessment area, NNSA officials said.
Scientists with the agency's Remote Sensing Laboratory out of Joint Base Andrews will use remote gamma radiation-sensing instrumentals to carry out the assessments.
Naturally occurring radiation is measured so that baseline levels can be established and used in security and emergency preparedness, scientists said.
In addition to monitoring radiation levels and responding to domestic radiological emergencies, the agency provides the U.S. Navy with nuclear propulsion and works to reduce the global danger of weapons of mass destruction, according to a news release.
Congress established the NNSA in 2000 as a semi-autonomous unit within the U.S. Department of Energy.