The low-flying helicopters heard and seen over the Washington, DC metro area are part of an radiation assessment by the National Nuclear Security Administration (NNSA), the federal agency that maintains the U.S.'s nuclear weapons stockpile.
The helicopters began flying over the region Thursday, measuring naturally occurring radiation, according to the group's website.
The flights will continue until Jan. 11 during daylight hours only.
Pilots fly at about 80 miles per hour at 150 feet or higher over the 70-square mile radiation assessment area, NNSA officials said.
Scientists with NNSA’s Remote Sensing Laboratory (RSL) 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 radiological emergencies on the home front, 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 release issued by the agency.
Congress established the agency in 2000 as a semi-autonomous unit within the U.S. Department of Energy.