Here are two photos taken on March 3, 2011 of a prescribed fire at Fort Indiantown Gap in Pennsylvania.

They are described as follows on flickr where several more photos can be found.

Following work by the Aerial Ignition Crew the prescribed burn progresses at Fort Indiantown Gap on March 3, 2011 . Fort Indiantown Gap conducted the 2,500 acre burn in the training area using an aerial ignition technique using ping pong sized balls filled with [snip] that are injected with [snip] and dropped from a helicopter over the area designated for prescribed burning. Prescribed burns help reduce the potential of major wildfires on Fort Indiantown Gap property and more than 100 prescribed burns have occurred at the installation in the past decade. These burns are always conducted by properly trained personnel with an approved plan and coordinated with the appropriate authorities. (U.S. Air Force photo by Staff Sgt. Ted Nichols/Released)

I wish people would not publicize the formula for creating these “ping pong balls”.