There's an obscure agency in the Defense Department with the unique distinction of costing the taxpayers nothing for its national security role. The members wear no uniforms, don't carry weapons and work from home. They are specially-licensed amateur radio operators who serve part-time in the Army and Air Force Military Auxiliary Radio Systems, known by the acronym MARS. They're the heroes of MARS at 90, a small (96-pages), illustrated non-fiction work. In the nine decades covered, the volunteer members pitched in to assist the military in hurricanes and other natural calamities, and most memorably during the wars in Korea, Vietnam and Iraq. Their new mission is keeping the country connected in case the internet and other communications are severed by hostile or natural action. It's the only survivable backup the U.S. has.