ARLINGTON, Va. The Defense Advanced Research Projects Agency (DARPA) launched a program called "Muons for Science & Security" (MuS2) with the aim of discovering a compact source of muons, which are deeply penetrating subatomic particles -- similar to electrons but about 200 times heavier -- that can be used in a variety of defense and scientific applications because they travel easily through dozens to hundreds of meters of water, solid rock, or soil.