The two side boosters of the SpaceX Falcon Heavy rocket carrying 24 satellites as part of the Department of Defense's Space Test Program-2 (STP-2) mission are seen as they begin their landing burns to return to Landing Zone 1, Tuesday, June 25, 2019 at NASA's Kennedy Space Center in Florida. Four NASA technology and science payloads which will study non-toxic spacecraft fuel, deep space navigation, "bubbles" in the electrically-charged layers of Earth's upper atmosphere, and radiation protection for satellites are among the two dozen satellites that will be put into orbit. Photo Credit: (NASA/Joel Kowsky)