Think “NASA,” and what comes to mind? Astronauts? Mars rovers? Voyager and the Golden Record? How about Earth? In fact, NASA has been studying and monitoring the health of our home planet for decades, using balloons, aircraft, satellites, and even the International Space Station in the effort. “Sky High,” the 16th documentary in the series “JPL and the Space Age,” traces the efforts of NASA’s Jet Propulsion Laboratory to measure greenhouse gases, from the pathfinding science instrument AIRS, through to today’s Orbiting Carbon Observatory 3 aboard the space station.