The details: The satellites, which were deployed atop a reused SpaceX Falcon 9 rocket that lifted off from Vandenberg Air Force Base in California at 3:47 p.m. ET., are capable of detecting changes in groundwater availability worldwide, giving early warning to security agencies about potential areas of political instability due to potential conflict over natural resources.

• The GRACE-FO mission, which is technically a joint mission between the U.S. and Germany, will also be used to answer critical questions about how quickly and extensively the planet's ice sheets are melting. The first GRACE mission helped scientists discover the quickening pace at which Greenland and parts of Antarctica are losing mass, which is raising sea levels worldwide.

Be smart: According to NASA, the twin satellites will follow each other in orbit around the Earth, separated by about 137 miles (220 km.) As they do so, they will send microwave signals to each other. Areas of slightly stronger gravity (more mass) as well as slightly weaker gravity will change this distance slightly. By using these changes, they will enable scientists to map water resources, ice sheets, and potentially discover other applications as well.

Go deeper: First map of global freshwater trends shows "human fingerprint."