We’ve found more than 1,800 pulsars today, which are the leftover cores of older stars that underwent supernova or stellar explosions. As they rotate, the pulsars send out signals at a highly precise rate. Their signals can come out in a range of wavelengths — anything from radio to X-ray to gamma ray.

Pulsar rotation rates are so precise, NASA says, that they are similar in accuracy to the atomic clocks used inside GPS satellites to help us navigate on Earth. So just like a lighthouse, pulsars are beacons that can help astronauts find their way in the universe. The concept of X-ray navigation, according to NASA, will potentially “enable sustained human presence throughout the solar system, as well as enhance and enable science in the outer solar system and beyond.”

But first we need to test pulsar navigation, which will require two ingredients installed on the International Space Station. The first is an instrument called the Neutron Star Interior Composition Explorer (NICER), which launches on the SpaceX mission. While not all neutron stars rotate rapidly (as pulsars do), pulsars and other types of neutron stars are all incredibly dense. Scientists aren’t sure how matter behaves inside this environment, and NICER will help them investigate.