NICER: The Neutron Star Interior Composition Explorer

The Neutron Star Interior Composition Explorer (NICER) is a NASA Explorer Mission of Opportunity that was launched in June 2017 to the International Space Station (ISS). NICER is a soft X-ray (0.2–12 keV) timing and spectroscopy mission originally dedicated to the study of neutron stars. NICER also conducted the first inflight demonstration of autonomous X-ray pulsar-based navigation. NICER detects photons with an energy resolution of approximately 2% and an absolute time resolution of better than 100 nanoseconds. Having completed its baseline mission in 2019, NICER has expanded its role to include a Guest Observer (GO) program that covers a broad range of science ranging from comets to black holes. Making use of established infrastructure on the ISS, NICER is very nimble and supports a wide range of coordinated observations with ground- and space-based observatories. Beginning in 2022, NICER will work with the Monitor of All-sky X-ray Image (MAXI) payload by JAXA that is also on the ISS in an automated way to localize and follow-up on X-ray transients in the sky.