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The Neutron Star Interior Composition Explorer (NICER) is a NASA mission launched in June 2017 and mounted on the International Space Station (ISS). Its primary objective is to study neutron stars—ultra-dense remnants of massive stars that have undergone supernova explosions. By observing X-ray emissions from these celestial objects, NICER aims to provide insights into their internal structures and the fundamental physics governing matter under extreme conditions.

NICER’s core component is the X-ray Timing Instrument (XTI), designed for high-precision timing and spectroscopy of soft X-rays in the 0.2–12 keV energy range. The XTI comprises 56 co-aligned X-ray concentrator optics, each paired with a silicon drift detector. These concentrators utilize grazing-incidence optics with 24 nested mirrors to focus incoming X-rays onto their respective detectors, enhancing sensitivity and resolution.

NICER is mounted on the ISS’s ExPRESS Logistics Carrier-2. It features a two-axis pointing system that allows the instrument to track celestial targets across the sky. An integrated star tracker ensures precise alignment, enabling NICER to observe multiple targets during each 92-minute orbit of the ISS.

To achieve its scientific goals, NICER incorporates a GPS-based timing system capable of tagging photon arrival times with sub-microsecond accuracy. This high temporal resolution is crucial for studying the rapid rotational periods of pulsars and other time-sensitive phenomena.

NICER has significantly advanced our understanding of neutron star interiors by providing precise measurements of their masses and radii. These observations have helped constrain the equation of state for ultra-dense matter, shedding light on the behavior of matter at densities exceeding those found in atomic nuclei.

An extension of NICER’s mission, known as SEXTANT (Station Explorer for X-ray Timing and Navigation Technology), successfully demonstrated the use of X-ray pulsars for autonomous spacecraft navigation. By measuring the timing of X-ray pulses from known pulsars, SEXTANT was able to determine the ISS’s position in space, paving the way for future deep-space navigation systems.

In 2018, NICER discovered an X-ray pulsar in the fastest known stellar orbit, with a companion star completing an orbit every 38 minutes. This finding provides valuable data on the dynamics of compact binary systems and the extreme gravitational environments in which they exist.

NICER observed the brightest X-ray burst ever recorded from the neutron star SAX J1808.4−3658. This event offered insights into thermonuclear processes on neutron star surfaces and the mechanisms driving such energetic emissions.

Although primarily focused on neutron stars, NICER has also contributed to black hole research. It mapped “light echoes” from the stellar-mass black hole MAXI J1820+070, revealing changes in the size and shape of the surrounding accretion disk and corona. These observations enhance our understanding of black hole accretion processes and their immediate environments.

In May 2023, NICER’s thermal shields developed a leak, allowing stray light to interfere with its X-ray detectors. To address this issue, NASA designed specialized patches delivered to the ISS via the Cygnus NG-21 resupply mission in August 2024. Astronauts successfully applied these patches during a spacewalk on January 16, 2025, restoring NICER’s full observational capabilities.

As of early 2025, NICER has contributed to over 300 scientific publications, underscoring its significant role in advancing astrophysical research. Its high-precision measurements continue to provide valuable data for the scientific community, enhancing our understanding of neutron stars and other cosmic phenomena.

Video credit: NASA Goddard