NASA’s X-ray telescope on the International Space Station, the Neutron star Interior Composition Explorer (NICER), has paused science after a steering motor stopped moving properly. Operations were halted on June 17, 2025, and the team is still troubleshooting.

The pause caps eight years of observations by an instrument originally built for an 18 month prime mission. It sits on the station’s starboard side and usually pivots to track targets across the sky each orbit.

NICER telescope problems

The immediate culprit is the instrument’s pointing system, hardware that rotates the telescope to lock on targets.

Robotic cameras outside the station have helped rule out external snags, and engineers are testing motions to isolate which internal part is misbehaving.

The team’s top priority is to move the telescope into a stowed configuration if the station needs the clearance. There is no published timeline to resume science.

Keith Gendreau leads the mission at NASA’s Goddard Space Flight Center (GSFC). He has guided NICER since its earliest planning and through years of intense observations.

The telescope remains safe for the station and crew. Engineers continue methodical tests so any recovery steps do not worsen the problem.

Why NICER telescope matters

NICER’s design centers on precise timing and spectral measurements of neutron stars, an extremely dense stellar core left after a supernova.

Their gravity and magnetic fields let scientists probe matter under pressures far beyond any lab on Earth.

NICER also hosted the Station Explorer for X-ray Timing and Navigation Technology, a demonstration of X-ray pulsar navigation, which is a technique that uses pulsar pulses as natural timing beacons for spacecraft.

An NTRS conference paper documented autonomous position estimates at the 10 kilometer level using pulsar signals and those results hint at future missions that can steer far from Earth without constant radio help. A spacecraft that reads pulsars could check its own position and time during long cruises.

NICER’s fast response and high throughput opened rich science beyond navigation. The instrument has clocked the spins of pulsars, caught explosive flashes on dead stars, and set new bounds on the size of objects so dense that a teaspoon would weigh billions of tons.

A fix already tested in spacewalks

NICER has fought through challenges before, including a 2023 thermal shield, a thin filter that blocks sunlight from reaching the detectors, that tore and let stray light leak in.

On January 16, 2025, astronaut Nick Hague installed nine patches during a spacewalk, then engineers reconfigured detector electronics to filter out residual sunlight during parts of the day.

“It was awesome to watch the training sessions and be able to debrief with the astronauts afterward,” said Gendreau. Training for that spacewalk drew the astronomy and human spaceflight teams closer.

Those patches and detector updates restored much of NICER’s daytime capability. The experience also built playbooks for inspecting the instrument with cameras and scheduling careful test motions.

That history matters today. Troubleshooting a stuck motor calls for the same slow, well-instrumented steps that kept NICER productive after the light leak.

What the data still offer

The pause comes after a run of results that continue to feed active research. In 2020, during a burst storm from the magnetar SGR 1935+2154, a NICER-led study reported more than 217 X-ray bursts in 1,120 seconds and traced how activity evolved in the weeks that followed.

A magnetar, a neutron star with an extreme magnetic field, can release short, powerful X-ray bursts and sometimes radio bursts. These events let scientists test how magnetic energy powers fast changes in high energy light.

NICER’s rapid scheduling made those observations possible. The telescope slewed quickly and recorded fine time tags on incoming X-ray photons.

Those detailed light curves and spectra now anchor models of how magnetars twist and snap their magnetic fields. They also guide new campaigns with other satellites when the same source flares again.

What happens next

Even while the telescope rests, NICER’s archive is open for new analyses. NASA has formally shifted near-term efforts toward data mining, as noted in the ROSES program notice.

The instrument’s location on the station adds both benefits and constraints. Power, data, and the robot arms are close by, yet station operations can limit when and how the payload moves.

If engineers can free the motor, NICER can go back to its normal plan of switching targets each orbit. If they cannot, the team may still squeeze science from fixed pointings during times when the current attitude matches a viable target.

For students and researchers, the pause is a chance to pull new signals from old photons. The archive holds thousands of hours on pulsars, black holes, and odd transients, ready for fresh questions and improved methods.

Image credits: NASA/Nick Hague.

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