NICER science not so nice as ISS telescope pauses operations

Cosmic research on hold while engineers investigate a problematic motor

NASA's NICER X-ray telescope is pausing operations just weeks after the US space agency boasted that a January repair and reconfiguration had improved its daytime measurements.

The agency pressed pause on its ops on June 17 when performance degradation in the motor began affecting science observations. NASA said, "Engineers are investigating the cause and potential solutions."

The Neutron star Interior Composition Explorer (NICER) telescope is mounted on the International Space Station (ISS) and its operations are being temporarily halted while engineers look into a problem with one of the motors used in the tracking of cosmic objects.

NICER uses a star-tracker-based pointing system with its X-ray Timing Instrument (XTI) to point to and track celestial targets. According to NASA, the design handles vibration from the ISS and enables "high-precision pulsar light-curve measurements through ultra-deep exposures."

The telescope was attached to the ISS's starboard solar array in 2017, following a launch on a SpaceX resupply mission. Its primary mission was 18 months, but it was extended. The telescope recently reached an eight-year milestone.

It was also never supposed to be serviced by astronauts. This caused controllers a headache in May 2023, when the telescope developed a "light leak" that allowed unwanted sunlight to enter its instrument during the orbit day. This didn't affect nighttime observations, but damage to the telescope's thermal shields, designed to block sunlight but allow cosmic X-rays, severely impacted daytime measurements.

Engineers devised a repair kit and sent it to the ISS in 2024. On January 16, 2025, astronaut Nick Hague installed patches, reducing sunlight inside NICER. However, engineers spotted more holes and cracks in the telescope's thermal shields during the spacewalk that were not previously visible.

Still, things did improve for a time following Hague's efforts. By April, NASA announced that, with some operational adjustments, NICER was back to collecting observations during more than 70 percent of ISS daytime.

That is, up until the motor, used for tracking, put a (hopefully temporary) end to the party.

In addition to the measurements of neutron stars, NASA noted that the mission had "successfully demonstrated a form of deep space navigation that could be used for travel to Mars and beyond."

Part of NICER's mission is to demonstrate the feasibility of pulsar-based spacecraft navigation, referred to as Station Explorer for X-ray Timing and Navigation Technology or SEXTANT (and people thought LOHAN was a stretch...) This uses observations of X-ray pulsars to determine an absolute position in space. ®