by Sahana Parker

Sahana Parker works in Human Exploration Development & Operations at the Marshall Space Flight Center, where she operates facilities on the International Space Station to support space-based life science and physical science experiments. She is passionate about science outreach and bringing stories of space across disciplines. Parker completed her undergraduate studies in Astrophysics and G.I.S. at the University of Georgia.
On January 16 at 8 am ET, astronauts Nick Hague and Suni Williams ventured outside a hatch on the International Space Station (ISS) into the vacuum of space. For the next 6 hours, with continuous support from the ground (including fellow astronaut Akihiko Hoshide), they maneuvered the sensitive instruments and robotic arms mounted on the outside of the ISS, set up their toolkits, and got to work on external repair tasks identified as critical by NASA ground teams. Among them was a patch for the Neutron star Interior Composition ExploreR (NICER), an X-ray telescope mounted to the zenith side of the ISS.
Launched in 2017, the NICER instrument uses X-ray timing to study the compositions and interiors of neutron stars. As the ISS orbits, a star tracker on the telescope guides the X-Ray Timing Instrument (XTI) to celestial objects of interest, with targets chosen and uploaded weekly by ground scientists at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Two or three targets are tracked over almost a full hemisphere, with individual observations for given targets compiled into larger datasets by the ground. In 2017, NICER also demonstrated the first in-space use of pulsar-based spacecraft navigation, in which the SEXTANT payload tested the feasibility of using pulsing neutron stars as a sort of in-space GPS.
The telescope wasn’t launched with longevity in mind. The mission duration at launch was predicted to be 18 months, with a goal of about two years of operations to meet science goals. After this time period, preliminary plans were developed to have the telescope removed and burned up on re-entry with other ISS trash. But seven years and almost 400 publications later, NICER has persevered.
It wasn’t until May 2023 that scientists noticed a problem. A persistent light leak warped data collected during ISS “daytime” observations. Imagery showed that the exterior thermal shields, the barriers that prevent sunlight from entering the X-ray detectors, were damaged, likely by errant space debris or micrometeoroids. While the telescope operated normally when the ISS orbited on the dark side of the Earth, data was consistently corrupted when exposed to the bright optical environment near the Sun. This left ground engineers with a puzzle: the telescope was never designed to be repaired in orbit, and no repair materials were present on the ISS. Was it worth it to repair?
NASA decided yes, and in January 2024, ISS boards approved an Extravehicular Activity (EVA) task to fix the thermal shields on the telescope. This decision kicked off months of EVA planning and modeling, including simulations in the 40-foot pool at NASA’s Neutral Buoyancy Laboratory. In May 2024, a repair kit designed specifically for NICER (Figure 1) was delivered to the Johnson Space Center in Houston and launched to the ISS in August.

While the repair was intended to be done in the fall, various issues with EVA equipment led to multiple spacewalks being postponed to January 2025. Nick Hague was selected to perform the repair, alongside other tasks, with Suni Williams, who may be recognized as one of the two “stranded astronauts” currently aboard the ISS.
Ultimately, the fix designed by engineers was relatively simple. The thin thermal shields on the telescope are topped with pie-shaped sunshades (Figure 2), which help protect from thermal overheating from the Sun. Engineers developed similarly pie-shaped wedges to be installed as patches to the tops of the sunshades, manually blocking sunlight and allowing the rest of the telescope to be used as designed. While about five areas with the worst damage (about the size of a U.S. postage stamp) were identified to be patched, Hague was equipped with extras as “targets of opportunity” just in case there were additional damaged areas outside of the Field of View of reference imagery. In total, all nine patches carried by Hague were installed.

Following the repair, Goddard scientists reported the patches have been working as designed, although some excess visible light is still present. Further calibrations are underway, but science capability has already increased beyond the damaged levels.
According to NASA, this was only the fourth science observatory in orbit patched up by astronauts, the previous ones being the Alpha Magnetic Spectrometer (AMS-02), the Hubble Space Telescope, and the Solar Maximum Mission. Among the other tasks performed during the EVA, Hague, and Williams set up the area around the AMS-02, an important particle physics detector, for an upgrade during a future space walk.
The successful repair mission of NICER represents the culmination of efforts of hundreds of people, from planners, console operators, and scientists on the ground to astronauts in space – efforts that will pay off in science.
Astrobite edited by Archana Aravindan
Featured image credit: NASA