NASA is sending astronauts out to fix an X-ray telescope on the International Space Station (ISS) after the instrument developed a "light leak."

The telescope in question is the Neutron star Interior Composition Explorer (NICER), which collects X-ray measurements. In May 2023, scientists found that the instrument had developed a "light leak" in which unwanted sunlight enters the instrument during daytime observations.

To mitigate the problem, the NICER team limited daytime observations to objects far away from the Sun's position in the sky and uploaded commands to lower the sensitivity of the telescope during the orbital day. Nighttime observations were unaffected.

Despite the measures taken, the data collected continued to be impacted by the light leak.

NICER was launched on a SpaceX Falcon 9 as part of the CRS-11 ISS resupply mission in 2017. It is an attached payload that was installed robotically, and deployed on June 15, 2017. The telescope has 56 aluminum X-ray concentrators, each with a set of mirrors. In front of the concentrator are thermal shields to block out sunlight.

It appears that some of the shields have been damaged, although the cause is unclear.

The problem is that NICER was never designed for mission servicing, even though it is attached to the ISS. "The possibility of a repair has been an exciting challenge," said Keith Gendreau, NICER's principal investigator.

The plan is to slot five wedges into the sunshades above the areas of greatest damage and lock them into place. The equipment will be launched on Cygnus NG-21, Northrop Grumman's resupply freighter for the ISS, later this year.

"While we worked hard to ensure the patches are mechanically simple, most repair activities in space are very complicated," said Steve Kenyon, NICER's mechanical lead. "We've been conducting tests to confirm the repair work will be both an effective fix for NICER's light leak and completely safe for the astronauts on the spacewalk and the space station."

NICER will be the fourth science observatory that NASA has risked astronauts to repair. The others include the Hubble Space Telescope, the Alpha Magnetic Spectrometer (AMS) - also attached to the ISS, and the Solar Maximum Mission. The latter was captured and repaired during a Space Shuttle Challenger mission 40 years ago.

While astronauts have regularly clambered around the exterior of the ISS to perform maintenance and assembly, there are risks involved, particularly when dealing with equipment that was not designed to be serviced.

More than half a century ago, astronauts Pete Conrad and Joe Kerwin ventured outside Skylab with an improvised tool to free a jammed solar array beam, which sprang into place, nearly pulling them into space but for their tethers.

The Skylab mission went on to be a huge success, but the spacewalk was also a demonstration of the risks and rewards of having astronauts fix equipment that was never designed for it. ®

https://www.theregister.com//2024/04/18/nasa_nicer_repair_spacewalk/