Scientists and engineers in Baltimore and Greenbelt are troubleshooting an electrical failure that has blinded the Hubble Space Telescope's "workhorse" instrument - the Advanced Camera for Surveys.
The ACS, which has provided spectacular images of distant galaxies and stellar nebulas since 2002, shut down Monday.
Scientific observations have continued on three other instruments, according to Mattias Mountain, director of the Space Telescope Science Institute in Baltimore.
But "always, when a major instrument has problems like this ... people are suitably nervous and worried," he said. "If we lost it altogether, it would be a disaster."
A fifth Hubble instrument, the Space Telescope Imaging Spectrometer, has been idle since an electronics failure in August 2004. Repairs are planned if NASA agrees to send shuttle astronauts on another servicing mission .
NASA Administrator Michael D. Griffin has said he won't make that decision until the shuttle fleet has safely flown two missions since the fatal Columbia disaster in February 2003. The second of those missions is scheduled for launch next Saturday.
Investigators at the space telescope institute and at the NASA Goddard Space Flight Center have worked since Monday to isolate the camera problem, focusing on a power supply to the camera's digital light sensors. Voltages apparently surged above safe limits, and the camera automatically shut down, NASA said.
It might have been a strike by cosmic radiation, Mountain said, "either that or the component came to the end of its life."
The ACS has backup electronics that can be activated as early as Friday if engineers determine the primary box can't be recovered.
"However, before we do that, we still have some homework to do ... to assure we understand the problem and make sure it's safe to make the change," said Edward Ruitberg, acting Hubble project manager at Goddard.
Monday's breakdown interrupted a number of scientific projects.
One of them was astronomer Massimo Stiavelli's observations of the Hubble Ultra Deep Field, containing some of the most distant objects ever observed. Light from these objects has been traveling toward Earth since the universe was just 5 percent of its present age.
Stiavelli, who works at the space telescope institute in Baltimore, was trying to get images of the objects in visible light to match and combine with images obtained last year in infrared wavelengths. "The time lost cannot be recovered. ... I'm essentially missing almost two-thirds of what I was expecting," he said.