The orbiting FUSE telescope, still in shakedown mode and not yet operating at the peak of its capability, is already providing uniquely detailed observations of stars and galaxies in far ultraviolet light, the mission leader said.
The Far Ultraviolet Spectroscopic Explorer, launched June 24, began scientific observations at the end of September as planned, even as mission engineers and scientists continued work on a series of shakedown problems, said Warren Moos, principal investigator on the Johns Hopkins-designed and -operated mission.
"We have in the bag close to 30 observations of a very wide range of targets, both from inside our galaxy to other nearby galaxies and even distant quasars," Moos said. Those studies have produced important results that will be reported in more than two dozen papers presented in January at the American Astronomical Society meeting in Atlanta, he said.
Even before final calibration of its instruments, FUSE has lived up to prelaunch expectations of its ability to produce the highest quality and most detailed information ever on far ultraviolet emissions from a wide range of celestial objects. Far ultraviolet is the high-energy, short wavelength light between ultraviolet light and X-rays, beyond the blue end of the visible light spectrum.
FUSE analyzes far ultraviolet light by spreading it out into a spectrum for analysis, much as a prism creates a rainbow from visible light. The more the light is spread out, the more information can be gleaned. So far, FUSE has been able to produce spectra that show far UV light at a resolution of 15,000, comparable to being able to see two edge-on sheets out of a stack of paper 10 feet high. That's 50 times better than the resolution achieved in 1995 on the second space shuttle flight of the Hopkins Ultraviolet Telescope, which also observed in this range of light. Further tweaking should improve the resolution by as much as a factor of two.
"It's like looking at a forest," Moos said. "If you take a picture at low resolution, all you see is green. If the photographer uses a different lens, you can see there are trees there. You get an even better lens, and you can tell there are oaks and maples."
The other advantage of FUSE is its sensitivity, or ability to detect faint light. The previous telescope most similar to FUSE was Copernicus, a satellite that flew in the mid-to-late 1970s. Checkout has confirmed the FUSE is 10,000 times more sensitive than Copernicus. That ability will allow it to furnish astronomers with a wealth of information that no other telescope can provide, on objects not only in our Milky Way galaxy, where Copernicus could "see," but out into the universe.
Though FUSE's scientific work began just about when expected in prelaunch schedules, Moos said that projects requiring the highest capabilities of FUSE will be put on hold until engineering obstacles are overcome and calibration is complete. That work is running up to two months behind what Moos had hoped, he said.
There have been several types of difficulties, some beyond the influence of FUSE controllers. In one case, operations were stalled for four weeks because it took longer than expected for gas to bleed out of composite materials in the telescope structure and vent into space, a process called "outgassing." That gas could have contaminated the telescope's optics or damaged its light detectors if they had been operational early on.
Two other problems required software patches, in one case ultimately involving a single line of code. But it took time to track down the problems, write software fixes, test them extensively on earthbound computers and then upload them to FUSE during one of its six to eight daily passes over ground stations in Puerto Rico and Hawaii.
Also, the Puerto Rico station, the primary avenue for communications with FUSE, has been erratic in performance. Though it has worked well in recent weeks, engineers are still trying to determine what went wrong on two earlier occasions so they can prevent it from happening again.
The last of the early problems was the unexpectedly serious impact of spikes in solar radiation. Those spikes, during FUSE passes over the South Atlantic, where the Earth's magnetic field is relatively low to the ground, can disrupt the programming of a certain electronic component in the light detectors. By mid-November, Moos said, there will be a program on FUSE allowing on-board computers to fix that disrupted programming without waiting for a ground station contact.
"All of these things together have put us about two months behind" getting to the point where the telescope's four light-reflecting mirrors can be fully aligned and focused, Moos said. And now that FUSE is finally at that point, another potential problem, anticipated in advance but not completely avoidable, has cropped up.
Engineers believe that two of the mirrors, because of their relative position in the spacecraft, may be more exposed than the other two to the extremes of heat and cold experienced by FUSE during an orbit. If those extremes shift a mirror as little as one- or two-hundredths of the width of a human hair out of alignment with the others, the light reflected from that mirror can miss the entrance hole into the spectrograph. The spectrograph is the component of FUSE that breaks light into its parts.
"We now have evidence that our overall optical system is sensitive to thermal effects, and we are in the process of learning how to deal with it," Moos said. Such sensitivities occur on most space observatories and cannot be fully assessed before launch, he said, because there are no facilities on earth that can perfectly simulate the environment of space.
Moos and George Sonneborn, NASA's project scientist for FUSE, said that engineers will be collecting data throughout November to pinpoint the problem and have many alternatives for resolving it, including adjusting FUSE's on-board heaters.
"We're in a data-gathering mode to characterize the problem," Moos said. "We're concerned, but not terrified, because we see the possible solutions."
Once the alignment problem is licked, the telescope can be fully focused and can begin observing with its designed maximum sensitivity and resolution. Moos said he has no reason to believe the thermal problem will have any implications for the focusing process.
Since observing is under way and the telescope's efficiency appears good, Moos said, the delay in final calibration need not affect the productivity of the three-year mission.
"There is no evidence yet of a science impact," he said. "We're very pleased with the amount of science time we're getting." Science observations are currently averaging about half of the available time in a given week of FUSE observations, Moos said. "On every journey, there are bumps in the road," he said, "but you still complete the journey. We expect to complete the journey as planned."
FUSE is the first NASA-funded project of its magnitude conceived, designed, built and operated by a university. Its primary scientific focus will be the study of hydrogen and deuterium (a different form of hydrogen), which were created shortly after the Big Bang. With FUSE's observations, astronomers in effect will be able to look back in time at the infant universe.
To learn more about FUSE and to keep up-to-date on its mission, log on to fuse.pha.jhu.edu.