Science Forum

This week’s column is a postmortem on the ill-fated Hubble Space Telescope (HST). What happened? How bad is the damage? Could it have been prevented? And, most important of all, can it be fixed? Very briefly, the answers are: not much, a lot, yes, and maybe. The following is a condensed version of what such scientific journals as Science, Nature, and Physics Today had to say on the subject.

What happened? A minute grinding error in Hubble’s main 2.4-meter main mirror left it with an imperfection amounting to about one-hundreth of a human hair. The defect, known as spherical aberration, is enough to cause star images to be surrounded by a fuzzy halo as if the telescope had glaucoma.

After a couple of weeks of intensive testing, some of the HST astronomers have professed to find rays of hope shining through the disaster. The defect is in the main mirror rather than the 0.3-meter secondary mirror. This is good news? Only in the sense that, if the imperfection were in the secondary mirror that collects light from the primary and directs it into the instrument package, the pictures would be much worse and the defect more difficult to fix. NASA engineers believe a series of 16 nickel-sized relay mirrors installed about the main mirror will correct the problem. Assuming that NASA’s shuttle program gets back on schedule, a likelihood equivalent to that of the Red Sox winning the pennant, the repairs may be done as early as 1993.

How bad is the damge? The damage varies depending upon the instrument being used. According to an assessment published in Science, about 30 percent of the instruments are among the “walking wounded,” another 30 percent will require intensive care, and the remaining 40 percent may have to be given up for lost. The wide field/ planetary camera, the HST’s workhorse instrument, is 95 percent devastated, says Sandra Faber from Lick Observatory. Bright objects, such as densely packed stars at the center of a cluster, will still give useful pictures even though they will be surrounded by a halo. Faint objects, however, will not show up any better than they do on ground-based telescopes. The one instrument that appears to be unaffected by the defect is the “Fine Guidance Sensors.” The task of this instrument is to accurately point the telescope and give very precise positions of the stars.

The sensors can lock on to a target within 0.007 arc second, an accuracy equivalent to hitting a dime in Boston with a laser based in Washington, D.C. William Jefferies of the University of Texas at Austin plans to use the sensors to precisely determine the distance of stars from Earth. This is done by a process called parallax, the fixing of a star’s position in the heavens at six month intervals and determining its apparent motion resulting from the Earth’s movement about the sun. Earthbound telescopes can measure parallax to within 0.01 arc second so the HST should better this by an order of magnitude. Like survivors of an earthquake, astronomers are sorting through the rubble and planning on ways to rebuild.

NASA, and its Space Telescope Science Institute located in Baltimore, had promised a select group of astronomers hundreds of free hours on the HST in return for spending the last 13 years helping NASA develop the telescope and instrument package. The group, known as “guaranteed time observers” (GTOs), was not only promised time but first crack at their choice of celestial targets. Now, with exposure times slated to be far longer than first supposed, how will NASA fulfill its obligations to the GTOs? “I don’t know,” says Edward Weiler, the NASA official in charge of the GTO program.

Suppose a GTO had asked to observe a certain object that is now impossible until the telescope is repaired. These are almost certain to be choice targets on the list of many astronomers. Can NASA forbid anyone else under its funding to look at it? Or suppose some astronomer comes along with a scheme to clean up the HST’s image of the object so it is useful. Does NASA refuse to let them try it? No matter how NASA tries to solve the GTO dilemma, the result is sure to be bitter scientific infighting.

Now the big question; Could all of this have been avoided? The defect is such a “textbook example,” according to deputy project manager Jean Oliver, that it must have been ground into the mirror during manufacture. The mirror has been ready for more than a decade so why was the flaw not picked up before the mirror went into space? Both Oliver and Bob O’Dell of Rice University said that the telescope design had been checked by so many people the fault had to lie with the mirror-testing procedure. The problem is that there is little agreement as to how extensive these tests were.

One immediate effect of the Hubble trouble is that NASA is rapidly losing support with legislators. The journal Science reports that senators went off on their July 4 break muttering about “techno-turkeys” and vowing to take a hard look at the NASA appropriations. NASA is trying to get Congress to approve a $15 billion budget for next year that includes the first installment on a grandiose plan for manned explorations of the moon and Mars as well as another payment on the manned space station. The latter has already had billions sunk into it and the plans for construction have never even been finalized. NASA’s case is not being helped by the ongoing problems with the shuttle fleet that could keep them grounded indefinitely.

NASA’s immediate challenge for the 1990s, says the Science article, is the same one it faced two decades ago in the 1970s: to get the shuttle operational. Until this happens the HST will continue to limp along as a half-blind, $1.5 billion boondoggle that is not as useful to astronomy as ground-based telescopes that were built at a fraction of the cost.

Clair Wood, a science instructor at Eastern Maine Technical College, is the NEWS science columnist.