Shuttle may soon fly, but Hubble stays dim.

Shuttle may soon fly, but Hubble stays dim

The damage is done but the saga continues. Lew Allen, chairman of a panel assembled by NASA to investigate the Hubble Space Telescope's defective optics, told Congress last week that the mirror imperfection could have been detected on the ground "using relatively simple and inexpensive characterization techniques." Allen, who directs NASA's Jet Propulsion Laboratory in Pasadena, Calif., told a House space subcommittee that the telescope's spherical aberration is large enough for technicians to have spotted it, even without trying out the two mirrors in a $100 million test facility. NASA claimed earlier this month that detecting the flaw would have required such a facility.

This week, prompted by Hubble's troubles and the grounding of all three space shuttles due to fuel-system leaks in two of the craft, the Bush administration directed NASA to form an independent task force to review the space program's future. And NASA announced that ongoing repairs might enable it to launch two shuttle missions before the scheduled Oct. 5 liftoff of the shuttle-borne Ulysses solar probe, which must fly by Oct. 23 or wait 13 months for another launch window.

If workers can fix a faulty flange that connects the orbiter Atlantis to its external fuel tank without moving the shuttle off its launch pad, NASA says, a secret military mission, originally scheduled for June, could fly in mid-August. NASA also says that the shuttle Columbia, newly outfitted with fuel lines borrowed from a shuttle still under construction, is now scheduled to launch the long-delayed Astro-1 -- a 10-day, three-telescope mission -- in mid-September.

On the Hubble front, scientists have begun work to sharpen the telescope's brighter camera images through ground-based computer enhancement, in hopes of minimizing the focusing problem that appears rooted in the primary mirror (SN: 7/7/90, p.4). Robert A. Gonsalves of Tufts University in Medford, Mass., says preliminary work indicates image processing can double or triple the resolution of high-contrast, bright objects. But the technique cannot improve the quality of weaker images, adds NASA's Edward J. Weiler of Washington, D.C.

Roger Angel, an expert mirror designer at the University of Arizona in Tucson and a member of the Hubble investigative panel, told SCIENCE NEWS his group's visit next week to the firm that built the primary and secondary mirrors will likely focus on the accuracy and adequacy of tests used to assess the mirrors' optical performance. Angel notes that a fabrication or placement error in a lens used to evaluate the mirrors could have led opticians at the Hughes Danbury Optical System, Inc., in Danbury, Conn., to mistakenly conclude that an imperfect mirror met its design specifications perfectly.

The test, known as a null lens test, uses a small lens to optically alter a mirror so that it appears spherical to incoming light beams. The laws of optics dictate that light striking a perfectly spherical surface reflects back along the same path as an incoming beam. Any mismatch between the incoming and outgoing beams indicates that the mirror deviates from the ideal and may require further polishing to meet specifications. But if the null lens itself was fashioned improperly or placed at the wrong distance from the mirror, the test could have masked significant imperfections in the primary mirror, Angel says. He notes that errors in the null lens test have led to mirror problems before, most recently in a mirror designed for the European Southern Observatory's New Technology Telescope in Chile. But a former member of the team that made the two Hubble mirrors told SCIENCE NEWS the null lens test is only "one of many, many possibilities" for errors that could have caused Hubble's optical woes.