Uncertainties surface over Hubble 'fix.' (space telescope)

Uncertainties surface over Hubble 'fix'

NASA wants a second opinion about treating its troubled eye in the sky.

Optical problems have impaired the Hubble Space Telescope's vision since NASA launcehd the instrument last April 12. A panel advising the space agency on how to compensate for the telescope's incorrectly shaped primary mirror and recurring "jitters" handed over its conclusions to NASA officials in a 120-page "strategy" report on Jan. 15. But now the space agency is assembling a second team of scientists and engineers to reevaluate the first group's unanimous recommendations.

Development of a package of corrective mirrors, which spacewalking astronauts would install in December 1993, figures prominently in the strategy panel's recommendations. This box of mirrors -- caleed COSTAR, for Corrective Optics Space Telescope Axial Replacement -- should correct the optical distortion afflicting three of the telescope's scientific instruments: its faint-object spectograph, hig-resolution spectograph and faint-object camera. However, uncertainties about COSTAR's potential cost and development time justify seeking a second opinion, says Charles J. Pellerin, head of astophysics at NASA's Office of Space Science and applications in Washington, D.C.

"We're moving as aggresively as we can to see if [COSTAR] should be done," he says, adding that there are at least two schools of though on the proposed project. While one side feels COSTAR would improve the trhee instruments' performance, the other side would rather reserve COSTAR's projected budget on the chance the money may be needed to guarantee the timely completion of two already planned "second-generation" instruments.

One of them, an imaging spectrometer slated for installation in 1996, would be five times as sensitive as any of the instruments that COSTAR would benefit, Pellerin says. (Sensitivity, however, is not always the only important trade-off. For instance, though Pellerin says the new spectrometer could resolve dimmer emissions than Hubble's existing faint-object camera, it will lack the camera's array of 48 interchangeable filters -- important for examining those objects at different wavelengths or degrees of polarization, notes F. Duccio Machetto, the camera's chief scientist at the Baltimore-based Space Telescope Science Institute.)

NASA's second-opinion panel should report back to the space agency in April. Pellerin notes that should this group back COSTAR, NASA should have time to complete the device in time to include it in a Hubble servicing already scheduled for 1993. During that space shuttle mission, astronautes will install another new Hubble instrument -- an improved wide-field and planetary camera.

Asked whether NASA really needs a second opinion on Hubble's recommended fixes--especially COSTAR -- Macchetto noted that the director of the Space Telescope Science Institute, the organization which coordinates Hubble's research, selected the first panel. "I truly support the idea of having a panel of people who ... are not directly connected with the telescope" reevaluate the need for COSTAR, he said.

The fist advisory panel also recommended fixing the telescope's jitters. This slight shaking, caused in part by temperature related expansion and contraction of Hubble's power-providing solar panels (SN: 11/10/90, p.295), also reduces the sharpness of the telescope's pictures and spectra. Engineers on the Hubble project have tried shaking the telescope (with its gyros) to better understand and possibly reduce the jitte-susceptibility of replacement solar panels. Results of this test, Macchetto says, are expected shortly, leaving time to modify the new panels so that shuttle astronauts can install them in 1993.