Worried engineers pored Friday over data from NASA's $1.5 billion Galileo Jupiter probe that indicates its critical 16-foot main antenna may have failed to unfold properly as planned Thursday.

Failure of the umbrellalike 16-foot "high-gain" antenna to open fully would be a major setback to the Galileo project, severely limiting the speed at which data about Jupiter could be transmitted to Earth and sharply reducing the number of photographs that could be beamed back to the inner solar system.But project manager William O'Neil at NASA's Jet Propulsion Laboratory in Pasadena, Calif., said by telephone Friday engineers are optimistic about coaxing the antenna open if, in fact, it did not deploy fully on Thursday as planned.

"The data that we have at this point does suggest there has been some degree of deployment, or unfurling," O'Neil said. "We can't even rule out the thing is entirely deployed and just didn't get to the full stop."

But, he said: "That's not a high likelihood. The more likely thing is we have some partial deployment condition. What we're doing is analyzing our data to understand that better."

The Galileo probe was launched from the shuttle Atlantis on Oct. 18, 1989. It is scheduled to slip into orbit around Jupiter in 1995.