CAPE CANAVERAL, DEC. 3 -- Columbia's astronauts and ground teams struggled to fix problems with an observatory aboard the space shuttle today as precious viewing time faded away and targets were scratched.

"There's a definite loss as we go," mission scientist Ted Gull said. "Some objects are just going to slip off the list."

The trouble was with NASA's instrument pointing system, needed by three of the four telescopes that constitute the $150 million Astro observatory. Two of the system's three star-trackers were demonstrating an oversensitivity to light, mission manager Jack Jones said.

While trying to overcome that problem, NASA was gearing up another star-tracker on board, attached to a telescope, to spot targets. That tracker would be more awkward and time-consuming to use, however, since it is designed for another purpose.

"Whichever one of these {systems} we demonstrate can work first, our plan is to immediately go into that mode of operation and start gathering science," Jones said.

NASA started having problems with the pointing system shortly after Columbia blasted off early Sunday on a 10-day mission to examine some of the hottest objects in the universe. The observatory in the cargo bay originally had been expected to look at targets including galaxies, quasars, a supernova and a comet, starting late Sunday. Of the 230 planned pointings, up to 20 were eliminated as of midday today because of the delay.