The space telescope, described as the "most important scientific instrument ever flown," will be launched a year later than planned and will cost $500 million more than expected, NASA says.

"This whole job was bigger and tougher than anybody thought it would be," Samuel W. Keller, NASA deputy associate administrator told The Washington Post in an interview.

"I think that developing and building the space telescope may have been the hardest thing this agency has ever tried to do."

Because the space telescope will orbit above the Earth's atmosphere and will have a primary mirror eight feet across, the telescope will be able to see objects 50 times fainter and seven times farther from Earth--and with three times greater clarity--than any glimpsed by telescopes on Earth. It is being built to last 15 years, has scientific instruments that can be replaced or repaired in orbit by trained astronauts and is even being designed to be brought home to Earth if major repairs are needed.

However, Keller said the space shuttle will not carry the 11-ton telescope into its orbit 320 miles above the earth until at least April, 1986--a year late--and possibly not until early fall of 1986.

Even if it flies in April, 1986, the space telescope will miss the voyage around the sun of Halley's Comet, which will begin approaching Earth in November, 1985, for the first time in 76 years and come closest in February, 1986.

"We would have liked to make an American commitment to Halley, which is one of the few long-period comets reappearing every 76 years we see here on Earth," Keller said with more than a little disappointment.

The telescope's delayed launch also means the space agency will not be able to observe the distant planet Uranus in time to identify targets of opportunity for the Voyager spacecraft's flight past Uranus in January, 1986.

The launch delay will bring the telescope's "runout" costs to about $1.1 billion, Keller said. The project's cost was estimated at $572 million when it began in 1977 and revised to $797.2 million late last year. Keller said NASA was forced to add $182.5 million in fiscal 1983 and $195.6 million in fiscal 1984 to keep the project under development.

Keller said the launch delay stems largely from the difficulty in designing three "fine guidance sensors" to lock the telescope onto distant stars and to sustain its lock as the telescope orbits the Earth in one direction while the star moves in another direction against a background of other moving stars.

So difficult was the design that the first three sensors delivered by Perkin-Elmer Corp. in Danbury, Conn., to Lockheed Corp. in California will not be built into the telescope but simply will serve as engineering test models so engineers can rebuild the devices that will fly in 1986.

Keller said the space agency will clean the telescope's primary mirror, a lightweight (1,800 pounds) mirror of a honeycomb construction unlike any other telescope mirror ever built. (A mirror of comparable size and power on Earth would weigh 8,000 pounds.) For the past 17 months, the telescope's mirror has been gathering dust in a "clean room" at Perkin-Elmer. It also is covered by lint left by workmen who used a jeweler's "rouge" to polish the mirror.

"The longer it sits in the clean room, the more dust it will gather but we'll clean it right before delivery by turning it upside down and blowing nitrogen on it," Keller said. "Believe me, the dust is only a nuisance, it's no show-stopper. All the dust does is cause light spots where starlight glitters onto the mirror."