As NASA officials expressed new optimism that the space shuttle will fly again in about one year, the Justice Department announced yesterday that it has decided not to take over a $4 billion lawsuit charging that Morton Thiokol Inc. defrauded the government in its production of the rocket boosters blamed for the Challenger disaster.

The suit was filed Jan. 28 by former Thiokol engineer Roger Boisjoly under the False Claims Act, which allows private parties to sue on behalf of the government and gives the government the right to join the action.

A review and investigation of Boisjoly's charges showed "there is not a sufficient basis" for the government to enter the suit, the Justice Department concluded, but noted that it can intervene later if it can show good cause.

"Obviously, I would have been a lot happier to have the government take on the case," said Boisjoly's attorney, Robert N. Levin. He said he is surprised that rather than dismissing the suit outright, the Justice Department allowed him the right to continue it independently, which he said he intends to do.

He speculated that they took this option because "this meant they {Justice} didn't have to take a position on the merits of the case."

Morton Thiokol spokesman Thomas Russell, at the company's Chicago headquarters, said, "We're very pleased, obviously."

Boisjoly was one of the Thiokol engineers who tried to warn against launching Challenger in cold weather. The shuttle exploded 73 seconds after launch on Jan. 28, 1986, killing all seven crew members.

Government documents filed in Boisjoly's suit were unsealed accidently by a court clerk in April, revealing that the Federal Bureau of Investigation was conducting a criminal probe of fraud allegations brought in January by other Thiokol employes in Utah. That investigation is continuing, sources said.

The decision was announced as the National Aeronautics and Space Administration approached the one-year anniversary of the release of the Rogers commission report, which made nine major recommendations. It required a one-year progress report to the president, which NASA officials said would be ready later this month.

NASA Administrator James C. Fletcher said at a news conference yesterday that agency officials "have met all of our commitments to the nine recommendations" in the report. He and other top officials acknowledged that the agency faces an "uphill and rocky road to recovery" but said there is "a great deal of confidence we'll resume our shuttle flights next June."

One result of the Rogers commission findings was the development of an extensive new safety office. Fletcher announced yesterday that NASA has also established an independent contractor system, patterned after one in use by the Federal Aviation Administration, for NASA and contractor employes who want to raise their concerns anonymously without fear of reprisal.