The Discovery's flight "was supposed to vault the shuttle fleet back into space after a prolonged grounding for repairs," said The New York Times in reference to the two and a half year hiatus in the US shuttle program following the February 2003 Columbia disaster.

"But given the repeat of the very problem that two years of retooling was supposed to resolve, the verdict is necessarily mixed," added the daily.

A large piece of foam insulation broke off the Discovery when it was launched on July 26. Although no damage was caused to the shuttle, NASA was distraught to see an almost identical flaw in the foam covering of the shuttle's external fuel tank that had caused the destruction of the Columbia and the death of its seven astronauts.

While the Discovery appears to have returned to Earth with fewer nicks and gouges than previous shuttle flights, showing that some improvements have been made over the past two years, the Times said, there is still room for concern.

"Successful as it was, this flight and the visible uncertainties of its managers left the unsettling impression that there is a lot NASA doesn't know about the performance of the spacecraft it has relied on for the past quarter-century," the editorial concluded.

More critical of the shuttle program was Homer Hickam, a retired NASA engineer who in a commentary published in The Wall Street Journal said the shuttle "is still not a reliable vehicle and never will be.

"You simply don't place a fragile bird at the base of a big, quaking nightmare of rocket engines and a massive, debris-shedding fuel tank and get anything but an engineering debacle," Hickam said before recommending the shuttle fleet to the junk heap.

"When your design stinks, Engineering 101 says admit your mistakes and go back to the drawing board," said Hickam, asserting that most of the engineers he knows at NASA "have wanted to retire the shuttle for a very long time and build a reliable spaceship worthy of our country."

The trouble, added the retired engineers, is that former and current astronauts -- "around 100, an awful lot..." -- "carry too much clout ... and they are mostly acolytes of the space shuttle."

The United States, said Hickam, "is presently fourth in the ability to put humans reliably into space, behind Russia, China, and Burt Rutan.

"So let's put the shuttles on the shelf right away and give engineers the gift of designing and building new ships to carry humans into space," he concluded.

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