Given their propensity for ill-chosen vehicles, it seems absurdly appropriate that Elliot Gould and James Brolin should bump into each other in the same one. The historic encounter occurs literally on the wing at the climax of " Capricorn One," a movie predicted on the demonstrably unworkable notion of a NASA plot to hoax the nation about a manned space flight to Mars.

Gould and Brolin serve as the perfunctory, now-you-see them, now-you-don't hero figures, who keep meandering in and out the moth-raten continuity that masquerades as a scenario.

Gould is supposed to be a TV reporter with a "Nose for News," and the big story he ever so slowly blunders upon concerns flight commander Brolin and fellow astronauts Sam Waterston and O.J. Simpson, who have been whisked from the launch site and persuaded to string along with a fake mission, supposedly to spare NASA the embarrassment of a cancellation.

Why this hoax, evidently mastermined by the director of the Manned Space Flight Program, played by Hal Holbrook, would serve NASA's immediate or long-range aims remains the prevailing mystery of the show.

"They're looking for a reason to cancel the program" whimpers Holbrook in the course of a long, sophistic speech meant to rationalize the hoax to the reluctant astronauts. The real message is "Gimme a break" It's writer-director Peter Hyams' plea to the public, which may or may not be complacent enough to oblige.

Backup mysteries complicate the central implausibility. Defying all bureaucratic logic, the hoax is sustained for almost a year before newshound Gould gets on the scent. He's not exactly tenacious. At least two months seem to pass between the time he gets a tip and bothers to follow through with the tipster, who disappears seconds after confiding his suspicions to Gould while they're shooting pools.

Unseen miscreants sabotage Gould's car for the sake of a stunt-driving sequence and then take pot shots at him on a later occasion. These episodes come out of thin air and fade into thin air. Each time Gould reappears, you think, "Long time, no see," and idly wonder how long it will be before another invisible assassin makes another freckless on his life.

Meanwhile, Brolin and his comrades sit around their hideaway, where a fake Mars landing sight has been constructed on a huge sound stage, and debate the pros and cons of making a break for it. Holbrook outdoes himself as a brilliant plotter by announcing that the astronauts perished upon reentry into the Earth's atmosphere. Evidently, this denouement to the mission would be just peachy for NASA, but don't ask how.

Miraculously deducing Holbrook's mad deception, the astronauts finally get off the dime but end up stranded in the desert. That's all "Capricorn One" needs. After nearly an hour-and-a-half of sedentary talking torsos, it suddenly becomes a lost-in-the-desert sage, mercifully terminated by 10 minutes of feverish stunt flying.

The second-unit work may be photogenic enough to pull Hyams' woeful melodramatic chestnuts out of the fire. The late, great stunt pilot Frank Tallman makes his last flights doubling for Telly Savalas in a monoplane.

How long has it been since action movies as expert and amusing as "Star Wars" and "Smokey and the Bandit?" Aren't they still in circulation? "Capricorn One" harks back to the old adventure serials, but Hyams doesn't have remotely enough wit or technique to achieve a fresh stylization of vintage formulas.

Like many a bad movie, "Capricorn One" often seems impelled to blab on speaking in lines like, "Something's very wrong here," or "He couldn't get over how something so fake could look so real." The latter seems particularly funny when you recall the glaring shadow thrown on a canvas backdrop by the fake landing vehicle on the fake Mars set. Evidently, this tell-tale slip-up is meant to go undetected by millions. At "Capricorn One" the wonder of it all is that something purporting to be the work of highly paid professionals could look so obstinately, amateruishly fake.