A moving target may be harder to hit, but astronaut-to-be Jake Garn is certainly taking his share of flak.

First, cartoonist Garry Trudeau blasted the senator's upcoming space shuttle trip in a wickedly funny series of "Doonesbury" strips.

Now, consumer advocate Ralph Nader has launched his own salvo.

In a two-page, single-spaced letter sent to Garn's Capitol Hill office Tuesday, Nader pointed out that for the past two weeks, the senator has been at the Johnson Space Center in Houston "engaged full time in training for the job of Payload Specialist for NASA's shuttle mission," taking him away from his official Senate duties.

Ever the public watchdog, Nader -- citing a federal statute requiring the secretary of the Senate to dock members a day's pay for each day out of the office -- said the Republican senator from Utah owes the American public part of his salary for the time he has missed.

Nader also accused Garn of violating the "incompatibility clause" of the U.S. Constitution, which prohibits members of Congress from being appointed to any civil office.

"It is understandable how a man of your special ambitions would want to become the first member of Congress to go into 64 full orbits literally rather than figuratively," Nader wrote. "What is not understandable is why you have ignored both a pertinent federal statute and the United States Constitution in pursuit of your new office on the NASA space mission."

Nader ended the letter by saying, "Your calm and detailed response to these afore-mentioned statutory and constitutional issues would be appreciated . . . In the meantime, may you have a safe trip, enjoy the clean environment and return to become an environmental health advocate."

Nader said yesterday that the letter was not tongue-in-cheek.

"It's quite serious," he protested. Although other members of Congress have been guilty of breaking both statutes, Nader says, Garn's "violation of them can give a new visibility, shall we say."

Jeff Bingham, Garn's adminstrative assistant, said yesterday from Houston that neither he nor the senator had seen the letter, but that he found the charges "laughable, frankly. But coming from Ralph Nader, it doesn't surprise me. And you can quote the senator as saying that."

Bingham defended his boss' moonlighting by saying, "Jake's not employed. He's doing it as part of his official duties." He said there may be a law prohibiting members of Congress from accepting other government posts, "but it doesn't apply to Jake.

"Frankly, it's ludicrous," Bingham laughed. "Ralph Nader has never been a fan of Jake's. They've had run-ins before."

Maybe that's why Nader was so confident of receiving an answer from the space-going senator.

"He'll respond," Nader said yesterday. "He's got a nice, sharp tongue."