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ALIEN SITCOMS -- LOVE 'EM AND LEAVE 'EM

Americans love aliens. The opening-weekend box office receipts for Men in Black prove that. But our feelings about alien sitcoms are a bit more complex. NBC successfully launched 3rd Rock From the Sun last year with an ad campaign pointing out there had been a hit alien sitcom in each of the past three decades -- My Favorite Martian in the '60s, Mork & Mindy in the '70s, and ALF in the '80s -- and positioning 3rd Rock as the alien sitcom for the '90s. What the ad campaign didn't point out is that all those shows had short shelf lives. After fast starts, none lasted longer than four seasons. Now 3rd Rock seems to be following the pattern.

It all started with My Favorite Martian, the bizarre story of a mild-mannered L.A. newspaperman (Bill Bixby) who befriends an antennaed spaceman (Ray Walston). The Sunday-night CBS sitcom was a breakout hit of 1963 -- in its first season, Martian finished No. 10, building on its Lassie lead-in and delivering a large audience to The Ed Sullivan Show, which followed.

Yet only a year later, Martian dropped to No. 24, then out of the top 25 altogether. The novelty had quickly worn off. When CBS announced its fall schedule for 1966, Martian was gone, replaced by It's About Time, a sitcom about astronauts who land in the Stone Age (it became extinct after one season).

The alien sitcom returned bigger than ever with Mork & Mindy's 1978 debut. Robin Williams skyrocketed to stardom, famously making the cover of TIME in M&M's first season, when it finished No. 3, tied with Happy Days and behind only Laverne & Shirley and Three's Company.

Then, in a classic case of trying to fix something that ain't broke, ABC tinkered with Mork before its second season, changing its time slot and jettisoning Mindy's straitlaced dad (Conrad Janis) and randy grandma (Elizabeth Kerr) in favor of a wisecracking deli owner (kiss of death Jay Thomas) and his spunky sister (Gina Hecht). Ratings immediately plummeted, and subsequent personnel changes -- Janis and Kerr returned, and Jonathan Winters joined the cast as Mork's ''baby'' -- didn't help. M&M bid its final ''Nanu nanu'' in 1982.

ALF -- short for ''alien life form'' -- landed on the planet four years later. Its twist: ALF was a fuzzy puppet with the voice of series cocreator Paul Fusco. ALF finished No. 10 for the 1987-88 season and hovered at No. 15 a year later. Then it fell out of the top 25 and was promptly axed by NBC. The moral of the story: It's hard to get adults to watch a puppet show for long (a lesson later learned by the makers of Fox After Breakfast).

3rd Rock's sudden smash-hit status took everyone by surprise last year, yet after studying this phenomenon, it seems predestined to burn out quickly. While its ratings dipped only slightly in its second season, it steadily lost momentum and now regularly gets its alien butt kicked by CBS' Touched by an Angel. This fall, NBC will move 3rd Rock to Wednesdays at 9 p.m., where it'll have to face ABC's The Drew Carey Show, a late bloomer with all the buzz that 3rd Rock used to have.