When the popular crime dramas "CSI" and "Law & Order" spawned spinoffs, nobody blinked. And it’s no surprise that the producers of "Friends" are hard at work on "Joey," a new sitcom starring one of the Central Perk gang.

But it’s worth noting when a veteran science fiction show such as "Stargate SG-1" gets cloned ("Stargate Atlantis" premieres at 8 p.m. Friday on the Sci Fi Channel). Because in today’s brutal TV climate, genre programs are far more likely to be canceled than copied.

But "Stargate SG-1," which began its eighth season on cable’s Sci Fi Channel last Friday, is quietly disproving the theory that genre shows can’t compete in the television marketplace. Unlike "Star Trek: Enterprise," which almost didn’t get renewed for a fourth season, or Joss Whedon’s acclaimed "Firefly," which only lasted a few months — to cite just two recent examples — "Stargate" is enjoying solid success.

Big cable numbers

"SG-1," Sci Fi’s highest-rated series, has been pulling in about 2.4 million viewers for each first-run episode, and the sci-fi adventure show is a bona fide top-10 hit in various countries around the world.

But maybe because the show hasn’t become a stateside media darling a la "The X-Files," the "Stargate" folks are modest about its success.

"We really are television’s best-kept secret," says Hank Cohen, president of television entertainment for MGM, the studio that released the 1994 film from which both programs are derived.

"One thing to keep in mind is that it is relative success," says Richard Dean Anderson, who’s a producer of "SG-1" and also stars as that show’s "Simpsons"-obsessed military leader, Jack O’Neill. "We are massively successful in the cable market, which is a smaller venue."

But "massive" would certainly describe the sales of "SG-1" box sets, which have been credited with helping shore up the bottom line of MGM. The strong sales of the DVD sets ("Stargate" occupies three of the top five slots in Amazon.com’s most recent Boxed Set Best Sellers chart), was a surprise to the producers of the show, as was the performance of "SG-1" when it moved to Sci Fi in 2002 after five years on Showtime.

"We had actually thought after season five, `Well, maybe this thing has run its course,’" says executive producer Robert C. Cooper, who said a move back to feature films was being contemplated at that point.

Instead, he and fellow executive producer Brad Wright got word last year that Sci Fi wanted the feature film idea — about a team of interstellar explorers finding the lost city of Atlantis — to become a spinoff series.

When it came to fleshing out the "Atlantis" idea, "the obvious pitfall is not to be seen as cannibalizing yourself and not being derivative," says Mark Stern, Sci Fi’s executive vice president of original programming. "How do you take what’s working [on `SG-1’] and still have a new approach?"

"Atlantis" wisely doesn’t mess much with "SG-1’s" success formula. As on "SG-1," there’s a "star gate" system that links hundreds of worlds, which are explored by a team headed by a gruff yet charismatic military man (on "SG-1" it’s the self-deprecating Anderson; on "Atlantis," hunky Joe Flanigan leads the way).

Can’t go home again

The difference on the new show, according to Wright, is that unlike the cast of "SG-1," the "Atlantis" team can’t go home again. They’re stranded on a new planet with a bunch of alien refugees, and the gate system on that world allows them to travel instantaneously to other planets — but not this one.

"This isn’t a few people going out on a mission and then coming back to Earth," says Wright. "This is akin to 19th Century expeditions to the other side of the world — it’s a large leap of faith. [The Atlantis team members] are very much on their own."

It might have seemed a leap of faith for Showtime and MGM to take a chance on a 3-year-old film franchise back in 1997, but creative thinking on the business end made the venture less risky. In addition to airing episodes on Showtime, MGM also sold the Vancouver-produced show in syndication, where it did extremely well.

"We were doing `repurposing’ and `cross-platforming’ before it was hip and cool," MGM’s Cohen says, referring to the now-common practice of having episodes of programs like "Monk" and "Charmed" appear on two different channels.

"It was just a way to offset higher production costs so we could do a better show," Cohen says. "Now it’s become the norm."

Because the business deal between Showtime and MGM worked "so much better" if they made a long-term investment in "SG-1," according to Cohen, before half a dozen episodes had aired, "SG-1’s" creative team found themselves with an almost unheard-of 44-episode commitment. (By comparison, last spring Fox’s acclaimed supernatural series "Wonderfalls" got on the air a grand total of four times before it was yanked).

"We had a great amount of freedom and certainly Brad [Wright] took that and ran with it, and created this amazing story arc without a lot of interference," says Amanda Tapping, who plays the brainy Samantha Carter on "SG-1."

The "Atlantis" cast and crew have the similar luxury of a whole season to develop the show’s stories, which will involve exploration of the city of Atlantis and ongoing battles with the villainous Wraith, who look like really angry backup singers for Marilyn Manson.

"We have the luxury of a 20-episode order on Sci Fi and it’s part of a very successful franchise," says Cooper. "It’s not like what happens on a network like NBC, where they give a show one or two or three episodes to succeed."

"We’re just hinting at where relationships could go at this point," says "Atlantis’" Torri Higginson, who’s halfway through shooting the show’s first season in Vancouver. "At this point, we’re still just trying to define everything."

And those definitions are still very much in flux, according to David Hewlett, who plays prickly scientist Rodney McKay on the new show.

`Horrible mistakes’

"We’re out there exploring and we don’t know what we’re supposed to be doing — we don’t have all the answers," says Hewlett. "We’re making horrible mistakes in some cases — we go onto planets and screw things up. In one case we went looking for food, and two or three episodes later [people from that planet] try to kill us."

Based on a viewing of an advance tape of "Atlantis," its characters are just as fallible as those on "SG-1," and the new show seems to share the older program’s quiet irreverence as well.

"That’s a waste of a perfectly good explanation," O’Neill says to an overly chatty "Atlantis" scientist in the show’s premiere.

"We don’t take ourselves that seriously, and I think that has helped us with mainstream audience," Wright notes. "Having said that, we’ve tackled some really big issues. And you never know, [a TV show or movie] with action and humor can actually move you sometimes."

For interviews with the casts of "Stargate SG-1" and "Stargate Atlantis," plus a poll and a message board, check out chicagotribune.com/stargate.