Like an alien planet with an environment too harsh to support human life, the major television networks have proved hostile to most science-fiction series.

Which is why two new shows are taking their stories of alien encounters and life beyond Earth to the expanding boundaries of the universe known as cable.

"The 4400" debuts tonight on USA Network; "Stargate Atlantis" premieres Friday on Sci Fi Channel.

"Cable certainly has more open arms in promoting something like this," says Scott Peters, executive producer of "The 4400," a so-called "special event series," which will run Sundays on USA through its Aug. 8 finale.

Brad Wright, executive producer of "Stargate Atlantis" and its parent show, "Stargate SG-1" (which kicked off its eighth season on Friday), says: "I would consider myself lucky if ’Atlantis’ is half as successful as ’SG-1.’ We think the brand itself is strong enough to get viewers the first few times."

In more than 50 years of network television, science fiction has always been something of a stepchild. The original "Star Trek" was almost canceled after its first season - and lasted only three. "Star Trek: The Next Generation" couldn’t find a network to finance it - then went on to become one of the most successful syndicated series of all time.

There hasn’t been a hit sci-fi series on the four major networks since "The X Files" went off the air in 2002. Shows such as "Dark Angel" and "Firefly" have come and gone, usually lasting no more than a season. "Star Trek: Enterprise" is entering its fourth season, but on the less-watched UPN. Which leaves cable as the destination of choice for most science-fiction programming.

"Some science fiction gets pretty narrow and isn’t accessible to a lot of people," says Steve McPherson, president of ABC-TV. "As a network, obviously, we’re looking for a broad audience."

"In terms of TV, it’s simply a matter of ratings," says Rick Berman, executive producer of "Enterprise" and other "Star Trek" series. "Science fiction, at least the kind that can be presented within the budget limits of television, has a somewhat niche audience. The people who watch Sci Fi Channel or sci-fi shows on Spike or USA Network get an hourly rating that’s acceptable to those networks but not to ABC or NBC or Fox. You’ve got a small but loyal group that’s going to go find those shows and watch them."

But Bonnie Hammer, president of Sci Fi and USA, rejects that "niche" classification. "SG-1," she points out, draws an average of 2.4 million viewers per episode - and has an audience that extends beyond the stereotypical technophile.

"When you say science fiction, what people think of is space operas and aliens," she says. "But it’s not just a geeky, culty thing. We try to stay away from the term ’niche’; we choose to call ourselves a branded channel - and the brand is the broad category of science fiction and speculative fiction. We’re building an audience by fueling their imagination. More women watch science fiction than anyone imagines."

Part of the process of attracting a wider audience involves telling stories that are accessible.

"The mass audience is looking for things they can identify with," says Robert Cooper, executive producer of "Atlantis" and "SG-1." "A lot of science fiction is not particularly identifiable to the average person."

"There’s a perception that science fiction is an egghead thing or, on the other hand, a comic-book thing," says Peters of "4400." What I wanted to come up with is a grounded story, dealing with people. These are real-world situations: divorce, home life."

"The 4400" uses alien abduction of Earthlings as its central plot device. The title refers to the number of humans abducted over the years - all of whom suddenly reappear as a group at a lake outside Seattle. They have been gone as long as 50 years or as little as three - and none has aged a minute from the moment of disappearance.

Quarantined by the government for several weeks, they are released after a civil-rights lawsuit and attempt to return to the lives they left behind. For some, however, that’s impossible, because their families and friends are either long dead or have moved on with their lives.

"It starts as a Rip Van Winkle story: You close your eyes and wake up 35 years later," Peters says. "What do you do? How do you reconnect with your life?"

Ira Behr, a "Star Trek" veteran who wrote for "The 4400," says: "The mandate on ’4400’ was: Life, interrupted. That’s what was said by the network and, as a writer, that really appealed to me. In most TV, plot is more important than characters. This show is 180 degrees opposite."

"The 4400" focuses on the fate of a half-dozen or so of the returnees, as well as the well-meaning government agents who track their re-integration into society. If the show captures an audience, Peters has plenty of ideas - about 4,400 of them - for turning it into a full-fledged series.

"The idea is to run six hours and see how the ratings are and then have a discussion," he says. "If audiences respond positively, we can get up and running quickly."

Peters admits that his initial idea was for a series in which he wouldn’t reveal what had happened to the 4400 until the 100th episode. But he found he had to wrap it up more quickly.

"I saw it as a five-year story arc, and now our ending will be in Episode 6," he says. "We had to wrap our minds around giving away the big finale - but if we don’t go further than this, we’ve already told a full story. If the series continues, there are a couple of avenues being left wide open."

Rene Echevarria, a writer for the show, adds: "It’s a tricky balance, to have a satisfying conclusion but to leave open the continuing mystery and have places to go. We’re answering some riddles, but posing new ones."

The creators and producers of "Stargate Atlantis" faced a different challenge. The two-hour script for the spinoff was shelved two years in a row because "SG-1" was so popular.

Beginning on Showtime in 1997, "SG-1" (itself a spinoff of the 1994 feature film "Stargate") deals with a team of Earth scientists and military men who explore the known - and unknown - galaxies through a stargate, a portal that sends Earthlings to other planets through wormholes in space, where they encounter both friendly and threatening cultures. As the program was entering its fifth season in 2001, it appeared to have run its course and Cooper and Wright wrote a spinoff pilot. Then the show was dealt by Showtime to Sci Fi Channel.

"The ratings took off," Cooper says. "We became a victim of our own success. Season 6 got double the ratings we’d had on Showtime. So Sci Fi ordered Season 7. That put a damper on the movie; we wound up incorporating some of its plot into the arc of the series. Season 7 did so well that they ordered Season 8. When we pitched them the spinoff, their response was, we want both. So we had to rethink everything."

"Stargate SG-1" started its eighth season on Friday and surrenders its time slot this Friday for the debut of "Stargate Atlantis." The two-hour pilot finds the cast of "SG-1" sending a new team of scientists and military men through the stargate to the lost city of Atlantis, which had been transported to a distant planet millennia earlier. Once on the new planet, however, the team finds it lacks the power to activate the stargate to return to Earth and so must deal with life in the deserted Atlantis and on the surrounding planet.

"The studio and the network want ’Atlantis’ to be new - but they want it to have a sense of familiarity," Wright says. "In other words, they want it absolutely the same and completely different."

Cooper adds: "We’re trying to wipe the slate clean and create the unexplored new frontier we had at the beginning of ’SG-1.’ We’ll create a world that will feel fresh and new."

The cost of a program with massive sets and special effects hasn’t been as problematic for the "Stargate" series because Showtime originally ordered 44 episodes - and had committed to all five years by the end of the second season. But cost is often a deterrent when it comes to getting science-fiction series on network television.

"Science fiction is a challenge, from a production standpoint," says ABC’s McPherson. "You spend $120 million or more for a feature film - look at ’The Matrix.’ It was fantastic - but you couldn’t do ’The Matrix’ every week on TV. It’s a question of expectation and execution."

Berman, who has produced "Star Trek" for TV and film, adds: "When you make a movie, you’re telling a single story. When you’re doing a TV series, you have to create a format on which you can hang 100 or 200 episodes. It’s a whole different ballgame."

"People seem to be able to buy into an alternate world in a two-hour movie," says Echevarria, another "Star Trek" veteran. "But to buy into it week in and week out is another story. You’re either a fan of the genre or you’re not."

Or you are without knowing it. "There are a host of science-fiction writers who work on ’CSI,’ " Cooper says. "In many ways, that’s a sci-fi show."

The other alternative is to wear your science-fiction label proudly - but to use it as a hook rather than as a reason for being. That’s what a show like "The 4400" is trying to do.

"I know people will be watching this whose instinct is not to watch anything with people in funny costumes or transporters," Behr says. "A large segment of the population turns off to that immediately. But this show has the feeling of reality - and people are fascinated with the abduction thing."

"The audience is out there for it," Peters says. "Even if you’re a fan of straight character stories, you’ll tune in and see that they’re not running around with laser guns and space ships."