Mimi to the rescue!

Diva supreme Mariah Carey assumed the daunting task this week of trying to right TV's biggest cruise ship, the SS American Idol, which has been taking on so much water of late that its lifeboats are swinging loose in their cradles.

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Just what did Fox get for $18 million? That's a matter of speculation.

"Mariah is famous primarily for her work in recording studios. Nobody really knows what to expect from her on live TV," says Rolling Stone columnist Rob Sheffield via e-mail. "She doesn't have a lot of experience talking off the top of her head in public, to say the least."

In a statement released the day her hiring was announced, Carey made the job sound like a mission of mercy. "As a singer, songwriter and producer, it's going to be fun and rewarding to help find new talent and give back to American Idol," she said.

But what's in it for the Grammy-laden singer with the supersonic range (besides a multimillion-dollar salary for five months of part-time work)?

To answer that, you have to go a little deeper into the statement:

"I am currently in the studio working on my new album and its first single, 'Triumphant,' which will be out early next month."

There you have a better clue to Carey's motivation. As Idol heads into its 12th season, the show has flipped its original mandate.

It has become less about finding unknown singing talent and more about providing a promotional platform for its famous, if faded, celebrity judges.

You'll hear a lot in coming months about Mariah's being one of the most successful recording artists of all time, having sold more than 200 million albums worldwide.

But bear in mind, she peaked in the 1990s. While still commercially viable, at 42, she's no longer the platinum princess she once was.

Carey is presumably hoping that Idol will provide her, as it did her immediate predecessors Steven Tyler and Jennifer Lopez, the kind of buzz you can't buy or manufacture.

In other words, at this point you take the Idol gig with the expectation that it will give you a career boost that will quickly make you too busy to be an Idol judge.

The networks have adopted a competitive policy of shelling out megabucks for celebrity judges on their reality talent shows.

Christina Aguilera will pull down $10 million for the third season of The Voice; Britney Spears is reportedly making $16 million for joining The X Factor panel.

The idea is that big names equal big ratings. But does this equation add up?

Well, it used to. The first year Tyler and J.Lo joined Idol in Season 10, they reversed a steady decline in viewership.

That slight but encouraging uptick was swamped by last season's disastrous decline, during the pair's second and, as it would turn out, final year as judges.

Season 11 saw the biggest drop in the show's history, down 23 percent from the previous season. The finale, in which Phillip Phillips prevailed over Jessica Sanchez, was the least-watched ever.

It has become evident, perhaps because the practice has become so widespread, that signing celebrated judges gives only short-term stimulus to ratings.

The first few episodes of America's Got Talent saw big tune-ins this summer with the arrival of Howard Stern (who is being paid a reported $20 million). But according to Nielsen data, overall viewership is actually down nearly 14 percent from last year's average when Piers Morgan occupied Stern's chair.

Still, if Idol was looking for a ratings jolt, Carey was a pretty savvy choice.

"I think Mariah is totally worth the gamble," says TV Guide columnist Damian Holbrook via e-mail. "Like J.Lo, she's a globally recognized commodity, has talent even on her worst days, and brings the star power needed to compete with the killer panel at The Voice. Plus, anyone who saw her Cribs on MTV knows she's just weird enough to deliver talk-worthy TV."

"She's a good fit," agrees Shirley Halperin, music editor for the Hollywood Reporter. "She brings a couple of things to the table, most importantly incredible vocal talent. She's an amazing singer and she can teach these kids.

"Then there's the personality part," Halperin says. "At first, people were saying she'll be the new Paula Abdul. But just from knowing her over the years, I think she's going to be sharp and witty. And talking to people in the [music] industry, they all say she's smarter than people give her credit for."

The key question for fans of the show is a variation on the one Glinda put to Dorothy in The Wizard of Oz: "What the Munchkins want to know is, are you a good witch or a bad witch?"

In other words, will Carey be the kind of judge who fawns over every performance, or will there be some bite to her comments?

"J.Lo and Steven were softies," says TV blogger Hilary Rothing. "People felt they were too easy on the contestants. Let's face it: The ghost of Simon Cowell is haunting the Idol judges' panel. We don't have a voice like that anymore. Is Mariah going to be an outspoken diva or the nurturing type?

"There's kind of a conflict of interest for star judges," she continues. "They're performers; they want to be likable. They can't go on and smash some kid's dream. If Mariah sees Idol as a stepping-stone for her career or as a way to sell records, it's not going to be that interesting."

Whether she's good or bad, she probably won't be there long.

"Clearly they're moving in the direction of disposable judges, and stepping back from the idea of keeping any kind of stable judges' panel from season to season," says Sheffield. "It seems likely that judging on Idol from now on will be a one-year guest gig for celebrities. If Idol can get a bigger news bump from replacing the judges every year, what's the upside of keeping them around?"

Even Fox's chief of entertainment, Kevin Reilly, acknowledges that Idol will mutate faster from here on out.

"I think change is going to be part of the show going forward," he told a gathering of television critics in Beverly Hills last week. "We're 12 years old. I think we've got to keep it fresh. It's a lot of work and not without some risk, but I think the audience wants to see the discovery of something new on the bench."

What's left of the audience, anyway. While Idol remains the top entertainment series on TV, it has shed nearly 45 percent of its viewership since its peak in Season 6, when it averaged 31 million viewers.

So Idol is motivated to evolve and experiment.

"Why not change?" Halperin asks. "For a long time, it leaned on 'If it ain't broke, don't fix it,' and I think they've lost a generation of viewers because of it. The young people have gone."

That's the other alarming aspect of Idol's audience - not only is it shrinking, but it's also rapidly aging, from an average of 32 years old initially to 49 today.

Carey is going to need more than her charm bracelet to turn all that around.

"Shows almost never improve their demographics as they age. That's just part of TV's natural life cycle," says Richard Rushfield, author of American Idol: The Untold Story. "In Idol's case, that is exacerbated by the fact that it feels like a very stodgy, older person's show. It's a loungey, pageanty spectacle and the music isn't contemporary.

"What they really needed was a game changer for their demographic problem," he says, "a wholesale revamp if they want to sell it to younger viewers as relevant. I don't see how changing judges will do that."

Others disagree, viewing the dynamic at the table as the most (excuse the pun) critical element on the show.

"The judges are essential," says Holbrook. "We forget the winners but we remember what the judges say. Insipid and empty as their comments may be, we wait and weigh in on their every insight. When they agree, when they fight, it doesn't matter . . . it's often the action at the judges' panel that's more entertaining than the performances."

Bring it, Mimi!

Contact David Hiltbrand at 215-854-4552 or dhiltbrand@phillynews.com, or follow on Twitter @daveondemand_tv. Read his blog, "Dave on Demand," at www.philly.com/dod.