Edge says they aren't wed to any single formula.
"Rick is methodical, and I'm excited about working in that style as well," he says, noting that the songs he and Bono have been writing for next year's Spider-Man: Turn Off the Dark Broadway musical require a more disciplined approach.
"There's no strict route to a U2 song," Edge says. "The only thing that's consistent is the search for inspiration. It can start from a drumbeat, a guitar part, a title, a lyric. An entire piece of music can suddenly arrive. We subscribe to the idea that there's no such thing as failure. There's just giving up. We do not give up. We are relentless."
U2's tenacity and artistic daring pay off in Horizon's towering splendor, says Blender editor Joe Levy.
"It combines two moments: the epic grandeur of The Joshua Tree and the experimental audio research of Achtung Baby and Zooropa," he says. "They're at a point where they can be the biggest band in the world and still be edgy, with a capital 'E' in this case. They haven't come out swinging this hard and reaching this high since Joshua. On the surface, it's classic U2. Put on the headphones, and you hear an album every bit as sonically ambitious as Achtung Baby."
Horizon's immediacy, nimble complexities and clear messages cement U2's standing as the only veteran rock band with consistent artistic relevance and commercial clout, he says.
"They don't do it by utilizing the same set of tricks or by having Justin Timberlake and Timbaland on speed-dial," Levy says. "None of their '80s contemporaries — Madonna, Bruce Springsteen, Prince or Michael Jackson — managed to continuously keep the focus on new music."
On the road, the band is eclipsed only by the Rolling Stones, whose Bigger Bang is history's top-grossing tour with $558 million, according to Billboard Boxscore. U2's Vertigo tour ranks second with $389 million, and the band will get another shot at the record book when it hits stadiums this summer, its first outdoor U.S. trek in 12 years.
"When U2 tours, it's a major global live entertainment event," says Ray Waddell, Billboard's touring editor. "Only a handful of bands have achieved that sort of international touring superstar status. Though you hate to say anyone is recession-proof, U2 is about as close to that as you can get. It's can't-miss entertainment.
"That said, any band would be foolish not to take into account economic conditions when mounting a major tour, and the U2 team is anything but foolish."
Mediocrity 'would kill us'
Sinking CD sales and the crumbling economy didn't dissuade U2 from gambling on stadiums (the band nearly went broke staging the extravagant 1997-98 PopMart tour) and plowing fresh turf on Horizon.
"The point was to get out of the comfort zone into uncharted territory," Edge says. "We love it when we don't know what we're doing. We're more alive. It has to be about discovery or we lose interest.
"Even so, no matter how far out we go, it always ends up sounding like U2."
Fan loyalty, critical acceptance and the industry's abiding support should fuel U2's nerve, but the band says its self-confidence is the first casualty during months of studio skydiving and spelunking.
"You don't get this much attitude if you're not insecure," Bono says with a laugh. "Insecurity is our best security, and the moment we lose that insecurity, we're in deep trouble. It's important to be out of our depth."