This space has been sorely missing pop posts, so let's start Wednesday with an Apocalypse-Fun face-melter — Britney Spears' new video, "Till the World Ends." Spears' comeback hasn't been earth-shaking, but it has been sneakily effective, thanks to the surgeon-precise hands behind the boards (Dr. Luke and Max Martin did this song and the excellent, dubstep-informed "Hold It Against Me"). And "Till the World Ends" adds Ke$ha (as a songwriter) into the fold, a star whose sleazy persona chips away at good taste until you realize she's the reason the party feels so right. Ke$ha's touch is all over "World," from her icy vocal inflections to the clipped "oh-oh-oh" chorus to the dirty talk ("Get you off with the touch, dancing in the dark"). Britney is merely the marionette, a costume she'll happily wear to reclaim relevance. The Ray Kay-helmed video is typical 2011 party-starter — sweaty bodies, futuristic DJ-gear, well-timed faulty sprinkler system — but it captures the track's raging mood perfectly. If the world is really ending next year, we should keep a close eye on Britney. We're going to want to be wherever she's gyrating.