What happened to Comet Holmes? Just-released observations by NASA's Spitzer Space Telescope define the mass and velocity of the explosion: "The energy of the blast was about 1014 joules and the total mass was of order 1010 kg," says Bill Reach of Caltech. In other words, Holmes exploded like 24 kilotons of TNT and ejected 10 million metric tons of dust and gas into space. These numbers fit a model favored by Reach in which a cavern of ice some hundred meters beneath the comet's crust changed phase, from amorphous to crystalline, releasing in transition enough heat to cause Holmes to blow its top.