Hartley 2 isn't the only such comet, but it's the first to be visited by a spacecraft during a flyby, researchers report in Friday's edition of the journal Science.
University of Maryland astronomer Michael A'Hearn says when warmed by the sun, dry ice deep in the comet's body turns to carbon dioxide gas jetting off the comet. The gas drags water ice along with it.
The researchers said the smooth waist of the peanut-shaped comet seems to have been re-deposited from other parts of Hartley 2. The comet both spins around one axis and tumbles around a different axis.
The photos were taken last fall by NASA's Deep Impact spacecraft.
