This is only the fifth time a spacecraft has ever come close enough to a comet to take pictures. What you see is not the standard image of a long, filmy tail; Hartley 2 is more than 100 million miles from the sun, and too cold to be streaming water vapor the way comets do when they come closer.
In the 1950s astronomer Fred Whipple theorized that comets were like "dirty snowballs," boulders of ice and dust that wander the outer reaches of the solar system, some of them diverted to pass close to the sun -- and within sight of earth. But their precise makeup has been hard to determine, and that is why Deep Impact was launched.
In 2005 Deep Impact passed another comet, Tempel 1, and sent a dishwasher-sized probe crashing into it. Chemical readings of the resulting blast helped flesh out Whipple's theory.