ASTRONOMY On Nov. 4, 2010, the Deep Impact spacecraft did a flyby of comet Hartley 2, coming within just 435 miles of it. The spacecraft, carrying out a University of Maryland-led EPOXI mission to study comets, encountered a bombardment of snowballs spewing from carbon dioxide jets on the rough ends of the 1.4-mile-long celestial body. This is the first time scientists have seen individual chunks of ice in the cloud surrounding a comet or jets powered by carbon dioxide.

The largest snowballs were the size of basketballs, and though the craft was hit by nine smaller balls, it incurred no damage. The snowballs have a very loose structure, weighing only about 0.2 milligram each — about the mass of an eyelash.

Studying images and videos taken by Deep Impact, EPOXI researchers determined that a layer of water ice is just beneath the surface of the peanutshaped comet. As carbon dioxide is shot out of both rough ends of the celestial body, it carries with it clumps of microscopic ice grains, essentially shooting giant snowflakes into space.

In the smooth middle section of the 4.5-billion-year-old comet, the ice evaporates into water vapor and seeps through the structure’s porous material. Hartley 2 loops around the sun every 6.5 years in its highly elliptical orbit, and on the journey, it passes within just 11 million miles of Earth. At its farthest, it’s beyond the orbit of Jupiter. Deep Impact met the comet on its way past Earth, where heat from the sun melts ice in the comet and converts it to gas, which creates fuzz around the nucleus and a streaming tail.