How Hartley 2 Creates "Snowstorm"

Comet's Snowstorm

Sharpened Blizzard

Ice Particles, Near and Far

Yellow circles mark the relatively large ice chunks surrounding comet Hartley 2 in a picture taken earlier this month by NASA's EPOXI spacecraft.

Comet Collision Risk

Yellow circles highlight two streaks—larger particles inside the halo of comet Hartley 2.

The particles are streaking because they are near the EPOXI spacecraft, which hurtled through the comet's halo at about 27,000 miles (43,000 kilometers) an hour

Comet's Dry-Ice Jets

An illustration shows the likely way frozen carbon dioxide, aka dry ice, inside comet Hartley 2 drives the highly active jets at the surface of the nucleus.

When the comet nears the sun, heat causes the dry ice to sublimate, or turn directly from a solid to a gas. The gas then shoots out of the surface, carrying particles of dirt and water ice.