Hubble Sees Comet Linear Blow its Top

1. What do the pictures show?

The orbiting observatory's Space Telescope Imaging Spectrograph tracked the streaking comet for two days, July 5 to 7, capturing a dramatic leap in its brightness [left image]; followed by seeing a wave of newly created dust from the outburst flowing into the coma, a shell of dust surrounding the core [middle image]; and culminating in the discovery of a castoff chunk of material from the nucleus sailing along its tail [the bright dot trailing behind the comet in the picture at right]. The white region represents the brightest part of the coma. The nucleus, which is only about a mile wide, cannot be seen in these images because it's too small for the Hubble telescope to see.

2. What caused the eruption?

Astronomers list several theories for the eruption. One possible reason is that a particularly volatile region of the core became exposed to sunlight for the first time and vaporized away very suddenly. Another possibility is that a buildup of gas pressure from sublimating ice (a change from ice to gas) trapped just below the comet's surface explosively "blew the lid off" a pancake-shaped layer of crust from its surface. The pressure from sunlight blew the fragment down the tail -- much like the wind propels a sailboat -- where it disintegrated into smaller and smaller pieces, eventually becoming too small to see.

Yet another possibility is that the observed fragment is one of the house-sized "cometesimals" that are thought to make up the nucleus. Evidence accumulated during the past decade suggests that comet nuclei are "rubble piles" of loosely held together cometesimals. Perhaps one of the "building blocks" comprising the core broke off and was blown down the tail by a gaseous jet shooting off the comet's surface like a garden hose spray.