Hubble Sees Comet Linear Blow its Top

July 28, 2000

Lackluster comet LINEAR (C/1999 S4) unexpectedly threw astronomers a curve. Using NASA's Hubble Space Telescope, researchers were surprised to catch the icy comet in a brief, violent outburst when it blew off a piece of its crust, like a cork popping off a champagne bottle.

The eruption, the comet's equivalent of a volcanic explosion (though temperatures are far below freezing, at about minus 100 degrees Fahrenheit in the icy regions of the nucleus or core), spewed a great deal of dust into space. This mist of dust reflected sunlight, dramatically increasing the comet's brightness over several hours. Hubble's sharp vision recorded the entire event and even snapped a picture of the chunk of material jettisoned from the nucleus and floating away along the comet's tail.

"We lucked out completely," says Hubble comet-watcher Harold Weaver of the Johns Hopkins University, Baltimore, Md. "In one surge of brilliance this under-performing comet showed us what it could have been. Comet LINEAR generally has not been as bright as we had hoped, but occasionally does something exciting."

For Weaver and his team, it was clearly a case of being at the right place at the right time. Apparently, ground-based telescopes did not clearly pick up the brief eruption — it required Hubble's pinpoint view. Though comet nuclei have been known to fragment, Hubble's sharp vision is revealing finer details of how they break apart. This unexpected glimpse at a transitory event may indicate that these types of "Mt. Saint Helens" outbursts occur frequently on the comet, because it would be unlikely that Hubble just happened to catch one isolated event, Weaver says. The astronomer noted similar "daughter" fragments when he used the Hubble telescope to observe comet Hyakutake (C/1996 B2) in 1996.

This outburst provides a rare insight into the structure and composition of volatile comet nuclei, commonly believed to be primeval agglomerations of dust and ice. When heated by sunlight, a comet's core spews gas and dust, creating its trademark gossamer tail. Even with its superb resolution, the Hubble telescope can't directly see comet LINEAR's core because it is probably only about a mile across and is veiled by dust particles comprising the shroud-like coma.

The orbiting observatory's Space Telescope Imaging Spectrograph tracked the streaking comet for two days, July 5 to 7, capturing the leap in brightness and discovering the castaway chunk of material sailing along its tail.

When the Hubble telescope first spied the comet 74 million miles (120 million km) from Earth, it watched the icy object's brightness rise by about 50 percent in less than four hours. By the next day, the comet was a third less luminous than it had been the previous day. On the final day, the comet was back to normal — one-seventh less bright than at peak level.

During the outburst's peak, the astronomers believe that the comet jettisoned the piece of its crust seen days later in the tail. The renegade fragment moved away from the core's weak gravitational grasp at an average speed of about six miles per hour, which is a little more than a brisk walking pace.

Weaver and his team 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.

"Observations of comet Hyakutake by the Hubble telescope and other observatories also showed fragments traveling down its tail, and some French researchers showed that those fragments might be these house-sized cometesimals," says team member Paul Feldman of Johns Hopkins University.

The cometary fireworks were an unexpected bonus to the astronomer's Hubble observations. Weaver and his team were observing the comet to measure its composition. They found a deficiency of carbon monoxide compared with other comets, which suggests that the comet originally formed much closer to the Sun at temperatures that would have depleted the carbon monoxide. The comet was then tossed out to the Oort cloud, a vast and distant "deep-freeze" reservoir of primordial comet nuclei.

Comet LINEAR was named for the observatory that originally discovered it in September 1999. LINEAR is the acronym for Lincoln Near Earth Asteroid Research, a project operated by the Massachusetts Institute of Technology's Lincoln Laboratory to search for Earth-approaching objects.

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.

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.