Enveloped in a billowing cloud of gas and dust known as a coma, Comet Borrelly is steaming past Earth and the Sun, harboring vital clues to the early solar system and possibly even the origin of lifeEnveloped in a billowing cloud of gas and dust known as a coma, Comet Borrelly is steaming past Earth and the Sun, harboring vital clues to the early solar system and possibly even the origin of life.
But like all comets, Borrelly is about as well known to scientists as the New World was before Columbus sailed.
Though comets have been seen, recorded and even feared for at least 3,000 years, researchers today only suspect what these frozen time capsules are made of. And they know very little about where comets come from even as they cruise the same solar system we inhabit.
Borrelly, discovered in 1904, is a lot like other comets. It spends the bulk of its life far from the Sun, its orbit reaching beyond that of Jupiter. But it loops around the Sun once every 6.9 years. And like any comet nearing the Sun, some of its icy outer shell is boiled away, offering a view of the surface of its core, or nucleus.
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 | A 1994 image of Comet Borrelly on one of its swings around the Sun.
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| Some comets are attracted to an abrupt ending. In 1994, a crumbling Comet P/Shoemaker-Levy 9 smashed into Jupiter, ripping holes in its atmosphere and leaving dark scars that lasted for weeks. This Hubble Telescope image shows icy fragments stretching across 710,000 miles (1.1 million kilometers) of space.
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With Borrelly currently in the neighborhood, scientists hope to probe its surface with a host of telescopes on the ground and in space. And as luck would have it, an ailing spacecraft has agreed to take on one last task and make a daring, data-gathering plunge into the comet's coma that is likely to destroy the craft.
The marriage of all this data could give researchers one of the most comprehensive views ever of the inside of a comet and help determine how representative comets are of the solar system's earliest years.
"We do think they're the most pristine objects in the solar system," said Harold Weaver, a Johns Hopkins University comet expert. "We think we're looking back 4.5 billion to 4.6 billion years ago and getting an indication of what solar system was like during its formation."
But how strong an indication remains unknown. "Have we frozen into the nuclei of comets the history of the solar system?" Weaver wonders. He and a host of other researchers aim to find out.
Bevy of observations scheduled
In a risky mission that NASA admits may not succeed, the Deep Space 1 spacecraft could provide important clues during a
flyby this Saturday, Sept. 22. The space probe will attempt to photograph the comet's nucleus from within the coma. It will also use an instrument designed to monitor the spacecraft's engine to instead study how charged particles and magnetic fields emanating from the comet react as they slam into their counterparts issued by the Sun.
The electrically charged particles are both electrons and charged atoms, or ions, and are collectively known as plasma. Learning how this cometary plasma interacts with the plasma known as the solar wind could reveal secrets about what material is on the surface of the comet.
Simultaneously, Weaver and his colleague Alan Stern of the Southwest Research Institute will study the comet in ultraviolet light with the Hubble Space Telescope. They hope to measure how much water burns off the comet's nucleus.
While comets have often been thought of as dirty snowballs, measurements of another comet earlier this year showed that they might be more like
snowy dirtballs. In the comet, called LINEAR, the density of water was no more than 30 kilograms per cubic meter, far less than the figure of 500 often assumed. Weaver and Stern are eager to learn if that measurement was an aberration, or if other comets tend to be drier than once thought.Combining the simultaneous observations, including some planned by ground-based observatories, would provide a more complete picture of Borrelly than has been obtained on most other periodic comets, Weaver said in a telephone interview.
And since scientists have bagged very few comets, each one tends to repaint the canvas of cometary knowledge.
When comets collide
Borrelly is a particular type of comet that astronomers call periodic, a class of comets that pass though the inner solar system on their trip around the Sun every 200 years or more frequently.
Exactly how and where periodic comets originate is not known.
Unlike asteroids, which are mostly confined to a belt between Mars and Jupiter, comets wander near and far from the Sun. Some reach out as much as a fifth of the way to the next nearest star. Most stay well beyond the orbit of Neptune, where a vast sea of objects orbit the Sun without ever getting too close.
Many comets have never entered the inner solar system.
But some, like Borrelly, are on strangely elliptical paths that bring them close to the Sun before shooting back out to the deep freeze.
Those comets that do enter the inner solar system could be created by collisions between larger primordial objects beyond the orbit of Neptune, Weaver says. When objects collide, fragments might be kicked into new, egg-shaped orbits that traverse the orbits of the inner planets.
Pluto and beyond
If true, then the study of a comet such as Borrelly would help researchers understand the larger objects, called Trans-Neptunian Objects, or TNOs -- dark, cold and distant things that can't otherwise be properly studied with current technology. More and more, scientists are learning that the space beyond Pluto, up to a fifth of the way to the next nearest star, is littered with TNOs, small and large objects that orbit the Sun.
Pluto, though designated a planet, is considered by some scientists to be a typical though large TNO. But other big and distant objects, possibly even bigger than Pluto, are expected to be found in coming years based on
recent studies that have spotted surprisingly large TNOs."Periodic comets may give us a connection to those things," Weaver said.
But Borrelly is just one comet, and because scientist don't know how varied comets might be in origin and composition, the entire shroud of mystery that cloaks these nomads of the solar system won't likely be completely lifted anytime soon.
Next page: Life, sex, death and comets: what's known and what's suspected
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