Comet ISON headed for 'Spacegiving Day' graze with sun

A comet that left the outer edge of the solar system more than 5.5 million years ago will pass close by the sun Thursday, becoming visible in Earth's skies in the next week or two – if it survives.

"There are three possibilities when this comet rounds the sun ... It could be tough enough to survive the passage of the sun and be a fairly bright, naked-eye object," Donald Yeomans, a NASA scientist, said in an interview posted on NASA's website.  

The second possibility is that the sun's gravity could rip the comet apart, creating several big chunks. Scientists say as long as there are pieces, it should be visible.

The third scenario: If the comet is very weak, it could break up into a cloud of dust and be a complete bust for viewing.

Comet ISON, as the object is known, was due to pass just 730,000 miles from the surface of the sun at 1:37 p.m. EST on Thanksgiving Day.

At that distance, the comet will reach temperatures approaching 5,000 degrees Fahrenheit – hot enough to vaporize not just ices in the comet's body, but dust and rock as well.

"While it may seem incredible that anything can survive this inferno, the rate at which ISON will likely lose mass is relatively small compared to the actual size of the comet's nucleus," Lowell Observatory astronomer Matthew Knight said in a NASA interview.

Scientists estimate that ISON needs to be about 219 yards across to survive its close encounter with the sun. The most recent measurements indicate it is more than twice that size, and perhaps as big as 0.75 miles.

It helps that ISON will not be staying in the solar furnace for long. When it zips around the sun, it will be moving at about 217 miles per second.

Comet ISON is especially interesting to NASA scientists, who launched a balloon to the upper levels of earth's atmosphere in September to study the object, because it has traveled from the outer reaches of the solar system.

That means it may still contain a primordial ratio of elements, which scientists could use to glean insights about the evolution of the solar system. Some scientists believe that comets delivered the building blocks of life – water, hydrogen, carbon, and oxygen – to Earth.

"You need comets in order to build the planets and this comet has been in deep freeze in the Oort Cloud for the last 4.5 billion years," said Carey Lisse, senior research scientist at Johns Hopkins Applied Physics Laboratory in Laurel, Md.

"Comet ISON is a relic. It's a dinosaur bone of solar system formation," he said.

The family of comets that ISON is from resides in the Oort Cloud, on the outskirts of the solar system, halfway to the next star.

Occasionally, one of these comets is gravitationally nudged out of the cloud by a passing star and into a flight path that millions of years later brings it into the inner solar system. Computer models show ISON is a first-time visitor.

The comet was discovered last year by two amateur astronomers using Russia's International Scientific Optical Network, or ISON.

It was extraordinarily bright at the time, considering its great distance beyond Jupiter's orbit, raising the prospect of a truly cosmic spectacle as it approached the sun.

Heat from the sun causes ices in a comet's body to vaporize, creating bright distinctive tails and fuzzy-looking, glowing bodies. The closer comets come to the sun, the brighter they shine, depending on how much ice they contain.

Reuters