Can 'comet of the century' survive close encounter with the Sun?
A comet that left the outer edge of the solar system more than 5.5 million years ago will pass close to the Sun today, becoming visible in Earth's skies within the next week or so – providing it survives.
Comet Ison is due to pass just 730,000 miles (1.2 million km) from the surface of the sun at 6.37pm GMT today.
Relatively recently, Ison was knocked out of the distant Oort cloud and began its journey towards the Sun. That light-year-long trip is now drawing to a close, and it remains to be seen if it will survive its journey.
The sun's gravity could rip the comet apart, breaking it into several chunks. If the comet has already broken apart, it should disintegrate completely before making a slingshot around our star.
Scientists are now watching through observatories to see if it has already been pulled into pieces by the gravitational forces of the Sun.
Another option for the comet is that it may not be destroyed completely as it will move past the sun so quickly. It has been hurtling towards the Earth at more than a million kilometres an hour.
Donald Yeomans, manager of Nasa’s Near-Earth Object Program Office at the Jet Propulsion Laboratory in Pasadena, California said: “It could be tough enough to survive the passage of the sun and be a fairly bright, naked-eye object.”
Nasa said if the comet does break up, this would provide a good opportunity for scientists to look at the insides of a comet and gain a better understanding of its composition.
Their observations over the last few days have at times suggested Ison was getting dimmer and might already be in pieces. However the comet brightened again over Tuesday and Wednesday.
In the early hours of Wednesday morning, the comet appeared in the view of the European Space Agency/NASA mission the Solar and Heliospheric Observatory in the Large Angle and Spectrometric Coronagraph instrument.
“As long as there are pieces there, we'll see something,” Carey Lisse, senior research scientist at Johns Hopkins Applied Physics Laboratory said on Tuesday.
Dr Dan Brown, a lecturer in astronomy at Nottingham Trent University, said: "If the comet does survive, it may have a huge and impressively long tail with a brightness that might make it an easy to spot object even with the naked eye". But if it is very weak, it could also break up into a cloud of dust and be a complete bust for viewing.
“This comet is giving us quite a ride. It's going to be hard to predict exactly what's going on,” Lisse said. “As a betting man, I think it's not going to survive solar passage,” he added.
Scientists estimate that Ison needs to be about 219 yards (200 meters) to survive its close encounter with the sun. The most recent measurements indicate the comet is more than twice that size, and perhaps as big as 0.75 miles (1.2 km).
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 (349 km per second.)
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.
Comets are believed to be frozen remains left over from the formation of the solar system some 4.5 billion years ago.