Solar visit threatened to vaporise the comet but the remnant may be visible from Earth in December

Comet Ison appears to have survived a close encounter with the sun that had threatened to vaporise it. The remnant could now go on to be visible from Earth in December, but astronomers do not know how bright it might become.

Travelling at more than 200 miles per second, Ison passed 730,000 miles above the sun's 6,000C surface on Thursday evening. This would have heated the comet to almost 3,000°C, enough to vaporise rock as well as ice.

"It would be an absolutely hellish environment, there's never been a better time to use the words 'snowball's chance in hell'," said Tom Kerss, astronomer at the Royal Observatory in Greenwich, south-east London.

The hope was that the comet would remain sufficiently bright to be visible with the unaided eye throughout December. However, as Ison sped towards the sun, it faded dramatically from view. This led some experts to assume it had disintegrated. "I'm not seeing anything that emerged from behind the solar disc. That could be the nail in the coffin," said astrophysicist Karl Battams, from the Naval Research Laboratory in Washington, during a live broadcast on Nasa TV.

Yet, rumours of the comet's demise may have been greatly exaggerated. Overnight, something following Ison's orbit re-emerged on the opposite side of the sun. Now it is brightening as it plunges back into deep space.

"To all intents and purposes it looked like it had gone, and then amazingly this thing appears out the other side," said Professor Tim O'Brien, associate director of the University of Manchester's Jodrell Bank observatory.

"What we don't know is whether the whole thing fell apart and whether the dust that was embedded within the ice is just basically in a big cloud and that is continuing to orbit," he explained. "The question is, is it just a cloud of dust … or is there still either one or more remnants of the nucleus."

Kerss said: "I would put a little bit of money on the fact that part of the nucleus, the actual core of the comet, is still intact."

The nucleus, a huge lump of rock and ice, was several miles wide on its approach to the sun, and brightened as the sun heated it to create an atmosphere, or coma, of ice and dust which was blown away from the sun to form a tail.

But radiation pressure, extreme heat and gravitational forces could have ripped the comet apart.

"I think it has had a very bad day," said Kerss. "But I think because the comet started to brighten again after passing the sun, that's a fairly good indication the nucleus is replenishing the coma."

If the remnant is a cloud of dust it will rapidly dissipate. If it is solid the sunlight will continue to vaporise its ice, creating a tail that may be visible from Earth.

Comets are ancient celestial objects. They date from 4.6bn years ago when Earth and other planets of our solar system were forming. They may even have brought the water that fills the oceans and the molecules necessary for life to our world.

Arriving from the Oort cloud, a comet-rich region about a light year from the sun, Ison was thought to be relatively "fresh", causing excitement among the scientific community.

"Those are quite exciting because if they haven't already given off a lot of their surface as a coma they're expected to be very bright on their first approach to the sun," said Kerss.

Before its encounter, Ison's tail stretched more than 5m miles through space. But it failed to brighten at the rate expected as it hurtled towards the sun. "We expect it to have received a gentle cooking over billions of years which has probably made the surface of the comet very volatile and that's why it was expected that it would perform very well in terms of brightness, said Kerss. "But actually the fact that Ison hasn't performed as expected perhaps is a sign that we don't understand the surface of comets as well as we'd like to," he said.

Astronomers are now awaiting sight of a new tail.

This will allow an estimate of whether Ison will be visible from Earth.

If it is, the best time to look will be around 6.30am, in the pre-dawn sky. Looking east, the comet will rise before the sun, with its tail pointing straight up into the sky.

O'Brien is optimistic. "It looks pretty bright in the images we are seeing from spacecraft but I wouldn't like to say whether it will be visible to the naked eye just yet. We'll know in the next few days."

The dramatic dimming is similar behaviour to a previous sun-grazing comet. In 2011 the comet Lovejoy skimmed the surface of the sun, passing eight times closer than Ison. It too faded dramatically but then survived to develop a new tail as it sped away from the sun.

For now, scientists are keeping a close eye on the situation as it unfolds. "We are watching these images coming in still and we are just seeing how it's going to develop," said O'Brien. "I think it's really a exciting thing that we are all following this thing in real time on the web via images being downloaded almost in real time from spacecraft observing the sun."