Astronomers are anxiously waiting to see if a comet survives its encounter with the Sun.
Comet Ison will reach its closest approach to our star at approximately 18:35 GMT on Thursday.
It has been billed as a potential "comet of the century", but the Sun's heat and gravitational tug could destroy it before it has a chance to light up the skies.
Some scientists believe it is already starting to buckle under the onslaught.
Prof Tim O'Brien, associate director of the UK's Jodrell Bank Observatory, said: "It's like throwing a snowball into fire. It's going to be tough for it to survive.
"But luckily, it's a big object and it moves fast, so it won't spend too much time close to the Sun. There is a lot of uncertainty."
Comet Ison came from the Oort Cloud, a mysterious, icy region at the furthest reaches of our Solar System.
It has been hurtling towards the Sun, travelling at more than a million kilometres an hour.
It will pass the Sun at a distance of just 1.2 million km, effectively grazing its surface.
Prof Mark Bailey, from Armagh Observatory in Northern Ireland, said: "It's going to be exposed to the worst that the Sun can throw at it.
"It will be getting exposed to more and more intense solar heat, and that will start to sublimate the ices (turning them into gas) at an increasing rate."
The Sun's intense gravitational field produces tidal forces that will also have a major effect on the comet.
Scientists fear it could follow the path of Comet Lovejoy, which broke apart after it passed near the Sun in 2011. Or it could run out of fuel and fizzle out. It is hoped Ison's large size could protect it.
Comet Ison will reach its closest approach to our star at approximately 18:35 GMT on Thursday.
It has been billed as a potential "comet of the century", but the Sun's heat and gravitational tug could destroy it before it has a chance to light up the skies.
Some scientists believe it is already starting to buckle under the onslaught.
Prof Tim O'Brien, associate director of the UK's Jodrell Bank Observatory, said: "It's like throwing a snowball into fire. It's going to be tough for it to survive.
"But luckily, it's a big object and it moves fast, so it won't spend too much time close to the Sun. There is a lot of uncertainty."
Comet Ison came from the Oort Cloud, a mysterious, icy region at the furthest reaches of our Solar System.
It has been hurtling towards the Sun, travelling at more than a million kilometres an hour.
It will pass the Sun at a distance of just 1.2 million km, effectively grazing its surface.
Prof Mark Bailey, from Armagh Observatory in Northern Ireland, said: "It's going to be exposed to the worst that the Sun can throw at it.
"It will be getting exposed to more and more intense solar heat, and that will start to sublimate the ices (turning them into gas) at an increasing rate."
The Sun's intense gravitational field produces tidal forces that will also have a major effect on the comet.
Scientists fear it could follow the path of Comet Lovejoy, which broke apart after it passed near the Sun in 2011. Or it could run out of fuel and fizzle out. It is hoped Ison's large size could protect it.
