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."