A comet believed to be the largest ever discovered has been exposed by a pair of US astronomers and is heading towards the sun.
This illustration shows the distant Comet Bernardinelli-Bernstein as it might look in the outer Solar System. Photo: Supplied / NOIRLab/NSF/AURA/J. da Silva
Comet Berardinelli-Bernstein, named after its founding astronomers, is estimated to measure at least 100km-long and 1000 times the mass of most comets.
That makes it arguably the largest comet discovered in modern times.
For comparison, the asteroid that slammed into Earth 66 million years ago and wiped out the dinosaurs was an estimated 10km across.
Astronomers Pedro Bernardinelli and Gary Bernstein from the University of Pennsylvania discovered the comet, which is a relic from the Oort Cloud - a bunch of leftovers kicked out of the Solar System by Jupiter about 4 billion years ago or so.
"Once in a while one of them gets knocked towards us and when it gets close to us, starts to evaporate, grows a tail and we call it a comet," Dr Bernstein told Morning Report.
It has to have a tail - cloud of gas and dust called a 'coma' - to be identified as a comet, otherwise it would be an asteroid.
The Dark Energy Survey first spotted the object in 2014, and for four years images collected of it did not show any tail.
However, fresh images of the object showed it had grown a coma in the past three years, officially making it a comet.
The closest the comet is expected to get to the sun is about the distance of Saturn, which will be in 2031.
Last month, it was estimated to be at the distance of Uranus from the sun.
"But if you're from the Oort Cloud that's very nearby, getting that close is rare," Dr Bernstein said.
"We discovered this guy as it was coming to us about the distance of Neptune, but it spends most of its life a thousand times further away than that.
"The last time it came through our neighbourhood here was more than three million years ago."
Because Oort Cloud objects tend to spend time far away from Earth, Dr Bernstein said researchers are usually unable to learn much about the Oort Cloud.
"But they're quite interesting because as I say they were relics from the early days of the solar System and this is kind of our chance to get some more information about what those things are made of, [and] as it evaporates, we can try to tell what kind of molecules and things are in it.
"Maybe we'll learn something about comets that the closer in ones, the smaller ones, aren't telling us."
However, despite its size, the comet would not be visible to the naked eye.
"Even though it's very big, it's also further away than most comets and further away wins ... but in telescopes it will be visible for decades.
"If it came within say instead of 10 times Earth's distance to the sun, if it came within two or three times Earth's distance to the sun, then it would potentially be one of the most spectacular naked eye comets ever seen, but unfortunately that won't happen."