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.