One of the mysteries of comet studies is why new comets, arriving straight from the oort cloud, show tails even when they are a very long way from the sun - beyond the orbit of saturn? comets are thought to be dirty snowballs - mixtures of ice and rock. comet tails are thought to be caused by water ice evaporating, and carrying away tiny grains of rock, which reflect the sunlight and produce the tails we see.
but how can a comet be warm enough to melt ice when still beyond the orbit of saturn? calculate the temperature of a comet surface when it is 10 astronomical units from the sun. an astronomical unit is 1.5× , the luminosity of the sun is 3.84× and the stefan-boltzmann constant =5.67×10−8w m−2 k−4 .