Asteroid Phaethon's mysterious tail may finally have an explanation

This illustration depicts asteroid Phaethon being heated by the Sun. The asteroid’s surface gets so hot that sodium inside Phaethon’s rock likely vaporizes and vents into space, causing it to brighten like a comet and form a tail.

For quite a while, an asteroid named Phaethon has presented astronomers with something of a conundrum. 

When it passes closest to the sun during its orbit, a long tail of material can be seen leaving the three-mile-wide (five-kilometer-wide) rock. However, if Phaethon's tail is the stuff of usual comets — ice and carbon dioxide — then it should be visible when the comet is as far away as Jupiter, too. But it's not.