Asteroid's tail composition revealed by solar observatories.
Solar observatories have revealed that the comet-like tail of asteroid 3200 Phaethon is not made of dust, as previously thought, but is actually made of sodium gas. The asteroid's tail is formed when sodium inside Phaethon's rock vaporizes and vents into space, causing it to brighten like a comet. Phaethon is the source of the annual Geminid meteor shower, even though it is an asteroid and not a comet. The discovery raises questions about other sunskirting 'comets' that may actually be rocky asteroids heated up by the sun.