Years of searching proved to no avail until finally, in 1983, NASA's Infrared Astronomical Satellite discovered a curious object moving in the same orbit as the Geminid meteoroid stream. The orbital match was so good that it had to be the source of the debris, but to the surprise of many it wasn't a comet. The source of the Geminids was apparently a rocky asteroid. It made its closest recent approach to Earth in December 1997 when it passed within 0.31 AU of our planet. Click on the image to take a virtual tour of the solar system, complete with the curious asteroid 3200 Phaethon, the source of the Geminid meteors.
Comets do it easily whenever they pass close enough to the sun to heat their frozen nucleus.
Tiny bits of ice and dust naturally bubble away into interplanetary space. Rocky asteroids are made of tougher stuff, however, so it is unclear how bits of 3200 Phaethon would break or boil off to form a meteoroid stream. One of the earliest ideas was that Phaethon might occasionally collide with other asteroids. Collisions would create a stream of pulverized rocks that would account for the Geminids meteor shower. Phaethon's orbit passes through the asteroid belt just beyond Mars, so at first this hypothesis seemed likely, but more detailed studies disagree.