Recent observations show that the asteroidal object 3200 Phaethon, the source of the Geminid meteor shower, creates a stubby dust tail each time it skirts searingly close to the Sun.
It's been almost exactly 30 years since the discovery of an oddball object named 3200 Phaethon. Mere days after NASA's Infrared Astronomical Satellite spotted it in October 1983, the late, great astronomer Fred Whipple realized that the orbit of this new object (then designated 1983 TB) matched the paths of Geminid meteors around the Sun. At last, astronomers had found the source of this robust annual shower.
Since most (all?) meteor showers are spawned by comets, it seemed logical that Phaethon must be one too. But observers scrutinized it for decades without detecting any cometary activity. Moreover, Phaethon's orbit is decidedly asteroidal — averaging 1.27 astronomical units from the Sun with a period of just 1.4 years. Perihelion occurs just 0.14 AU from the Sun — about 15 solar diameters — too close to observe the object with ground-based telescopes.
A breakthrough came in June 2009, when David Jewitt and Jing Li (University of California, Los Angeles) used NASA's STEREO A spacecraft to image Phaethon just as it passed through perihelion. They found an unexpected doubling in the object's brightness, presumably due to the sudden escape of particles from the nucleus to form a cloudlike coma. They reasoned that this couldn't be due to deep-seated water vapor gushing from the 3-mile-wide nucleus, nor the aftermath of an impact.
Instead, the two researchers speculated that the outburst most likely resulted from the sudden cracking or breakdown of clay minerals into dust as the searing sunlight heated its surface to roughly 1,300°F (700°C). The Sun's radiation pressure then drives them off into space.
So is Phaethon really a "rock comet"? Apparently. Li and Jewitt have enlisted the help of STEREO A again to observe Phaethon two more times when it skirted the Sun. By carefully processing those images, plus revisiting their initial observations, they've detected a stubby dust tail that pointed directly away from the Sun just after perihelion in 2009 and 2012 (but not in 2010).
The tail grew rapidly, reaching its full length — 150,000 miles (250,000 km) — in a single day. This implies that the dust particles are very small, perhaps only a micron across, which allows the pressure of solar radiation to push them rapidly away from the nucleus.
The two researchers, together with Jessica Agarwal (Max Planck Institute for Solar System Research) detail their findings in a paper submitted to Astrophysical Journal Letters. Jewitt also presented these results at the just-concluded European Planetary Science Conference in London.
A final thought: Dynamical modeling shows that the Geminid stream should dwindle away in less than about 1,000 years, so it must be getting replenished somehow. But Phaethon's brief outbursts yield only about 300 tons of fine dust — hardly enough gritty bits to create all those bright, beautiful "shooting stars" every December. So something more vigorous must have occurred in the recent past.