The asteroid 3200 Phaethon is an enigma, and not just because it passes closer to the Sun than any other asteroid we've given a name. The light it reflects is unusually polarized light, and while the paper announcing this fact has some possible explanations, all would indicate something odd about this worldlet. The Japan Aerospace Exploration Agency (JAXA) is planning a mission past Phaethon, but it won't even launch until at least 2022, so answers may take some time.
If you've had the joy of witnessing a Geminid meteor shower you have Phaethon to thank. The asteroid is shedding dust grains and the Earth passes through the debris every December, producing one of the best annual meteor showers (check out our guide to 2018's meteor showers and how to watch them here), distinguished by its unusually slow-moving meteors. This is a strange thing in itself. Most meteor showers are produced by particles released by comets as the ices that bind their dust together melt on approaches to the Sun. Only one other asteroid is known to shed in a similar way.
It's not clear whether these unusual characteristics are related to the specific feature Dr Takashi Ito of the National Astronomical Observatory of Japan has investigated, Phaethon's unusual blueish color. One way to study this was to look at how polarized (vibrating in a common plane) the light we see from Phaethon is, depending on the relative positions of the Earth, Sun, and asteroid. Previous research has shown a correlation between the maximum polarization of light reflected off small objects within the Solar System and the size of the grains of dust on their surfaces.
In Nature Communications, Ito has announced Phaethon's light is more than 50 percent polarized, the highest recorded for a small Solar System object.
The paper offers three explanations. The particles on Phaethon's surface may be unusually large, which would also explain the bluish tinge, as bigger particles reflect more blue light. However, the authors note some past observations aren't consistent with this theory. A porous surface could also produce this polarization.
The more intriguing scenario is that Phaethon is darker than it looks, with light bouncing off it in a relatively straightforward manner, rather than the multiple reflections that are common from the loose collections of rubble that make up most asteroids.
The correct explanation probably reveals the effects of Phaethon passing seven times as close to the Sun as the Earth ever gets.
At 5.8 kilometers across (3.6 miles) Phaethon is no Ceres or Vesta, but is larger than other asteroids that pass inside Mercury's orbit. It was the first asteroid to be discovered using images from a spacecraft, after analysis from the Infrared Astronomical Satellite revealed its presence in 1983. Although it sometimes makes close approaches to Earth, it presents no danger at least within the next 400 years.