Blue asteroids are rare and blue comets are almost unheard of, but one blue space object in our solar system is stranger still: Phaethon is known as a “rock comet” because it blurs the line between asteroids and comets. It gets closer to the sun than Mercury, is mysteriously blue, and it leads an army of meteors, some of which we see on earth as shooting stars each year around December 14th. Not only is Phaethon one of the only non-comets to cause a major recurring meteor shower but, get this: at some angles, the light reflected from Phaethon is the most polarized light ever observed among small bodies in the Solar System. Here’s an illustration of the polarized light reflected by Phaethon.
You may recall from high school physics that polarized light is light that is reflected or transmitted through certain media so that all vibrations are restricted to a single plane.
Natural light sources have usually only random polarization, but polarized light does occur in nature.
When light strikes an interface between two materials, the reflected light is generally partially polarized.
Polarized light is created in nature principally via the process of reflection from a specular surface, such as from a lake.
Is the surface of Phaethon a double layer and/or microscopically smooth?
Phaethon is dark and blue. Could it have surface water or ice? It gets hotter than mercury but could water repopulate the surface from the inside to form a dark carbon-rich ice after it passes the sun every 1.4 years and gets baked? The light it emits does not indicate water.
Whatever it is made of, Phaethon is 5.8 km (3.6 mi) in diameter. It orbits the sun at 45,000 mph, making the trip to get super heated every 1.433 years while spinning on its axis every 3.604 hours.
An international team led by Teddy Kareta, a doctoral student at the University of Arizona’s Lunar and Planetary Laboratory, investigated (3200) Phaethon, a bizarre asteroid that sometimes behaves like a comet, and found it even more enigmatic than previously thought.
Using telescopes in Hawaii and Arizona, the team studied sunlight reflected off Phaethon, which is known to be blue in color. Blue asteroids, which reflect more light in the blue part of the spectrum, make up only a fraction of all known asteroids. A majority of asteroids are dull grey to red, depending on the type of material on their surface.
Phaethon sets itself apart for two reasons: it appears to be one of the “bluest” of similarly colored asteroids or comets in the solar system; and its orbit takes it so close to the sun that its surface heats up to about 800 degrees Celsius (1,500 degrees Fahrenheit), hot enough to melt aluminum.
Astronomers have been intrigued by Phaethon for other reasons, too. It has the qualities of both an asteroid and a comet based on its appearance and behavior.
Phaethon always appears as a dot in the sky, like thousands of other asteroids, and not as a fuzzy blob with a tail, like a comet. But Phaethon is the source of the annual Geminid meteor shower, easily seen in early-to-mid December.
Here’s the best detail I could find on the shape. It looks like a shaved potato here:
Phaethon also releases a tiny dust tail when it gets closest to the sun in a process that is thought to be similar to a dry riverbed cracking in the afternoon heat. This kind of activity has only been seen on two objects in the entire solar system — Phaeton and one other, similar object that appears to blur the line traditionally thought to set comets and asteroids apart.
The team obtained several new insights about Phaethon after analyzing data obtained from NASA’s Infrared Telescope Facility on Mauna Kea in Hawaii and the Tillinghast telescope, operated by the Smithsonian Astrophysical Observatory on Mount Hopkins in Arizona. They think Phaethon might be related or have broken off from (2) Pallas, a large blue asteroid farther out in the solar system.
Since its discovery, several other objects were found exhibiting mixed cometary and asteroidal features, such as 133P/Elst–Pizarro.
What could make Phaethon blue?