Although a blue moon has nothing to do with color, the same can't be said for asteroids.

An international team studying 3200 Phaethon, a 3.6-mile-long rock believed responsible for the annual Geminid meteor shower, has revealed that the asteroid's surface exhibits a blue color. First discovered 1983, Phaethon has long been considered a "dead comet" based on its eccentric orbit, a characteristic that brings it closer to the sun than any other known asteroid. The fact that it's blue, however — a color that defines only a fraction of known asteroids and never a comet — has the team once more stumped on Phaethon's true identity.

"At the time, the assumption was that Phaethon probably was a dead, burnt-out comet," team leader Teddy Kareta, a graduate student at the University of Arizona, said in a statement, "but comets are typically red in color, and not blue. So, even though Phaeton's highly eccentric orbit should scream 'dead comet,' it's hard to say whether Phaethon is more like an asteroid or more like a dead comet."

The research team was able to discover new insights about Phaethon after training telescopes in Hawaii and Alaska on it during a close flyby of Earth in December 2017. It then continued on its orbital path around the sun (a circuit that typically takes just over 1.4 years to complete) where it experienced temperatures exceeding 1,500 degrees Fahrenheit (815 Celsius) — hot enough to melt aluminum.

Unlike comets, which feature dust tails sometimes millions of miles long as they approach the sun, Phaethon's approach is much less dramatic.

"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," Kareta wrote. "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."

Prelude to a visitor

Japan Space Agency mission Destiny Japan's space agency, JAXA, will study interplanetary dust with a new mission called DESTINY, which stands for Demonstration and Experiment of Space Technology for INterplanetary voYage. It's scheduled to launch in 2022. (Photo: JAXA)

While new research has only deepened the mystery surrounding Phaethon, scientists won't have to wait long to pull back the curtain a bit further. In 2022, Japan's space agency, JAXA, will launch a spacecraft called DESTINY+ with a mission to fly by Phaethon, analyze its dust and photograph and map its surface.

Of particular interest will be Phaethon's low-reflectivity, a recent discovery that places it at odds with theories that it was once part of a larger blue asteroid named Pallas.

"Interestingly, we found Phaethon to be even darker than had been previously observed, about half as reflective as Pallas," Kareta added. "This makes it more difficult to say how Phaethon and Pallas are related."

Perhaps soon we'll finally how to be best categorize this bizarre blue traveler.

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.

Extremely rare blue asteroid stumps scientists
Phaethon, the source of the Geminids meteor shower, sometimes behaves like an asteroid and sometimes like a comet.