Enigmatic Rare Blue Asteroid 'Phaethon' Baffles Astronomers

Phaethon Asteroid Dust2-small
Artist representation of Phaethon. Teddy Kareta

A very weird and very rare blue asteroid recently flew by Earth and astronomers observing discovered something weird—it might not be an asteroid at all.

The space rock, known as (3200) Phaethon, is responsible for the Geminid meteor shower—one of the most prolific shooting star displays of the year, which peaks in the middle of December.

Researchers have been intrigued by Phaethon since it was first discovered in 1983. Until this point, scientists thought all meteor showers were formed by comets—not asteroids. But this object appeared to have characteristics of both an asteroid and comet. Comets produce meteor showers when Earth passes through the trail of dust they leave behind.

Phaethon's blue hue made things very confusing. Blue asteroids make up just a tiny fraction of all known asteroids—most are a dull grey or red color, depending on the material that makes up their surface.

"At the time, the assumption was that Phaethon probably was a dead, burnt-out comet," Teddy Kareta, from the University of Arizona's Lunar and Planetary Laboratory, 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."

Kareta and colleagues used telescopes in Hawaii and Arizona to study sunlight reflected from Phaethon. They presented their findings at the 50th meeting of the American Astronomical Society's Division for Planetary Sciences in Tennessee.

Phaethon, they found, appears to be one of the bluest asteroids or comets in the solar system. They believe it broke away from Pallas—a large blue asteroid located at the farther reaches of the solar system.

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

When it flies past the Sun, Phaethon's surface heats up to around 800 degrees Celsius. This extreme heat, the researchers believe, is responsible for the blue color being even across the surface.

Researchers now plan to carry out observations of another blue asteroid that may be related to Phaethon. Hopefully this will provide more clues about what Phaethon actually is.

As NASA explains: "It is possible that Phaethon is a 'dead comet' or a new kind of object being discussed by astronomers called a 'rock comet.' Phaethon's comet-like highly elliptical orbit around the sun gives credence to this hypothesis.

"However, scientists are not certain how to define Phaethon. When Phaethon passes by the sun it does not develop a cometary tail, and its spectra looks like a rocky asteroid. Also, the bits and pieces that break off to form the Geminid meteoroids are also several times denser than cometary dust flakes."