Blue asteroids are rare, and blue comets are almost unheard of. An international team of astronomers investigated 3200 Phaethon, a bizarre blue asteroid that sometimes behaves like a comet, and found it even more enigmatic than they’d previously thought.
On December 16, 2017, the asteroid made its closest approach to Earth since 1974, passing within 6.4 million miles (10.3 million km). The team analyzed data from the flyby from several telescopes around the world to learn more about the mysterious object which has puzzled astronomers since its discovery in 1983. The researchers presented the results of their study on October 23, 2018, at the annual meeting of the American Astronomical Society’s Division for Planetary Science in Knoxville, Tennessee.
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 1,500 degrees Fahrenheit (800 degrees C), 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.
Meteor showers occur when Earth passes through the trail of dust left behind on a comet’s orbit. When they occur and where they appear to originate from depends on how the comet’s orbit is oriented with respect to the Earth. Phaethon is thought to be the “parent body” of the Geminid meteor shower because its orbit is very similar to the orbit of the Geminid meteors.
Until Phaethon was discovered in 1983, scientists linked all known meteor showers to active comets and not asteroids.
At the time, the assumption was that Phaethon probably was a dead, burnt-out comet, but comets are typically red in color, and not blue. So, even though Phaethon’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.
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 – Phaethon 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 in Hawaii and the Tillinghast telescope 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. Karate said:
Interestingly, we found Phaethon to be even darker than had been previously observed, about half as reflective as Pallas. This makes it more difficult to say how Phaethon and Pallas are related.
The team also observed that Phaethon’s blue color is the same on all parts of its surface, which they say indicates it has been cooked evenly by the sun in the recent past.
Bottom line: New insights on 3200 Phaethon, an asteroid that often acts like a comet.
Eleanor Imster has helped write and edit EarthSky since 1995. She was an integral part of the award-winning EarthSky radio series almost since it began until it ended in 2013. Today, as Lead Editor at EarthSky.org, she helps present the science and nature stories and photos you enjoy. She also serves as one of the voices of EarthSky on social media platforms including Facebook, Twitter and G+. She and her husband live in Tennessee and have two grown sons.