A large near-Earth asteroid called (3200) Phaethon is known to be blue in color and sometimes act like a comet. Using NASA’s Infrared Telescope Facility on Mauna Kea in Hawaii and the 5-foot (1.5 m) Tillinghast telescope on Mount Hopkins in Arizona, University of Arizona astronomer Teddy Kareta and co-authors studied sunlight reflected off Phaethon and found the asteroid even more enigmatic than previously thought.
An artist’s illustration of what the near-Earth asteroid Phaethon might look like up close. Image credit: Heather Roper.
Also known as 1983 TB, Phaethon was discovered on October 11, 1983 by NASA’s Infrared Astronomical Satellite (IRAS).
With a diameter of about 3.2 miles (5.1 km), the object is the third largest near-Earth asteroid classified as ‘potentially hazardous’ after asteroids (53319) 1999 JM8 and 4183 Cuno.
“Blue asteroids, which reflect more light in the blue part of the spectrum, make up only a fraction of all known asteroids,” Kareta and colleagues said.
“A majority of asteroids are dull gray 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 Celsius), hot enough to melt aluminum.”
Phaethon is also unusual in that it has the qualities of both an asteroid and a comet based on its appearance and behavior.
It 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 it is the parent body of December’s Geminid meteor shower.
“Meteor showers occur when Earth passes through the trail of dust left behind on a comet’s orbit,” the astronomers said.
“When they occur and where they appear to originate from depends on how the comet’s orbit is oriented with respect to the Earth.”
These colorized radar images show the near-Earth asteroid Phaethon. At time of closest approach on December 16, 2017, Phaethon was about 1.1 million miles (1.8 million km) away. The encounter is the closest the asteroid will come to Earth until 2093. Image credit: Arecibo Observatory / NASA / NSF.
Phaethon is thought to be the source of the Geminid meteor shower because its orbit is very similar to the orbit of the Geminid meteors.
Until its discovery, 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,” Kareta said.
“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.
Kareta and co-authors think Phaethon might be related or have broken off from (2) Pallas, a large blue asteroid farther out in 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.”
The team also observed that Phaethon’s blue color is the same on all parts of its surface, which indicates it has been cooked evenly by the Sun in the recent past.
The findings will be published in the Astronomical Journal.
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Theodore Kareta et al. 2018. Rotationally-Resolved Spectroscopic Characterization of near-Earth object (3200) Phaethon. AJ, in press; arXiv: 1810.11157
