asteroid
Having studies countless asteroids in near-Earth space, astronomers have come to understand that the majority of these rocks fall into one of two categories: S-type (grey) and C-type (red). These are defined by the types of materials on their surfaces, with S-type asteroids being primarily composed of silicate rock and C-type asteroids being made up of carbon materials.

However, there is also what are known as blue asteroids, which make up only a fraction of all known Near-Earth Objects (NEO). But when an international team astronomers observed the blue asteroid (3200) Phaeton during a flyby of Earth, they spotted behavior that was more consistent with a blue comet. If true, then Phaeton is of a class of objects that are so rare, they are almost unheard of.

The team's findings were presented at the 50th annual meeting of the American Astronomical Society's Division for Planetary Science, which is taking place this week (Oct. 21st to Oct. 26th) in Knoxville, Tennessee. The presentation, titled "Physical Characterization of (3200) Phaethon: Target of the DESTINY+ Mission", was led by Theodore Kareta of the Lunar and Planetary Laboratory (LPL).

Phaethon
© NASA JPL
This diagram shows the highly eccentric orbit of 3200 Phaethon.
As they stated during the course of the presentation, the team analyzed data from NASA's Infrared Telescope Facility (located atop Mauna Kea in Hawaii) and the Smithsonian Astrophysical Observatory's Tillinghast telescope, which is located on Mount Hopkins in Arizona. What they found was that Phaeton's appearance and behavior indicate that it has the characteristics of both an asteroid and a comet.

For instance, like all asteroids, Phaeton is known to reflect more light in the blue part of the spectrum than other classes (hence the name). However, Phaeton sets itself apart by being one of the bluest, and having the same color over all of its surface. This is an indication that it may have recently been uniformly heated by the Sun.

"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."

Its orbit is also one of the very eccentric, taking it so close to the Sun that it reaches temperatures of up to about 800 °C (1500 °F). Similarly, it appears like an asteroid in the sky (as a small dot versus a cloudy blotch), but also releases a tiny tail of dust when it gets closest to the sun. This is a indication that Phaeton's composition includes volatile elements (such as water, carbon dioxide, methane, ammonia, etc.) that sublimate as it warms.


Geminids
© SteedJoy.
The 2015 Geminids over the LAMOST observatory in China.
Lastly, Phaeton is thought to be the "parent body" of the annual Geminid meteor shower because of how its orbit is similar to those of the Geminid meteors. Prior to Phaeton's discovery in 1983, scientists believed that all meteor showers were due to active comets. As Kareta explained:

"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 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."

This kind of activity has only been seen twice in the history of astronomical observations, with Phaeton and one similar object that defies classification as either an asteroid or a comet. For these reasons, the research team theorizes that Phaeton might be related to, or have broken off from, (2) Pallas, one of the larger objects in the Main Asteroid Belt (and also a blue asteroid).

Currently, the team is conducting observations of 2005 UD, another small blue asteroid which may related to Phaethon. By determining if it and Phaethon share the same properties, they will gain valuable insight into what this comet/asteroid's true nature is. In addition, the study may have implications on future asteroid-rendezvous missions, like the Japanese Aerospace Exploration Agency's (JAXA) DESTINY+ technology demonstrator.

Artist's impression of JAXA's Demonstration and Experiment of Space Technology for INterplanetary voYage, Phaethon fLyby with reUSable (DESTINY+) probe . Credit: JAXA

This mission, which stands for Demonstration and Experiment of Space Technology for INterplanetary voYage, Phaethon fLyby with reUSable probe, is scheduled to conduct a flyby with several NEOS, including Phaeton, after launching in 2022. The purpose of this mission will be investigate the origin and nature of cosmic dusts, which are key sources of organic compounds on Earth - and therefore intrinsic to life.

In addition, the demonstrator will observe dust from Phaeton and map its surface to gain a better understanding of the mechanisms behind dust ejection. In this respect, this mission could help us to better understand the differences between comets and asteroids. Moreover, the highly unique object it will study could help us to better understand the origins of life in our Solar System.

The work was funded by the NASA Near-Earth Object Observation program (NEOO) grant. In addition to Karten, the team included multiple members of the LPL, the NASA Johnson Space Center, the Planetary Exploration Research Center at the Chiba Institute of Technology, and the Planetary Science Institute (PSI).
Further Reading: UANews, AAS