Asteroids or Comets? Discover the strange and fascinating objects frequently found among giant planets.

Learning Objective:

Learn a specific class of smaller bodies found in the solar system

Overview:

The unique characteristics of Centaur objects have captured the interest of scientists, and further research on them will provide insights into the fundamental components of our solar system.

Specifics: 

Centaurs belong to a class of minor celestial bodies with mixed characteristics. They exhibit asteroid-like appearances but can also display comet-like features such as a coma and tail. This dual nature inspired the class’s name, paying homage to the mythological creatures that were half-human and half-horse in Greek mythology.

Centaurs reside in the region where the giant planets are located and are periodically perturbed by their orbits. This instability can result in them being ejected to other locations within the solar system or even colliding with other celestial objects.

Orbits of some Centaurs compared with giant planets and dwarf planets (Roen Kelly/ Duncan Steel/Astronomy Magazine)

The first Centaur officially discovered was the object (2060) Chiron in 1977. Another object, (944) Hidalgo, discovered in 1920 and initially classified as an asteroid, was later recognized as a member of this newly defined category based on the characteristics of its orbit observed over time. 

Examples of Centaurs

(2060) Chiron – This object, estimated to be over 200 km in diameter, displays occasional comet-like activity. Observations using the stellar occultation method have revealed the presence of material surrounding it. The origin and nature of which remain unknown.

False-color image of 2060 Chiron taken in 1998 (Adrian Silva / Sergio Cellone / Sci-News)

(5335) Damocles – The case of (944) Hidalgo was not the only one. When discovered in 1991, Damocles was not classified as a centaur, but subsequent observations made it integrated into this class, displacing the object (5145) Pholus, discovered in 1992 and considered until then the second recognized Centaur. With an estimated diameter of 10 km, its particularities later created a specific category of minor objects with orbits very similar to periodic comets, the Damocloids.

(10199) Chariklo – Discovered in 1997, it is recognized as the largest member of the Centaurs, boasting a confirmed diameter exceeding 300 km. Its orbit extends beyond 3 billion kilometers, farther than Saturn’s orbit. In 2013, using the stellar occultation method, astronomers made the groundbreaking discovery of rings encircling Chariklo. These rings are situated approximately 400 kilometers from the object, with an estimated width ranging between 3 and 7 kilometers.

Illustration of  Centaur (10199) Chariklo and its ring system (James Webb Space Telescope/NASA, ESA, CSA, Leah Hustak/STScI)

Some short-period comets are classified as Centaurs, including 29P/Schwassmann-Wachmann 1 and 39P/Oterma. Phoebe, one of Saturn’s moons, is speculated to be a former Centaur that was captured by the planet’s gravitational pull at some point in its history.

Mission to Centaurs

Studying Centaurs is crucial because they are exceptionally well-preserved celestial objects that offer valuable insights into the chemical composition and physical processes of the early Solar System. Moreover, their comet-like characteristics present a unique opportunity to witness the evolution of a comet nearly in real-time.

Several missions aimed at studying Centaur objects have been proposed, such as Centaurus and Chimera, but have not received approval for future development by NASA. The European Space Agency is advancing with the Comet Interceptor mission, slated to be positioned at the Sun-Earth L2 Lagrange point. Initially targeting a long-period comet, this mission could potentially extend its scope to include Centaur objects. 

Other missions focusing on sample collect, such as one targeting comet 29P/Schwassmann-Wachmann, which is also classified as a Centaur object, are being proposed

Image composition of comet 29P/Schwassmann-Wachmann 1 during its passage in 2013 (Damian Peach/Sky & Telescope)

Learn more about this subject by visiting these websites:  

LEARN – Can Asteroids Have Rings Like Saturn?

LEARN – What are active asteroids?

LEARN – What are Trojan asteroids?

ESA’s Comet Interceptor mission

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