Themis family
Themis Family
The Themis family is a prominent group of asteroids located in the outer portion of the asteroid belt. Named after the asteroid 24 Themis, this family is one of the largest and most well-studied asteroid families in the Solar System. The family is characterized by its members' similar orbital elements, which suggest a common origin from a larger parent body that underwent a catastrophic collision.
Discovery and Naming
The Themis family was first identified in 1918 by the Japanese astronomer Kiyotsugu Hirayama, who used statistical methods to group asteroids based on their orbital elements. Hirayama's work led to the identification of several asteroid families, with the Themis family being one of the most significant. The family is named after its largest member, 24 Themis, which was discovered by Annibale de Gasparis in 1853.
Orbital Characteristics
Members of the Themis family share similar orbital elements, including semi-major axes, eccentricities, and inclinations. The semi-major axes of Themis family asteroids range from approximately 3.08 to 3.24 AU, with eccentricities between 0.12 and 0.19, and inclinations from 0.75° to 3.0°. These orbital parameters suggest that the family originated from a single parent body that was shattered in a collision.
Physical Properties
Themis family asteroids are predominantly C-type, which means they have a carbonaceous composition. This composition is indicative of primitive, unaltered material from the early Solar System. The surfaces of these asteroids are dark, with low albedo values typically ranging from 0.05 to 0.08. Spectroscopic studies have revealed the presence of water ice and organic compounds on some family members, suggesting that they may have played a role in delivering these materials to the early Earth.
Notable Members
The largest and most well-known member of the Themis family is 24 Themis, which has a diameter of approximately 200 kilometers. Other significant members include 90 Antiope, a binary asteroid system, and 379 Huenna. These asteroids have been the subject of numerous observational and theoretical studies aimed at understanding the family's formation and evolution.
Formation and Evolution
The Themis family is believed to have formed from the disruption of a larger parent body, estimated to have been around 400 kilometers in diameter. This catastrophic collision likely occurred over a billion years ago, producing a large number of fragments that now make up the family. Over time, the orbits of these fragments have been altered by gravitational interactions with Jupiter and other asteroids, as well as by non-gravitational forces such as the Yarkovsky effect.
Scientific Significance
The Themis family is of great interest to planetary scientists because it provides insights into the processes that shaped the early Solar System. The family's carbonaceous composition and the presence of water ice and organic compounds make it a valuable target for studying the delivery of volatiles and prebiotic materials to the inner Solar System. Additionally, the family's well-defined orbital characteristics make it an ideal case study for understanding the dynamics of asteroid families.
Exploration
Several missions have targeted or are planning to target Themis family asteroids. The most notable of these is the OSIRIS-REx mission, which, although primarily focused on the near-Earth asteroid Bennu, has provided valuable data on the composition and properties of carbonaceous asteroids that are relevant to the Themis family. Future missions may aim to directly explore members of the Themis family to further our understanding of their composition, structure, and history.