Hilda family
An ancient collisional family of dark asteroids in Jupiter's 3:2 resonance.
The Hilda family is a collisional asteroid family that is at least 4 billion years old, making it one of the oldest such families known. It contains at least 409 identified asteroids and takes its name from its largest member, 153 Hilda, which measures about 170 kilometers across. This family belongs to the broader Hilda dynamical group, a collection of asteroids locked in a 3:2 orbital resonance with Jupiter. Every member of the family is a dark P-type asteroid with an albedo of roughly 0.06. Unlike a dynamical group, whose members share orbits solely due to gravitational interactions, an asteroid family consists of fragments from a single collision. The Hilda family is considered non-catastrophic because 153 Hilda still accounts for nearly three-quarters of the family’s total mass, rather than being just the largest among many small fragments. The impact that created the family likely occurred during the Late Heavy Bombardment, a period of much higher impact rates in the early Solar System. This explanation helps resolve a puzzle: models of the modern Solar System suggest that a collision large enough to produce this family from an asteroid the size of 153 Hilda is extremely improbable, even over billions of years. Another asteroid family within the same dynamical group is the Schubart family, named after its largest member, 1911 Schubart.
- Number of known members
- at least 409
- Largest member
- 153 Hilda
- Diameter of largest member
- 170 kilometres (110 mi)
- Albedo
- around 0.06
- Spectral type
- P-type
- Estimated age
- likely 4 billion years
- Mass fraction of largest member
- nearly 3/4 of the family's total mass
Lore & Background
The Hilda family is considered a non-catastrophic asteroid family because 153 Hilda, its largest member, makes up nearly three-quarters of the family's total mass, rather than simply being the largest of a number of fragments each making up a small fraction of the original destroyed asteroid. The family is believed to be one of the oldest collisional families ever discovered, with an estimated age of likely 4 billion years. Models based on the modern Solar System indicate it being very unlikely for an asteroid as large as 153 Hilda to suffer a collision large enough to create its family, even over the course of billions of years of encounters with other asteroids, leading to a 2011 study to propose that the family-creating impact event happened during the Late Heavy Bombardment, when impact rates in the Solar System were briefly much higher. Another asteroid family in the Hilda dynamical group is the Schubart family, named for its largest member, 1911 Schubart.
Reader's Guide
The Hilda family is significant as one of the oldest collisional families known, with an age likely reaching back 4 billion years. Its existence challenges models of the modern Solar System, which suggest that a collision large enough to create such a family from a body as massive as 153 Hilda is extremely improbable under current impact rates. This has led to the proposal that the family-forming impact occurred during the Late Heavy Bombardment, a period of much higher impact rates. The family is also notable for being non-catastrophic: 153 Hilda retains nearly three-quarters of the family's total mass, indicating the parent body was not completely shattered. All members are dark P-type asteroids with low albedos, and the family resides within the broader Hilda dynamical group of asteroids locked in a 3:2 orbital resonance with Jupiter. The family's ancient age and unusual formation conditions provide a window into the early collisional history of the asteroid belt.
Did You Know?
- All members of the Hilda family are dark P-type asteroids with albedos of around 0.06.
- The family-creating impact may have occurred during the Late Heavy Bombardment, when impact rates were briefly much higher.
More in Notable Asteroids, Part 4 1-24
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