Flora family
A large, poorly defined stony asteroid family near Earth.
The Flora family, also called the Ariadne family, is a large group of stony asteroids in the inner main belt. With over 13,000 known members, it makes up roughly 3.5% of all main-belt asteroids, but its boundaries are poorly defined and it gradually blends into the surrounding background population. The family’s origin and properties remain relatively unclear. It is very broad and contains several distinct subgroups, possibly formed by later secondary collisions, leading some researchers to describe it as an asteroid clan. Only a few interlopers have been identified. This family may be the source of the impactor that created the Chicxulub crater, which likely caused the extinction of the dinosaurs.
The largest member, 8 Flora, is about 140 km in diameter and accounts for roughly 80% of the family’s total mass. The next largest, 43 Ariadne, makes up another 9%. Because Flora sits near the family’s edge and the boundaries are so indistinct, some analyses (such as the 1995 WAM study) have excluded Flora and referred to the group as the Ariadne family. The remaining members are all fairly small, below 30 km in diameter. A significant fraction of the original parent body’s mass has been lost since the initial impact, likely due to secondary collisions. Estimates suggest that Flora itself contains only about 57% of the parent body’s mass, yet it holds about 80% of the mass now present in the family. The reason the largest members lie near the family’s edge is unknown.
The medium-sized member 951 Gaspra, part of the family’s core, was visited by the Galileo spacecraft. Studies of its crater density indicate the family is roughly 200 million years old, and a high abundance of olivine suggests the parent body was at least partially differentiated. Family members are considered strong candidates for the parent bodies of L chondrite meteorites, which account for about 38% of all meteorites that hit Earth. This idea is supported by the family’s proximity to the unstable ν6 secular resonance and by the spectral match between family members and this meteorite type.
The Flora family was one of the five original Hirayama families identified. It has many early-discovered members, partly because S-type asteroids are highly reflective and partly because it is the closest major asteroid grouping to Earth.
- Largest member
- 8 Flora (140 km diameter)
- Total known members
- More than 13,000
- Percentage of main-belt asteroids
- Approximately 3.5%
- Family mass held by 8 flora
- About 80%
- Family mass held by 43 ariadne
- About 9%
- Age estimate
- Of the order of 200 million years
- Number of core members (zappalà 1995)
- 604
Lore & Background
The Flora family is a very broad family which gradually fades into the surrounding background population. While the largest members, 8 Flora and 43 Ariadne, are located near the edge, there are several distinct groupings within the family, possibly created by later, secondary collisions. Due to this complex internal structure and the poorly defined boundaries, the Flora family has also been described as an asteroid clan. Only a few interlopers have been identified.
The largest member is 8 Flora, which measures 140 km in diameter and comprises about 80% of the total family mass. 43 Ariadne makes up much of the remaining mass (about a further 9%). Because of the family's poorly defined boundaries, and the location of Flora itself near the edge, it has also been called the 'Ariadne family', when Flora did not make it into the group during an analysis. The remaining family members are fairly small, below 30 km in diameter. A noticeable fraction of the parent body has been lost from the family since the original impact, presumably due to later processes such as secondary collisions.
951 Gaspra, a medium-sized core family member, was visited by the Galileo spacecraft on its way to Jupiter, and is one of the most extensively studied asteroids. Studies of Gaspra suggest that the family's age is of the order of 200 million years (indicated by the crater density), and that the parent body was at least partially differentiated (indicated by the high abundance of olivine). The Flora family members are considered good candidates for being the parent bodies of the L chondrite meteorites, which contribute about 38% of all meteorites impacting the Earth.
Reader's Guide
The Flora family holds significance as one of the five original Hirayama families first identified, and it is the closest major asteroid grouping to Earth. Its high number of early discovered members is due both to S-type asteroids tending to have high albedo and to its proximity. The family's location close to the unstable zone of the ν6 secular resonance, combined with spectral properties consistent with L chondrite meteorites, supports the theory that it is the parent body of these meteorites, which account for about 38% of all meteorites impacting Earth. The family may also be the source of the impactor that formed the Chicxulub crater, the likely culprit in the extinction of the dinosaurs. Despite its large size, the origin and properties of the Flora family are relatively poorly understood, and its boundaries are very indistinct. The largest members are located near the edge of the family, a distribution whose reason is unknown. The family's legacy includes the extensive study of 951 Gaspra by the Galileo spacecraft, which provided insights into the family's age and the differentiation of its parent body.
Did You Know?
- 951 Gaspra, a member of the Flora family, was visited by the Galileo spacecraft.
- The family's age is estimated to be of the order of 200 million years.
More in Notable Asteroids, Part 4 1-24
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