Massalia family
A young, two-lobed S-type family in the inner asteroid belt.
The Massalia family (also called Massalian; FIN: 404) is a group of asteroids in the inner main belt, centered on its largest member, 20 Massalia. These are S-type asteroids with very low orbital inclinations, and their orbits straddle the 1:2 mean-motion resonance with Mars. Over 6,000 Massalian asteroids are currently known.
This family formed from a cratering impact on 20 Massalia, which is by far the biggest body at roughly 150 km across. The next largest known member, (7760) 1990 RW3, is only about 7 km in diameter. All the smaller fragments together make up less than about 1% of the family’s total mass. The family is relatively young, likely created by an impact between 100 and 200 million years ago.
In proper orbital element space (semi-major axis vs. eccentricity), the family has a distinct two-lobed shape. One lobe is centered near 2.38 AU, the other near 2.43 AU, with 20 Massalia itself sitting between them. Asteroids in the lobes tend to be smaller on average than those in the central region. This structure is thought to result from the slow drift of semi-major axes due to the Yarkovsky and YORP effects, and the lobe details have been used to estimate the family’s age. A strong 1:2 resonance with Mars at 2.42 AU crosses the family and appears to cause some members to drift away into higher-inclination orbits.
The Massalia family, or a recent minor collision within it, is a likely source for the prominent α dust band. The other candidate is a recent collision within the Themis family.
The family’s asteroids have very low inclinations. A 1995 HCM analysis by Zappalà identified 42 core members, whose proper orbital elements fall within certain ranges (core members only reach an eccentricity of 0.170, though more modern data show the family extends to at least e=0.175). A later 2014 HCM analysis by Nesvorný, using a catalog of 398,000 bodies, found 6,424 member asteroids.
Some objects share the same orbital elements as true family members but are considered interlopers because their spectral (compositional) properties differ. For example, 2946 Muchachos and several others were identified as interlopers in a detailed study; 2316 Jo-Ann also shows a mismatched spectrum. Muchachos is larger than any true family member except Massalia itself.
- Parent body
- 20 Massalia
- Number of known members
- more than 6,000
- Largest member diameter
- about 150 km
- Second largest member
- (7760) 1990 RW3, about 7 km in diameter
- Age estimate
- 100 to 200 million years
- Core members (zappalà 1995)
- 42
- Core members (nesvorný 2014)
- 6,424
Lore & Background
The Massalia family is a definite cratering family consisting of 20 Massalia and a mass of small fragments excavated from Massalia's surface by an impact. Massalia is by far the largest member with a diameter of about 150 km, while the next largest body, (7760) 1990 RW3, is only about 7 km in diameter. The mass of all the small members is negligible, less than about 1%, compared to Massalia. The family is fairly young, estimated to have been created by an impact 100 to 200 million years ago.
It has a distinctly two-lobed appearance in proper a–e space, with one lobe centered at semi-major axes of 2.38 AU, the other at about 2.43 AU, with the parent body Massalia itself located in between. The bodies in the lobes tend to be smaller on average than those in the central region. This structure is likely caused by slow drift of the semi-major axis caused by the Yarkovsky and YORP effects. Details of the lobes were used to calculate the age of the family. A strong 1:2 orbital resonance with Mars crosses the family at 2.42 AU, and appears responsible for some leakage of family members away from the area into higher inclination orbits.
The Massalia family or a recent minor collision within it may be the source for the prominent α dust band, the other candidate being a recent collision within the Themis family. A number of interlopers have been identified, which share the same orbital elements as the true family members, but cannot have come from the same cratering event because of spectral differences. 2946 Muchachos and some other bodies were noted as interlopers during a detailed study of the family, while 2316 Jo-Ann is seen to have the wrong spectrum by inspection of the PDS asteroid taxonomy data set. Muchachos is larger than any of the true family members apart from Massalia itself.
Reader's Guide
The Massalia family is significant as a well-studied example of a young cratering family, providing insights into impact processes and the subsequent dynamical evolution of asteroid fragments. Its two-lobed structure in proper orbital elements, shaped by the Yarkovsky and YORP effects, has been used to estimate the family's age at 100 to 200 million years, making it a benchmark for understanding how small bodies drift in semi-major axis over time. The family's location straddling the 1:2 resonance with Mars also demonstrates how resonances can cause leakage of members into higher inclination orbits, influencing the long-term distribution of asteroid populations. Additionally, the Massalia family is a candidate source for the prominent α dust band, linking it to the production of interplanetary dust in the inner Solar System. The identification of interlopers such as 2946 Muchachos and 2316 Jo-Ann highlights the importance of spectral analysis in confirming family membership, ensuring that compositional differences are accounted for when reconstructing the family's origin. With over 6,000 known members, the Massalia family serves as a key laboratory for studying the collisional history and dynamical processes of the inner asteroid belt.
Did You Know?
- The Massalia family consists of S-type asteroids with very low inclinations.
- The family has a distinctly two-lobed appearance in proper a–e space, with lobes centered at 2.38 AU and 2.43 AU.
More in Notable Asteroids, Part 4 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced