A Family Adrift

Title: Discovery of an asteroid family linked to (22) Kalliope and its moon Linus

Authors: M. Brož, M. Ferrais, P. Vernazza, P. Ševeček, M. Jutzi

First Author’s Institution: Charles University, Faculty of Mathematics and Physics, Institute of Astronomy, Prague, Czech Republic

Status: Accepted at A&A

An artist’s concept of an asteroid being broken apart.
Credit: NASA / JPL – Caltech

Up until now, the asteroid known as (22) Kalliope was deemed to be rather lonely. Well, not entirely lonely, as it actually has a moon in orbit: Linus

Linus is around five times smaller than Kalliope, which at a diameter of 167 km, is the second largest M-type asteroid in the main asteroid belt, after (16) Psyche. Linus, however, is by far the largest asteroid moon known and it circles Kalliope at a mean distance of 1109 km in 3.6 days. Kalliope is exceptionally dense. This, along with its low radar albedo, suggests that the interior of the asteroid is differentiated, meaning that the materials the body is made from have been geochemically segregated, most likely due to a very strong heating event in the past. In fact, Kalliope’s past may be similar to Mercury’s, in the sense that the original body underwent a massive collision which stripped it of its outer layers and now only the dense core material remains. This giant collision could also explain the existence of Linus.

Unidentified Kin

The authors of today’s paper have asked themselves why there is no further asteroid family associated with Kalliope, since such a massive, moon-creating collision should have left a large number of fragments. These fragments would have been ejected during the break-up of the original body, in a stable orbit around the asteroid. They started looking for Kalliope’s family and concluded, that while it was a little difficult to pin down exactly, Kalliope is in fact not a lonely asteroid with a singular companion in the form of Linus, but actually part of a bigger family, dispersed and a little depleted due to age. Running simulations of the orbital and collisional evolution, the authors are convinced that the asteroid family previously associated with the body (7481) San Marcello is actually Kalliope’s family. 

Why was this not figured out earlier? Kalliope is located at a 4-4-1 resonance with Jupiter and Saturn. The resonance causes Kalliope to have a significant offset in eccentricity in comparison to the rest of the family, due to something called “chaotic diffusion”: the resonance of Kalliope with Jupiter and Saturn causes small variations in the proper elements of the asteroid to accumulate slowly over time. Thus, Kalliope is shifted in eccentricity by 0.01 with respect to the family, separating it from the other bodies and thus making it hard to associate it with this particular family (see Fig. 1). However, the semi-major axis is not affected by chaotic diffusion and Kalliope is located close to the center of the family in this regard. 

Fig. 1: Kalliope (indicated by “22”) and its observed family in terms of their proper elements semimajor axis (a_p), eccentricity (e_p) and inclination (sin(I_p)). The geometric albedo is indicated in colors from blue to yellow. Bodies with unknown albedo are shown in gray. Mean motion resonances are indicated by the dashed lines. The large dispersion of eccentricity and inclination angle on the left hand side of the 17:7 resonance to Jupiter is explained by Yarkovsky drift across the resonance. This means, because the “afternoon” side of an asteroid is warmer than the “morning” side of an asteroid (since the latter has cooled over its night time), the radiation pressure differs marginally on these sides. This causes a force to act on the asteroid which may change the asteroid’s speed and orbital parameters. If the asteroid is rotating retrograde (against the direction of its orbit), it is pushed inwards, as can be observed in this plot with many bodies having moved to smaller semimajor axis amplitudes. Source: Figure 2 in today’s paper.

A Turbulent Past

Assuming the parent body was axially symmetric and differentiated after being broken up, the internal structure of the fragments, including Kalliope, would have been highly asymmetric to the extent of affecting the moon’s orbit substantially. There wouldn’t be an orderly layering of lighter elements to heavier elements going from the mantle to the core. The authors predict that the asteroid’s iron core is close to the surface on one side of Kalliope, likely the one flatter in +y-direction (see Fig. 2)

Fig. 2: The shape of (22) Kalliope according to the ADAM (“All Data Asteroid Model”) model, a tool to reconstruct the 3D shape of an asteroid. The two hills in -y direction are visible (indicated by “1” and “2”) as well as the flatter part of the body in +y direction. Source: Figure 7 in today’s paper

Using simulations, the authors conclude that Kalliope’s parent-body was broken up around 900 +/- 100 million years ago by a projectile, which was between 29 and 45 km wide. The mantle was mostly broken apart and ejected and only the dense core material remained in present-day Kalliope, explaining its high density. Interestingly, Kalliope has two very distinct hills, which can be explained very well by reaccumulation of eject fragments, according to the simulations. However, something the authors could not recreate with simulations is a scenario which forms the moon Linus in the shape and form we observe it to be today in a bound orbit to Kalliope. But they have narrowed down the parameter space for the parent body and collision event, which should contain the proper solution for the system’s formation. 

The Legacy of a Giant Impact

The authors suggest further investigation into the family of Kalliope, which can be difficult due to the fragments being rather small. If the average density of the bodies within the family is greatly below that of Kalliope, it would independently confirm the fact that these asteroids formed from the mantle material ejected after the original body was broken up. 

Kalliope’s family may quite literally not be as tight as those of other asteroids in the main belt. However, today’s authors have shown that there is in fact a large number of bodies, which are the legacy of a large collision event, breaking apart Kalliope’s parent body and creating its moon, Linus. This is pretty much the same story we believe to be the explanation for Earth’s moon, in line with the well-accepted statement: “A giant moon requires a giant impact.” Kalliope’s family has drifted apart over the millenia. However, it still tells the story of its past and may even hold further evidence of its origins. 

Astrobite edited by Maryum Sayeed

Image credits: NASA/JPL Caltech, Today’s Paper

About Jana Steuer

I spent almost two years at the LMU Munich, working for the University Observatory (USM), which owns the 2.1m Fraunhofer Telescope Wendelstein. My field of research is exoplanets. I hunt for traces of them in data from big surveys, like the TESS mission and then follow them up, using spectrography and photometry. Mainly, I focus on long period planets that may potentially harbor life. Now, I am a full-time science communicator, working in a public observatory and making content in all its form to tell people about astronomy! I'm a huge Lord of the Rings fan and act as a DM for several Dungeons and Dragons groups. I love cats and do kickboxing in my free time.


  1. Psyche is not the largest asteroid in the Main Belt. It is the 16th largest by diameter and the 10th largest by mass. Ceres is the largest in both categories; if you want to regard it as a dwarf planet, then Vesta is the largest and most massive asteroid in the Main Belt.

  2. You are absolutely correct, it was supposed to say “… second largest M-type asteroid in the Main Belt after (16) Psyche”
    Thank you for the comment, I have corrected the text.


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