Potential New Fission Cluster?: (23637) 1997 AM6, (580635) 2015 CG73, and 2015 BC618

Orbital data

                             a            e          i         om          w
580635 (2015 CG73)  2.30532288 0.0834925047 7.85348058 101.326491 314.803897
 23637 (1997 AM6)   2.30528533 0.0834721545 7.85472149 101.329196 314.770434
       (2015 BC618) 2.30526674 0.0834717493 7.85456627 101.329981 314.775063

read from JPL Horizons

Orbital similarity
Same-epoch osculating elements (epoch=2461000.5) are extremely similar, and pairwise Drummond (1981) D-criteria are very small:

(23637) 1997 AM6 – (580635) 2015 CG73: D_D ≈ 1.2e-4

(580635) 2015 CG73 – (2015 BC618): D_D ≈ 1.2e-4

(23637) 1997 AM6 – (2015 BC618): D_D ≈ 5.0e-6

Mercury6 backward simulation

)O+_06 Integration parameters  (WARNING: Do not delete this line!!)

) Lines beginning with `)' are ignored.

)---------------------------------------------------------------------

) Important integration parameters:

)---------------------------------------------------------------------

 algorithm (MVS, BS, BS2, RADAU, HYBRID etc) = BS

 start time (days)= 2461221.5

 stop time (days) = -1e8

 output interval (days) = 100

 timestep (days) = 0.05

 accuracy parameter=1.d-12

Backward integration results

Nominal backward integrations taking into account all planets + Ceres + Pallas + Vesta (gravity only; no Yarkovsky) show two distinct pair-like minima:

• (23637) 1997 AM6 – (580635) 2015 CG73: convergence at ~ -46.6 kyr, V_rel ≈ 0.22 m
• (580635) 2015 CG73 – (2015 BC618): convergence at ~ -24.9 kyr, V_rel ≈ 0.18 m/s

 

However, the direct integration of (23637) 1997 AM6– (2015 BC618) does not show a comparable young convergence, finding only a broad higher-velocity minimum near -245 kyr (V_rel ~ 1.5 m/s).

Discussion of origins

two possible interpretations:

 

Scenario 1: Cascade / intermediate parent
(23637) 1997 AM6 splits -> "Object A", which later splits almost in half -> (580635) 2015 CG73 and 2015 BC618.

This matches the nominal timeline (primary event at ~ -46 kyr, secondary event at ~ -25 kyr) and is consistent with the nearly identical H magnitudes of the two smaller bodies.

Difficulty: it is dynamically counterintuitive that (580635) 2015 CG73 is offset from the primary (D_D ~ 1e-4) while 2015 BC618 remains essentially "on top of" the primary (D_D ~ 1e-6). In this scenario, both fragments would be expected to inherit the dispersion/drift of the intermediate parent ("Object A"), so it is not obvious how 2015 BC618 ended up with an osculating orbit so close to (23637) 1997 AM6.

In spite of this, (580635) 2015 CG73 can be traced back to the primary in the nominal integration, while this is not possible for 2015 BC618.

 

Scenario 2: Repeated fission from the primary
(23637) 1997 AM6 ejected both fragments directly in separate events.

This naturally explains why 2015 BC618 matches the primary so closely (it could be a very recent fragment), while the ~ -46 kyr convergence for (23637) 1997 AM6 - (580635) 2015 CG73 would represent a distinct older event.

Difficulties:

(1) it is surprising that the gravity-only integration fails to link (23637) 1997 AM6 and 2015 BC618 at a young epoch

(2) The two small bodies have nearly identical H (and thus may be similar in size if their albedos are comparable) : a curious coincidence for two indipendent ejection events.

 

Conclusion
At present, neither interpretation is fully conclusive.

I would welcome any comments on the dynamical picture.

Are there any physical observations (colors/spectra/albedo/rotation) available for these bodies?