Shape modelling contact binaries 2024 ON and 2000 RS11 with ground-based radar

Introduction

Bilobed objects (contact binaries) appear in both asteroid and comet populations with a bimodal mass distribution and bifurcated shape. They are common in both near-Earth asteroid populations, estimated from radar imaging to be near 30% contact binaries [1], but also further out in the Kuiper belt, where optical observations suggest that upwards of 40-50% of Plutinos could be bilobate or elongated in shape [2,3]. Additionally, spacecraft imaging and radar observations of comet nuclei, while few in number, suggest that the majority are contact binaries.

The NASA Lucy mission discovered and imaged Selam, moon of (152830) Dinkinesh and the first known contact-binary satellite orbiting another asteroid in 2024 [4]. In April Lucy performed a flyby of its second target, asteroid (52246) DonaldJohanson, confirming the presence of another contact binary in the main asteroid belt. Now, 24 known contact binary objects have been imaged by spacecraft or modelled using radar to confirm their shapes [5]. Notable examples of these objects include (25143) Itokawa (NEA), visited by the Hayabusa mission; (486958) Arrokoth (KBO), visited by the New Horizons mission; and 67P/Churyumov–Gerasimenko, visited by the Rosetta mission.

The current sample of known contact binaries can be visually categorised into different types of objects, with the level of bifurcation varying dramatically between objects such as (4179) Toutatis and (388188) 2006 DP14. By modelling more of these objects, we can investigate whether particular morphologies are more prevalent, giving more insight into the methods behind contact binary formation.

Targets

We are modelling two targets with radar and optical data to investigate the differing shapes and morphologies within the sample of modelled contact binaries.

We are refining the shape model of asteroid (275677) 2000 RS11, henceforth RS11, which was preliminarily modelled in 2014 with only radar data [6], with the inclusion of the optical lightcurve data, for which the current model does not strongly agree. RS11 appears to have a ‘rubber duck’ shape similar to 67P, with the smaller lobe nearer to the other's shortest principal axis. This contrasts with most of the other modelled contact binaries, which tend to have their lobes aligned with the longest principal axis of the larger lobe. The data used to model RS11 are the same archival radar observations from the Goldstone DSN antenna and Arecibo collected during the asteroid's close approach in 2014 and used in the previous modelling. We combine these with optical lightcurves collected in 2014 and one lightcurve collected in 2023 with the Danish telescope at La Silla. The addition of the 2023 lightcurve allows for a better constraint on the target's rotational period and provides a different viewing geometry to help define the rotational pole solution.

We are also shape modelling asteroid 2024 ON using optical and radar data collected over a period of 3 months in late 2024, when it was discovered. This object is therefore a good analogue for possible planetary defence efforts, such as the recent concern over 2024 YR4, which demonstrated that potential impactors may only be known to us one or two apparitions before impact. The capability to quickly model the shape of an asteroid and determine estimates of additional properties like composition and bulk density is relevant to planning any planetary defence missions, such as the NASA DART mission. Despite the data for this target concentrated in one close approach of the target, the object was discovered before its closest approach, so a wide variety of viewing geometries could be collected in a short period. Some example radar images of this target are displayed in Fig. 1.

Fig 1. Radar observations collected on the 16th of September 2024 at the Goldstone DSN antenna of asteroid 2024 ON during its close approach.

Modelling

To create a radar shape model, we first use the optical data to constrain the spin state of the object. Using convex inversion [7,8], we can estimate the periods of 2000 RS11 and 2024 ON to be 4.4456 +/- 0.0005 h and 6.014 +/- 0.001 h, respectively (estimates of RS11’s period in 2014 were 4.444 +/- 0.001 h). With a strong initial period, we use the SHAPE modelling software to combine the optical and radar observations [9]. We created simple bi-ellipsoid models and performed fits at different fixed rotational pole solutions to investigate the rough location of the rotation pole before refining this with higher-resolution pole scans. Once the rotational pole is well defined, we create a vertex shape model for the best solutions to allow the model to replicate more complex features in the data. Preliminary results suggest that both objects have a pole solution in the southern hemisphere, as is common among near-Earth asteroids. With the addition of the optical lightcurves for 2000 RS11, the pole solution is likely closer to the southern pole of the ecliptic than previously thought. This emphasises the need for optical observations to be used to support radar modelling.

Conclusion

We are refining the shape model of RS11 and creating a new shape model for the recently discovered 2024 ON using a combination of optical and radar data. Using the SHAPE modelling software, we will model the spin state and morphology of both objects to investigate how they compare to previously modelled contact binaries and the implications they hold for their formation histories and the population as a whole.

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