Dr. Rabus, Markus

Contact binaries are found throughout the solar system. The recent discovery of Selam, the satellite of MBA (152830) Dinkinesh, by the NASA LUCY mission has made it clear that the term ‘contact binary’ covers a variety of different types of bi-modal mass distributions and formation mechanisms. Only by modelling more contact binaries can this population be properly understood. We determined a spin state and shape model for the Apollo group contact binary asteroid (388188) 2006 DP14 using ground-based optical and radar observations collected between 2014 and 2023. Radar delay-Doppler images and continuous wave spectra were collected over two days in February 2014, while 16 lightcurves in the Cousins R and SDSS-r filters were collected in 2014, 2022 and 2023. We modelled the spin state using convex inversion before using the SHAPE modelling software to include the radar observations in modelling concavities and the distinctive neck structure connecting the two lobes. We find a spin state with a period of (5.7860±0.0001) hours and pole solution of 𝜆 = (180±121)◦ and 𝛽 = (−80±7)◦ with morphology indicating a 520 m long bi-lobed shape. The model’s asymmetrical bi-modal mass distribution resembles other small NEA contact binaries such as (85990) 1999 JV6 or (8567) 1996 HW1, which also feature a smaller ‘head’ attached to a larger ‘body’. The final model features a crater on the larger lobe, similar to several other modelled contact binaries. The model’s resolution is 25 m, comparable to that of the radar images used.

Between 2010 and 2017, we have collected new optical and radar observations of the potentially hazardous asteroid (2102) Tantalus from the ESO NTT and Danish telescopes at the La Silla Observatory, and from the Arecibo planetary radar. The object appears to be nearly spherical, showing a low-amplitude light-curve variation and limited large-scale features in the radar images. The spin-state is difficult to constrain with the available data; including a certain light-curve subset significantly changes the spin-state estimates, and the uncertainties on period determination are significant. Constraining any change in rotation rate was not possible, despite decades of observations. The conv e x light curv e-inv ersion model, with rotational pole at λ = 210 ◦ ± 41 ◦ and β = −30 ◦ ± 35 ◦, is more flattened than the two models reconstructed by including radar observations: with prograde ( λ = 36 ◦ ± 23 ◦, β = 30 ◦ ± 15 ◦), and with retrograde rotation mode ( λ = 180 ◦ ± 24 ◦, β = −30 ± 16 ◦). Using data from WISE , we were able to determine that the prograde model produces the best agreement in size determination between radar and thermophysical modelling. Radar measurements indicate possible variation in surface properties, suggesting one side might have lower radar albedo and be rougher at the centimetre-to-decimetre scale than the other. However, further observations are needed to confirm this. Thermophysical analysis indicates a surface co v ered in fine-grained regolith, consistent with radar albedo, and polarisation ratio measurements. Finally, geophysical investigation of the spin-stability of Tantalus shows that it could be exceeding its critical spin-rate via cohesive forces.