Thermophysical model of asteroid (269) Justitia: a main-belter out of place

Context
Asteroid (269) Justitia is a main-belt asteroid with unusual surface properties. Its reflectance spectrum is extremely red, unlike any other taxonomic type found in the main belt, and not even within the Jupiter Trojan population (Hasegawa et al. 2021). However, such a spectrum closely resembles spectra of trans-Neptunian Objects. Also polarimetric properties of (269) Justitia are unusual (Gil-Hutton & García-Migani 2017), with both very small depth of negative polarisation branch and also small angle of minimum polarisation. These findings suggest Justitia origin within the trans-Neptunian population, making it an easy-reach target for a space mission, MBR Explorer (El-Maarry et al. 2023, Filacchione et al. 2023).

Aims
In preparation to the mission a possibly most detailed knowledge of the target properties like spin, shape, size, and properties of the surface are essential. For these aims a detailed thermophysical model of Justitia was created.

Methods
By a fortunate coincidence asteroid Justitia has been on our list of Slow Rotators since a few years already, when the MBR Explorer mission was announced (see Marciniak et al. 2015 and 2021 for the project description and example results). As such, it has been observed for lightcurves by our team in each apparition since the year 2019. These data combined with literature lightcurves enabled obtaining a unique model using lightcurve inversion (Kaasalainen & torppa 2001, Kaasalainen et al 2001). Next, these dense lightcurve data have been combined with sparse data from ATLAS survey (Tonry et al. 2018) and thermal data from three infrared missions (IRAS, AKARI and WISE). We used all these data as an input to simultaneous optimisation using Convex Inversion Thermophysical Model (CITPM, Durech et at. 2017).

Results
We obtained a thermophysical of this asteroid that well fits all types of data, both in the visible and in the infrared range. Thanks to high quality data the shape model is smooth (Fig. 1), and obtained parameters reliable (Tab. 1). Together with the shape model we determined spin axis orientation, sidereal period, thermal inertia and size. Our model of Justitia was confirmed by an extensive stellar occultation campaign in August 2023 (Buie et al., in prep.).

Fig. 1. Shape model of asteroid (269) Justitia from simultanous fit to visible lightcurves and thermal data. Solution for pole 1. Two equatorial views on the left and in the middle, pole-on view on the right.

Model prameter	value
λp	73°+/- 11°
βp	-81° +/- 15°
Psid	33.12962 +/- 0.00001 h
Deq	58 +/- 2 km
pV	0.058 +/- 0.006
Γ	41+110-40 SIu

Tab. 1. Parameters of the model: spin axis longitude (λ_p) and latitude (β_p), sidereal rotation period (P_sid)_, diameter of equivalent surface sphere (D_eq)_, geometric albedo (p_V)_, and thermal inertia (Γ). All the parameters are for pole solution 1, mirror pole not shown here.

 

Acknowledgement
This work was supported by the National Science Centre, Poland, through grant no. 2020/39/O/ST9/00713.

References

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