Polarimetric Characterisation of Minor Planets with VSTPOL

VSTPOL, the new polarimetric mode of the OmegaCAM 1 square degree imager at the VLT Survey Telescope is planned to be commissioned in December 2026 . The polarimetric mode of the VST has a 1 square degree Field of View with a 2.5 meter diameter primary mirror and an expected systematic instrumental polarization controlled to about 0.1%. VSTPOL aims to improve one or more of the limitations of current polarimeters: (i) limited field of view; (ii) small light collecting area; (iii) difficult calibration of (spatially varying) instrumental polarization. 

Near-Earth Asteroids, main-belt asteroids, Jupiter Trojans, Centaurs, Transneptunian objects and comets have shown to have linear polarization levels. These can vary between subpercent to few percent at small phase angles (up to order ten degree) and increase to values of up to 10%-30% at 90 degrees. Decreasing again for larger phase angle. The details of the polarization phase curve (e.g., the value of the phase angle at which the polarization changes sign, and the slopes of the curve) depend on the topology and structure (e.g., craters), size and structure of regolith / dust particles and the refractive index of the polarizing materials. 

The combination of low systematic floor in the calibration and the large collecting area of VSTPOL has the potential to characterize the details of the polarization phase curve even for distant minor bodies (i.e., small range of phase angles) by achieving systematic errors well below the measured signal.  

In case of rocky bodies such as asteroids, one of the quantities that can be determined with polarimetric techniques is the geometrical albedo of the object, which is proportional to the slope of the polarization phase curve. Theoretical modeling and lab experiments indicate that the variation in the physical mineral composition of asteroids leads to average refractive indices in the optical range that vary in the range ∼1.6-4 and can vary as a function of optical wavelength. The wide field of view opens the possibility to get sufficiently large samples of diverse populations to establish statistically the diversity and demography of the types of weathered surfaces on especially asteroids, Near-Earth Objects, Centaurs and Transneptunian objects. 

For comets the detailed features of the polarization phase curves depend on the composition and size of the dust in conglomerates averaged over the inner regions of the comet coma. The wide field of view allows us to follow the evolution of the dust structure in the comet coma and in the comet tail. This could represent a game changer in the field. Specifically, VSTPOL could contribute to help with target identification and with the follow up with ground based observations during the encounter of ESA’s Comet Interceptor mission (to be launched around 2029) when it obtains observations from within the comet coma, using its instrument ENViSS which has polarimetric capabilities.

By combining VST’s optical polarimetric results with optical and thermal infrared photometry plus optical/near-IR spectroscopy, one can lift degeneracies between object size and albedo to provide definitive constraints on surface topology, physical composition and object size. This to increase our understanding of the evolutionary path of the minor body populations since their formation. 

In this talk we describe in more detail these anticipated advances in minor body characterization to be achieved with survey programs with VSTPOL. We also describe the instrument design and the calibration approaches to achieve the subpercent polarization measurements across 1 square degree field of view.