Optical-to-NIR magnitude measurements of the Starlink LEO Darksat satellite and effectiveness of the darkening treatment

Abstract

Aims. We aim to measure the Sloan r′, Sloan i′, J, and Ks magnitudes of Starlink’s STARLINK-1130 (Darksat) and STARLINK-1113 low Earth orbit (LEO) communication satellites and determine the effectiveness of the Darksat darkening treatment from the optical to the near-infrared (NIR). Methods. Four observations of Starlink’s LEO communication satellites, Darksat and STARLINK-1113, were conducted on two nights with two telescopes. The Chakana 0.6 m telescope at the Ckoirama observatory (Chile) observed both satellites on 5 Mar. 2020 (UTC) and 7 Mar. 2020 (UTC) using a Sloan r′ and Sloan i′ filter, respectively. The ESO VISTA 4.1 m telescope with the VIRCAM instrument observed both satellites on 5 Mar. 2020 (UTC) and 7 Mar. 2020 (UTC) in the NIR J-band and Ks-band, respectively. Results. The calibration, image processing, and analysis of the Darksat images give r ≈ 5.6 mag, i ≈ 5.0 mag, J ≈ 4.2 mag, and Ks ≈ 4.0 mag when scaled to a range of 550 km (airmass = 1) and corrected for the solar incidence and observer phase angles. In comparison, the STARLINK-1113 images give r ≈ 4.9 mag, i ≈ 4.4 mag, J ≈ 3.8 mag, and Ks ≈ 3.6 mag when corrected for range, solar incidence, and observer phase angles. The data and results presented in this work show that the special darkening coating used by Starlink for Darksat has darkened the Sloan r′ magnitude by 50%, Sloan i′ magnitude by 42%, NIR J magnitude by 32%, and NIR Ks magnitude by 28%. Conclusions. The results show that both satellites increase in reflective brightness with increasing wavelength and that the effectiveness of the darkening treatment is reduced at longer wavelengths. This shows that the mitigation strategies being developed by Starlink and other LEO satellite operators need to take into account other wavelengths, not just the optical. This work highlights the continued importance of obtaining multi-wavelength observations of many different LEO satellites in order to characterise their reflective properties and to aid the community in developing impact simulations and developing mitigation tools.

Plaskett 1.8 metre Observations of Starlink Satellites

“…We, unfortunately, cannot confirm that the satellites indicated as Starlink-Vs do indeed have visors or that the visors are correctly deployed. Starlink-1436 is the first visorsat (e.g., see discussion in Tregloan-Reed et al 2021), placed on orbit as a test case and widely discussed on social media. Additional Starlink satellites were launched without visors until the tenth Starlink mission, which according to SpaceX (SpaceX 2020), included deployable visors for all 57 Starlink satellites on board the 6 August 2020 Falcon 9 launch.…”

“…Several groups have reported on LEO satellite magnitudes under various conditions (Tyson et al 2020;Tregloan-Reed et al 2021;Mallama 2021a). This includes efforts to focus on specific satellites to assess mitigation attempts, such as 'DarkSat' and 'Visorsats', representing albedo and reflection modifications, respectively.…”

Analysing the Impact of Satellite Constellations and ESO’s Role in Supporting the Astronomy Community

“…These results showed that a) the reduction in brightness for DarkSat was 50.8% ± 3.5%, b) the brightness magnitude of DarkSat (6.10 magnitudes) was still within the range of the naked-eye limiting magnitude of an experienced observer (Bortle, 2001) and c) DarkSat was still brighter than the limiting magnitude of 7 that has been recommended to minimise unwanted effects in sensitive astronomical cameras (Walker et al, 2020;IAU 2020). Subsequent observations of these satellites were conducted in various spectral bands to assess their brightness and the effectiveness of the darkening treatment used in DarkSat, in various astronomical spectral bands from visible to near-infrared (Tregloan-Reed et al, 2021). The results of these observations, conducted using the Chakana telescope (Char et al, 2016) and VIRCAM (Emerson, McPherson & Sutherland, 2006;Dalton et al, 2006) on the VISTA telescope, are summarised in Figure 4.…”

Short timescale imaging polarimetry of geostationary satellite Thor-6: The nature of micro-glints

“…Most of the current efforts have focussed on obtaining (multi-colour) broadband flux lightcurves (e.g. Horiuchi et al, 2020;Tregloan-Reed et al, 2021;Mróz et al, 2022), and basic models have been created to evaluate expected brightness as a function of sun-observer-satellite angle (e.g. Hainaut & Williams, 2020;Mallama, 2020;Cole, 2021;Bassa et al, 2022;Lawler et al, 2022).…”