A thermal control surface for the Solar Orbiter Kevin A.J. Doherty a , James G. Carton b , Andrew Norman c , Terry McCaul d , Barry Twomey b,n , Kenneth T. Stanton a a UCD School of Mechanical and Materials Engineering, University College Dublin, Belfield, Dublin 4, Ireland b ENBIO Ltd., NOVA UCD, Belfield Innovation Park, Dublin 4, Ireland c European Space Agency, ESTEC, Noordwijk, South Holland, Netherlands d Airbus Defence & Space, Stevenage, Hertfordshire SG1 2AS, UK article info Article history: Received 8 May 2015 Received in revised form 1 August 2015 Accepted 11 September 2015 Available online 21 September 2015 Keywords: Solar Orbiter Thermal Control Flat Absorber Heat Shield SolarBlack abstract A high-absorptivity/high-emissivity (flat absorber) bone char-based thermal control surface known as SolarBlack has been developed for use on rigid and flexible metallic substrates, including titanium, aluminium, copper, stainless steel, Inconel and magnesium alloys. This work describes the thermo-optical properties, stability, and qualification of this surface for use on the European Space Agency’s Solar Orbiter mission. SolarBlack is deposited using a proprietry coating technique known as CoBlast and currently stands as the baseline coating for the spacecraft’s front surface heat-shield, which is composed of 50 mm titanium foils (1.3  0.3 m) that have been constructed to cover the 3.1  2.4 m 2 shield. The heat shield makes use of the material’s highly stable ratio of solar absorptance to near-normal thermal emissivity (α s /ε N ) as well as its low electrical resistivity to regulate both temperature and electrostatic dissipation in service. SolarBlack also currently stands as the baseline surface for the High-gain and Medium-gain antennae as well as a number of other components on the spacecraft. The thermo-optical stability of SolarBlack was determined using the STAR Facility space environment simulator in ESTEC., Material characterisation was carried out using: SEM, UV/Vis/NIR spectrometry, and IR emissometry. The coating performance was verified on the Structural Thermal Model using ESA's Large Space Simulator. & 2015 IAA. Published by Elsevier Ltd. All rights reserved. 1. Introduction The Solar Orbiter (SolO) is an M-class mission, currently under collaborative development by the European Space Agency (ESA) and the National Aeronautic and Space Administration (NASA) [1–4]. The mission's primary launch window is set for October of 2018 and aims to answer several questions about our sun and how it produces the heliosphere via in-situ measurements in the inner solar system in conjunction with remote-sensing observation of the Sun and the Corona [4]. The SolO mission summary is outlined in Table 1. The mission had originally been intended to reach perihelion of 0.22 AU [4], however this was increased due to growing concerns surrounding the thermal challenges posed by the mission. Depending on the mission launch date, SolO (depicted in Fig. 1) is now expected to achieve a perihelion of 0.28 AU [3] following multiple Gravitational Assist Manoeuvres (GAMs) at Earth and Venus. SolO is due to launch alongside its American counterpart, the Solar Probe Plus (SPP or SP þ ) (0.06 AU, expected), in 2018, with both spacecraft expected to surpass Helios-2 (0.29 AU [5]) in terms of proximity to the Sun. Though the hostile thermal environment of the mission is with precedent, as evidenced by the Helios mission, new technologies were required to ensure Solar Orbiter’s sur- vival over far longer periods under such conditions. SolO will never reach see the same Solar Flux to be encountered Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/actaastro Acta Astronautica http://dx.doi.org/10.1016/j.actaastro.2015.09.004 0094-5765/& 2015 IAA. Published by Elsevier Ltd. All rights reserved. n Corresponding author. E-mail address: barry.twomey@enbio.eu (B. Twomey). Acta Astronautica 117 (2015) 430–439