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English: The ESA-JAXA BepiColombo mission to Mercury has been firing its electric propulsion thrusters since December in the first of 22 ‘thruster burn arcs’ that will steer the spacecraft to its destination. While taking routine images with the Mercury Transfer Module (MTM)’s solar array-pointing camera before and after firing the thrusters, an interesting – but expected – effect has been observed.

Close and careful inspection of the image pair – this set taken on 27 December 2018 – reveals a slight ‘bending’ of the 15 m-long solar array, linked to the before and after temperature of the array during the operation. Little power is drawn from the large solar arrays when not thrusting, but once the operation begins, and with power consumption increasing, the temperature of the four outermost panels decreases. The temperature for the panel closest to the spacecraft (and camera) doesn't change, as power is drawn from it already when not thrusting. As the front and the back side of the array have different properties when it comes to thermal expansion, and the temperature difference between ‘off’ and ‘on’ amounts to 10-20ºC, this leads to a slight bending of the array, estimated to be approximately 2.8 cm at the tip – corresponding to about two pixels in the image.

This effect is regularly observed, because, in general, there is a weekly interruption of thrusting to gather data for orbit determination. Orbit determination for deep space missions requires highly precise data – and for BepiColombo, accurate knowledge of the orbit is key to knowing when to stop the thrust arc (the current thrust phase is planned to be completed mid-February). But since firing the thrusters results in ‘noisy’ doppler and ranging data needed to determine the orbit, they have to be temporarily switched off. In addition, it is sometimes the case that the orientation of the spacecraft during thrusting means that the high-gain antenna – needed to download the data – is not pointing at Earth. So a weekly interruption is also needed to send the data back to home.

The transition to electric propulsion mode can take up to six hours and requires about 650 commands, so this weekly stop-start makes operations during a thrust arc very demanding.

The images in this pair were taken a few hours apart: the first once the solar array was moved to the position required for firing the thrusters – they point directly towards the Sun to soak up the maximum amount of solar energy to power the thrusters – and the second after electric propulsion was initiated.

BepiColombo will complete a series of 22 thruster arcs and a total of nine gravity assist flybys at Earth, Venus and Mercury in order to reach the innermost planet in 2025. The MTM is carrying two science orbiters: JAXA’s Mercury Magnetospheric Orbiter and ESA’s Mercury Planetary Orbiter, which together will study the planet from its internal structure to surface processes, to its interaction with the solar wind.

BepiColombo is a prime example of the long-term planning needed to ensure technological and scientific return in the future. The mission was conceived in the 1990s, launched two decades later, and will arrive at its destination during the 2020s. The scientists and engineers dedicated to the mission so far have invested in the future of the next generation. Some of the technologies developed for BepiColombo are already being used for ESA’s Solar Orbiter mission launching next year. Furthermore, the science that BepiColombo will return in the next decades will also provide context for understanding the formation of solar systems beyond our own. This rapidly evolving field of exoplanet studies will see ESA launching three dedicated missions – Cheops, Plato and Ariel – in the next decade to build on answering the fundamental question at the heart of our Solar System exploration missions: what are the conditions for planet formation and the emergence of life?