In December 2025, BepiColombo will be captured in a polar orbit around the planet Mercury. As Europe’s first voyage to the solar system’s innermost planet, the mission will provide key data relating to Mercury’s weak magnetic field, as well as the first high-resolution images of suspected water-ice deposits in its northern polar region. The mission consists of three spacecraft: the Mercury Transfer Module (MTM) and Mercury Planetary Orbiter (MPO) developed by ESA, and the Mercury Magnetospheric Orbiter (MMO) developed by JAXA.
To survive the extreme thermal environment around Mercury, the MPO combines an intricate patchwork of high-temperature MLI with a network of almost 100 heat pipes. These are then coupled to a radiator that is itself shielded by an array of specially-shaped titanium fins. This carefully crafted thermal control system operates at the edge of material capabilities, allowing the spacecraft to endure extreme temperatures as high as 450 ºC.
On the 12th November 2021 - three years after launch - a mission-critical component on-board the MPO experienced a sudden step-wise temperature increase only weeks after its first Mercury flyby at 199 km. This anomaly has been traced to the partial loss-of-performance of a pair of heat pipes which would ordinarily transfer heat dissipation away from the component to the radiator panel, for rejection to space. Whilst partial loss of performance of the heat pipes was the cause of the initial temperature increase, their complete failure would cause the mission-critical component to exceed its operating temperature limit and thereby threaten the mission. It was deemed an urgent task to determine the cause of the anomaly to establish whether or not the mission was at risk.
After a number of flight-spare heat pipes — manufactured over 10 years ago in preparation for launch — were located and delivered by the manufacturer to ESTEC, I was assigned to perform a test campaign aiming to establish the reason for the anomaly. This involved building and operating three separate test rigs for non-condensable gas measurement and performance testing of the flight-spare heat pipes, and regular delivery of test results to the BepiColombo investigation committee involving representatives from the European Space Operations Centre (spacecraft operator), Airbus (spacecraft prime), and the heat pipe manufacturer.
Because of Non-Disclosure Agreements and the Intellectual Property Rights of the manufacturer, there is not much I can explain here about the work conducted. But a paper is currently in preparation for publication, and I can share a few pictures and videos.
Performance test set-up
Test data showing reproduction of sudden loss of performance phenomenon as suspected to have occurred in flight
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Preparing the non-condensable gas measurement set-up
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Non-condensable gas measurement set-up in Mechanical Systems Laboratory, ESTEC
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Unconventional approach to evaluating effect of tilt on heat pipe performance!