The spacecraft thermal control system is responsible for maintaining all spacecraft equipment within the allowed temperature range during all phases of the mission. The equipment falls into two categories:

• Collectively controlled units, for which the heat rejection and heating capabilities are provided by the spacecraft thermal control system
• Individually controlled units, provided with their own thermal control features (coatings, heaters, and insulation, for example) for which the spacecraft design maintains the thermal interfaces within the required limits

The thermal design of Venus Express uses a passive control concept having the maximum possible commonality with Mars Express. However, some system and design modifications have been implemented to cope with Venus's inner orbit and hotter environment.

Since Venus is closer to the Sun than Mars, the heating of the spacecraft due to solar infrared radiation is increased. While the infrared flux due to Venus is far lower than that from Mars (and remains constant), the albedo flux is significant during the operation phase around orbital pericentre. To cope with the harsher thermal environment, the following changes were made in the design of the thermal control system on Venus Express as compared to that on Mars Express:

• Low solar absorptance and low ageing coatings were used wherever possible
• Radiator areas have been increased and their efficiency has been improved by the replacement of Indium Tin Oxide (ITO) Second Surface Mirror (SSM) technology with Optical Surface Radiator (OSR) materials
• Multi-layer Insulation (MLI) efficiency has been improved: the external layer coating is white, as opposed to black, and a twenty three layer blanket is used
• Surface treatment of the Launch Vehicle Adapter (LVA) was changed to minimise the LVA ring temperature when Sun illuminated
• Heat pipes are used to connect the Power Control Unit and Power Distribution Unit to an enlarged radiator
• OSR materials are used on the rear of the solar array panels and between the cells on the front of the panels

An additional design change was made to take account of the fact that Venus is an inferior planet (its orbit lies inside that of the Earth), while Mars is a superior planet. For a spacecraft in orbit around Mars, the Earth vector is always within ± 40 degrees of the Sun vector, which allows the spacecraft cold face to be kept pointed away from the Sun during Earth communication. In orbit around an inner planet such as Venus, it is not possible to perform Earth pointing of a single, fixed antenna for all planetary configurations while keeping the spacecraft cold face in permanent shadow. To overcome this problem, a second High Gain Antenna has been added to Venus Express for communication when the spacecraft is in the part of its orbit near inferior conjunction (closest to the Earth).

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