Satellites operate in the harsh environment of space, which differs vastly from the environment here on Earth. In space, satellites are exposed to temperature differences that can reach up to several hundreds of degrees Celsius depending on whether they are in a sunlit or non-sunlit area. To ensure they can function properly in such an environment, thermal design is used to control and maintain the temperature of satellites at an appropriate level. But what exactly does this entail?

The thermal environment of space

The biggest difference between space and the Earth is that space is a near-perfect vacuum without any air. On Earth, heat can be dissipated through air convection—for example, using fans or ventilation. However, in space, convection is not possible. In the vacuum that is space, heat must be transferred by thermal radiation and thermal conduction.

In near-Earth space, the thermal environment is mainly influenced by the following four factors:

Among these, factors (1) to (3) vary in intensity depending on the satellite’s position, while (4) remains constant.
For instance, the intensity of solar radiation (factor 1) is approximately: 2 times stronger for the Venus Climate Orbiter "Akatsuki," 10 times stronger for the Mercury Magnetospheric Orbiter “Mio,” and 0.5 to 1.5 times for the Asteroid Explorer "Hayabusa 2."

(Strictly speaking, objects orbiting planets like Earth are called “(artificial) satellites,” while those traveling to the Moon or asteroids are called “probes” or “explorers.”)

Thermal environment of satellites

The orientation of an artificial satellite depends on its mission. For example, a communications satellite always keeps a fixed side (such as an antenna) facing Earth, while an observation satellite constantly points its telescope toward space. As a result, some surfaces may receive continuous sunlight, while others may experience changing exposure. This leads to varying thermal environments across different parts of the satellite. Additionally, satellites contain numerous electronic devices that generate heat. The amount of heat that needs to be dissipated into space depends on factors such as the level of heat generation and the duration of operation for each device.

Thermal design of satellites

The fundamental principles of satellite thermal design are as follows:

The key point here is "appropriately." Since space is at -270°C, excessive cooling is also a risk.
To achieve this balance, satellite thermal design incorporates specialized materials and components.

Verifying the thermal design

To verify the effectiveness of thermal design, engineers conduct simulations using analytical models (thermal mathematical models) and perform thermal vacuum tests that replicate the space environment. However, no test can perfectly reproduce every condition a satellite encounters in space. This makes the approach to testing a crucial challenge for engineers. At NEC, there is a dedicated testing facility large enough to accommodate an entire satellite for thermal vacuum testing.

Even after a satellite has been launched, temperature data is transmitted back to Earth to ensure the appropriate temperature is maintained. In some cases, engineers must recalculate the satellite’s thermal behavior if unexpected orbital or attitude adjustments occur. Thermal engineers continue to monitor and manage the satellite’s thermal performance until the end of its mission.

Future initiatives

The electronic devices installed on artificial satellites are becoming increasingly advanced, leading to higher heat generation and greater component density. Since space lacks air, heat cannot be dissipated through convection. To address this challenge, NEC is developing spacecraft cooling systems that enclose heat-generating components in an insulating coolant and utilize single-phase or two-phase loop mechanisms (with numerous related patents currently filed). By incorporating new technologies, NEC will continue developing numerous satellites and contributing to solving social issues.

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