Space is an environment that’s very different from Earth, which means satellites need to be constructed carefully to survive and operate there. In today’s post, we’re going to look at some of the problems satellites face in space, and at how these problems are tackled.
Here are a few of the challenges that you need to bear in mind when you’re building a satellite:
We’ve already talked about the space debris problem in our posts ‘What is space debris?’ and ‘How are satellites powered?’, so we’ll look at the other issues here.
A satellite’s orientation in space is also called its ‘attitude’, and attitude is crucial. For example, a satellite might need to face Earth in order to take photographs, while keeping its solar panels facing the sun whenever possible. If it’s facing the wrong direction, it can’t do its job or keep itself powered.
Fortunately, satellites can use sensors to tell them what direction they’re facing in space. For example, they can use infrared sensors to detect the location of Earth, or use sensors containing solar cells to keep track of the sun. With the use of these sensors, a satellite can ensure it’s facing the correct direction at all times.
Because it’s difficult to refuel a satellite, it’s best if a satellite can adjust its attitude without the use of fuel-powered thrusters. This may be done using reaction wheels: rotating wheels set at different angles on the satellite. Increasing or decreasing the speed of a wheel will change the rotation of the satellite. Because these wheels are controlled by an electric motor, they can be operated using plentiful solar energy rather than limited fuel.
Without the protection of Earth’s atmosphere, high temperatures become higher and low temperatures become lower. The moon, for example, can swing from day temperatures of over 100°C to night temperatures of below -100°C. This means satellites need to be able to withstand conditions that most Earth-based machinery will never encounter.
Space doesn’t experience temperature in the same way the Earth does, because there aren’t enough air molecules close together in space to transmit heat, in the same way there aren’t enough to transmit sound. This means conduction and convection, which are both heat transfer mechanisms that involve particles colliding, aren’t possible in space.
However, radiation involves electromagnetic waves rather than particles, which means heat can still travel through a vacuum in the form of radiation. This is why the heat of the sun is able to reach Earth, despite having to pass through the vacuum of space first. In other words, although the satellite isn’t surrounded with air for the sun to heat up, the sun’s radiation can still connect with the satellite and heat it directly.
You might have noticed that, in pictures, satellites sometimes look like they’re covered in some sort of shiny, crinkly material. These are multi-layer insulation (MLI) blankets, and their job is to reduce heat loss from inside the satellite and reflect back heat radiated from the sun. In this way, they simultaneously help keep the satellite from getting too warm and help keep it from getting too cold.
The Parker Solar Probe is a satellite that was designed with heat protection strongly in mind, as it orbits the sun on a path that takes it through the sun’s upper atmosphere. It withstands heat by using a coolant circulation system and a thick but lightweight carbon-based heat shield called the Thermal Protection System (TPS), preventing most of the satellite from heating up. You can see the TPS demonstrated in this video from NASA.
Another interesting element of the Parker Solar Probe is the design of the Solar Probe Cup, a Faraday cup used to measure details of the solar wind. As the Solar Probe Cup is not protected by the heat shield, it needed to be designed to survive very high temperatures by itself. Because of this, it was built using materials with high melting points; for example, the grids that produce the cup’s electric field were made from tungsten, with a melting point of nearly 3,500°C. However, this created new challenges; the high melting point of the grids made it impossible to draw gridlines on them using lasers, so these lines were drawn with acid instead.
Cosmic radiation can have health effects for humans, so radiation shielding is hugely important for the International Space Station and any other satellites that people might inhabit.
However, uncrewed satellites also need to be protected from radiation in order to, for example, preserve delicate machinery or control testing conditions. Radiation can cause crashes, memory changes or other issues in computers and other electronics, which is a problem if you rely on electronics to keep your satellite functioning.
The electronics used in satellites are often radiation-hardened. ‘Radiation-hardened’ is a slightly misleading phrase, as it suggests that they’ve been hardened through exposure to radiation. In fact, radiation-hardened electronics are just electronics that have been designed to resist radiation damage. Radiation exposure is used to test radiation-hardened electronics, but it’s not part of the process of making them radiation-resistant.
For example, a radiation-hardened device may have multiple memory backups that can be checked against each other. If one backup differs from the others, it may have been altered by radiation, and it can be corrected by changing it to match the other backups. The Orion spacecraft used in the Artemis I launch had four sets of flight computers to handle the possibility of radiation issues.
Instruments can also be physically shielded against radiation to some extent, for example by using layers of metal. In addition to using up resources in the shield itself, however, adding shielding to a satellite will make the satellite heavier, meaning more fuel will be needed to get it into orbit.
On Earth, if you have a vacuum inside a container and you create a hole in that container, air will rush to fill the vacuum. This can create the impression that the vacuum pulls on the air, dragging it into the container. If you conclude from this that a vacuum pulls on whatever’s around it, you might expect the vacuum of space to pull apart whatever’s there.
In fact, the vacuum doesn’t pull at the air at all. The air is pushed into the vacuum by the pressure of the air surrounding it.
It’s similar to opening a hole in the side of a tank of water. If you do this, water will immediately start to pour out of the side of the tank. However, this obviously isn’t because the air outside the tank is pulling on the water; the water is being pushed out of the hole by the pressure of the rest of the water in the tank.
Because of this, a satellite in space won’t be pulled apart by the vacuum. Even on Earth, a metal box can contain a vacuum without collapsing. A less sturdy container, such as a plastic bottle, might collapse, but that’s because the air is pressing in from the outside without any air inside the bottle to counteract it; it’s not because the vacuum is pulling on the bottle. It’s similar to the way a plastic bottle filled with air might collapse underwater because of the weight of the water outside. In both cases, the bottle is pushed inwards by external pressure, rather than being pulled by its contents.
This doesn’t mean you can ignore the fact that your satellite will be operating in a vacuum, of course. The fact remains that the vacuum of space creates different conditions from the atmospheric pressure of Earth, so you’ll want to take it into account when building satellites. For example, satellite materials can give off gas in a vacuum, a process called ‘outgassing’, which can cause problems if the gas then condenses on lenses or sensitive components.
It’s important to make sure satellites are well prepared for the challenges of space. If a satellite fails after launch, it could become space debris, ultimately posing more problems for other satellites. Both NASA and ESA operate large vacuum chambers that can be used to test satellites before they’re launched.
Cover image: NASA
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