20 July 2023


Blog, Engineering, Media Updates, Science, Space

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The 2020s are seeing a surge of major space exploration missions. High profile launches mean that names such as James Webb, Juice and Artemis are known beyond the space, science and engineering communities. However, the success of these missions relies on huge amounts of research and development that often goes unnoticed.

Here, RHEA explains how mission scientists and engineers tackle the challenges presented by the many types of radiation in space, including cosmic rays, planetary radiation belts and solar wind.

Mitigating against various types of radiation is a particularly complicated aspect of space exploration.

The radiation environment varies according to the type of mission (including whether or not it carries a human crew), the destination, the route taken to reach the target and its final orbit. Among the issues that have to be considered are the type and energy of the particles, and their abundancy.

Solar flares and planetary radiation belts

One major source of radiation is the Sun. Solar flares can emit energetic protons, heavy ions and electrons that can penetrate shielding and damage components.

Then there are two radiation belts around the Earth, known as the Van Allen Belts, formed of particles trapped by the Earth’s magnetosphere: an inner one of protons and an outer one which is mainly electrons. Radiation belts are also found around Uranus, Neptune, Mercury, Jupiter and Saturn, and this has to be taken into account for any mission travelling to those planets or passing close by during flybys.

Jupiter’s magnetic field is the strongest in the solar system, making its radiation belts much more dangerous than Earth’s; the most energetic parts of these belts are effectively no-go zones for humans or robotic explorers. Jupiter’s moon Ganymede also has a magnetic field that interacts with Jupiter’s, which complicated the modelling for the Juice mission.

Jupiter and its moons, with lines indicating Jupiter's magnetosphere
Jupiter’s magnetic field is nearly 15 times bigger than the Sun. This traps radioactive particles in ‘belts’, which are thousands of times more intense than those surrounding the Earth. Image © ESA

The threat posed by galactic cosmic rays

Another source of potentially damaging radiation is galactic cosmic rays.

Most are atomic nuclei: protons are most common, but nuclei of elements up to uranium have been detected. Not only does this make them far more massive than electrons, but they move at nearly the speed of light and can have energies up to 10 giga-electron volts.

The complexity of shielding a spacecraft

“Different particles interact with materials in different ways, so the shielding strategies have to be optimized depending on the environment,” notes Marco Vuolo, RHEA Space Environment and Effects Engineer who works at ESA.

“Even one high energy ion can produce a densely ionized track in a semiconductor. This ionization may cause a highly localized effect in the component, such as a glitch in the output, errors in memories and potentially destructive events such as latch-up burnout. In the design we have to take into account all of the potential failure rates for each component for each different effect, and mitigate this by using additional shielding, redundancy and/or radiation tolerant components, or by improving the design.

“Often there is no zero failure option, but we want to keep the rate as low as possible. For this we design according to ECSS [European Cooperation for Space Standardization] standards that set the margins we should adhere to, and all possible failure probabilities have to be included and propagated in the FMECA [failure mode, effects and criticality analysis].”

Catastrophic latch up damage to transistor due to heavy ion
Close-up of a transistor that has catastrophically failed due to a ‘single event effect’ caused by the charge on one heavy ion inducing a ‘latch-up’. Image © ESA

Why lead is not the best shield in space

Finding materials that can efficiently stop the different kinds of particles is not an easy task. For example, the heavy ions from cosmic rays can produce fragments and a shower of secondary particles when they hit ‘high-Z’ (high atomic number) materials like lead. Instead, research shows that lighter elements, such as carbon and hydrogen, are better for shielding due to the ratio between the stopping power and the generation of secondary particles. However, a larger shielding volume would then be required because of the lower density of such materials.

Marco Vuolo explains that one potential approach is to use polymers, for instance polyethylene, which has a high hydrogen content. Another solution that is being researched is to use materials that would provide a secondary function in the spacecraft, perhaps as a structural component, or more generally for the mission, such as water, fuel supplies or Martian/lunar regolith (for surface missions).

Radiation is also an issue for missions that rely on solar panels, which can degrade over time and gradually produce less energy – something that has to be built into the calculations. The solar panels for Juice will have to withstand its very high radiation environment, which necessitated thicker cover glass, impacting the overall mass of the spacecraft.

What is the problem with solar wind?

Heading in the other direction, solar exploration missions, such as Solar Orbiter, or those that head in that direction for flybys, face significant challenges not only from solar flares but also from the ‘solar wind’, which is the result of expanding plasma emitted from the Sun’s corona. This can create a charge on the surface of a spacecraft, including solar panels, which can cause a damaging electrostatic discharge.

Micrometeoroids – major impact

One further problem is micrometeoroids. Depending on their size and speed, they can perforate materials and even change the attitude or rotation of a spacecraft. Just one microparticle impacting solar panels, an exposed pipe or harness can cause a major problem.

Holes in ESA built solar cells retrieved from the Hubble Space Telescope in 2002 caused by micrometeoroids
Holes caused by micrometeoroids in ESA-built solar cells retrieved from the Hubble Space Telescope in 2002. Image © ESA

How do we protect humans in space?

Radiation issues cause other challenges for crewed missions. Particles in galactic cosmic rays, for example, can cause irreparable damage to human DNA, leading to cancer or leukaemia, and the risk factor rises the longer you are exposed. Solar flares are less prevalent, but when they do happen, they can release huge numbers of particles that can lead to acute radiation syndrome.

Providing shielding that can protect humans completely for a trip to Mars would make the mass of a habitation unit so high that it would not be possible to launch it. Such missions will therefore rely on a combination of dosimeters to monitor the ongoing radiation received by each individual and an area with additional shielding where astronauts can shelter when a warning of a solar flare is received. Another option is radiation vests, which are being researched within the Artemis programme. Once on the Moon or Mars, there are proposals to use in situ resources, i.e. regolith, to build shelters.

Main image: © ESA-ATG