“This is space. It does not cooperate.” This quote – by stranded astronaut Mark Watney in The Martian – encapsulates the challenge of the new era of space industry and the NASA-led journey back to the Moon and on to Mars, known as Project Artemis.
The Martian demonstrated – at least through literature and cinema – that the farther a spacecraft travels in space, the less opportunity there will be for the crew to repair, refuel or regulate that craft. Watney had to overcome several roadblocks to eventually get back to Earth, particularly by utilising available resources and tools in some unconventional ways to survive on Mars.
Sure, it was only a movie, but it illustrated the extreme measures required to mitigate the rigours of space travel for a crew to complete their mission and get back home safely. So why was it so difficult for Mark Watney to survive, and why is it so challenging for us to build for distant space?
Withstanding the extremes of space
Fundamentally, space is a realm of extremes, from temperature – with hot and cold cycles of +288°C to -101°C – to ultra-high vacuum (i.e. effectively zero pressure) environments. Then there are the incredibly fast micro-meteorites – essentially tiny grains of space dust travelling at about 72 km/second, tens of times faster than the speed of sound on Earth. Materials used for spacecraft and space travel have no protective layer of atmosphere available to shelter them. If a micro-meteorite collides with the surface of a spacecraft at hyper velocities, it can lead to microcracks in space materials and eventually the failure of parts of the craft. For instance, the International Space Station (ISS) has reported incidents of flying tiny paint flakes causing damage to windows.
To protect against such debris, shielding materials can be added to spacecraft. The Gemini and Apollo missions employed a heat shield made of fibreglass honeycomb filled polymer resin, whereas SpaceX’s Dragon is reported to use resin-impregnated carbon fibre as its heat shield material. Multicomponent impact-resistant materials have also been developed, consisting of mounted aluminium and bulletproof Kevlar layers bonded with resins to reduce the penetration of small debris. However, repairing these multi-component designs has always been an issue. In many cases, the resins or polymer binders used in such composites or coatings are permanently set, so any damage cannot be reversed or repaired. The only solution is replacement of the whole structure, which is too expensive and can be challenging in space.
Fibreglass honeycomb filled polymer resin: Let’s break this one down a bit! A polymer is a substance made from many repeating subunits bonded together. In this case, the polymer forms a resin, a type of viscous liquid that becomes solid under UV or heat, and is mixed with fibreglass, a kind of glass-reinforced plastic. The honeycomb refers to the regular physical structure of the material (see image).
Resin-impregnated carbon fibre: ‘Carbon fibre’ here refers to fabrics containing tiny, hair-like fibres made out of carbon. Carbon-fibre-reinforced polymers are plastics reinforced with tiny carbon fibres, somewhat analogous to fibreglass (see above). ‘Impregnating’ the polymer with resin makes a more durable material.
Space materials to take us to Mars
The new era of space exploration will see human crews undertake the same interplanetary journey as The Martian in a mind-blowing translation of science fiction concepts into reality.
At its minimum distance from Earth – known as Mars Close Approach – Mars is about 62 million kilometres away. That’s more than 200 times the distance of the Moon. To send a crew off on this journey with the best chance of success, we will have to combine electronic miniaturisation and automation with advanced, multi-functional materials to push the boundaries of space travel.
You have to try to imagine future spacecraft that will include dynamic features, such as self-preservation (the ability to heal or repair themselves), recyclability and multifunctionality, along with new materials and design paradigms that make it possible to support vessels capable of adapting to their journey.
Slimming down space materials
The challenges are manifest. Spacecraft design has always aimed at light weight whilst maintaining strength, safety and durability. For example, NASA used the first lightweight propellent tank in Space Shuttle mission STS-3 in March 1982. That yielded a weight saving of 272 kilograms from its previous missions. This weight was further reduced significantly over time – the super-lightweight external tank used in the final shuttle mission to the Mir space mission, STS-91, in June 1998, was the same size as tanks used in previous missions but 3400kg lighter. Lightweight spacecraft can have smaller, more efficient engines with less fuel, which directly translates to significant cost savings.
Hybrid material composites such as carbon fibre, or carbon nanotube reinforced polymer composites (CFRP), promise dramatic improvements over metal alloys by bringing down weight threefold without sacrificing strength. For example, the landing deck panels of the Mars 2020 Perseverance rover were prepared using carbon fibre prepreg (pre-impregnated with resin) manufactured by leading carbon fibre supplier Toray. And the now-famous Mars helicopter, Ingenuity, has rotor blades and legs made from carbon CFRPs; the whole structure weighs only 1.8kg.
Carbon nanotube reinforced polymer composites: Carbon nanotubes are tiny tubes formed by curling sheets of graphene (a form of carbon one nanometre thick). These nanotubes are very strong and stay stable at high temperatures. Using carbon nanotubes to reinforce a polymer (see above) creates a lightweight but still strong material.
Self-healing spacecraft inspired by biology
Self-preserving spacecraft will include materials such as self-healing plastics, composites, metals or ceramic. These will overcome any structural damage due to microcracking, crazing, or other types of mechanical failure.
Microcracks: Tiny (micron-sized) damage that form on the surface or within materials. Because of their size, damage from microcracks is usually not visible to the naked eye.
Crazing: A network of fine cracks formed on the surface of a material.
To inform this work, we are learning from biological healing systems such as those found in the human body – but self-healing materials in space will need to respond to damage on a timeframe vastly shorter than what is needed to heal a cut in human skin.
For example, spacecraft structural parts can be made with microcapsules and hollow microfibres containing autonomous self-healing agents that can bleed out into a mechanical crack or other damage as soon as it occurs. This mechanism mimics the human body’s vascular system, which carries platelets to the site of an injury to stop bleeding. Work is still required to assess the applicability of these self-healing materials – including a critical investigation of how they are affected by radiation, temperature fluctuations and vacuum effects.
Microfibre: A very fine synthetic fibre or yarn.
Even better than self-healing is avoiding harm in the first place. Breakthrough materials only one atom thick (also known as two-dimensional nanomaterials), such as graphene, have demonstrated extraordinary energy absorption and dissipation efficiency against micro-bullets in experimental impact trials. These trials give us an idea of how materials will stand up to micro-meteorites in space. Some two-dimensional nanomaterials have twice the stopping power of Kevlar, and about 10 times the stopping power of steel plates.
Creating a spacecraft from a hybrid of these materials would provide both high shielding efficiency and self-repair functionality that would ensure that stranded astronauts of the future can get home safe and sound.
The future of space materials
While material fabrication and development, as well as novel engineering approaches, are making significant progress, there remain many challenges on the path to interplanetary exploration by humanity to the Moon and Mars.
We continue to research and develop advanced materials for cosmic radiation shielding, ultra-reliable power sources; self-adaptive, self-healing spacecraft components; and highly flexible and sensitive sensors and actuators. For example, SpaceX’s DragonEye was equipped with a Laser Imaging Detection and Ranging (LIDAR) sensor, which can be used to create high-resolution maps of the planet surface and control navigation of autonomous vehicles. The DragonEye LIDAR sensor can provide 3D images for range and bearing information from the spacecraft to the ISS.
It’s clear there are many opportunities and challenges in the development of highly efficient, autonomous, self-healing, adaptive, lightweight and multifunctional materials to help realise our goals of space travel.
That we can develop and use these materials on Earth before we ever see them deployed on Mars by a future Mark Watney just makes their value even greater. We’re striving to create materials that will stand up to the rigours of uncooperative space – just imagine what their Earthbound uses will be.
To quote a real astronaut this time, the late but still inspiring Christa McAuliffe: “Space is for everybody. It’s not just for a few people in science or math, or for a select group of astronauts. That’s our new frontier out there, and it’s everybody’s business to know about space.”