NASA’s Mars 2020 rocket (launching this July), and the International Space Station (ISS), are providing an invaluable test-bed for cutting-edge coatings and application techniques.
In-space testing is currently underway on coatings to protect astronauts and equipment from razor sharp dust particles, and to prevent the growth of superbugs on the International Space Station. The upcoming Mars mission, meanwhile, will be showing off high-tech coating application techniques on the new Mars Rover.
Anti-dust surface
A test underway on the International Space Station (ISS) is looking at an advanced coating for use on satellite components that could also help solve one of NASA’s biggest challenges: how to keep the Moon’s irregularly shaped, razor-sharp dust grains from adhering to virtually everything they touch, including astronauts’ spacesuits.
Bill Farrell, at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, says that lunar dust is among the top challenges to establishment of sustainable exploration of the Moon by 2028 under its Artemis Program, and to future exploration of Mars. In the words of Harrison “Jack” Schmitt, Apollo 17 astronaut: “Dust is going to be the environmental problem for future missions, both inside and outside habitats”.
The new coating was initially designed to help ‘bleed off’ electrical charges that build up when spacecraft fly through plasma found in Earth’s magnetosphere, which can destroy spacecraft electronics. It is now also being looked at as protection for Moon rovers, habitats, and the fibres in spacesuit material.
The application technique creates super-thin films of indium tin oxide onto dry pigments of paint. Once mixed, the paint can be applied to radiators and other spacecraft components to help mitigate dust and electrical problems. The thin layers are achieved through atomic layer deposition, where a substrate is placed inside a reactor chamber and subjected to pulsing of different types of gases, resulting in an ultra-thin film no thicker than a single atom. The technique can be applied on virtually anything, including three-dimensional objects, and is effective against Lunar dust travelling at hurricane-like speeds.
The testing involves coated coupons or wafers, which are now being exposed to plasma from an experiment pallet aboard the International Space Station.
Farrell continues: “We have conducted a number of studies investigating lunar dust. A key finding is to make the outer skin of the spacesuits and other human systems conductive or dissipative. We, in fact, have strict conductivity requirements on spacecraft due to plasma. The same ideas apply to spacesuits. A future goal is for the technology to produce conductive skin materials, and this is currently being developed.”
Superbugs
Another trial currently underway on the ISS was sparked by last year’s discovery of some highly resistant bacteria in crew areas of the ISS. The problem was seen as particularly pressing since the crew’s immune defences are generally lowered in space due to stress and isolation.
Research in Germany showed that the extreme conditions of spaceflight were in fact serving to toughen up some strains. A solution currently being tested on the ISS is a new silver- and ruthenium-based antimicrobial coating called AGXX, developed at Beuth University in Berlin. The coating has been applied to patches affixed to the Station’s toilet door, an obviously contamination-prone surface.
A study published in Frontiers in Microbiology shows that the AGXX dramatically reduced the number of bacteria on contamination-prone surfaces – and could help protect future astronauts beyond the moon and Mars. “Spaceflight can turn harmless bacteria into potential pathogens,” explains Senior study author Prof Elisabeth Grohmann. “Just as stress hormones leave astronauts vulnerable to infection, the bacteria they carry become hardier – developing thick protective coatings and resistance to antibiotics – and more vigorous, multiplying and metabolising faster.”
To make matters worse, says Grohmann, the genes responsible for these new traits can be readily shared among different species of bacteria, via direct contact or in the ‘matrix’ of slime they secrete. “AGXX contains both silver and ruthenium, conditioned by a vitamin derivative, and it kills all kinds of bacteria as well as certain fungi, yeasts and viruses. The effects are similar to bleach – except the coating is self-regenerating so it never gets used up.”
She continues:”After six months exposure on the ISS, no bacteria were recovered from AGXX-coated surfaces. Even at 12 and 19 months, a total of just 12 bacteria was recovered – a reduction of 80% compared to bare steel. A regular silver coating tested for comparison had only a slight antimicrobial effect, reducing the number of bacteria by 30% versus steel.”
A significant advantage of the technology is that the redox (oxidation and reduction) reactions and micro-electric field are generated by the multiple microgalvanic cells at the AGXX surface, solely powered by the oxygen in aqueous solutions, the humidity in the air and by the oxidation of the microorganisms themselves. This means that the coating functions independently of any external energy supply.
Initial commercial applications are in the field of microbial decontamination of aqueous solutions, cooling and processing waters. The coating also has applications in other fields such as in the food industry, healthcare, biomedicine and drinking water conservation, and different strains of multi-resistant microbes taken from patients of a Spanish hospital have been successfully killed by the coating.
Thermal barrier
More testing on the ISS involves innovative materials for radiation shielding and thermal barrier coatings, currently attached to panels outside the ISS. These are hoped to lead to applications for lunar habitation, long-term deep space missions such as Mars, and other unspecified defence applications.
Louisiana Technology and Engineering firm Geocent won a NASA grant to develop multifunctional lightweight composite materials for use as an integral part of spacecraft or habitat structures, to shield crew and critical avionics against Galactic Cosmic Rays and secondary particles.
A second grant covered development of material for Thermal Barrier Coatings for direct application on hot rocket and missile structures, such as nozzles and leading edges, also now on the International Space Station for evaluation.
“We have matured these materials to the extent that NASA has selected Geocent and its partners to fly and test them on the Materials International Space Station Experiment platform, which is mounted externally to the International Space Station,” explains Dr Subhayu Sen, Geocent’s Principal Investigator.
Two batches of shielding and thermal barrier material were launched last year for mounting outside the ISS by astronauts, and testing will continue for a year.
Going a-roving
In the meantime, July’s Mars 2020 flight will be carrying an updated wheeled rover built and managed by NASA’s Jet Propulsion Laboratory in Pasadena, California. Coating the new rover has been carried out with considerable thought and painstaking precision.
“Paint that is formulated for space use is quite different from what you see in your day to day life,” states David Agle of NASA. “We use a range of paint binder chemistries (including epoxy and polyurethane, but also more exotic ones like silicone and silicate), with specialised additives and pigments. Each paint has a specific job – some are great at absorbing heat from the sun, others are great at radiating heat, and there’s even a few that dissipate static electricity.”
NASA Paint technician John Campanella says: “We do a thousand different paint jobs a year at JPL, from components as small as a pill to entire spacecraft fuselages. I have worked on the first Mars rover Pathfinder, the Deep Impact mission, GRAIL, Juno and Cassini, and I think my paint is just about everywhere in the solar system.”
Construction of the rover last year involved 20 freshly-machined, large, shiny chunks of 7050 and 7075 aluminium making up the primary structure of the chassis, along with about a hundred smaller secondary parts, all requiring 610 rivets, 730 washers, 644 nuts and 964 mechanical fasteners to hold them together. A computer-controlled cutter makes sure each piece is exactly the right size and shape.
More than 600 pieces of masking tape for the rover came in all shapes and sizes, many smaller than a coin. All were applied by hand and smoothed out to prevent bubbling, along with one hundred and thirty-five temporary sheet-metal stencils.
“With any spacecraft, after you get the static wrap off and inspect the rover, you start with surface prep — abrading the surface with sand paper so the paint will adhere better,” explains Campanella. “To prevent any chance of corrosion or oxidation, the rule is, once you start sanding, you have six hours to complete everything — sanding, priming and painting.”
The small team painted the top deck first, then allowed it to cure for a day before coating the sides. The primer and paint have to adhere to aluminium through the jolts, vibrations, UV rays and cold of a trip to Mars, while not outgassing organic compounds and other materials that could affect the mission’s science experiments.
“You can’t think about where it’s going or how much history it can make,” says Campanella. “We use the same paint guns on the same setting and fire them from the same distance and move at the same speed each and every time.”
A second coat was applied with the first coat still tacky, to make a stronger bond between layers, followed by third and fourth coats.
Oven-ready
The chassis was then sheathed in antistatic wrap and transported for baking in a vacuum chamber, cooking the chassis at 230°F (110°C) in a vacuum for three days to harden the paint and bake out contaminants from the paint that might outgas in flight. Since Mars 2020 is an astrobiology mission, ensuring samples have not been contaminated is paramount.
Mars 2020 is scheduled to launch from Space Launch Complex 41 at Cape Canaveral Air Force Station in Florida and expected to reach Mars in February 2021.