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By NASA
5 min read
Atomic Layer Processing Coating Techniques Enable Missions to See Further into the Ultraviolet
Astrophysics observations at ultraviolet (UV) wavelengths often probe the most dynamic aspects of the universe. However, the high energy of ultraviolet photons means that their interaction with the materials that make up an observing instrument are less efficient, resulting in low overall throughput. New approaches in the development of thin film coatings are addressing this shortcoming by engineering the coatings of instrument structures at the atomic scale.
Researchers at the NASA Jet Propulsion Laboratory (JPL) are employing atomic layer deposition (ALD) and atomic layer etching (ALE) to enable new coating technologies for instruments measuring ultraviolet light. Conventional optical coatings largely rely on physical vapor deposition (PVD) methods like evaporation, where the coating layer is formed by vaporizing the source material and then condensing it onto the intended substrate. In contrast, ALD and ALE rely on a cyclic series of self-limiting chemical reactions that result in the deposition (or removal) of material one atomic layer at a time. This self-limiting characteristic results in a coating or etchings that are conformal over arbitrary shapes with precisely controlled layer thickness determined by the number of ALD or ALE cycles performed.
The ALD and ALE techniques are common in the semiconductor industry where they are used to fabricate high-performance transistors. Their use as an optical coating method is less common, particularly at ultraviolet wavelengths where the choice of optical coating material is largely restricted to metal fluorides instead of more common metal oxides, due to the larger optical band energy of fluoride materials, which minimizes absorption losses in the coatings. Using an approach based on co-reaction with hydrogen fluoride, the team at JPL has developed a variety of fluoride-based ALD and ALE processes.
(left) The Supernova remnants and Proxies for ReIonization Testbed Experiment (SPRITE) CubeSat primary mirror inside the ALD coating facility at JPL, the mirror is 18 cm on the long and is the largest optic coated in this chamber to-date. (right) Flight optic coating inside JPL ALD chamber for Pioneers Aspera Mission. Like SPRITE, the Aspera coating combines a lithium fluoride process developed at NASA GSFC with thin ALD encapsulation of magnesium fluoride at JPL. Image Credit: NASA-JPL In addition to these metal-fluoride materials, layers of aluminum are often used to construct structures like reflective mirrors and bandpass filters for instruments operating in the UV. Although aluminum has high intrinsic UV reflectance, it also readily forms a surface oxide that strongly absorbs UV light. The role of the metal fluoride coating is then to protect the aluminum surface from oxidation while maintaining enough transparency to create a mirror with high reflectance.
The use of ALD in this context has initially been pursued in the development of telescope optics for two SmallSat astrophysics missions that will operate in the UV: the Supernova remnants and Proxies for ReIonization Testbed Experiment (SPRITE) CubeSat mission led by Brian Fleming at the University of Colorado Boulder, and the Aspera mission led by Carlos Vargas at the University of Arizona. The mirrors for SPRITE and Aspera have reflective coatings that utilize aluminum protected by lithium fluoride using a novel PVD processes developed at NASA Goddard Space Flight Center, and an additional very thin top coating of magnesium fluoride deposited via ALD.
Team member John Hennessy prepares to load a sample wafer in the ALD coating chamber at JPL. Image Credit: NASA JPL The use of lithium fluoride enables SPRITE and Aspera to “see” further into the UV than other missions like NASA’s Hubble Space Telescope, which uses only magnesium fluoride to protect its aluminum mirror surfaces. However, a drawback of lithium fluoride is its sensitivity to moisture, which in some cases can cause the performance of these mirror coatings to degrade on the ground prior to launch. To circumvent this issue, very thin layers (~1.5 nanometers) of magnesium fluoride were deposited by ALD on top of the lithium fluoride on the SPRITE and Aspera mirrors. The magnesium fluoride layers are thin enough to not strongly impact the performance of the mirror at the shortest wavelengths, but thick enough to enhance the stability against humidity during ground phases of the missions. Similar approaches are being considered for the mirror coatings of the future NASA flagship Habitable Worlds Observatory (HWO).
Multilayer structures of aluminum and metal fluorides can also function as bandpass filters (filters that allow only signals within a selected range of wavelengths to pass through to be recorded) in the UV. Here, ALD is an attractive option due to the inherent repeatability and precise thickness control of the process. There is currently no suitable ALD process to deposit aluminum, and so additional work by the JPL team has explored the development of a custom vacuum coating chamber that combines the PVD aluminum and ALD fluoride processes described above. This system has been used to develop UV bandpass filters that can be deposited directly onto imaging sensors like silicon (Si) CCDs. These coatings can enable such sensors to operate with high UV efficiency, but low sensitivity to longer wavelength visible photons that would otherwise add background noise to the UV observations.
Structures composed of multilayer aluminum and metal fluoride coatings have recently been delivered as part of a UV camera to the Star-Planet Activity Research CubeSat (SPARCS) mission led by Evgenya Shkolnik at Arizona State University. The JPL-developed camera incorporates a delta-doped Si CCD with the ALD/PVD filter coating on the far ultraviolet channel, yielding a sensor with high efficiency in a band centered near 160 nm with low response to out-of-band light.
A prototype of a back-illuminated CCD incorporating a multi-layer metal-dielectric bandpass filter coating deposited by a combination of thermal evaporation and ALD. This coating combined with JPL back surface passivation approaches enable the Si CCD to operate with high UV efficiency while rejecting longer wavelength light. Image credit: NASA JPL Next, the JPL team that developed these coating processes plans to focus on implementing a similar bandpass filter on an array of larger-format Si Complementary Metal-Oxide-Semiconductor (CMOS) sensors for the recently selected NASA Medium-Class Explorer (MIDEX) UltraViolet EXplorer (UVEX) mission led by Fiona Harrison at the California Institute of Technology, which is targeted to launch in the early 2030s.
For additional details, see the entry for this project on NASA TechPort
Project Lead: Dr. John Hennessy, Jet Propulsion Laboratory (JPL)
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Last Updated Mar 18, 2025 Related Terms
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By NASA
Although NASA’s Lucy spacecraft’s upcoming encounter with the asteroid Donaldjohanson is primarily a mission rehearsal for later asteroid encounters, a new paper suggests that this small, main belt asteroid may have some surprises of its own. New modeling indicates that Donaldjohanson may have been formed about 150 million years ago when a larger parent asteroid broke apart; its orbit and spin properties have undergone significant evolution since.
This artist’s concept compares the approximate size of Lucy’s next asteroid target, Donaldjohanson, to the smallest main belt asteroids previously visited by spacecraft — Dinkinesh, visited by Lucy in November 2023, and Steins — as well as two recently explored near-Earth asteroids, Bennu and Ryugu. Credits: SwRI/ESA/OSIRIS/NASA/Goddard/Johns Hopkins APL/NOIRLab/University of Arizona/JAXA/University of Tokyo & Collaborators When the Lucy spacecraft flies by this approximately three-mile-wide space rock on April 20, 2025, the data collected could provide independent insights on such processes based on its shape, surface geology and cratering history.
“Based on ground-based observations, Donaldjohanson appears to be a peculiar object,” said Simone Marchi, deputy principal investigator for Lucy of Southwest Research Institute in Boulder, Colorado and lead author of the research published in The Planetary Science Journal. “Understanding the formation of Donaldjohanson could help explain its peculiarities.”
“Data indicates that it could be quite elongated and a slow rotator, possibly due to thermal torques that have slowed its spin over time,” added David Vokrouhlický, a professor at the Charles University, Prague, and co-author of the research.
Lucy’s target is a common type of asteroid, composed of silicate rocks and perhaps containing clays and organic matter. The new paper indicates that Donaldjohanson is a likely member of the Erigone collisional asteroid family, a group of asteroids on similar orbits that was created when a larger parent asteroid broke apart. The family originated in the inner main belt not very far from the source regions of the near-Earth asteroids Bennu and Ryugu, recently visited respectively by NASA’s OSIRIS-REx and JAXA’s (Japan Aerospace Exploration Agency’s) Hayabusa2 missions.
“We can hardly wait for the flyby because, as of now, Donaldjohanson’s characteristics appear very distinct from Bennu and Ryugu. Yet, we may uncover unexpected connections,” added Marchi.
“It’s exciting to put together what we’ve been able to glean about this asteroid,” said Keith Noll, Lucy project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “But Earth-based observing and theoretical models can only take us so far – to validate these models and get to the next level of detail we need close-up data. Lucy’s upcoming flyby will give us that.”
Donaldjohanson is named for the paleontologist who discovered Lucy, the fossilized skeleton of an early hominin found in Ethiopia in 1974, which is how the Lucy mission got its name. Just as the Lucy fossil provided unique insights into the origin of humanity, the Lucy mission promises to revolutionize our knowledge of the origin of humanity’s home world. Donaldjohanson is the only named asteroid so far to be visited while its namesake is still living.
“Lucy is an ambitious NASA mission, with plans to visit 11 asteroids in its 12-year mission to tour the Trojan asteroids that are located in two swarms leading and trailing Jupiter,” said SwRI’s Dr. Hal Levison, mission principal investigator at the Boulder, Colorado branch of Southwest Research Institute in San Antonio, Texas. “Encounters with main belt asteroids not only provide a close-up view of those bodies but also allow us to perform engineering tests of the spacecraft’s innovative navigation system before the main event to study the Trojans. These relics are effectively fossils of the planet formation process, holding vital clues to deciphering the history of our solar system.”
Lucy’s principal investigator is based out of the Boulder, Colorado, branch of Southwest Research Institute, headquartered in San Antonio. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, provides overall mission management, systems engineering, and safety and mission assurance. Lockheed Martin Space in Littleton, Colorado, built the spacecraft. Lucy is the 13th mission in NASA’s Discovery Program. NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Discovery Program for the agency’s Science Mission Directorate in Washington.
By Deb Schmid and Katherine Kretke, Southwest Research Institute
Media Contact:
Karen Fox / Molly Wasser
Headquarters, Washington
202-358-1600
karen.c.fox@nasa.gov / molly.l.wasser@nasa.gov
Nancy N. Jones
NASA’s Goddard Space Flight Center, Greenbelt, Md.
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Last Updated Mar 17, 2025 EditorMadison OlsonContactNancy N. Jonesnancy.n.jones@nasa.govLocationGoddard Space Flight Center Related Terms
Goddard Space Flight Center Lucy Missions Planetary Science Planetary Science Division Explore More
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By European Space Agency
Video: 01:08:00 Watch the replay of our Hera mission Mars flyby event. On 12 March 2025, ESA’s Hera mission came to within 5000 km of the surface of the red planet and 300 km of Mars’s more distant and enigmatic moon Deimos. During this flyby, Hera performed observations of both Mars and the city-sized Deimos. Hera then needed to swing its High Gain Antenna back to Earth to transmit its data home. On Thursday, 13 March, these images were premiered by Hera’s science team from ESA’s ESOC mission control centre in Darmstadt, Germany, explaining what they reveal, during our public webcast starting at 11:50 CET. The team was joined by ESA astronaut Alexander Gerst and renowned science fiction writer Andy Weir, author of The Martian and Project Hail Mary, as well as a surprise special guest!
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By European Space Agency
While performing yesterday’s flyby of Mars, ESA’s Hera mission for planetary defence made the first use of its payload for scientific purposes beyond Earth and the Moon. Activating a trio of instruments, Hera imaged the surface of the red planet as well as the face of Deimos, the smaller and more mysterious of Mars’s two moons.
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By European Space Agency
Video: 00:03:21 Meet Hera, our very own asteroid detective. Together with two CubeSats – Milani the rock decoder and Juventas the radar visionary – Hera is off on an adventure to explore Didymos, a double asteroid system that is typical of the thousands that pose an impact risk to planet Earth.
In September 2022 NASA’s DART spacecraft tested if it was possible to divert an asteroid by giving it a shove – and found out that it was! Important knowledge, should we wish to avoid going the same way as the dinosaurs. Astronomers can observe from afar how the smaller asteroid’s orbit has shifted since DART’s impact, but there is still a missing piece of the puzzle if we want to fully understand how ‘kinetic impacting’ works in practice. Suitable for kids and adults alike, this episode of ‘The Incredible Adventures of Hera’ explains what ESA’s asteroid detective and its CubeSat assistants are doing on their cosmic roadtrip through space towards the asteroid, and why it involves skimming close to Mars.
Watch the other episodes of The Incredible Adventures of the Hera Mission
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