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Jupiter's Comet Collision Sites As Seen in Visible and Ultraviolet Light
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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
5 min read
February’s Night Sky Notes: How Can You Help Curb Light Pollution?
Light pollution has long troubled astronomers, who generally shy away from deep sky observing under full Moon skies. The natural light from a bright Moon floods the sky and hides views of the Milky Way, dim galaxies and nebula, and shooting stars. In recent years, human-made light pollution has dramatically surpassed the interference of even a bright full Moon, and its effects are now noticeable to a great many people outside of the astronomical community. Harsh, bright white LED streetlights, while often more efficient and long-lasting, often create unexpected problems for communities replacing their old street lamps. Some notable concerns are increased glare and light trespass, less restful sleep, and disturbed nocturnal wildlife patterns. There is increasing awareness of just how much light is too much light at night. You don’t need to give in to despair over encroaching light pollution; you can join efforts to measure it, educate others, and even help stop or reduce the effects of light pollution in your community.
Before and after pictures of replacement lighting at the 6th Street Bridge over the Los Angeles River. The second picture shows improvements in some aspects of light pollution, as light is not directed to the sides and upwards from the upgraded fixtures, reducing skyglow. However, it also shows the use of brighter, whiter LEDs, which is not generally ideal, along with increased light bounce back from the road. City of Los Angeles Amateur astronomers and potential citizen scientists around the globe are invited to participate in the Globe at Night (GaN) program to measure light pollution. Measurements are taken by volunteers on a few scheduled days every month and submitted to their database to help create a comprehensive map of light pollution and its change over time. GaN volunteers can take and submit measurements using multiple methods ranging from low-tech naked-eye observations to high-tech sensors and smartphone apps.
Globe at Night citizen scientists can use the following methods to measure light pollution and submit their results:
Their own smartphone camera and dedicated app Manually measure light pollution using their own eyes and detailed charts of the constellations A dedicated light pollution measurement device called a Sky Quality Meter (SQM). The free GaN web app from any internet-connected device (which can also be used to submit their measurements from an SQM or printed-out star charts) Night Sky Network members joined a telecon with Connie Walker of Globe at Night in 2014 and had a lively discussion about the program’s history and how they can participate. The audio of the telecon, transcript, and links to additional resources can be found on their dedicated resource page.
Light pollution has been visible from space for a long time, but new LED lights are bright enough that they stand out from older street lights, even from orbit. The above photo was taken by astronaut Samantha Cristoforetti from the ISS cupola in 2015. The newly installed white LED lights in the center of the city of Milan are noticeably brighter than the lights in the surrounding neighborhoods. NASA/ESA DarkSky International has long been a champion in the fight against light pollution and a proponent of smart lighting design and policy. Their website (at darksky.org) provides many resources for amateur astronomers and other like-minded people to help communities understand the negative impacts of light pollution and how smart lighting policies can not only help bring the stars back to their night skies but make their streets safer by using smarter lighting with less glare. Communities and individuals find that their nighttime lighting choices can help save considerable sums of money when they decide to light their streets and homes “smarter, not brighter” with shielded, directional lighting, motion detectors, timers, and even choosing the proper “temperature” of new LED light replacements to avoid the harsh “pure white” glare that many new streetlamps possess. Their pages on community advocacy and on how to choose dark-sky-friendly lighting are extremely helpful and full of great information. There are even local chapters of the IDA in many communities made up of passionate advocates of dark skies.
DarkSky International has notably helped usher in “Dark Sky Places“, areas around the world that are protected from light pollution. “Dark Sky Parks“, in particular, provide visitors with incredible views of the Milky Way and are perfect places to spot the wonders of a meteor shower. These parks also perform a very important function, showing the public the wonders of a truly dark sky to many people who may have never before even seen a handful of stars in the sky, let alone the full, glorious spread of the Milky Way.
More research into the negative effects of light pollution on the health of humans and the environment is being conducted than ever before. Watching the nighttime light slowly increase in your neighborhood, combined with reading so much bad news, can indeed be disheartening! However, as awareness of light pollution and its negative effects increases, more people are becoming aware of the problem and want to be part of the solution. There is even an episode of PBS Kid’s SciGirls where the main characters help mitigate light pollution in their neighborhood!
Astronomy clubs are uniquely situated to help spread awareness of good lighting practices in their local communities in order to help mitigate light pollution. Take inspiration from Tucson, Arizona, and other dark sky-friendly communities that have adopted good lighting practices. Tucson even reduced its skyglow by 7% after its own citywide lighting conversion, proof that communities can bring the stars back with smart lighting choices.
Originally posted by Dave Prosper: November 2018
Last Updated by Kat Troche: January 2025
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By European Space Agency
In a landmark achievement, ESA successfully migrated Galileo’s Control Centres in a highly complex operation involving over 200 people from the Agency, industry and EUSPA. This milestone marked the beginning of a global operation to upgrade Galileo’s entire Ground Segment, a vast network of stations spread around the world.
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By NASA
NASA, ESA, and M. Wong (University of California – Berkeley); Processing: Gladys Kober (NASA/Catholic University of America) This NASA Hubble Space Telescope image shows the planet Jupiter in a color composite of ultraviolet wavelengths. Released on Nov. 3, 2023, in honor of Jupiter reaching opposition, which occurs when the planet and the Sun are in opposite sides of the sky, this view of the gas giant planet includes the iconic, massive storm called the “Great Red Spot.” Though the storm appears red to the human eye, in this ultraviolet image it appears darker because high altitude haze particles absorb light at these wavelengths. The reddish, wavy polar hazes are absorbing slightly less of this light due to differences in either particle size, composition, or altitude.
Learn more about Hubble and how this type of data can help us learn more about our universe.
Image credit: NASA, ESA, and M. Wong (University of California – Berkeley); Processing: Gladys Kober (NASA/Catholic University of America)
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By NASA
NASA On April 21, 1972, NASA astronaut John W. Young, commander of the Apollo 16 mission, took a far-ultraviolet photo of Earth with an ultraviolet camera. Young’s original black-and-white picture was printed on Agfacontour professional film three times, with each exposure recording only one light level. The three light levels were then colored blue (dimmest), green (next brightest), and red (brightest), resulting in the enhanced-color image seen here.
Dr. George Carruthers, a scientist at the Naval Research Laboratory, developed the ultraviolet camera – the first Moon-based observatory – for Apollo 16. Apollo 16 astronauts placed the observatory on the Moon in April 1972, where it sits today on the Moon’s Descartes highland region, in the shadow of the lunar module Orion.
Image credit: NASA
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