From Johns Hopkins University Applied Physics Lab : “Using ‘Charon-light’ Researchers Capture Pluto’s Dark Side”

The Johns Hopkins University Applied Physics Lab

From Johns Hopkins University Applied Physics Lab

October 27, 2021

NASA’s New Horizons spacecraft made history by returning the first close-up images of Pluto and its moons.

National Aeronautics Space Agency(USA) New Horizons(US) spacecraft

Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and The Southwest Research Institute (US) for The National Aeronautics and Space Agency (US).

Now, through a series of clever methods, researchers led by Tod Lauer of the National Science Foundation’s NOIRLab in Tucson, Arizona, on the New Horizons team have expanded that photo album to include the portion of Pluto’s landscape that wasn’t directly illuminated by sunlight — what the team calls Pluto’s “dark side.”

National Science Foundation(US) NOIRLab NOAO Kitt Peak National Observatory on the Quinlan Mountains in the Arizona-Sonoran Desert on the Tohono O’odham Nation, 88 kilometers (55 mi) west-southwest of Tucson, Arizona, Altitude 2,096 m (6,877 ft).

After flying within 7,800 miles (12,550 kilometers) of Pluto’s icy surface on July 14, 2015, New Horizons continued at a rapid 9 miles per second (14.5 kilometers per second) on to the Kuiper Belt Object Arrokoth and beyond. But while departing Pluto, the spacecraft looked back at the dwarf planet and captured a series of images of its dark side.

Backlit by the distant Sun, Pluto’s hazy atmosphere stood out as a brilliant ring of light encircling the Pluto’s dark side.

The image shows the dark side of Pluto surrounded by a bright ring of sunlight scattered by haze in its atmosphere. But for a dark crescent zone to the left, the terrain is faintly illuminated by sunlight reflected by Pluto’s moon Charon. Researchers on the New Horizons team were able to generate this image using 360 images that New Horizons captured as it looked back on Pluto’s southern hemisphere. A large portion of the southern hemisphere was in seasonal darkness similar to winters in the Arctic and Antarctica on Earth, and was otherwise not visible to New Horizons during its 2015 flyby encounter of Pluto. Credit: NASA/Johns Hopkins APL/ The Southwest Research Institute (US)/NOIRLab

From its vantage point when this experiment was conducted, New Horizons was mainly able to see Pluto’s southern hemisphere, a large portion of which was transitioning to its winter seasonal darkness — something much like the dark, months-long Arctic and Antarctic winters on Earth, except on Pluto each season lasts 62 Earth years.

Fortunately, a portion of Pluto’s dark southern hemisphere was illuminated by the faint sunlight reflecting off the icy surface of Pluto’s largest moon, Charon, which is about the size of Texas. That bit of “Charon-light” was just enough for researchers to tease out details of Pluto’s southern hemisphere that could not be obtained any other way.

“In a startling coincidence, the amount of light from Charon on Pluto is close to that of the Moon on Earth, at the same phase for each,” said Tod Lauer, an astronomer at the National Science Foundation’s National Optical Infrared Astronomy Research Observatory in Tucson, Arizona, and the study’s lead author. “At the time, the illumination of Charon on Pluto was similar to that from our own Moon on Earth when in its first-quarter phase.”

The researchers published the resulting image and the scientific interpretation of it on Oct. 20 in The Planetary Science Journal.

Recovering details on Pluto’s surface in faint moonlight wasn’t easy. When looking back at Pluto with the New Horizons Long Range Reconnaissance Imager (LORRI), scattered light from the Sun (which was nearly directly behind Pluto) produced a complex pattern of background light that was 1,000 times stronger than the signal produced by Charon-reflected light, according to New Horizons coinvestigator and Project Scientist Hal Weaver, at the Johns Hopkins Applied Physics Laboratory. In addition, the bright ring of atmospheric haze surrounding Pluto was itself heavily over-exposed, producing additional artifacts in the images.

“The problem was a lot like trying to read a street sign through a dirty windshield when driving towards the setting Sun, without a sun visor,” said John Spencer, New Horizons co-investigator and planetary scientist at the Southwest Research Institute in Boulder, Colorado, a study co-author.

It took the combination of 360 images of Pluto’s dark side, and another 360 images taken with the same geometry but without Pluto in the picture, to produce the final image with the artifacts subtracted out leaving only the signal produced by Charon-reflected light. Alan Stern, the New Horizons principal investigator at the Southwest Research Institute, added that “the image processing work that Tod Lauer led was completely state of the art, and it allowed us to learn some fascinating things about a part of Pluto we otherwise would not have known.”

The resulting map, while still containing digital noise, shows a few prominent features on Pluto’s shadowed surface. The most prominent of these is a dark crescent zone to the west, where neither sunlight nor Charon-light was falling when New Horizons took the images. Also conspicuous is a large, bright region midway between Pluto’s south pole and its equator. The team suspects it may be a deposit of nitrogen or methane ice similar to Pluto’s icy “heart” on its opposite side.

Pluto’s south pole and the region of the surface around it appears to be covered in a dark material, starkly contrasting with the paler surface of Pluto’s northern hemisphere. The researchers suspect that difference could be a consequence of Pluto having recently completed its southern summer (which ended 15 years before the flyby). During the summer, the team suggests that nitrogen and methane ices in the south may have sublimated from the surface, turning directly from solid to vapor, while dark haze-particles settled over the region. Future Earth-based instruments could eventually verify the team’s image and confirm their other suspicions, but it would require Pluto’s southern hemisphere to be in sunlight — something that won’t happen for nearly 100 years. “The easiest way to confirm our ideas is to send a follow-on mission,” Lauer said.

See the full article here .


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Founded on March 10, 1942—just three months after the United States entered World War II— The Johns Hopkins University Applied Physics Lab (US) -was created as part of a federal government effort to mobilize scientific resources to address wartime challenges.

The Applied Physics Lab was assigned the task of finding a more effective way for ships to defend themselves against enemy air attacks. The Laboratory designed, built, and tested a radar proximity fuze (known as the VT fuze) that significantly increased the effectiveness of anti-aircraft shells in the Pacific—and, later, ground artillery during the invasion of Europe. The product of the Laboratory’s intense development effort was later judged to be, along with the atomic bomb and radar, one of the three most valuable technology developments of the war.

On the basis of that successful collaboration, the government, The Johns Hopkins University, and APL made a commitment to continue their strategic relationship. The Laboratory rapidly became a major contributor to advances in guided missiles and submarine technologies. Today, more than seven decades later, the Laboratory’s numerous and diverse achievements continue to strengthen our nation.

The Applied Physics Lab continues to relentlessly pursue the mission it has followed since its first day: to make critical contributions to critical challenges for our nation.

Johns Hopkins University opened in 1876, with the inauguration of its first president, Daniel Coit Gilman. “What are we aiming at?” Gilman asked in his installation address. “The encouragement of research … and the advancement of individual scholars, who by their excellence will advance the sciences they pursue, and the society where they dwell.”

The mission laid out by Gilman remains the university’s mission today, summed up in a simple but powerful restatement of Gilman’s own words: “Knowledge for the world.”

What Gilman created was a research university, dedicated to advancing both students’ knowledge and the state of human knowledge through research and scholarship. Gilman believed that teaching and research are interdependent, that success in one depends on success in the other. A modern university, he believed, must do both well. The realization of Gilman’s philosophy at Johns Hopkins, and at other institutions that later attracted Johns Hopkins-trained scholars, revolutionized higher education in America, leading to the research university system as it exists today.

The Johns Hopkins University (US) is a private research university in Baltimore, Maryland. Founded in 1876, the university was named for its first benefactor, the American entrepreneur and philanthropist Johns Hopkins. His $7 million bequest (approximately $147.5 million in today’s currency)—of which half financed the establishment of the Johns Hopkins Hospital—was the largest philanthropic gift in the history of the United States up to that time. Daniel Coit Gilman, who was inaugurated as the institution’s first president on February 22, 1876, led the university to revolutionize higher education in the U.S. by integrating teaching and research. Adopting the concept of a graduate school from Germany’s historic Ruprecht Karl University of Heidelberg, [Ruprecht-Karls-Universität Heidelberg] (DE), Johns Hopkins University is considered the first research university in the United States. Over the course of several decades, the university has led all U.S. universities in annual research and development expenditures. In fiscal year 2016, Johns Hopkins spent nearly $2.5 billion on research. The university has graduate campuses in Italy, China, and Washington, D.C., in addition to its main campus in Baltimore.

Johns Hopkins is organized into 10 divisions on campuses in Maryland and Washington, D.C., with international centers in Italy and China. The two undergraduate divisions, the Zanvyl Krieger School of Arts and Sciences and the Whiting School of Engineering, are located on the Homewood campus in Baltimore’s Charles Village neighborhood. The medical school, nursing school, and Bloomberg School of Public Health, and Johns Hopkins Children’s Center are located on the Medical Institutions campus in East Baltimore. The university also consists of the Peabody Institute, Applied Physics Laboratory, Paul H. Nitze School of Advanced International Studies, School of Education, Carey Business School, and various other facilities.

Johns Hopkins was a founding member of the American Association of Universities (US). As of October 2019, 39 Nobel laureates and 1 Fields Medalist have been affiliated with Johns Hopkins. Founded in 1883, the Blue Jays men’s lacrosse team has captured 44 national titles and plays in the Big Ten Conference as an affiliate member as of 2014.


The opportunity to participate in important research is one of the distinguishing characteristics of Hopkins’ undergraduate education. About 80 percent of undergraduates perform independent research, often alongside top researchers. In FY 2013, Johns Hopkins received $2.2 billion in federal research grants—more than any other U.S. university for the 35th consecutive year. Johns Hopkins has had seventy-seven members of the Institute of Medicine, forty-three Howard Hughes Medical Institute Investigators, seventeen members of the National Academy of Engineering, and sixty-two members of the National Academy of Sciences. As of October 2019, 39 Nobel Prize winners have been affiliated with the university as alumni, faculty members or researchers, with the most recent winners being Gregg Semenza and William G. Kaelin.

Between 1999 and 2009, Johns Hopkins was among the most cited institutions in the world. It attracted nearly 1,222,166 citations and produced 54,022 papers under its name, ranking No. 3 globally [after Harvard University (US) and the Max Planck Society (DE) in the number of total citations published in Thomson Reuters-indexed journals over 22 fields in America.

In FY 2000, Johns Hopkins received $95.4 million in research grants from the National Aeronautics and Space Administration (US), making it the leading recipient of NASA research and development funding. In FY 2002, Hopkins became the first university to cross the $1 billion threshold on either list, recording $1.14 billion in total research and $1.023 billion in federally sponsored research. In FY 2008, Johns Hopkins University performed $1.68 billion in science, medical and engineering research, making it the leading U.S. academic institution in total R&D spending for the 30th year in a row, according to a National Science Foundation (US) ranking. These totals include grants and expenditures of JHU’s Applied Physics Laboratory in Laurel, Maryland.

The Johns Hopkins University also offers the “Center for Talented Youth” program—a nonprofit organization dedicated to identifying and developing the talents of the most promising K-12 grade students worldwide. As part of the Johns Hopkins University, the “Center for Talented Youth” or CTY helps fulfill the university’s mission of preparing students to make significant future contributions to the world. The Johns Hopkins Digital Media Center (DMC) is a multimedia lab space as well as an equipment, technology and knowledge resource for students interested in exploring creative uses of emerging media and use of technology.

In 2013, the Bloomberg Distinguished Professorships program was established by a $250 million gift from Michael Bloomberg. This program enables the university to recruit fifty researchers from around the world to joint appointments throughout the nine divisions and research centers. Each professor must be a leader in interdisciplinary research and be active in undergraduate education. Directed by Vice Provost for Research Denis Wirtz, there are currently thirty two Bloomberg Distinguished Professors at the university, including three Nobel Laureates, eight fellows of the American Association for the Advancement of Science (US), ten members of the American Academy of Arts and Sciences, and thirteen members of the National Academies.