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By NASA
NASA’s Jet Propulsion Laboratory used radar data taken by ESA’s Sentinel-1A satellite before and after the 2015 eruption of the Calbuco volcano in Chile to create this inter-ferogram showing land deformation. The color bands west of the volcano indicate land sinking. NISAR will produce similar images.ESA/NASA/JPL-Caltech A SAR image — like ones NISAR will produce — shows land cover on Mount Okmok on Alaska’s Umnak Island . Created with data taken in August 2011 by NASA’s UAVSAR instrument, it is an example of polarimetry, which measures return waves’ orientation relative to that of transmitted signals.NASA/JPL-Caltech Data from NASA’s Magellan spacecraft, which launched in 1989, was used to create this image of Crater Isabella, a 108-mile-wide (175-kilometer-wide) impact crater on Venus’ surface. NISAR will use the same basic SAR principles to measure properties and characteristics of Earth’s solid surfaces.NASA/JPL-Caltech Set to launch within a few months, NISAR will use a technique called synthetic aperture radar to produce incredibly detailed maps of surface change on our planet.
When NASA and the Indian Space Research Organization’s (ISRO) new Earth satellite NISAR (NASA-ISRO Synthetic Aperture Radar) launches in coming months, it will capture images of Earth’s surface so detailed they will show how much small plots of land and ice are moving, down to fractions of an inch. Imaging nearly all of Earth’s solid surfaces twice every 12 days, it will see the flex of Earth’s crust before and after natural disasters such as earthquakes; it will monitor the motion of glaciers and ice sheets; and it will track ecosystem changes, including forest growth and deforestation.
The mission’s extraordinary capabilities come from the technique noted in its name: synthetic aperture radar, or SAR. Pioneered by NASA for use in space, SAR combines multiple measurements, taken as a radar flies overhead, to sharpen the scene below. It works like conventional radar, which uses microwaves to detect distant surfaces and objects, but steps up the data processing to reveal properties and characteristics at high resolution.
To get such detail without SAR, radar satellites would need antennas too enormous to launch, much less operate. At 39 feet (12 meters) wide when deployed, NISAR’s radar antenna reflector is as wide as a city bus is long. Yet it would have to be 12 miles (19 kilometers) in diameter for the mission’s L-band instrument, using traditional radar techniques, to image pixels of Earth down to 30 feet (10 meters) across.
Synthetic aperture radar “allows us to refine things very accurately,” said Charles Elachi, who led NASA spaceborne SAR missions before serving as director of NASA’s Jet Propulsion Laboratory in Southern California from 2001 to 2016. “The NISAR mission will open a whole new realm to learn about our planet as a dynamic system.”
Data from NASA’s Magellan spacecraft, which launched in 1989, was used to create this image of Crater Isabella, a 108-mile-wide (175-kilometer-wide) impact crater on Venus’ surface. NISAR will use the same basic SAR principles to measure properties and characteristics of Earth’s solid surfaces.NASA/JPL-Caltech How SAR Works
Elachi arrived at JPL in 1971 after graduating from Caltech, joining a group of engineers developing a radar to study Venus’ surface. Then, as now, radar’s allure was simple: It could collect measurements day and night and see through clouds. The team’s work led to the Magellan mission to Venus in 1989 and several NASA space shuttle radar missions.
An orbiting radar operates on the same principles as one tracking planes at an airport. The spaceborne antenna emits microwave pulses toward Earth. When the pulses hit something — a volcanic cone, for example — they scatter. The antenna receives those signals that echo back to the instrument, which measures their strength, change in frequency, how long they took to return, and if they bounced off of multiple surfaces, such as buildings.
This information can help detect the presence of an object or surface, its distance away, and its speed, but the resolution is too low to generate a clear picture. First conceived at Goodyear Aircraft Corp. in 1952, SAR addresses that issue.
“It’s a technique to create high-resolution images from a low-resolution system,” said Paul Rosen, NISAR’s project scientist at JPL.
As the radar travels, its antenna continuously transmits microwaves and receives echoes from the surface. Because the instrument is moving relative to Earth, there are slight changes in frequency in the return signals. Called the Doppler shift, it’s the same effect that causes a siren’s pitch to rise as a fire engine approaches then fall as it departs.
Computer processing of those signals is like a camera lens redirecting and focusing light to produce a sharp photograph. With SAR, the spacecraft’s path forms the “lens,” and the processing adjusts for the Doppler shifts, allowing the echoes to be aggregated into a single, focused image.
Using SAR
One type of SAR-based visualization is an interferogram, a composite of two images taken at separate times that reveals the differences by measuring the change in the delay of echoes. Though they may look like modern art to the untrained eye, the multicolor concentric bands of interferograms show how far land surfaces have moved: The closer the bands, the greater the motion. Seismologists use these visualizations to measure land deformation from earthquakes.
Another type of SAR analysis, called polarimetry, measures the vertical or horizontal orientation of return waves relative to that of transmitted signals. Waves bouncing off linear structures like buildings tend to return in the same orientation, while those bouncing off irregular features, like tree canopies, return in another orientation. By mapping the differences and the strength of the return signals, researchers can identify an area’s land cover, which is useful for studying deforestation and flooding.
Such analyses are examples of ways NISAR will help researchers better understand processes that affect billions of lives.
“This mission packs in a wide range of science toward a common goal of studying our changing planet and the impacts of natural hazards,” said Deepak Putrevu, co-lead of the ISRO science team at the Space Applications Centre in Ahmedabad, India.
Learn more about NISAR at:
https://nisar.jpl.nasa.gov
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Andrew Wang / Jane J. Lee
Jet Propulsion Laboratory, Pasadena, Calif.
626-379-6874 / 818-354-0307
andrew.wang@jpl.nasa.gov / jane.j.lee@jpl.nasa.gov
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This artist’s illustration represents the results from a new study that examines the effects of X-ray and other high-energy radiation unleashed on potential exoplanets from Wolf 359, a nearby red dwarf star. Researchers used Chandra and XMM-Newton to study the impact of steady X-ray and energetic ultraviolet radiation from Wolf 359 on the atmospheres of planets that might be orbiting the star. They found that only a planet with greenhouse gases like carbon dioxide in its atmosphere and at a relatively large distance away from Wolf 359 would have a chance to support life as we know it.X-ray: NASA/CXC/SAO/S.Wolk, et al.; Illustration: NASA/CXC/SAO/M.Weiss; Image processing: NASA/CXC/SAO/N. Wolk Planets around other stars need to be prepared for extreme weather conditions, according to a new study from NASA’s Chandra X-ray Observatory and ESA’s (European Space Agency’s) XMM-Newton that examined the effects of X-rays on potential planets around the most common type of stars.
Astronomers found that only a planet with greenhouse gases in its atmosphere like Earth and at a relatively large distance away from the star they studied would have a chance to support life as we know it around a nearby star.
Wolf 359 is a red dwarf with a mass about a tenth that of the Sun. Red dwarf stars are the most common stars in the universe and live for billions of years, providing ample time for life to develop. At a distance of only 7.8 light-years away, Wolf 359 is also one of the closest stars to the solar system.
“Wolf 359 can help us unlock the secrets around stars and habitability,” said Scott Wolk of the Center for Astrophysics | Harvard & Smithsonian (CfA), who led the study. “It’s so close and it belongs to such an important class of stars – it’s a great combination.”
Because red dwarfs are the most prevalent types of stars, astronomers have looked hard to find exoplanets around them. Astronomers have found some evidence for two planets in orbit around Wolf 359 using optical telescopes, but those conclusions have been challenged by other scientists.
“While we don’t have proof of planets around Wolf 359 yet, it seems very possible that it hosts multiple planets,” Wolk added. “This makes it an excellent test bed to look at what planets would experience around this kind of star.”
Wolk and his colleagues used Chandra and XMM to study the amounts of steady X-rays and extreme ultraviolet (UV) radiation – the most energetic type of UV radiation – that Wolf 359 would unleash on the possible planets around it.
They found that Wolf 359 is producing enough damaging radiation that only a planet with greenhouse gases like carbon dioxide in its atmosphere – and located at a relatively large distance from the star – would likely be able to sustain life.
“Just being far enough away from the star’s harmful radiation wouldn’t be enough to make it habitable,” said co-author Vinay Kashyap, also of CfA. “A planet around Wolf 359 would also need to be blanketed in greenhouse gases like Earth is.”
To study the effects of energetic radiation on the habitability of the planet candidates, the team considered the star’s habitable zone – the region around a star where liquid water could exist on a planet’s surface.
The outer limit of the habitable zone for Wolf 359 is about 15% of the distance between Earth and the Sun, because the red dwarf is much less bright than the Sun. Neither of the planet candidates for this system is located in Wolf 359’s habitable zone, with one too close to the star and the other too far out.
“If the inner planet is there, the X-ray and extreme UV radiation it is subjected to would destroy the atmosphere of this planet in only about a million years,” said co-author Ignazio Pillitteri of CfA and the National Institute for Astrophysics in Palermo, Italy.
The team also considered the effects of radiation on as-yet undetected planets within the habitable zone. They concluded that a planet like the Earth in the middle of the habitable zone should be able to sustain an atmosphere for almost two billion years, while one near the outer edge could last indefinitely, helped by the warming effects of greenhouse gases.
Another big danger for planets orbiting stars like Wolf 359 is from X-ray flares, or occasional bright bursts of X-rays, on top of the steady, everyday output from the star. Combining observations made with Chandra and XMM-Newton resulted in the discovery of 18 X-ray flares from Wolf 359 over 3.5 days.
Extrapolating from these observed flares, the team expects that much more powerful and damaging flares would occur over longer periods of time. The combined effects of the steady X-ray and UV radiation and the flares mean that any planet located in the habitable zone is unlikely to have a significant atmosphere long enough for multicellular life, as we know it on Earth, to form and survive. The exception is the habitable zone’s outer edge if the planet has a significant greenhouse effect.
These results were presented at the 245th meeting of the American Astronomical Society in National Harbor, Maryland, and are being prepared for publication in a journal. NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.
Read more from NASA’s Chandra X-ray Observatory.
Learn more about the Chandra X-ray Observatory and its mission here:
https://www.nasa.gov/chandra
https://chandra.si.edu
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Megan Watzke
Chandra X-ray Center
Cambridge, Mass.
617-496-7998
mwatzke@cfa.harvard.edu
Lane Figueroa
Marshall Space Flight Center, Huntsville, Alabama
256-544-0034
lane.e.figueroa@nasa.gov
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A previous experiment aboard station exposed three species of wood to the space environment to help researchers determine the best option for LignoSat. The final design used 10 cm long honoki magnolia wood panels assembled with a Japanese wood-joinery method.
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