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Final images before Aeolus demise
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By European Space Agency
Week in images: 08-12 September 2025
Discover our week through the lens
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By European Space Agency
Week in images: 01-05 September 2025
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By NASA
Explore This Section Overview Science Science Findings Juno’s Orbits Spacecraft People Stories Multimedia JunoCam Images Jupiter hosts the brightest and most spectacular auroras in the Solar System. Near its poles, these shimmering lights offer a glimpse into how the planet interacts with the solar wind and moons swept by Jupiter’s magnetic field. Unlike Earth’s northern lights, the largest moons of Jupiter create their own auroral signatures in the planet’s atmosphere — a phenomenon that Earth’s Moon does not produce. These moon-induced auroras, known as “satellite footprints,” reveal how each moon interacts with its local space environment.
Juno capturing the marks on Jupiter of all four Galilean moons. The auroras related to each are labeled Io, Eur (for Europa), Gan (for Ganymede), and Cal (for Callisto). NASA/JPL-Caltech/SwRI/UVS team/MSSS/Gill/Jónsson/Perry/Hue/Rabia Before NASA’s Juno mission, three of Jupiter’s four largest moons, known as Galilean moons — Io, Europa, and Ganymede — were shown to produce these distinct auroral signatures. But Callisto, the most distant of the Galilean moons, remained a mystery. Despite multiple attempts using NASA’s Hubble Space Telescope, Callisto’s footprint had proven elusive, both because it is faint and because it most often lies atop the brighter main auroral oval, the region where auroras are displayed.
NASA’s Juno mission, orbiting Jupiter since 2016, offers unprecedented close-up views of these polar light shows. But to image Callisto’s footprint, the main auroral oval needs to move aside while the polar region is being imaged. And to bring to bear Juno’s arsenal of instruments studying fields and particles, the spacecraft’s trajectory must carry it across the magnetic field line linking Callisto and Jupiter.
These two events serendipitously occurred during Juno’s 22nd orbit of the giant planet, in September 2019, revealing Callisto’s auroral footprint and providing a sample of the particle population, electromagnetic waves, and magnetic fields associated with the interaction.
Jupiter’s magnetic field extends far beyond its major moons, carving out a vast region (magnetosphere) enveloped by, and buffeted by, the solar wind streaming from our Sun. Just as solar storms on Earth push the northern lights to more southern latitudes, Jupiter’s auroras are also affected by our Sun’s activity. In September 2019, a massive, high-density solar stream buffeted Jupiter’s magnetosphere, briefly revealing — as the auroral oval moved toward Jupiter’s equator — a faint but distinct signature associated with Callisto. This discovery finally confirms that all four Galilean moons leave their mark on Jupiter’s atmosphere, and that Callisto’s footprints are sustained much like those of its siblings, completing the family portrait of the Galilean moon auroral signatures.
An international team of scientists led by Jonas Rabia of the Institut de Recherche en Astrophysique et Planétologie (IRAP), CNRS, CNES, in Toulouse, France, published their paper on the discovery, “In situ and remote observations of the ultraviolet footprint of the moon Callisto by the Juno spacecraft,” in the journal Nature Communications on Sept. 1, 2025.
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Last Updated Sep 02, 2025 Related Terms
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By European Space Agency
Week in images: 25-29 August 2025
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By NASA
This graphic features data from NASA’s Chandra X-ray Observatory of the Cassiopeia A (Cas A) supernova remnant that reveals that the star’s interior violently rearranged itself mere hours before it exploded. The main panel of this graphic is Chandra data that shows the location of different elements in the remains of the explosion: silicon (represented in red), sulfur (yellow), calcium (green) and iron (purple). The blue color reveals the highest-energy X-ray emission detected by Chandra in Cas A and an expanding blast wave. The inset reveals regions with wide ranges of relative abundances of silicon and neon. This data, plus computer modeling, reveal new insight into how massive stars like Cas A end their lives.X-ray: NASA/CXC/Meiji Univ./T. Sato et al.; Image Processing: NASA/CXC/SAO/N. Wolk The inside of a star turned on itself before it spectacularly exploded, according to a new study from NASA’s Chandra X-ray Observatory. Today, this shattered star, known as the Cassiopeia A supernova remnant, is one of the best-known, well-studied objects in the sky.
Over three hundred years ago, however, it was a giant star on the brink of self-destruction. The new Chandra study reveals that just hours before it exploded, the star’s interior violently rearranged itself. This last-minute shuffling of its stellar belly has profound implications for understanding how massive stars explode and how their remains behave afterwards.
Cassiopeia A (Cas A for short) was one of the first objects the telescope looked at after its launch in 1999, and astronomers have repeatedly returned to observe it.
“It seems like each time we closely look at Chandra data of Cas A, we learn something new and exciting,” said Toshiki Sato of Meiji University in Japan who led the study. “Now we’ve taken that invaluable X-ray data, combined it with powerful computer models, and found something extraordinary.”
As massive stars age, increasingly heavy elements form in their interiors by nuclear reactions, creating onion-like layers of different elements. Their outer layer is mostly made of hydrogen, followed by layers of helium, carbon and progressively heavier elements – extending all the way down to the center of the star.
Once iron starts forming in the core of the star, the game changes. As soon as the iron core grows beyond a certain mass (about 1.4 times the mass of the Sun), it can no longer support its own weight and collapses. The outer part of the star falls onto the collapsing core, and rebounds as a core-collapse supernova.
The new research with Chandra data reveals a change that happened deep within the star at the very last moments of its life. After more than a million years, Cas A underwent major changes in its final hours before exploding.
“Our research shows that just before the star in Cas A collapsed, part of an inner layer with large amounts of silicon traveled outwards and broke into a neighboring layer with lots of neon,” said co-author Kai Matsunaga of Kyoto University in Japan. “This is a violent event where the barrier between these two layers disappears.”
This upheaval not only caused material rich in silicon to travel outwards; it also forced material rich in neon to travel inwards. The team found clear traces of these outward silicon flows and inward neon flows in the remains of Cas A’s supernova remnant. Small regions rich in silicon but poor in neon are located near regions rich in neon and poor in silicon.
The survival of these regions not only provides critical evidence for the star’s upheaval, but also shows that complete mixing of the silicon and neon with other elements did not occur immediately before or after the explosion. This lack of mixing is predicted by detailed computer models of massive stars near the ends of their lives.
There are several significant implications for this inner turmoil inside of the doomed star. First, it may directly explain the lopsided rather than symmetrical shape of the Cas A remnant in three dimensions. Second, a lopsided explosion and debris field may have given a powerful kick to the remaining core of the star, now a neutron star, explaining the high observed speed of this object.
Finally, the strong turbulent flows created by the star’s internal changes may have promoted the development of the supernova blast wave, facilitating the star’s explosion.
“Perhaps the most important effect of this change in the star’s structure is that it may have helped trigger the explosion itself,” said co-author Hiroyuki Uchida, also of Kyoto University. “Such final internal activity of a star may change its fate—whether it will shine as a supernova or not.”
These results have been published in the latest issue of The Astrophysical Journal and are available online.
To learn more about Chandra, visit:
https://science.nasa.gov/chandra
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
Visual Description
This release features a composite image of Cassiopeia A, a donut-shaped supernova remnant located about 11,000 light-years from Earth. Included in the image is an inset closeup, which highlights a region with relative abundances of silicon and neon.
Over three hundred years ago, Cassiopeia A, or Cas A, was a star on the brink of self-destruction. In composition it resembled an onion with layers rich in different elements such as hydrogen, helium, carbon, silicon, sulfur, calcium, and neon, wrapped around an iron core. When that iron core grew beyond a certain mass, the star could no longer support its own weight. The outer layers fell into the collapsing core, then rebounded as a supernova. This explosion created the donut-like shape shown in the composite image. The shape is somewhat irregular, with the thinner quadrant of the donut to the upper left of the off-center hole.
In the body of the donut, the remains of the star’s elements create a mottled cloud of colors, marbled with red and blue veins. Here, sulfur is represented by yellow, calcium by green, and iron by purple. The red veins are silicon, and the blue veins, which also line the outer edge of the donut-shape, are the highest energy X-rays detected by Chandra and show the explosion’s blast wave.
The inset uses a different color code and highlights a colorful, mottled region at the thinner, upper left quadrant of Cas A. Here, rich pockets of silicon and neon are identified in the red and blue veins, respectively. New evidence from Chandra indicates that in the hours before the star’s collapse, part of a silicon-rich layer traveled outwards, and broke into a neighboring neon-rich layer. This violent breakdown of layers created strong turbulent flows and may have promoted the development of the supernova’s blast wave, facilitating the star’s explosion. Additionally, upheaval in the interior of the star may have produced a lopsided explosion, resulting in the irregular shape, with an off-center hole (and a thinner bite of donut!) at our upper left.
News Media Contact
Megan Watzke
Chandra X-ray Center
Cambridge, Mass.
617-496-7998
mwatzke@cfa.harvard.edu
Corinne Beckinger
Marshall Space Flight Center, Huntsville, Alabama
256-544-0034
corinne.m.beckinger@nasa.gov
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Last Updated Aug 28, 2025 EditorLee MohonContactCorinne M. Beckingercorinne.m.beckinger@nasa.govLocationMarshall Space Flight Center Related Terms
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