NASA's Europa Clipper captured a unique ultraviolet view of 3I/ATLAS

Objects from beyond our solar system do not come calling very often. This explains why comet 3I/ATLAS generated so much speculation and has been under observation by several space and ground telescopes since its discovery. And while it was an accident, NASA’s Europa Clipper spacecraft, too, observed 3I/ATLAS on November 6, when the comet was about 102 million miles, or roughly 164 million kilometers, away, the agency informed. 

A big hurdle during November was that Earth-based observations were being blocked by the Sun. However, with 3I/ATLAS passing between Europa Clipper and our host star at the time, the spacecraft was uniquely positioned to snap an image of the comet, thereby bridging the gap between Mars-based observations in late September and those made from Earth later. And so it seized the opportunity, capturing the comet over seven hours with its Europa Ultraviolet Spectrograph (Europa-UVS). As the spacecraft happened to be facing the Sun at the time, it caught a rather unique downstream view of the comet's two tails while looking back toward the comet’s core and the cloud of gas around it. 

"We're hopeful that this new view, along with observations from Earth-based assets and other spacecraft, will help us to piece together a more complete understanding of the tails' geometries," said SwRI's Dr. Thomas Greathouse, co-deputy principal investigator of Europa-UVS, per Phys.org. The UVS picked up signs of oxygen, hydrogen, and dust around the comet. Together, these findings suggest that Comet 3I/ATLAS was releasing large amounts of gas and material shortly after it made its closest pass to the Sun.

Launched in October 2024, the spacecraft will not arrive at Jupiter until April 2030. Its primary mission is to study Europa, one of Jupiter’s largest moons and a world thought to hide a vast ocean beneath its icy surface. Kurt Retherford, a staff scientist at the Southwest Research Institute in San Antonio and the principal investigator for Europa-UVS, had previously explained the process used by the UVS in everyday terms. Light enters through the telescope, reflects off a mirror, and passes through a narrow slit. It then hits a grating that spreads the light out before it finally reaches a detector. "It’s like a digital image you take with a cell phone camera," Retherford explained. "But unlike a cell phone, Europa-UVS sees ultraviolet light, and records more than pictures."

By scanning the sky and splitting ultraviolet light into its component wavelengths, the instrument can identify chemical fingerprints left behind by different elements. By stacking several observations together and converting them into visible colors, the team can even produce an image of an object that normally cannot be seen by the eye. This technique made it possible to capture a visual record and the chemical fingerprint of 3I/ATLAS last month.

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