Interstellar comet 3I/ATLAS on Wednesday reached its closest point to the Sun, a point known as perihelion. Although this cosmic visitor hasn’t been visible from Earth since September, our space-based observatories were able to track it on its path toward our star, witnessing it as it grew brighter and brighter.
As the comet approached the Sun, the stars heat caused the comet’s icy surface to rapidly transform from a solid to a gas—so fast that it bypassed a liquid phase. The resulting gas surrounded the comet’s nucleus in a bright, glowing cloud called a coma, creating a visible tail.
Interstellar comets like 3I/ATLAS offer exceptionally rare opportunities to study other solar systems, briefly illuminating the distant reaches of our galaxy. Astronomers are particularly interested in studying the comet at perihelion because the gases and dust emanating from its nucleus can reveal its composition.
The findings of NASA’s and the European Space Agency’s STEREO-A and SOHO space observatories, as well as NASA’s GOES-19 satellite, could help illuminate new details about this interstellar object—only the third of its kind ever spotted. The observations are detailed in a new study by scientists Qicheng Zhang of Lowell Observatory and Karl Battams of the U.S. Naval Research Laboratory, and posted to the preprint server arXiv on Wednesday.
3I/ATLAS’s last hurrah (for now)
The scientists found that, at perihelion, 3I/ATLAS brightened to about magnitude 9—bright enough that if it were in Earth’s line of view, it would be detectable using just a backyard telescope.
Zhang and Battams also found that 3I/ATLAS appeared “distinctly bluer than the Sun,” a coloration consistent with the gas emissions that contributed to the comet’s visible brightness near perihelion.
Interestingly, 3I/ATLAS brightened far more rapidly than a more familiar class of comets that originate from the Oort cloud, a mysterious, icy sphere that lies beyond the Kuiper Belt at the outer reaches of our solar system that researchers believe is filled with comet-like objects—although no one has directly observed it, so no one is entirely sure.
In their paper, Battams and Zhang speculate as to why 3I/ATLAS brightened so quickly. One possibility is that “oddities” in its nucleus properties, like composition, shape, or structure, may be behind it. The comet could have acquired these unusual characteristics from its host star system or during its long interstellar journey to our own, they hypothesize.
The show’s not over yet
This interstellar traveler is now traveling away from the inner solar system. Astronomers expect 3I/ATLAS to briefly pop back into Earth’s view in late November or early December, before making its closest approach to our planet around December 19.
From that point on, 3I/ATLAS will gradually fade from sight as it continues on its journey away from our solar system. But spacecraft could continue to offer new insights. It’s possible that NASA’s Psyche mission to the asteroid of the same name, or the Lucy mission to Jupiter’s Trojan asteroids, might observe the comet again, offering us more clues to its origins and makeup.
Scientists have already found that 3I/ATLAS is highly unusual, with one of the highest carbon dioxide to water ratios ever observed in any comet. It’s likely this cosmic interloper will continue to surprise as researchers sift through the perihelion data.
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