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On October 30, the interstellar comet 3I/ATLAS is set to reach perihelion — its closest point to the Sun — at a distance of 1.35 astronomical units, or about 125 million miles (202 million kilometres). Perihelion marks the orbital point where an object is nearest to the Sun, a crucial phase for comets with highly eccentric orbits like 3I/ATLAS.
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As comets draw closer to the Sun, rising temperatures cause the sublimation of surface ice, leading to outgassing and the formation of a bright coma around the nucleus. This activity also gives rise to two distinct tails: a dust tail and an ion tail, composed of charged particles swept away by solar wind. These processes peak during perihelion, significantly enhancing the comet’s brightness.
Unlike most comets in the solar system, 3I/ATLAS is not bound to the Sun’s orbit. It is currently making a one-time pass through the inner solar system before heading back into interstellar space. The comet entered solar conjunction at the end of September, becoming obscured by the Sun’s glare and invisible from Earth. It is expected to reappear in late November or early December, limiting opportunities for observation from Earth-based and near-Earth telescopes, including those positioned at the L2 Lagrange point.
Despite this, a network of spacecraft across the solar system continues to monitor the comet from more favourable angles. NASA missions on Mars tracked 3I/ATLAS during its closest approach to the Red Planet on October 3, when it passed at a distance of 0.19 AU (17.6 million miles or 28.4 million kilometres). These coordinated observations will help scientists better understand the comet’s composition and activity as it reaches perihelion.
Additional missions observing 3I/ATLAS include NASA’s Psyche, which recently began its journey to a metal-rich asteroid, and Lucy, en route to study Jupiter’s Trojan asteroids. The European Space Agency’s Jupiter Icy Moons Explorer (JUICE), currently on its way to the Jovian system, will also be well positioned to monitor the comet. However, JUICE is using its main antenna as a shield against sunlight and will not transmit data back to Earth until February.
Researchers are particularly focused on analysing 3I/ATLAS’s chemical composition during perihelion. Preliminary data suggest unusually high concentrations of carbon dioxide and nickel compared to typical solar system comets. These differences offer clues about the molecular cloud that gave birth to 3I/ATLAS and its parent star system roughly seven billion years ago, providing a rare opportunity to compare its chemistry with that of our own solar system.
Scientists are also watching closely to see whether iron emissions — previously found to be low — increase as the comet heats up. When 3I/ATLAS re-emerges from behind the Sun at the end of November, it is expected to remain mildly active. With an estimated brightness of magnitude 12, it will likely appear dim to the naked eye, but advanced observatories such as Hubble and the James Webb Space Telescope should be able to capture it in striking detail.
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