In 1995, he and Thomas Bopp spotted, from different states, the same mysterious object in the sky. What turned out to be a comet was named after them: Hale-Bopp.

The NSF–DOE Vera C. Rubin Observatory has yet to begin its 10-year Legacy Survey of Space and Time, but it’s already keeping an eye on the sky. Astronomers recently described Rubin’s serendipitous spotting of 3I/ATLAS days before the interstellar object’s discovery.

Welcome, 3I/ATLAS

Our solar system has been playing host to a rare interstellar tourist: 3I/ATLAS. Just the third known interstellar object to have passed through our star system, 3I/ATLAS was discovered on 1 July 2025 by the robotic telescope network of the Asteroid Terrestrial-impact Last Alert System (ATLAS). After making its closest approach to the Sun in October 2025, 3I/ATLAS is now journeying out of the solar system and is currently located beyond the orbit of Jupiter.

While astronomers would likely grumble about hosting a human guest for nearly a year, they’ve been thrilled with 3I/ATLAS’s lengthy visit and have turned countless telescopes toward the rare interstellar object. They’ve also turned to the archives, searching for pre-discovery sightings of the object — and finding them in the commissioning data from one of the most highly anticipated observatories on the planet: Rubin Observatory.

A Serendipitous Sight

In the months since the release of Rubin’s exceptional first images, its 8.4-meter telescope and array of instruments have undergone rigorous testing to ensure they can meet the high standards required for the upcoming survey.

For most of 2025, Rubin was in its commissioning phase, carrying out science validation observations — a critical period during which instruments, procedures, and pipelines are fine tuned, and the quality and quantity of data collection can vary. On 20 June 2025, the very first night of its science validation survey, Rubin happened to turn toward the small patch of sky containing the as-yet undiscovered 3I/ATLAS.

More to Come

In a recent article, Colin Orion Chandler (LSST Interdisciplinary Network for Collaboration and Computing Frameworks; University of Washington) and collaborators described how they tracked down 3I/ATLAS in Rubin’s commissioning data after the object’s discovery was announced. In total, the observatory serendipitously spotted 3I/ATLAS nine times between 21 June and 2 July, then several more times — some on purpose, some by chance — between 3 and 20 July.

Analyzing Rubin’s observations of 3I/ATLAS required the development of custom techniques, since the observatory’s pipeline for handling observations of rapidly moving solar system objects was not in operation at that time. Despite these challenges, Chandler’s team successfully made astrometric measurements, collected photometry, determined 3I/ATLAS’s orbit, and studied the comet’s structure and nucleus. Among other findings, these data clearly show the presence of the object’s fuzzy coma, providing the earliest high-resolution evidence of cometary activity.

In their analysis of Rubin’s observations of 3I/ATLAS, Chandler and collaborators found that had the science validation survey started sooner, and had the analysis pipelines been operational, Rubin might have been the first to welcome humanity’s third interstellar visitor. Findings like these heighten the anticipation for the start of the Legacy Survey of Space and Time, during which Rubin will surely build upon its already impressive record of interstellar object observations.

Citation

“NSF-DOE Vera C. Rubin Observatory Observations of Interstellar Comet 3I/ATLAS (C/2025 N1),” Colin Orion Chandler et al 2026 ApJL 1001 L35. doi:10.3847/2041-8213/ae4b3a

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Researchers tracked the tail of comet C/2023 P1 (Nishimura) as it interacted with a string of coronal mass ejections, leading to the first-ever quantitative analysis of a cometary tail detachment event.

A Comet’s Journey

When comets journey into the inner solar system, they tend to do so sporting two tails: a dust tail that sweeps back from the comet along its curved trajectory and an ion tail that points away from the Sun, in the direction of the interplanetary magnetic field.

The ion tail, which forms when ultraviolet light from the Sun ionizes gas in a comet’s fuzzy coma, interacts with structures in the solar wind, causing it to shift, sputter, and sometimes even disconnect entirely. Now, for the first time, researchers have quantified the timescales involved when a comet loses — and regrows — its tail.

The Tale of a Tail

Shaheda Begum Shaik (George Mason University; US Naval Research Laboratory) and collaborators studied this phenomenon in observations of the comet C/2023 P1 (Nishimura) from 1 to 14 September 2023. In high-resolution images from the Solar Orbiter Heliospheric Imager, Shaik’s team analyzed the dynamics of C/2023 P1’s tail as the comet braved blustery solar wind conditions in the inner solar system.

In a two-week period, the comet underwent four separate tail disconnection events, in which the connection between the ion tail and the comet was severed. Each of these events coincided with the passage of a coronal mass ejection: a tangled mass of solar plasma and magnetic fields ejected from the Sun’s outer atmosphere.

Lizard-Like Regrowth

Focusing on the tail disconnection event with the highest-resolution and highest-cadence data, Shaik’s team observed constant small-scale fluttering of the ion tail, reflecting the buffeting of the tail by the solar wind. The tail then developed a kink, which the team speculated is due to compressed solar wind plasma piling up in front of an oncoming coronal mass ejection. The wide-field images show the coronal mass ejection advancing upon the comet and the comet’s tail seemingly being sliced in two.

The free-floating tail segment sped away from the comet at roughly 295 km/s, likely indicating that the tail became caught up in and was transported by the flank of the coronal mass ejection as it barreled past the comet. The timescale and geometry of the event suggest that the interaction of the coronal mass ejection’s magnetic field with the comet’s tail was responsible for the disconnection.

Over the following 24 hours, the comet’s tail slowly regrew at a rate of 86 km/s to its original length of 1.9 million kilometers. The rate of regrowth is likely determined by several factors, such as the rate at which the comet produces ions and the local magnetic field configuration. This work represents a first look at the quantitative behavior of a tail disconnection event, paving the way for future investigations of cometary behavior and a greater understanding of the complex magnetic and plasma environment of the inner solar system.

Citation

“The First Quantitative Study of Cometary Tail Regrowth Following a Coronal Mass Ejection-Driven Disconnection Event,” Shaheda Begum Shaik et al 2026 ApJ 999 60. doi:10.3847/1538-4357/ae3bdb

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Editor’s Note: After a short hiatus, the Monthly Roundup is back! This series of articles, which began in 2023, examines multiple perspectives or findings on a single topic.

The discovery of interstellar object 3I/ATLAS on 1 July 2025 was one of last year’s top astronomy stories. First identified by the Asteroid Terrestrial-impact Last Alert System (ATLAS), 3I/ATLAS is just the third interstellar object that astronomers have seen speeding through our solar system; it was preceded by 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019.

Last summer, professional and amateur astronomers worldwide rolled out the red carpet in 3I/ATLAS’s honor, enlisting a pack of ground-based and spacefaring paparazzi to capture its every move. Observations in the first few months after 3I/ATLAS’s discovery revealed cometary features, including a fuzzy coma of gas and dust, a tail that trailed behind the object, and a plume of dust on the side facing the Sun.

With 3I/ATLAS having now made its closest approach to the Sun, venturing within 1.36 au of our star on 29 October 2025, the comet is wrapping up its tour of the solar system and heading back out to interstellar space — but the research continues. Today, we’re taking a look at five of the more than three dozen research articles published in the AAS journals that have examined 3I/ATLAS and its journey through our solar system. As always, this Monthly Roundup contains only a short summary of each article, so be sure to check out the full research articles linked at the bottom of this post!

Early Spectra of 3I/ATLAS

Bin Yang (Diego Portales University and the Planetary Science Institute) and collaborators obtained spectra of 3I/ATLAS less than two weeks after it was discovered, when it was 4.0–4.4 au from the Sun. Using the Gemini South telescope and the Infrared Telescope Facility (IRTF), Yang’s team sought to characterize its coma, the dusty envelope of gas that puffs out from the icy nucleus as the comet warms up.

These observations showed that 3I/ATLAS has a relatively featureless, red-sloped spectrum from 0.5 to 0.8 microns, similar to certain asteroids and active comets in our solar system. In the near-infrared, 3I/ATLAS’s spectrum was flatter, with a broad absorption feature around 2.0 microns.

Yang and coauthors found that the near-infrared spectrum was well fit by a model including a blend of amorphous carbon and water ice (63% carbon and 37% water ice by volume) at a temperature of 120K. 3I/ATLAS’s spectrum bears certain similarities to well-studied solar system comets like C/2006 W3 and 6P/d’Arrest, which suggests that the particles in these objects’ comae are similar in size or composition. Ultimately, the discovery of abundant water ice in 3I/ATLAS’s coma, along with the carbon dioxide spotted in other studies, suggests that the comet formed in the cold, volatile-rich outer regions of its home system.

Unique Polarization Properties

3I/ATLAS is the second interstellar object for which astronomers have obtained polarization information. The first, 2I/Borisov, showed a high degree of positive polarization, comparable to the exceptional solar system comet Hale–Bopp.

Using the Very Large Telescope, the Nordic Optical Telescope, and the 2m Ritchey-Chrétien-Coude telescope to study 3I/ATLAS’s polarization between 17 July and 28 August 2025, Zuri Gray (University of Helsinki) and coauthors showed that 3I/ATLAS is also exceptional — but in the opposite direction. In contrast to 2I/Borisov’s strong positive polarization, 3I/ATLAS is strongly negatively polarized, with an unusually deep and narrow polarization profile. The narrowness of 3I/ATLAS’s polarization profile is similar to that of certain rare asteroids and cometary nuclei, but 3I/ATLAS is more strongly negatively polarized than these objects.

Gray’s team suggested that 3I/ATLAS’s extreme polarization behavior could be evidence for a mixture of large icy and dark particles in its coma. It’s also possible that it’s the first identified member of a new class of comets, distinct from both solar system comets and interstellar comet 2I/Borisov.

Is 3I/ATLAS Truly a Pristine Relic of the Star System It Came From?

Interstellar objects like 3I/ATLAS have been hailed as pristine beacons that grant us a rare glimpse into the conditions of other star systems. But as new work by Romain Maggiolo (Royal Belgian Institute for Space Aeronomy) and collaborators shows, 3I/ATLAS was likely greatly altered by its journey through interstellar space.

Spectroscopic measurements from JWST suggest that 3I/ATLAS has extremely high abundance of carbon dioxide and carbon monoxide compared to water. A comet’s chemical abundance ratios can be either inherited from the material from which the comet formed, or they can be due to processing after its formation. The team finds that comet 3I/ATLAS’s extreme abundance ratios can be attributed to the comet being pummeled by galactic cosmic rays — high-energy charged particles — as it cruised through interstellar space.

This means that 3I/ATLAS’s current composition doesn’t match its initial composition at the time of its formation, and the cosmic-ray-altered layer of the comet’s nucleus may be 15–20 meters thick. Could volatile outgassing and dust ejection scratch away enough of the comet’s outer layers to expose its pristine interior? This depends on the level of activity the comet experienced as it passed close to the Sun in October 2025. If the comet outgassed strongly, it could have shed tens of meters from its irradiated shell, exposing the pristine core material. Under low to moderate outgassing scenarios, however, the pristine inner material remains hidden beneath the processed shell. Now past the comet’s closest approach to the Sun, we’ll soon know more about whether the composition of the outgassed material has changed as the comet has moved through the solar system.

3I/ATLAS as a Template for Future Encounters: Planning a Potential Rendezvous

It’s only a matter of time before the next interstellar object pays our solar system a visit, and researchers are already thinking about how they’ll study the next visitor. One of the best ways to learn about an interstellar interloper would be to send a spacecraft to rendezvous with it and collect data from up close.

To plan for possible future rendezvous, Atsuhiro Yaginuma (Michigan State University) and collaborators considered what it would have taken for an existing spacecraft to meet up with 3I/ATLAS shortly after it was discovered. The team considered ready-to-launch spacecraft on Earth as well as operational spacecraft currently orbiting Mars.

Their calculations showed that meeting up with 3I/ATLAS from Mars would have required less energy than launching a new spacecraft from Earth, with Mars-originating flybys potentially possible with existing technologies. For Earth-originating flybys launched on or after the discovery date of 1 July, linking up with 3I/ATLAS is likely beyond current capabilities — but if the comet had been discovered earlier, a rendezvous launched from Earth may have been possible.

This highlights the importance of detecting interstellar objects as early as possible, as well as the advantage of stationing well-fueled spacecraft at key locations around the solar system; these spacecraft could be deployed rapidly after an interstellar object is discovered, enabling the collection of valuable data. (This is similar to the premise of the European Space Agency’s Comet Interceptor mission, which will wait at one of the Sun–Earth Lagrange points for the arrival of a long-period comet; such a mission could be repurposed to track down an interstellar comet.)

So, About Those Aliens…

Like its predecessor 1I/ʻOumuamua, 3I/ATLAS prompted speculation that it’s not a natural object, though there’s no compelling evidence that it’s anything but a comet. If 3I/ATLAS were an interstellar probe, we might be able to unmask it as such by intercepting its radio communications. In a recent Research Note, Ben Jacobson-Bell (University of California, Berkeley) and coauthors described their search for narrow-band radio signals — the communication medium for all spacecraft in our solar system — from 3I/ATLAS.

The Breakthrough Listen project pointed the 100-meter Green Bank Telescope in 3I/ATLAS’s direction, just one day before the object made its closest approach to Earth at a distance of 1.8 au. The team identified nine possible “events” in their data, all of which were ruled to be radio-frequency interference. Based on this non-detection, Jacobson-Bell and collaborators ruled out the presence of isotropic, continuously outputting radio transmitters with power greater than 0.1 watt. (For comparison, a cellphone puts out approximately 1 watt, and spacecraft radios tend to put out a few dozen watts.) Other searches for radio transmissions from 3I/ATLAS have similarly detected no signals.

Citation

“Spectroscopic Characterization of Interstellar Object 3I/ATLAS: Water Ice in the Coma,” Bin Yang et al 2025 ApJL 992 L9. doi:10.3847/2041-8213/ae08a7

“Extreme Negative Polarization of New Interstellar Comet 3I/ATLAS,” Zuri Gray et al 2025 ApJL 992 L29. doi:10.3847/2041-8213/ae0c08

“Interstellar Comet 3I/ATLAS: Evidence for Galactic Cosmic-Ray Processing,” Romain Maggiolo et al 2026 ApJL 996 L34. doi:10.3847/2041-8213/ae2fff

“The Feasibility of a Spacecraft Flyby with the Third Interstellar Object 3I/ATLAS from Earth or Mars,” Atsuhiro Yaginuma et al 2025 ApJ 995 64. doi:10.3847/1538-4357/ae11b2

“Breakthrough Listen Observations of 3I/ATLAS with the Green Bank Telescope at 1–12 GHz,” Ben Jacobson-Bell et al 2025 Res. Notes AAS 9 351. doi:10.3847/2515-5172/ae3083

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Editor’s Note: For the remainder of 2025, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 2nd.

Hubble Space Telescope Observations of the Interstellar Interloper 3I/ATLAS

Published August 2025

Main takeaway:

David Jewitt (University of California, Los Angeles) and collaborators used the Hubble Space Telescope to observe the interstellar object 3I/ATLAS just a few weeks after it was discovered. These observations allowed the team to constrain the object’s mass-loss rate and the size of its nucleus, placing its radius between 0.22 and 2.8 kilometers.

Why it’s interesting:

3I/ATLAS is just the third known object to visit our solar system from another planetary system. With such a small sample size, every interstellar object discovered is a source of fascination. Are these objects more like comets or asteroids? How many of them roam the space between the stars? What, exactly, launches them into interstellar space? Using all available tools, including powerful observatories like Hubble, researchers can extract as much information as possible when interstellar objects make their brief journeys through our solar system, and get us closer to answering these key questions.

How these observations were planned, and what happened afterward:

After interstellar objects 1I/ʻOumuamua and 2I/Borisov zipped through the solar system in 2017 and 2019, respectively, researchers knew it was only a matter of time before the next interstellar interloper paid a visit. In preparation for the next arrival, Jewitt’s team proposed a target-of-opportunity observation with Hubble. This allowed them to disrupt the telescope’s planned observing schedule once 3I/ATLAS was discovered, getting an early high-resolution look that could guide further observations. Since then, researchers have published more than 30 articles in the AAS journals alone about 3I/ATLAS. In these articles, researchers have sought to understand where 3I/ATLAS came from, investigated its polarization properties, collected JWST data of the object, found it in pre-discovery data, and much more, crafting a comprehensive view of a rare visitor to our neighborhood.

Citation

David Jewitt et al 2025 ApJL 990 L2. doi:10.3847/2041-8213/adf8d8

The post Selections from 2025: Hubble Observes 3I/ATLAS appeared first on AAS Nova.

The James Webb Space Telescope observed a large amount of methane around 3I/ATLAS, revealing just how different it is from comets from our solar system.

The discovery of interstellar object 3I/ATLAS raises speculation about its nature, with some suggesting it could be an alien spacecraft. However, most astronomers believe it is a comet. The question highlights differing belief systems regarding the origins of life, with critics emphasizing the necessity of extraordinary evidence for such claims.

Jupiter dominates the Solar System’s architecture. At about 318 times Earth’s mass and orbiting far beyond Mars, it acts as a gravitational gatekeeper, tugging on asteroids and comets that might otherwise stay put or never reach the inner planets. Remove Jupiter from the picture and Earth’s impact environment would change in complicated ways. It would […]

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