Comet 3I/ATLAS is the 3rd known interstellar object (ISO) to have visited our Solar System.

Interstellar objects are leftover fragments (comets and asteroids) of the process of the birth of a solar system that are ejected out of their home star systems and whose path takes them through our Solar System. These are important for science because it gives us the opportunity to study a sample from another solar system “up close” in a way that would not otherwise be possible.

Discovery

3I/ATLAS (also known as C/2025 N1) was discovered 2025 July 1 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) station in Río Hurtado, Chile. On July 2, the International Astronomical Union’s (IAU) Minor Planet Center announced that with the object’s high eccentricity (e=6.137; a term which describes the shape of the orbit) that it was not orbiting our Sun, but coming from outside the Solar System. This was an extremely challenging observation because the path of 3I was projected against the dense star background towards the center of our galaxy.

3I/ATLAS is the third known interstellar object (ISO) to have visited our Solar System. The first, 1I/ʻOumuamua, was discovered by the Pan-STARRS1 telescope in Maui, Hawaiʻi on 2017 October 19, and the second, 2I/Borisov, was discovered 2019 September 30 by amateur astronomer Gennadiy Borisov.

Why are these interstellar objects just now being discovered? Most likely these have passed through our Solar System since its formation, but we have only recently developed sufficiently large telescopes designed to frequently survey the entire night sky for faint moving objects.

Orbit

3I/ATLAS made its closest approach to the Sun on 2025 October 29 at a distance of 1.35 au from the Sun (about 126 million miles). At perihelion, it was behind the Sun as seen from Earth and thus not visible to Earth-based telescopes. The comet will start to become visible again in mid-November 2025. However, there were many NASA assets in space that could observe it even at perihelion: 3I’s orbital inclination is close to the plane of the orbits of the Solar System planets, which meant that it has (and will) fly relatively close to some of the missions exploring our planetary neighbors.

Credit: NASA/JPL-Caltech/AP

Astronomers, including David Tholen at the IfA, have been carefully measuring the position of 3I as it moves along its orbit, searching for evidence of non-gravitational acceleration. This represents deviations in the orbital path of a small body caused by non-uniform outgassing from the comet’s surface, which acts like small thrusters. This has now been detected in the orbit of 3I/ATLAS—something that is unsurprising given the data that shows that the outgassing is not coming uniformly from the surface.

Visibility

3I will be coming out from behind the Sun and become visible in Hawaiʻi in mid-November and should be visible to ground-based telescopes through May 2026 when it will be in solar conjunction again. At that time it will have moved outward beyond the orbit of Jupiter. It will again be visible to telescopes from September 2026 (when it will be beyond the orbit of Saturn) to May 2027 (out to the orbit of Uranus). From September 2027 through the end of 2028 only the largest space telescopes will be able to observe 3I, at which point it will be too faint for even JWST to observe. Scientists will be excited to see how the comet has changed as a result of the heating from perihelion.

Observations

Ground-based observations

A number of ground-based facilities have been used to detect and observe the gas flowing from 3I. The University of Hawaii 2.2m telescope was used to obtain spectra from July 3 through September 2, led by IfA graduate student Willem Hoogendam. This data started to show the presence of cyanide (CN) and nickel (which breaks off from compounds on the surface of the comet) on August 18. CN had been detected as early as August 14 with the Very Large Telescope in Chile (Rahatgaonkar et al, accepted at ApJL).

Data from Maunakea’s Keck Observatory, again led by Willem Hoogendam, showed that the spatial distribution of the CN and nickel around the comet was different: with the nickel uniformly distributed, and the CN appearing in two jets (Hoogendam et al, arXiv preprint). Searches for another organic molecule, hydrogen cyanide (HCN) and carbon monoxide, were conducted using the James Clerk Maxwell Telescope (JCMT) by former IfA graduate student Jason Hinkle between July 16-21. However, no HCN was detected with JCMT until September 14 when the comet approached close enough to the Sun for more volatiles to escape (Coulson et al, arXiv preprint).

Infrared observations led by former IfA postdoc Bin Yang, using the NASA Infrared Telescope Facility (IRTF) on Maunakea, detected water ice grains in the cloud of material surrounding 3I from data taken in early July (Yang et al, accepted at ApJL); but gaseous water, likely coming from the icy grains, was not detected until late July from the space-based Swift Observatory (Xing et al, submitted to ApJL).

Space-based observations

The Hubble Space Telescope observed 3I/ATLAS on 2025 July 21, and the data allowed astronomers to image the dust cloud coming off of the solid icy nucleus and to estimate that the size of the nucleus was less than 5.6 km across (3.5 miles; Jewitt et al, arXiv preprint).

The James Webb Space Telescope (JWST), using its NIRSpec instrument while the comet was inbound toward the Sun, detected carbon dioxide (CO₂) in the gas cloud surrounding the nucleus, in addition to water. The ratio of CO₂/H₂O ≈ 8 is one of the highest ratios measured in any comet. Smaller amounts of CO, H₂O, OCS, water-ice and dust were also seen (Cordiner et al, accepted at ApJL). The early brightness data obtained from IfA facilities, including data found in the ATLAS survey data incidentally taken prior to its discovery, were used in models which suggested that carbon dioxide outgassing was controlling the activity observed in 3I. Gaseous water was detected by the Swift space observatory in late July 2025, likely coming from the icy grains seen in the cloud of material surrounding the nucleus as noted above.

Observations from spacecraft

Many NASA spacecraft have (and will) observe the comet, including SPHEREx; the PUNCH mission; the Parker Solar Probe; the Perseverance and Curiosity rovers on Mars; the Mars Reconnaissance Orbiter; the Lucy, Psyche, and Europa Clipper spacecraft; and ESA/NASA’s SOHO and Juice spacecraft. Two of ESA’s Mars orbiters, ExoMars Trace Gas Orbiter (TGO) and Mars Express, observed 3I in early October 2025 (ESA press release). However, because of the US government shutdown, any data that have already been taken by NASA have not yet been fully analyzed and released as of November 7.

It should be noted that none of these missions was originally designed to observe faint moving comets. In particular, the cameras for the Mars missions were designed to look down on a nearby bright planet; so while the spacecraft data could provide information when the comet was not visible from Earth, the science that can be accomplished is limited.

What have we learned about 3I?

Because these objects (so far) have been very rare, and they present a unique opportunity to study in detail the chemistry and physical conditions at the birth of another solar system, astronomers quickly devote telescope time to study them. At the Institute for Astronomy the telescope time has been a combination of projects that had requested time in the event of a new discovery, while others have re-purposed existing time to take advantage of the exciting opportunity.

Data from the ATLAS, ASAS-SN, and Pan-STARRS surveys are being used to understand the composition of the ices that are controlling the development of the tail (carbon dioxide).

The Gemini 8m telescopes in Hawaiʻi and Chile have been used by our UH team, led by Karen Meech, to obtain images and spectra to properly study the dust and gas coming from the comet. Images of the comet (below) taken from the 8m Gemini North telescope through multiple color filters on July 3 (shortly after discovery) are combined into a “true color” image which shows the characteristic reddish color of the dust. The red color comes from the interplay between the variety of different particle sizes and the dust’s organic-rich composition.

Comet 3I/ATLAS streaks across a dense star field in this image captured by the Gemini North telescope’s Gemini Multi-Object Spectrograph (GMOS-N). The left panel captures the comet’s colorful trail as it moves through the Solar System. The image was composed of exposures taken through three filters, shown here as red, green, and blue. The right inset zooms in to reveal the comet’s compact coma—a cloud of gas and dust surrounding its icy nucleus.

Image credit: International Gemini Observatory/NOIRLab/NSF/AURA/K. Meech (IfA/U. Hawaii)
Image Processing: Jen Miller & Mahdi Zamani (NSF NOIRLab)

One set of observations was conducted in conjunction with students in Chile and Hawaii in the “Shadow the Scientists” program. The Gemini South images taken during the “Shadow the Scientists” event on August 27 (see image at top of page) show the growing tail of the comet and provided some early detections of cyanide gas (CN) coming from the comet.

The most common ices found in comets, which when heated turn into gas and push the dust from the surface, are water (H₂O), carbon dioxide (CO₂), and carbon monoxide (CO). The image below shows the signature of water ice in the infrared wavelengths for 3I/ATLAS from the NASA IRTF.

The black line shows the observed data in the infrared; the data are noisy due to the the comet’s faintness at the time. The blue line shows what a laboratory spectrum of small water ice grains looks like. This almost matches the data; material needed to be added to make the modeled reflected light slightly more red (typically this is from carbon-rich compounds; orange line). The red line is the best fit model to the data.

There are many other minor species from organic compounds that also get dragged out from the comet with these other gases. Cyanide (CN), while not abundant, is often the first gas detected as it reacts very strongly with sunlight, causing it to glow.

Cyanide (CN) emission is a useful diagnostic tool to study comets, and this includes the remarkable latest interstellar visitor: 3I/ATLAS. While CN is not the most abundant molecule in comets, it is one of the first to sublimate because it reacts strongly to sunlight. This causes CN to fluoresce brightly, even at faraway distances from the Sun. Initial spectroscopy of 3I/ATLAS using the University of Hawaii 2.2m telescope found no CN emission; however, subsequent observations from the same telescope revealed CN emission as the comet approached the Sun. Furthermore, integral field spectroscopy by UH/IfA astronomers using the Keck Observatory showed that CN exhibited a distinct spatial distribution around the comet and appeared in two jets rather than uniformly around the comet. This is the first time such observations have been performed for any comet using the Keck Observatory, and future observations will continue to leverage this unique capability.

This is an unprocessed image downloaded from the Gemini South telescope on August 27, showing the spectrum of the light coming from the dust (bright horizontal band), and the emitted light from CN (extended brightness above and below the dust, circled in red).

Comparing the 3 interstellar objects we have seen so far

All three interstellar objects show differences from each other, but are very similar to our solar system’s comets, but with some chemical differences. Only once we have statistical information on a large number of these will we understand what they are telling us about the diversity of planet formation environments. The 8m Rubin Telescope’s Large Synoptic Survey which is beginning this year from the southern hemisphere will likely find many more ISOs during its 10 years of operation.

Institute for Astronomy