The comet 3I/ATLAS (C/2025 N1) was discovered on 1 July 2025 by the ATLAS survey telescope in Chile. Early astrometric measurements revealed that the object was moving on an unusual hyperbolic orbit, far too fast to be gravitationally bound to the Sun. This raised the exciting possibility that it could be an interstellar visitor!
As additional observations were collected by a global network of professional and citizen scientist telescopes, and pre-discovery images were identified in sky surveys, it became certain that 3I/ATLAS originated outside our Solar System. This made 3I/ATLAS the third confirmed interstellar object ever observed entering the Solar System, following 1I/ʻOumuamua and 2I/Borisov.
The interstellar object 3I/ATLAS still behaved like a conventional active comet. As it approached the Sun, It developed a bright coma and an extended tail as solar heating led to the outgassing of volatile material. This combination of an interstellar origin and familiar cometary activity made 3I/ATLAS an important target for precision astrometry and orbit determination.
The Challenge of Precision Comet Astrometry
As noted in guidance circulated by the “Minor Planet Center” (the international body responsible for collecting observational data for asteroids, comets, and other small Solar System bodies), comets present unique challenges for accurate astrometric measurements.
Unlike asteroids, which appear as compact point sources, comets are extended and evolving objects. Their comae and tails distort centroid measurements, pulling them away from their central brightness peak, systematically shifting the measured position away from the true nucleus. If not handled properly, these effects introduce systematic errors that bias orbit solutions.
For long-period and interstellar objects, even small systematic errors in astrometric measurements can significantly affect trajectory estimates. Thus, precision astrometry of interstellar objects is less about discovery and more about controlling subtle measurement errors.
As additional observations were collected by a global network of professional and citizen scientist telescopes, and pre-discovery images were identified in sky surveys, it became certain that 3I/ATLAS originated outside our Solar System. This made 3I/ATLAS the third confirmed interstellar object ever observed entering the Solar System, following 1I/ʻOumuamua and 2I/Borisov.
The interstellar object 3I/ATLAS still behaved like a conventional active comet. As it approached the Sun, It developed a bright coma and an extended tail as solar heating led to the outgassing of volatile material. This combination of an interstellar origin and familiar cometary activity made 3I/ATLAS an important target for precision astrometry and orbit determination.
The Challenge of Precision Comet Astrometry
As noted in guidance circulated by the “Minor Planet Center” (the international body responsible for collecting observational data for asteroids, comets, and other small Solar System bodies), comets present unique challenges for accurate astrometric measurements.
Unlike asteroids, which appear as compact point sources, comets are extended and evolving objects. Their comae and tails distort centroid measurements, pulling them away from their central brightness peak, systematically shifting the measured position away from the true nucleus. If not handled properly, these effects introduce systematic errors that bias orbit solutions.
For long-period and interstellar objects, even small systematic errors in astrometric measurements can significantly affect trajectory estimates. Thus, precision astrometry of interstellar objects is less about discovery and more about controlling subtle measurement errors.
[Picture]
Figure 1: Comet 3I Atlas imaged on 19 December 2025. Luminance filter; 60 s exposure optimized for SNR without trailing.
IAWN Campaign: Improving Comet Astrometry Using 3I/ATLAS
To address these challenges, the International Asteroid Warning Network (IAWN) organized a focused international observing effort between November 27, 2025 and January 27, 2026. [Campaign Link] to improve astrometric measurements of comet 3I/ATLAS.
I participated in the mandatory preparatory workshop, which introduced best practices for measuring comet position. The training emphasized accurate timing calibration, careful centroiding of extended sources, and standardized data submission procedures to ensure consistent methodologies across contributors. Over 100 participants took part in the workshop.
To address these challenges, the International Asteroid Warning Network (IAWN) organized a focused international observing effort between November 27, 2025 and January 27, 2026. [Campaign Link] to improve astrometric measurements of comet 3I/ATLAS.
I participated in the mandatory preparatory workshop, which introduced best practices for measuring comet position. The training emphasized accurate timing calibration, careful centroiding of extended sources, and standardized data submission procedures to ensure consistent methodologies across contributors. Over 100 participants took part in the workshop.
My Observations and Submissions on 3I/ATLAS
I observed comet 3I/ATLAS and submitted multiple astrometric measurements as part of the IAWN observing effort. This was my first experience of performing comet astrometry. 3I/ATLAS proved to be an ideal initial target for my telescope, as it was bright, active, spatially extended, and observable over a long time span.
Observations
Imaging observations were obtained on multiple nights, including 19 December 2025 and 31 December 2025. See Figure 1, where the comet’s bright central condensation and surrounding coma are clearly visible. FITS images of the comet were collected using short exposure times to limit trailing to less than 0.5× the full width at half maximum (FWHM). For typical seeing conditions of ~2 arcseconds, this ensured trailing remained below ~1 arcsecond.
A sufficient number of images were acquired to achieve a signal-to-noise ratio (SNR) greater than 10 in stacked data products. When stacking was performed, individual frames were aligned on the comet using its known motion and motion vector obtained from JPL Horizons database. Care was taken to ensure that individual images were not reused across multiple stacks.
Astrometric Methodology
Zero-aperture photometry was used to determine precise astrometric positions of comet 3I/ATLAS. Circular apertures with radii decreasing from 7 to 2 pixels were placed on the comet, and the corresponding right ascension (RA) and declination (Dec) were measured for each aperture. These measurements were then extrapolated to a zero-pixel aperture to estimate the most accurate comet position for each image. This process was applied to 3–5 images on each observing night.
Astrometric calibration was performed using the GAIA DR3 star catalog. For each image, the extrapolated zero-aperture RA, Dec, and apparent magnitude of the comet were recorded.
Figure 2 illustrates how I used the aperture-dependent RA and Dec measurements, along with the linear extrapolation used to derive the zero-aperture astrometric solution.
I observed comet 3I/ATLAS and submitted multiple astrometric measurements as part of the IAWN observing effort. This was my first experience of performing comet astrometry. 3I/ATLAS proved to be an ideal initial target for my telescope, as it was bright, active, spatially extended, and observable over a long time span.
Observations
Imaging observations were obtained on multiple nights, including 19 December 2025 and 31 December 2025. See Figure 1, where the comet’s bright central condensation and surrounding coma are clearly visible. FITS images of the comet were collected using short exposure times to limit trailing to less than 0.5× the full width at half maximum (FWHM). For typical seeing conditions of ~2 arcseconds, this ensured trailing remained below ~1 arcsecond.
A sufficient number of images were acquired to achieve a signal-to-noise ratio (SNR) greater than 10 in stacked data products. When stacking was performed, individual frames were aligned on the comet using its known motion and motion vector obtained from JPL Horizons database. Care was taken to ensure that individual images were not reused across multiple stacks.
Astrometric Methodology
Zero-aperture photometry was used to determine precise astrometric positions of comet 3I/ATLAS. Circular apertures with radii decreasing from 7 to 2 pixels were placed on the comet, and the corresponding right ascension (RA) and declination (Dec) were measured for each aperture. These measurements were then extrapolated to a zero-pixel aperture to estimate the most accurate comet position for each image. This process was applied to 3–5 images on each observing night.
Astrometric calibration was performed using the GAIA DR3 star catalog. For each image, the extrapolated zero-aperture RA, Dec, and apparent magnitude of the comet were recorded.
Figure 2 illustrates how I used the aperture-dependent RA and Dec measurements, along with the linear extrapolation used to derive the zero-aperture astrometric solution.
[Picture]
Figure 2. Zero-aperture astrometric extrapolation for comet 3I/ATLAS. Measured right ascension (top) and declination (bottom) offsets are shown as a function of photometric aperture radius
Data Submission
The final astrometric results were saved in IAU ADES format and submitted to the IAWN campaign coordinators. Multiple independent positions were reported for each night in accordance with campaign guidelines.
Scientific Value of the Observations
The final astrometric results were saved in IAU ADES format and submitted to the IAWN campaign coordinators. Multiple independent positions were reported for each night in accordance with campaign guidelines.
Scientific Value of the Observations
Although comet 3I/ATLAS posed no risk to Earth and has since passed through the inner Solar System and returned to interstellar space, the observations obtained have lasting scientific value. Accurate comet astrometry enables reliable orbit determination, supports modeling of non-gravitational forces caused by outgassing, and aids interpretation of cometary evolution and internal structure.
As an interstellar object, 3I/ATLAS offers a rare opportunity to study material formed around other stars. The astrometric data collected during this campaign will continue to inform investigations of comet physics, planetary system formation, and the dynamic exchange of material between stellar systems.
As an interstellar object, 3I/ATLAS offers a rare opportunity to study material formed around other stars. The astrometric data collected during this campaign will continue to inform investigations of comet physics, planetary system formation, and the dynamic exchange of material between stellar systems.