Visitor 3I/ATLAS shines close to the Sun: Brazilians will be able to observe with equipment from November

Interstellar comet 3I/ATLAS, the third confirmed object outside of Sistema Solar, reached its closest point to Sol on October 29, 2025, 1.36 astronomical units away. The approach of Essa, known as perihelion, occurred at around 8:47 am, the time at which Astrônomos has been monitoring the phenomenon since July, when the comet was detected by the ATLAS telescope in Chile.

The temporary occultation of Sol prevented ground-based observations in the days before perihelion, however space missions such as ESA’s ExoMars. The object, with a core estimated to be up to 5.6 kilometers in diameter, follows a hyperbolic trajectory at 221 thousand kilometers per hour, confirming its external origin.

For observers at Brasil, visibility depends on specific clear sky conditions and appropriate equipment, as the comet will not reach brightness with the naked eye. Rede Internacional of Alerta of Asteroides coordinates a global monitoring campaign between November and January to analyze its composition and trajectory.

• Initial apparent magnitude: about 12, requiring telescopes with an aperture of at least 200 mm.
• Post-perihelion position: in the constellation of Virgem, rising above the eastern horizon before dawn.
• Minimum distance to Terra: 1.8 astronomical units on December 19, 2025.

Trajectory and discovery of 3I/ATLAS

Comet 3I/ATLAS entered Sistema Solar in June 2025, crossing the constellation of Sagitário near the center of Via Láctea. Telescópios and Chile have recorded cometary activity since May, with a reddish mane indicative of carbon-rich dust.

Observations of Telescópio Espacial Hubble in July revealed a faint 30-arcsecond tail formed by dust particles driven by solar radiation.

Chemical composition revealed by space missions

NASA’s SPHEREx mission detected an extensive carbon dioxide coma in August, with a radius of at least 348,000 kilometers, visible only in the near-infrared. Essa the high ratio of CO2 to water differentiates the comet from local objects.

The Telescópio Gemini Sul captured deep images showing an irregular nucleus and diffuse coma, with reddish tones confirmed by spectroscopy.

Initial observations and initial activities.

The comet’s activity began at 6.4 astronomical units of Sol, further away than expected for metal vaporization. Observações from Nordic Optical Telescope in July confirmed active coma and tail elongation of 3 arc seconds.

Preparation for monitoring in the southern hemisphere

Brasil astronomers adjust the telescopes to increase elongation starting on November 3, when the comet appears 9 degrees above the eastern horizon. Visibility improves until November 17th in the constellation of Virgem, with an altitude of three degrees.

Amateur equipment with an aperture of 150 to 200 mm allows detection as a diffuse point, however interference from sunlight requires hours close to dawn. Observatórios and Brasília integrate global data to refine orbital models.

The IAWN campaign prioritizes accurate measurements despite the coma, which complicates calculations of the nucleus’ exact position and size.

Visibility in Brasil: challenges and windows

For Brazilian latitudes, close to the equator, the initial position favors low observation in the morning sky, however it requires a horizon free of obstacles. In December, the increased elongation reduces solar interference, making detection more viable with medium-sized telescopes.

The projected brightness of magnitude 11.5 allows for confusion with nearby stars, requiring tracking software for precise location.

Global tracking campaign

International Asteroid Warning Network organizes coordinated observations from November 27 to January 27, 2026, with the participation of ground-based and space telescopes. NASA’s Europa Clipper probe could cross the tail in November, collecting samples of interstellar particles for the first time.

Missions like ESA’s Juice schedule sessions on November 2 and 25 to obtain high-resolution images of the tail structure.

Differences with previous visitors

Compared to 1I/Oumuamua, which had no tail detected, 3I/ATLAS has a clear coma and tail, similar to 2I/Borisov, but with greater diameter and activity at cold distances. The retrograde orbit of Sua, unlike most solar comets, requires adjustments in projections.

Studies indicate a possible age greater than three billion years, prior to Sistema Solar, offering data on planetary formation in other systems.

Scientific potential of interstellar visitor

The comet provides the third interstellar sample for multiwavelength radio-ultraviolet analysis, refining models of external chemical composition. Observações of James Webb Space Telescope in August exposed atypical nickel emissions, detected at distances where temperatures do not vaporize base metals.

Preliminary data suggests an evolving surface, with the coma turning red throughout July due to the release of dust.