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

The interstellar comet 3I/ATLAS, the third object confirmed from outside the Solar System, reached its closest point to the Sun on October 29, 2025, 1.36 astronomical units away. This approach, known as perihelion, occurred at around 8:47 am Brasília time and marked the moment when the visitor gained greater brightness due to the sublimation of ice in its composition. Astronomers have been monitoring the phenomenon since July, when the comet was detected by the ATLAS telescope in Chile.

Temporary occultation by the Sun prevented ground-based observations in the days before perihelion, but space missions such as ESA’s ExoMars Trace Gas Orbiter captured images from Mars’ orbit on October 3. 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 in Brazil, visibility depends on specific clear sky conditions and suitable equipment, as the comet will not reach brightness for naked eye viewing. The International Asteroid Warning Network coordinates a global tracking campaign between November and January to analyze their composition and trajectory.

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

Trajectory and discovery of 3I/ATLAS

Comet 3I/ATLAS entered the Solar System in June 2025, crossing the constellation of Sagittarius near the center of the Milky Way. Telescopes in Chile have recorded cometary activity since May, with a reddish coma indicative of carbon-rich dust.

Hubble Space Telescope observations 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. This high ratio of CO2 to water differentiates the comet from local objects.

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

Initial observations and early activity

Comet activity began 6.4 astronomical units from the Sun, further away than expected for metal vaporization. Observations from the Nordic Optical Telescope in July confirmed active coma and tail elongation of 3 arc seconds.

Southern Hemisphere Tracking Preparation

Astronomers in Brazil adjust telescopes for increasing elongation starting November 3, when the comet appears 9 degrees above the eastern horizon. Visibility improves until November 17th in the constellation Virgo, with an altitude of three degrees.

Amateur equipment with an aperture of 150 to 200 mm allows detection as a diffuse point, but interference from sunlight requires times close to dawn. Observatories like the one in 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 Brazil: challenges and windows

For Brazilian latitudes, close to the equator, the initial position favors low observation in the morning sky, but requires a horizon free of obstacles. In December, greater 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 monitoring campaign

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

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

Differences from previous visitors

Compared to 1I/Oumuamua, with no tail detected, 3I/ATLAS displays a clear coma and tail, similar to 2I/Borisov, but with greater diameter and activity at cold distances. Its retrograde orbit, opposite to most solar comets, requires adjustments in projections.

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

Scientific potential of interstellar visitor

The comet provides the third interstellar sample for multi-wavelength, radio-to-ultraviolet analysis, refining models of external chemical composition. Observations by the James Webb Space Telescope in August revealed unusual nickel emissions, detected at distances where temperatures do not vaporize common metals.

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