Nasa announced this Monday, December 1, 2025, confirmation that comet 3I/Atlas is a natural object originating from another star system, with no evidence of artificial activity. The comet, detected in July this year, crosses Sistema Solar on a hyperbolic trajectory and poses no threat to Terra. In parallel, the Organização of the Nações Unidas began a planetary defense exercise to monitor its passage, involving global space agencies. The action takes place at Genebra, at Suíça, and aims to improve protocols for tracking celestial bodies.
Experts from the American space agency based the conclusion on observations from more than 20 missions, including telescopes Hubble and James Webb. The object exhibits characteristics typical of comets, such as a gaseous coma and dust tails. Onu, through Rede Internacional of Alerta of Asteroides, selected 3I/Atlas as the simulation target due to its prolonged visibility.
The comet heads towards the constellation of Gêmeos after perihelion in October.
- Current speed: around 246 thousand km/h.
- Minimum distance to Terra: scheduled for December 19th, 270 million km.
- Brightness: magnitude 11.3, observable with amateur telescopes.
Discovery and initial trajectory
Comet 3I/Atlas appeared in the telescopes of the Asteroid Terrestrial-impact Last Alert System system, in Chile, between June 14 and 29, 2025.
The hyperbolic trajectory indicates that the object does not return to Sol, moving at speeds that exceed 220,000 km/h at initial detection. Observatórios in Havaí and África in Sul contributed to the first data, revealing a core estimated to be between 440 meters and 5.6 km in diameter.
In July and August, missions such as Hubble and James Webb captured images of the coma and tail, highlighting atypical but natural gaseous emissions.
Scientific confirmation of Nasa
The American space agency gathered data from orbiters at Marte and ground-based telescopes to validate the composition of 3I/Atlas. Análises chemistries show a predominance of carbon dioxide in the coma, with traces of carbon monoxide and water vapor.
Nickel emissions, initially seen as anomalous, align with sublimation processes in ancient comets, exposed to cosmic rays for billions of years. The surface crust, 15 to 20 meters deep, preserves original materials from its home star system.
Nasa dismissed acceleration variations as evidence of technology, attributing them to irregular gas jets.
Images from the Perseverance rover on October 3 refined the orbit by a factor of ten, eliminating uncertainties.
Start of the exercise of Onu
Rede Internacional of Alerta of Asteroides, linked to Onu, activated the protocol on November 27, 2025, for a period of two months. The focus lies on tracking diffuse objects, such as comets, which defy precise measurements due to their extended appearance.
Member countries, including Estados Unidos and members of União Europeia, coordinate observations via Escritório from Nações Unidas to Assuntos from Espaço Exterior. The exercise tests global communication and orbital modeling without real risk of impact.
Experts highlight that 3I/Atlas offers a unique opportunity to simulate responses to interstellar visitors.
Training closes on January 27, 2026, with preliminary reports expected in February.
Observed chemical characteristics
The comet’s composition reveals a coma rich in CO₂, with sublimation that releases cyanide and carbonyl sulfide. Gelo of water forms the core, altered by cosmic bombardment over about 7 billion years.
The anti-tail, a phenomenon of heavy dust crossing the orbital plane, explains the post-perihelion glow. Erupções of fresh ice increased luminosity in November, reaching magnitude 11.
Spectroscopic analyzes confirm the absence of iron associated with nickel, a common trait in solar comets but explainable by external origin.
Observations from international agencies
Agência Espacial Europeia contributed data from ExoMars Trace Gas Orbiter, capturing sharp images in October. The Juice mission, en route to Júpiter, adjusted trajectory for timely observations in March 2026.
Telescopes such as SOHO and STEREO have recorded the blue ion tail, stretching hundreds of thousands of kilometers. Global collaboration has improved predictions, reducing errors in astrometric positions.
- Key contributions: Hubble to coma structure; James Webb for infrared emissions.
- Missions in Marte: MAVEN detected halo of gas and dust.
- Terrestrial observatories: MeerKAT prepares radio listening in December.
Comet 3I/Atlas represents the most detailed study of an interstellar visitor, with data that compares our Sistema Solar to others on Via Láctea. Sua passage on December 19, at 1.8 astronomical units, allows continued observations before exit into interstellar space. Astrônomos amateurs in the northern hemisphere see the object at Virgem and Leão, using 150 mm aperture equipment. The magnitude weakens to 13, calling for dark skies. Telescópios automated systems such as ATLAS continue monitoring until March 2026, when the comet passes 50 million km from Júpiter. Essa gravitational interaction can slightly alter the final trajectory, providing lessons for future modeling of hyperbolic objects. Onu emphasizes that the exercise strengthens the global warning network, preparing for real threat detections.
Visibility and upcoming milestones
3I/Atlas appears in the morning sky, rising from Virgem to Leão in December. Sua ionic tail, bluish, contrasts with the yellowish dust, visible in long exposures.
Astronomers recommend apps like Stellarium for accurate tracking, taking into account urban light pollution. Peak observability occurs between December 10th and 20th, before closest approach.
In March, the passage through Júpiter offers a chance for measurable orbital deviations, refining theories of non-gravitational acceleration.
Contributions from space missions
Nasa integrated data from Lucy and Psyche, capturing images in September 445 million km from Terra. SPHEREx measured gas columns, confirming 95% CO₂.
Esa’s Comet Interceptor, planned for the decade, uses lessons from 3I/Atlas for future missions. Orbitadores in Marte paused routines for photos, revealing details of the coma.
This network of global observations guarantees precision in ephemeris, essential for defense exercises.