Astronomers confirmed the presence of an anti-tail in the interstellar comet 3I/ATLAS through observations conducted in October 2025. The phenomenon, first detected in August, occurs when the object is approximately 3.8 astronomical units from the Sun. This elongated structure points toward the central star, differing from typical tails that extend away due to solar radiation pressure.
The comet, discovered in July 2025 by the NASA-funded ATLAS system, is the third known interstellar visitor to the Solar System, following 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. Its hyperbolic trajectory, with a speed exceeding 200,000 kilometers per hour, indicates an origin outside the system. Preliminary analyses suggest larger particles follow the nucleus’s orbit, creating the inverted visual effect.
- The anti-tail extends approximately 30 arcseconds, equivalent to 56,000 kilometers.
- Observations from the WM Keck Observatory in Hawaii recorded asymmetric emission in the comet’s coma.
- The composition includes nickel concentrated near the nucleus and cyanide dispersed in the periphery.
- The object will reach perihelion on October 30, 2025, at 1.4 AU from the Sun.
Origin and trajectory of the stellar visitor
The comet 3I/ATLAS follows a path that brings it close to Mars before crossing Earth’s orbital plane. Telescopes like the Nordic Optical Telescope in the Canary Islands captured images showing the anti-tail transitioning to a conventional tail in September. This shift results from the sublimation of volatile materials under increasing thermal influence.
Its hyperbolicity, with an eccentricity of 1.2, rules out a permanent gravitational link to the Sun. Estimates suggest a mass exceeding 33 billion tons, based on the absence of detectable non-gravitational acceleration.
Formation of the coma and material ejection
The coma of 3I/ATLAS forms from the sublimation of frozen gases as the nucleus heats up. Dust and ice particles released create the enveloping bubble, influenced by solar wind. Observations from the Hubble Space Telescope in July revealed a faint tail pointing eastward, initially away from the Sun.
At distances like 6.4 AU, activity was already evident, with water vapor and hydroxyl detected by the Swift Observatory.
The process releases about 150 kilograms of material per second, predominantly carbon dioxide at 87% and carbon monoxide at 9%. This rate increases as the comet approaches perihelion, altering the observed morphology.
Data from the James Webb Space Telescope confirm emissions of CO2, water, and CO, common in comets, but with atypical nickel concentrations.
Differences from conventional tails
Typical tails form as solar radiation repels fine dust, extending opposite the Sun. In the anti-tail’s case, Earth’s perspective or the orbital dynamics of larger particles creates the illusion of inverted orientation.
Studies indicate this variant is not solely optical but stems from asymmetric ejections in the elongated coma.
The phenomenon was previously recorded in comets like Kohoutek in 1974, where larger grains traced the orbit ahead of the nucleus.
Chemical composition revealed in observations
Spectroscopic analysis from the Keck Cosmic Web Imager detected cyanide and nickel at elevated rates, surpassing solar comet standards by orders of magnitude. Nickel concentrates near the nucleus, suggesting selective release processes.
Cyanide, conversely, spreads widely in the coma, indicating dispersion by solar winds. This asymmetry points to heterogeneities in the comet’s nucleus.
Images from probes orbiting Mars
Probes from the European Space Agency, such as ExoMars Trace Gas Orbiter and Mars Express, captured 3I/ATLAS near Mars in October. The images, with a resolution of 30 kilometers per pixel, show the coma and anti-tail without direct solar interference.
These data complement ground-based observations, limited by the current solar conjunction. NASA’s Europa Clipper mission may interact with the ionic tail between October 30 and November 6.
The potential interaction with solar wind will generate magnetic draping, measurable by onboard instruments.
Changes in tail structure over time
The anti-tail observed in July and August gave way to an antisolar tail in September, as shown by Nordic Optical Telescope images. This reversal occurs due to the ejection of ice fragments under rising temperatures, inverting the particle flow.
Initially, slow dust grains scattered sunlight, mimicking the inverted structure. Later, lighter particles dominate, aligning with the conventional direction.
This evolution supports comet activity models, with surface losses below 0.00005 of the total mass.
Future observations and continuous monitoring
After perihelion on October 30, the comet will approach Earth on December 19, 2025, at 1.8 AU. Ground-based telescopes will resume tracking in November, focusing on potential brightness surges or fragmentation.
The James Webb Space Telescope will continue chemical analyses, seeking signatures of organic compounds. The object’s speed of 58 kilometers per second will keep it visible until September 2026.
