Avi Loeb

On August 27, 2025, deep imaging of the interstellar object 3I/ATLAS by the Gemini South 8.2-meter telescope — aided by the Gemini Multi-Object Spectrograph (GMOS), revealed a weak tail with a teardrop shape in the anti-Sun direction (reported here). At that time, 3I/ATLAS was at a distance from Earth of 2.59 times the Earth-Sun separation. The Gemini South Observatory is located on a mountain called Cerro Pachón in the Chilean Andes.

The images were taken in the u (upper left), g (upper right), r (lower left) and i (lower right) spectral bands, centered on wavelengths of 0.365, 0.467, 0.616 and 0.747 micrometers, respectively. They show clear evidence for a teardrop shaped tail in the anti-Sun direction behind 3I/ATLAS. The observed tail is about 30 arcseconds, or equivalently 56,400 kilometers long, pointing towards the South East. The coma is about 10 arcseconds or equivalently 18,800 kilometers wide, significantly more extended than its compact appearance in images of 3I/ATLAS from early July 2025.

Data collected on August 6, 2025 by the Webb Telescope (accessible here) confirmed the existence of a carbon dioxide (CO2) gas plume around 3I/ATLAS with an order of magnitude lower levels of water (H2O) and carbon monoxide (CO), as reported earlier by the SPHEREx space observatory team (here). SPHEREx mapped the CO2 plume out to 348,000 kilometers around 3I/ATLAS. The inferred mass loss rates from 3I/ATLAS are 130 kilograms per second for CO2, 6.6 kilograms per second for H2O and 14 kilograms per second of CO. The H2O mass loss rate is only 5% of the CO2 output, unlike expectations from a water-rich comet.

It therefore comes as no surprise that the plume of gas around 3I/ATLAS is shaped by the solar wind and the solar radiation pressure to a teardrop configuration. Before my morning jog at sunrise, I calculated today that the outer edge of the CO2 plume observed by SPHEREx, is bounded by the distance where the ram-pressure of the solar wind equals the ram-pressure of the CO2 outflow.

As of now, it is unclear whether the scattering of sunlight in the glow and tail around 3I/ATLAS is from dust particles or icy fragments made of CO2, CO and H2O that broke off the surface of the nucleus.

Early data from NASA’s Transiting Exoplanet Survey Satellite (TESS), taken on May 7 to June 2, 2025 (accessible here), suggests that 3I/ATLAS may have been active with a surrounding glow of scattered sunlight already at a much larger heliocentric distance of 6 times the Earth-Sun separation. At that distance, the warming of water ice by sunlight is insufficient to trigger cometary activity.

The flux detected by the SPHEREx space observatory at a wavelength of 1 micrometer from 3I/ATLAS on August 8–12, 2025 suggests a huge nucleus or alternatively a compact scattering cloud, with a diameter of 46 kilometers (as reported here). If made of solid material, this size implies that the mass of 3I/ATLAS is a million times larger than that of the previous interstellar comet 2I/Borisov. This huge gap in mass is surprising since we should have discovered numerous objects of the size of 2I/Borisov before discovering a 46-kilometer interstellar object. Moreover, as I noted when 3I/ATLAS was discovered (in a paper accessible here), the amount of rocky material per unit volume in interstellar space is much too small than the value needed to deliver into the inner Solar system one giant rock of this size over the decade-long survey conducted by the ATLAS telescope. In another puzzling tidbit, the trajectory of 3I/ATLAS is anomalously aligned with the ecliptic plane of the planets around the Sun (as discussed here).

Recent spectroscopic data from the Very Large Telescope in Chile (accessible here), reported the surprising detection of cyanide and nickel without iron in the plume of gas around 3I/ATLAS with steeply increasing rates as the object approaches the Sun. Nickel without iron is a signature of industrial production of nickel alloys. Natural comets generically show iron and nickel simultaneously, as both elements are produced simultaneously in supernova explosions.

All this anomalous data raises once again the fundamental questions: what is the nature and origin of 3I/ATLAS?

As 3I/ATLAS will approach perihelion on October 29, 2025, its surface will get warmer and its enhanced outgassing will encounter stronger radiation and wind pressures from the Sun. As is well known from interrogation tactics, a high stress environment elicits confessions. For that reason, 3I/ATLAS might reveal its nature and origin in the coming months.

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https://avi-loeb.medium.com/detection-of-an-anti-solar-tail-for-3i-atlas-0b80eb529ead