The Red Visitor from Beyond: Unlocking the Secrets of Interstellar Object 3I/ATLAS

Introduction

Astronomers have recorded their first detailed observations of the third confirmed interstellar object, 3I/ATLAS (C/2025 N1), using the Goodman High Throughput Spectrograph on the 4.1 m SOAR Telescope in Chile. Following in the footsteps of 1I/’Oumuamua and 2I/Borisov, 3I/ATLAS presents a unique opportunity to probe the composition and behavior of material formed around distant stars (1).

3I/ATLAS approaching the Sun © Images4U -chronicles-stock.adobe.com

Discovery and Context

3I/ATLAS was discovered on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey telescope in Rio Hurtado, Chile (1). At the time of discovery, the object was 4.4 astronomical units from the Sun on an inbound trajectory. Initial orbit calculations revealed an extremely hyperbolic path, making 3I the fastest and most inclined interstellar visitor yet observed in our solar system, destined to leave the solar system after its perihelion passage on October 29, 2025 (1). Such an extreme path and dynamics suggest that 3I may be a sample of ancient material from a thick-disk star system in the Milky Way (1).

Observations and Methods

On July 3, 2025, Thomas H. Puzia, Rohan Rahatgaonkar, Juan Pablo Carvajal, Prasanta K. Nayak, and Baltasar Luco, used the Goodman High Throughput Spectrograph (HTS), 4.1 m SOAR Telescope in Cerro Pachón, Chile to obtain optical spectra of 3I/ATLAS (1). The spectrograph’s blue camera covers 3200–8500 Å (320–850 nm), and the team employed a 10" long slit with a 400 lines/mm grating to achieve a spectral resolution of R ≃ 850 at 5500 Å (550 nm). Observing conditions were near ideal: 4% humidity, 20 °C air temperature, and clear skies with 1–1.7" seeing. Multiple 600 s integrations allowed precise tracking of 3I’s rapid motion across the sky (1).

Solar analog calibration was a crucial step in measuring reflectance. The researchers used HD 168595 (a G2V star) and the SOLSPEC space-based solar spectrum to correct for instrumental and atmospheric effects. This dual calibration approach ensured robust assessment of the continuum slope and subtle spectral features (1).

Spectral Characteristics

The optical reflectance spectrum from 3700–7000 Å (370–700 nm) revealed a steep red continuum, comparable to extreme trans-Neptunian objects, and only a weak ultraviolet (UV)-optical turnover, consistent with complex carbonaceous compounds and irradiated organics on the surface (1,2). Notably, no gas emissions from CN, C₃, C₂, CO⁺, or [O I] were detected, even though the object displayed a growing dust coma (1). This lack of volatiles aligns with the team’s thermal evolution model, which predicts that sublimation-driven activity should begin as 3I passes nearer to the Sun (1).

The measured spectral slopes were steep: 27.4 ± 1%/kÅ at 4000–5500 Å and varying from 16.4 ± 0.4% to 26.7 ± 0.7%/kÅ at 5500–7000 Å (550–700 nm) depending on solar calibration (1). These reflectance patterns mirror those of D-type asteroids and other interstellar visitors like 1I/’Oumuamua, suggesting 3I’s surface is dominated by refractory organics with minimal ice or mineral absorption features (1–3).

Thermal Modeling and Coma Formation

To understand the lack of sublimation signatures, the team simulated 3I’s thermal evolution using a 1D conduction model (1). They found that water ice remains stable at depths beyond 15–20 cm, with only shallow subsurface volatiles potentially sublimating near perihelion. Consequently, dust ejection likely results from mechanisms other than ice sublimation, including solar-wind sputtering, UV desorption, and other surface processing events experienced over eons in interstellar space (1).

Significance and Future Work

3I/ATLAS exemplifies the value of interstellar objects as probes of extrasolar planetary systems. Its red, organic-rich surface and early coma challenge conventional expectations based on solar system comets. Continued spectroscopic and photometric monitoring as 3I approaches perihelion will provide critical data to test models of dust ejection and thermal evolution, and to compare the composition of interstellar visitors across different evolutionary states (1).

References

(1) Puzia, T. H.; Rahatgaonkar, R.; Carvajal, J. P.; Nayak, P. K.; Luco, B. Spectral Characteristics of Interstellar Object 3I/ATLAS from SOAR Observations. Astrophys. J. Lett. 2025, 990, L27. DOI: 10.3847/2041-8213/adfa0b

(2) Jewitt, D.; Seligman, D. Z.; Moro-Martín, A. Interstellar Objects. Contemp. Phys. 2022, 63 (3), 200–232. DOI: 10.1080/00107514.2023.2203976

(3) Opitom, C.; Snodgrass, C.; Jehin, E.; Bannister, M. T.; Bufanda, E.; Deam, S. E.; Dorsey, R. C.; Ferrais, M.; Hmiddouch, S.; Knight, M. M.; Kokotanekova, R. Snapshot of a New Interstellar Comet: 3I/ATLAS Has a Red and Featureless Spectrum. Mon. Not. R. Astron. Soc.: Lett. 2025, slaf095. DOI: 10.48550/arXiv.2507.05226