Recent data from the Zwicky Transient Facility (ZTF) camera has unveiled significant findings regarding the interstellar object 3I/ATLAS. The object was first identified on May 15, 2025, when it was located at a distance from the Sun that was 6.1 times greater than the Earth-Sun separation, measured in astronomical units (AU).
According to the ZTF data, 3I/ATLAS exhibited a steady brightening starting in early June 2025. This phenomenon is attributed to the mass loss from the object, which caused a glow of scattered light, as evidenced by an image captured by the Hubble Space Telescope on July 21, 2025. At that time, 3I/ATLAS was located 3.8 AU from the Sun.
Interestingly, the Hubble image revealed that the glow was extended towards the Sun by a factor of 10 compared to other directions. This suggests that the scattered sunlight was generated by ice fragments evaporating under solar radiation, as discussed in a paper co-authored with Eric Keto.
Between May 15 and early June 2025, the ZTF data indicated no significant increase in the reflecting area of 3I/ATLAS, suggesting a "plateau" in its effective size over time. The authors of this study calibrated the growth in size against a model that estimates the nucleus diameter of 3I/ATLAS at approximately 5.6 kilometers, based on the Hubble image. This estimate, however, is model-dependent.
The ZTF data revealed a consistent enhancement factor of 5 to 10 during the plateau period, indicating that the actual diameter of 3I/ATLAS could be larger than the estimated 5.6 kilometers. This implies a potential diameter of around 15 kilometers, making it approximately 20 times larger than the previous interstellar object, 2I/Borisov.
This significant disparity in size raises intriguing questions about the population of interstellar objects. If both 3I/ATLAS and 2I/Borisov have similar solid densities, then 3I/ATLAS's nucleus mass would be greater by a factor of 8,000. This leads to the pivotal question: why have we not detected more objects like 2I/Borisov before encountering one as substantial as 3I/ATLAS?
Researchers speculate that the reservoir of rocky material in interstellar space may only yield massive objects of this scale once every few millennia. These considerations were explored in a paper I authored shortly after the discovery of 3I/ATLAS.
One theory posits that 3I/ATLAS was on a selective trajectory towards the inner solar system, a concept further analyzed in a recent paper co-authored with Oem Trivedi. This trajectory could stem from unknown astrophysical origins or might suggest a technological fine-tuning of its path, given the unusual alignment of 3I/ATLAS with the ecliptic plane of the solar system.
Given the uncertainties in the early ZTF data, it is also possible that the observed plateau might be misleading, indicating that the nucleus size of 3I/ATLAS could be considerably smaller. However, the trajectory’s alignment brings 3I/ATLAS into close proximity to Mars, where NASA, ESA, and the Chinese Space Agency have deployed orbiters equipped with advanced cameras.
On October 3, 2025, 3I/ATLAS will approach within 29 million kilometers of the HiRISE camera aboard the Mars Reconnaissance Orbiter. This camera, with a 0.5-meter aperture, will capture images of 3I/ATLAS at a resolution of 30 kilometers per pixel. The glowing cloud around the object is optically thin, allowing for precise measurements of the nucleus's brightness and size, potentially offering insights two orders of magnitude better than the Hubble observations.
Engaging in evidence-based science is akin to solving a detective story. The high-resolution images from the HiRISE camera will serve as a crucial tool in deducing the nucleus diameter of 3I/ATLAS. As Sherlock Holmes famously stated, “It is a capital mistake to theorize before you have all the evidence.” This principle remains vital when studying anomalous interstellar objects like 1I/`Oumuamua and 3I/ATLAS.
