The Third Interstellar Visitor Captured Also at Skalnaté Pleso
The calm waters of asteroid and comet research have been stirred by the recent discovery of an object entering the inner Solar System and coming relatively close to Earth from the distant reaches of space. It is already clear that this is the third candidate for an interstellar object, which has recently been designated as 3I/ATLAS. Some may still remember the previous visitors – ‘Oumuamua and Comet Borisov. This new space object has also been observed by astronomers of the Slovak Academy of Sciences (SAS).
The new celestial body was discovered on Tuesday, July 1, 2025, at the Asteroid Terrestrial-impact Last Alert System (ATLAS) station in Río Hurtado, Chile. At the time of discovery, the object was 4.5 astronomical units (670 million km) from the Sun. A relatively quick influx of new astrometric data revealed that the object is moving on a very peculiar, extremely hyperbolic orbit with an orbital eccentricity of 6.30. Calculations also showed that it is hurtling through space at a relative speed of 61 km/s. These data attracted increasing attention from observers. The object quickly shed its temporary designation A11pl3Z, and today it is known not only as 3I but also as C/2025 N1 – a designation used for comets.
Observations, especially from large telescopes, revealed clearly visible cometary activity. The estimated size of the nucleus is currently around 24 km, though this will likely be revised to somewhere between 4 and 6 km. Other physical details remain unknown for now.
Astronomers from the Skalnaté Pleso Observatory (part of the Astronomical Institute of the SAS in Tatranská Lomnica) did not want to miss the chance to observe Comet 3I. The image shown is the first capture of this interstellar visitor, taken with a 0.61-m telescope. However, observing conditions from Slovakia are far from ideal. The comet rises only about 20 degrees above the horizon, which means observations are heavily affected by the atmosphere and the limited time window for collecting longer, more complex data series. Another challenge is the low brightness of the object. The fact that it is currently in the constellation Sagittarius, close to the galactic plane and center, also complicates observations.
The comet is still approaching the inner Solar System. It will reach its closest distance to the Sun at the end of October, when it will still be over 206 million km away. However, it will not be visible from Earth at that time. The comet is expected to become visible again from Earth in early December 2025, on the eastern morning sky (from Slovakia). It is expected to reach a brightness of around 12 magnitudes then.
Prepared by: Marek Husárik, Astronomical Institute of the Slovak Academy of Sciences
Photo: Astronomical Institute of the Slovak Academy of Sciences