November 19, 2025
The scientists from Physical Research Laboratory (PRL) observed the interstellar comet 3I/ATLAS currently on its way out of the inner Solar system after perihelion passage. Observations were carried out in imaging and spectroscopic (analysis of the constituent wavelengths of the light emitted) modes with PRL’s 1.2m telescope. The images (false colour representation in Fig. 1) show a near-circular coma. The coma of a comet is the large, glowing atmosphere of gas and dust that forms around its nucleus as it gets closer to the Sun. It is created when the Sun's heat causes the frozen ices on the nucleus to vaporize, or "sublimate," releasing gas and dust that form a large, diffuse cloud. In the present observing geometry, the dust tail, if present, would be pointing away from the Sun behind the comet as seen from the Earth, while deep wide field multiband images may show the ion tail.
Fig. 1: False colour image of Comet 3I/ATLAS with the 1.2m telescope at Mt Abu using a wide area imager in the red filter. Two fainter background stars are also seen in this image.
Apart from imaging, scientists also acquired a spectrum of the light from the comet, before the start of morning twilight. The result shows prominent emission features commonly seen in Solar system comets - the CN, C2 and C3 bands in the shorter wavelength side of the spectrum (Fig. 2).
Fig. 2: Flux-normalised spectrum of Comet 3I/ATLAS using the LISA spectrograph.
In the terminology of comet, the rate of release (or sublimation) of gas from the nucleus into the coma is referred to as the ‘production rate’, which is a measure of the comet's activity level. The production rate varies significantly throughout the comet's orbit and is driven by factors like distance from the sun and the comet's internal composition.
In case of 3I/ATLAS, the production rates for the prominent bands (emissions pertaining to the constituent molecules) were computed with limiting values around 1025 molecules/sec. The production rate ratios seem to place this comet in the class of ‘typical comets’ of the solar system. Further observations will be continued as the comet gradually comes into the darker part of the night.
The Physical Research Laboratory (PRL) 1.2 m telescope at Mount Abu, located at an altitude of 1680 m near Gurushikhar, is a ground-based observatory that is used for astronomical research, including exoplanet hunting, high-energy phenomena, and solar system studies. These observations were made during November 12-15, 2025.