There interstellar comet 3I/ATLAS never ceases to amaze the scientific community and continues to be talked about months after its discovery: among the latest news is the fact that when the comet “re-emerged” from behind the Sun it immediately showed a green colour, while when it “disappeared” behind the solar disk the color was reddish. There is also the persistence of aanti-tailthat is, an apparent “tail” directed however towards the Sun and not in the opposite direction, which does not seem to be due to a perspective effect but to a genuine emission of material. The anti-tail had already been discovered months ago and, although not unheard of, also for comets Solar systemis important because it could provide large amounts of information about the interstellar object.
With the emergence of these new elements, the astrophysicist Avi Loeb highlighted his theories on the alleged alien nature of 3I/ATLAS: in reality, there is a clear scientific explanation for both the new color and the presence of the anti-tail. All this happens as 3I/ATLAS prepares to reach the point of closest proximity to Earth, expected for December 19th at a distance of approximately 270 million km.
What causes the chromatic change of comet 3I/ATLAS
As also confirmed by INAF (National Institute of Astrophysics), at the moment the coma of 3I/ATLAS appears green color: on the contrary, before disappearing behind the Sun in recent months, it appeared reddish. According to experts, the green color color would be conferred by diatomic carbon present in the foliage. It is a molecule composed of two carbon atoms (C2) which emits light precisely at green wavelengths.
The change in color would be due to the heat of the Sun received by the comet during its maximum proximity to the Sun, which activated the sublimation of the innermost layers and ended up giving it this color.
What is the anti-tail of a comet and how that of 3I/ATLAS was formed
More interesting is the anti-tail of the interstellar comet. But what is it? As it travels, solar heat causes the sublimation (i.e. the direct transition from the solid state to the gaseous state) of its volatile materials, which can create mechanical tensions in the structure of the celestial body favoring the loss of debris. While the finest dust is “pushed backwards” by the solar wind, forming the tail of the comet, any larger debris they do not receive enough push from the solar wind, and as a result they remain around the comet thus creating a weak disc: “cut” view, the front of this disc looks just like an anti-tail that precedes the comet, instead of following it.
This is a phenomenon observed in several comets in our Solar System, so it is not strange in itself. However, this is a phenomenon which for 3I/ATLAS does not seem to be due to a prospective effect. The Harvard astrophysicist Avi Loeb – now known for his “alternative” theories on 3I/ATLAS – stressed that NASA was not concerned about this anomaly during his press conference on 3I/ATLAS in late November.
In reality, the anti-tail of this interstellar comet has been studied by the scientific community: already in September, in fact, one scientific study published on the platform ArXiv (therefore not yet peer-reviewed) managed to explain the characteristics of this anti-tail based on the comparison between observations made by the Hubble telescope and theoretical models. In particular, the development of this “opposite” tail is due to one non-uniformity in the emission of the cometary nucleus – due in turn to a non-uniformity in solar illumination on the comet’s surface – which leads to different emission rates in different directions thus triggering the loss of larger debris in some directions rather than others. Precisely the latter create a sort of visible anti-tail in front of the comet and in the direction of the Sun, creating that elongated shape that we see in the images.
