Interstellar comet 3I/Atlas comes closest to Earth this Friday and will leave forever

• Astronomers concerned: Satellite proliferation will compromise photos from space telescopes, NASA says
• New images: Interstellar comet 3I/ATLAS emits a greenish glow

When this object was detected in July, little was known about it other than that it had an interstellar origin. In other words, from its trajectory it was possible to know that it was not in orbit around the Sun and that it originated in another star system, not the solar system.

The astronomers then carried out a first analysis by spectrometry (decomposition of light) and observed that 3I/Atlas had a metallic composition slightly different from that of a common comet. This led to speculation that it was a technological artifact, but as we got closer it became clearer that it was indeed a comet, although not quite an ordinary one.

— Overall, it’s more similar than different — says Cyrielle Opitom, an astronomer at the University of Edinburgh, involved in several studies of 3I/Atlas. — Its behavior is very similar to that of a comet from our own solar system, with its gas dispersion, surrounding atmosphere and tail. What there is is a small difference in its composition.

• Caves on the moon? : New discovery strengthens plans for future human bases beneath the lunar surface

The interstellar visitor, preliminary observations point out, is very rich in carbon dioxide, which is found in few known comets, in addition to having a different proportion of iron and nickel (with a slight excess of this second metal).

This anomaly is not completely new in the solar system, but it was enough for astrophysicist Abraham Loeb, of Harvard University, to launch the hypothesis himself in July that the object in question would have an extraterrestrial technological origin.

In the scientist’s most recent study, however, he no longer seems to be concerned about this possibility. The work is dedicated to understanding the process of forming the tail of the object, already treated as a comet. The word “alien” is not reused in research.

Speculation aside, 3I/Atlas had a strange characteristic when it arrived in the solar system. When it was detected in July, near the orbit of Jupiter, its tail was more or less pointed toward the Sun. This is atypical for an ordinary comet, whose tail normally turns outward.

The tail forms when the solar wind (electrically charged particles emitted by the Sun) “blows” gases from the comet’s atmosphere in the opposite direction. Therefore, it became clear that 3I/Atlas was actually a comet when it approached the Sun and a conventional tail formed.

Loeb and other scientists believe that the “anti-tail” of the interstellar comet appeared when it was still very distant, and only its side closest to the Sun was hot enough to sublimate. This material detaches from the comet only on its illuminated side, it begins to be ejected in the form of ice grains still too large to be carried by the solar wind. It was only when solar radiation melted these cometary “snowflakes” that the conventional comet tail began to form.

The most recent work on 3I/Atlas, including that of Loeb and Opitom, is all published as “preprints,” studies that have not yet been independently reviewed because scientists want to interact soon while the object is still in the solar system. Once these analyzes are sifted through, it will be easier to know which ones are most likely to be correct.

• Path: When will comet 3I/Atlas approach the Sun?

However, the most precise observations on the 3I/Atlas are not yet described in the studies.

— The comet is getting closer to us, but it is also moving away from the Sun, which makes it less active, said Opitom. — This is why I think that the best observations will be made before Friday, or perhaps even already made.

The scientist is involved in analyzing both images from NASA’s James Webb Space Telescope and those from the Very Large Telescope (VLT), the world’s largest, built by the European Southern Observatory (ESO) in Chile.

— This is a unique opportunity to study very closely a piece of material formed around another star, says Opitom. — The other methods of doing this rely on observing things very, very far away, and they are really difficult.

However, it was not thanks to these supertelescopes that 3I/Atlas was discovered. The person who discovered it is Project Atlas (Asteroid Terrestrial-impact Last Alert System), also from NASA, which has a smaller telescope in Chile dedicated to exploring the sky and looking for stars at risk of colliding with Earth. This is not the case for 3I/Atlas, which will pass 270 million kilometers from us and move away forever.

NASA space probes orbiting Mars have already taken images when 3I/Atlas passed by the Red Planet, and there is a distant hope that the Juno probe, orbiting Jupiter, might also capture something.

Saying goodbye to this comet leaves some melancholy among astronomy enthusiasts, because it was only the third object of interstellar origin ever identified. The other two were Oumuamua, an elongated rock measuring around 300 meters discovered in 2017, and Borisov, an interstellar comet very similar to a solar comet, discovered in 2018.

Experts, however, believe there is a good chance of finding more such visitors in the future. The Vera Rubin Observatory in Chile, home to the Legacy Survey of Space and Time (LSST) project, has the world’s largest digital camera telescope dedicated to exploring the sky for years to come.

With Brazil participating, the company is now the best bet for finding these bits of matter from other corners of the galaxy.

— This vast study is expected to discover one to five interstellar objects per year, — explains Brazilian astronomer Felipe Ribas, professor at the Federal Technological University of Paraná (UTFPR).

A good sign is that 3I/Atlas itself was discovered later in the first LSST images, from when the telescope was still being commissioned and tested, in mid-year. Ribas is part of the team analyzing these observations.

According to the scientist, as more interstellar objects are discovered, we will have a better idea of ​​whether the things we see around our Sun are similar to those generated in other stars.

Those who were frustrated that this comet was not an alien spacecraft might be excited by the realization that, perhaps, the place it came from has a planet similar to Earth. The fact that 3I/Atlas is a somewhat different comet is telling in this sense.