The discoveries and mysteries of the only comet ever visited by humanity: 67P

Many people are already anxiously preparing a nice outfit to welcome the visit of comet C/2023 A3 (Tsuchinshan-ATLAS), which promises to put on a show in the night sky starting this month. But 10 years ago, we were the ones visiting a comet. 67P/Churyumov-Gerasimenko is the only comet to have ever been visited by a space probe. The Rosetta Mission, which orbited the comet between 2014 and 2016, made this space traveler, shaped like a “rubber duck”, an astronomical celebrity and the most studied comet in history, but it has not stopped being an intriguing celestial body.

What makes 67P so special that it has mobilized a team of scientists to send a probe to encounter this space ice rock is the secrets it holds about the formation of the Solar System and, who knows, even the origin of life on Earth!

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We humans first learned about 67P in 1969, when Ukrainian astronomers Klim Churyumov and Svetlana Gerasimenko were studying comets at the Kiev University Astronomical Observatory. Churyumov, searching for Comet Solà in the images captured by Gerasimenko, noticed something unusual: another comet, previously unknown, in the corner of one of the photos. Thus, following tradition, it was named after the discoverers, also receiving the designation “67P”, as it is the 67th periodic comet recorded.

Periodic comets are those that return to the inner Solar System in less than 200 years. 67P, for example, takes just 6.4 years to complete its orbit, but this was not always the case…

This comet belongs to the so-called “Jupiter Family,” made up of short-period, low-inclination comets whose orbits are shaped by the gas giant’s powerful gravity. Studies show that before 1840, it took 8.5 years to complete one orbit around the Sun, passing at least 600 million kilometers away. However, a close encounter with Jupiter that year brought it closer, shifting its perihelion to about 450 million kilometers. In 1959, another encounter with Jupiter reduced its orbit to just 6.5 years, with a perihelion of 193 million kilometers from the Sun.

While it poses no immediate threat to our planet, this new trajectory has made it a Near-Earth Object (NEO), disproving the idea that Jupiter merely protects us from collisions. In fact, the gas giant sometimes even throws boulders our way. But in the case of Churyumov-Gerasimenko, this gravitational influence ended up being useful, because it made 67P an ideal target for a space mission!

The Rosetta mission, launched in 2004, aimed to study 67P up close, entering the comet's orbit and sending a probe, Philae, to land on its surface. Comets are like freezers that preserve virtually intact the primordial material that gave rise to the Solar System. For this reason, scientists were determined to understand their composition, internal structure, activity and the mysteries of their origin.

And Rosetta did not disappoint! The spacecraft spent two years orbiting 67P, returning spectacular images and scientific data that revolutionized our understanding of comets. The mission revealed that Churyumov-Gerasimenko is a complex and dynamic celestial object, with nearly 4 kilometers of steep cliffs, deep craters, dusty plains and jets of gas that were expelled from its surface as it approached the Sun.

Upon its arrival in 2014, scientists were surprised by its “rubber duck” shape. It was thought that the duck’s “neck” could have been sculpted by jets of gas and dust emanating from the comet. But studies have shown that 67P is actually a contact binary, that is, a celestial body formed by the gentle impact of two other bodies.

Among Rosetta’s most important discoveries is the presence of complex organic molecules, such as amino acids, which are the building blocks of life! The mission also detected water in 67P, but with a different isotopic composition than water found in Earth’s oceans, suggesting that our planet’s water may have come from other sources, such as hydrated asteroids.

The mission also revealed that 67P has an extremely low density, equivalent to half the density of water! This means that if the comet were placed in a giant bathtub filled with water, it would float like a rubber ducky… But it also indicates that our star is porous like a sponge, composed of water ice, frozen gases and dust, but with large empty spaces inside.

And despite all of Rosetta’s discoveries, 67P still holds many mysteries! We still don’t know for sure how the comet formed, what its chemical composition is, or how its gas jets are produced. Another mystery is the “active pits” on the surface of 67P, large depressions that appear to be expelling material from the comet’s interior. All of this has left scientists at the European Space Agency wanting more!

Not surprisingly, the agency is leading the Comet Interceptor Mission, which is due to launch in 2029 and will wait for the passage of a long-period comet, such as Tsuchinshan-ATLAS, but which can be intercepted by the probe before its first approach to the Sun.

But while we wait for the next mission that will take us to visit a comet, we can appreciate from here the beauty and mystery of these icy travelers that, from time to time, visit us and cross our sky, reminding us of the vastness and beauty of the Universe that surrounds us.