Perseids 2023: what the meteor shower tells us about our origins and our future

I confess, I have looked little at the sky. Actually it is not something so rare, since there are many professional astrophysicists who are not very fond of looking through an eyepiece or an amateur telescope camera: we directly write commands on a computer console and see images on ds9 (one of the most used professional programs to visualize astrophysical images). There are also astrophysicists who started this by looking at the starry sky one night in August and enjoying one of the astronomical shows that are repeated every so often; Thanks in part to this repetition, these phenomena teach us a lot about what the universe is like and where we came from. In a few days we will be able to witness two of these shows, which invite you to enjoy your holidays looking at the skies.

One of the great astronomical events of this summer is the opposition of Saturn, which will occur on August 27. This phenomenon occurs when Saturn, the Earth and the Sun are aligned almost in a straight line; and when, in addition, our planet is between the two stars on that line, with the Sun looking to one side and Saturn to the opposite side. It is one of the best moments to observe the most beautiful of the planets, because its distance from Earth reaches a minimum; Although this minimum distance is not always the same, it depends on the year. To be able to observe Saturn’s rings well, you need a telescope.

But without waiting until the end of the month, before that will come an astronomical event that is best observed without binoculars or a telescope: the shower of shooting stars known as the Perseids. For a few days now, the Earth has begun to transit, in its annual journey, through the trail of material left on its own journey by a comet called 109P/Swift-Tuttle. The densest of that cloud of debris left behind by the comet will occur around August 12, but we will be passing through it until August 24.

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First, service information: the Moon will be in the last quarter on that date, the new moon is on August 16, so 2023 will be very favorable to see the great summer meteor shower. You have to look towards the constellation of Perseus, which will rise before the Moon at about midnight, quite to the north (northeast) in peninsular and insular latitudes of Spain; and that it will be visible all night, remaining above the Andromeda galaxy, which is well worth observing with binoculars.

Now, astrophysical data: what we see every year are particles from a cloud left behind by comet 109P. The P in the names of the comets means that they are periodic, that is, that they have a closed orbit: they travel from the outer zones of the Solar System to the inner ones in periods of less than 200 years. 109P was discovered in July 1862 by Lewis Swift and Horace Parnell Turtle, hence its more prosaic name. It has only passed close to us twice since its identification as a comet; the second and last so far, in 1992. The third will be in 2125 and perhaps someone who has already been born can enjoy it. It comes closest to the Sun very close to our orbit, then recedes past Pluto (which is 50 times farther from the Sun than Earth). It seems that the Chinese already observed this celestial object 23 centuries ago, and there is a document (from a Jesuit in China, dated 1737) that speaks of a comet that could be this very one.

Comet 109P/Swift-Tuttle is unique in several respects. With almost twice the size of the famous Halley, it has a very inclined orbit, that is, very far from the plane in which all the planets move (with quite small deviations). That plane is what is known as the ecliptic. If we consider the plane of the ecliptic and the line that joins a body with the Sun at its point of maximum distance from the plane, we can define the orbital inclination. An inclination of 0 degrees implies that the body moves in the plane of the ecliptic (that is what the Earth does, because its orbit is the one that precisely defines the ecliptic). A 90 degree tilt implies that the orbit is perpendicular to the ecliptic (said to pass through the ecliptic poles). 109P/Swift-Tuttle has a bank of nearly 114 degrees; This angle greater than 90 degrees means that its orbit is retrograde: it revolves around the Sun in the opposite direction to that of the planets. Comet 109P/Swift-Tuttle only produces one shower of stars each year, while there are others (such as Halley) that produce two annual showers, since its orbit intersects Earth’s at two points.

Long exposure photography during a night of shooting stars in Saint-Cergue (Switzerland). SALVATORE DI NOLFI (EFE)

It is curious that, despite its rather long period (coupled with that of Jupiter, the king of comets), its great inclination and its retrograde orbit, this comet passes very close to Earth. It is estimated that in 2126 it will pass through the vicinity of Earth’s orbit at the end of July, just 15 days before the Earth passes through roughly the same area, so we hope that the period calculations are very accurate or that there will be a hero like that then. the one played by Bruce Willis in the movie Armageddon (1998). But, indeed, the calculations invite peace of mind: they indicate that there is no danger that both orbits will coincide in the following millennia.

Where does comet 109P/Swift-Tuttle come from? Has it always been there since the origin of the Solar System? What is their destiny (and ours)? We don’t have concrete answers, but the comets we have discovered teach us a lot about our origins.

There must be a large number of icy objects (which is what comets are) beyond at least 50 times the orbit of Pluto: between 2,000 and 100,000 times the distance between the Sun and Earth, in what is known as the Oort cloud, which has not been directly observed. In our origins, these objects were swept out of the inner parts of the Solar System by Jupiter, perhaps at the same time that the Earth could receive water from our oceans. Some comets can have very elongated orbits (eccentric, it is said) that bring them closer from that deep space to Earth orbit in periods of millennia.

Others may stay there, in the Oort cloud, with stable orbits and we will never see them approach. And others can be disturbed by nearby massive objects (such as the hypothetical Planet X) and be redirected towards the inner Solar System, with orbits that are called open: they will pass by and never return. Some of these will be able to interact with more stars (especially Jupiter) and end up having a closed orbit, with shorter periods. This last type of comet, periodic, should not last long (on astronomical scales), because comets are losing mass, especially when they pass through the closest point to the Sun. For example, Halley is thought to disappear, in a few millennia. The fate of these comets, the P type, is to evaporate like this or hit another star, as was the case of Comet Shoemaker-Levy, which collided with Jupiter in 1994. That may be our fate.

Perhaps at some point we will come across a type C comet (not periodic, but with an open orbit, which can reach higher speeds), as in the movie Don’t Look Up (2021). May the mention of that apocalyptic fiction serve as an invitation to enjoy the skies, shooting stars and other summer worlds, and as a reminder that they will not be there forever.

Cosmic Void is a section in which our knowledge about the universe is presented in a qualitative and quantitative way. It is intended to explain the importance of understanding the cosmos not only from a scientific point of view, but also from a philosophical, social and economic point of view. The name “cosmic vacuum” refers to the fact that the universe is and is, for the most part, empty, with less than one atom per cubic meter, despite the fact that in our environment, paradoxically, there are quintillions of atoms per meter cubic, which invites us to reflect on our existence and the presence of life in the universe. The section is made up of Pablo G. Pérez González, researcher at the Center for Astrobiology, and Eva Villaver, research professor at the Instituto de Astrofísica de Canarias.

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