There are not one or two films that begin with an amateur astronomer making a great discovery that allows humanity to be saved. From Armageddon to Don’t look up, passing through Deep impact. Much of what is depicted in these films is real: from the great role of amateurs in the discovery of comets and asteroids to the destiny that may one day come true on our planet (being optimistic with the tense). What, however, is fiction is being able to determine the solutions that concern the future of humanity by doing some accounts on a blackboard, that can only be admitted in a fiction starring Leonardo DiCaprio. The matter is more complicated, we will get to that shortly.
The most important thing, in any case, is the need to continuously explore the sky to detect those wandering objects that at some point may pass more or less close to our home and endanger it. We are talking today about the discovery of comets, which is something to which everyone (with patience) can contribute.
It gives rise to this that a comet is visiting us these days, the so-called C/2023 P1 (Nishimura), which we may be able to see with the naked eye and which was discovered only a few weeks ago, on August 11-12. It was thanks to the Japanese amateur astronomer, Hideo Nishimura, an expert in discovering new astronomical objects, from comets to novae, that he alone uses a good camera and commendable patience and dedication. Current calculations, which we’ll get to in a moment, tell us that it will pass the closest point to Earth on September 13. Like all comets, it will be brightest when it is closest to our star, on the 17th of this month, so we can surely see it with the naked eye, especially if the tail becomes very bright and the comet does not break up, which is a possibility. And comets break up and disappear, the so-called solar wind is very erosive. It will be visible at sunset or sunrise near Venus, in the direction of the Sun, which is now in the constellation of Leo going towards Virgo, the only thing the horoscope is good for (to know where the Sun is throughout the year –at least at a certain time). In a few days it will be very difficult to contemplate it from our hemisphere. And eventually it will be lost in space, its type (C) indicates that it is not periodic, it will only pass through here once (unless something happens to it) or it will take several centuries to return, we still don’t know exactly.
But let’s go to what a cometary visit like Nishimura’s teaches us. We are talking about a basic physical concept that, although fundamental, is complicated on a quantum scale, and involves other mathematical concepts that we have all studied at some point. Since ours are large scales, we can forget about quantum indeterminacy and trust this astronomical principle: the complete trajectory of an object orbiting around another can be known just by measuring two physical magnitudes at any instant: position and velocity . This statement has several direct implications, although not easy to understand, and an implicit detail, which is important to keep in mind.
Starting with the last thing, we must consider that both position and speed are vectors, do you remember them? So knowing the position and speed is not just determining two numbers with their units, but in a three-dimensional space we are talking about six measurements, which would define these two vectors.
Measuring the position and velocity vectors of a comet is not easy, it requires many observations. First of all, identifying a comet implies detecting that something in the sky moves during one night and between different nights in a way that is different from almost everything we see. In fact, what we must do is distinguish between two types of celestial objects. On the one hand, there are the objects that hardly move with respect to the Earth, stars or galaxies, and during one night we only see that they travel through the sky because the Earth rotates on its axis, in such a way that we see how they draw circles around the Earth. axis of rotation. This axis is identified in the sky, and in the northern hemisphere, with the North Star (although they do not coincide exactly). On the other hand, there are objects that move very fast with respect to that rotation, so that their trajectory is not those circles, but different curves in the sky. These objects must be much closer, from a plane or satellite, to a planet or other star in the Solar System.
Identifying a comet then means taking images of the sky and looking for objects whose movement does not follow those circles, it is said that they have a non-sidereal movement. They move across the sky, changing constellations, we could say using a reference system that is already millennia old. Once an object with non-sidereal movement has been identified, we have information about its position in the sky, 2 coordinates, but we would need a third, the distance, which is tremendously difficult to obtain directly. The same for speed, we only have a speed projected in the sky, we would lack an angle with respect to a reference axis. Again, very very difficult to measure. So how do we determine where a newly discovered comet comes from and where it goes?
If we know the position and velocity (its vectors) of a star, there is only one orbit, of all the infinite ones we could imagine, that is possible and gives rise to those values. Said with an example, a satellite that we want to rotate around the center of the Earth with exactly a period of one day, that is, that it rotates every 24 hours, as a point on the Earth’s surface does, can only be one height of 35786 kilometers (on the Earth’s equator) and have a speed of 3.07 kilometers per second (with a direction perpendicular to the line that joins it with the center of the planet). This satellite, which we call geostationary (it would be seen at the same point in the sky throughout the day), cannot have any other speed and height. If it were a little faster, its orbit would move away from the Earth’s surface at some point and its period would be shorter. Or if it were at that same speed, but at a higher altitude, its orbit would also move away at some point.
This concept is basic for the human devices that we send to orbit around our planet or to explore space. To go from one orbit to another, we only have to make these ships change speed, that is, give them an acceleration (positive or negative) at a given moment and for a short period of time. Controlling the direction, yes.
And this basic concept is what we can apply, and should apply, to safeguard the integrity of our planet (at least as a first step towards it). But since we were missing 2 essential means to know the position and speed of the comet, the solution is to observe it for several nights and apply orbit models, in which, through half a dozen parameters, they can reproduce the position of the comet in the sky. The more different positions are used and the further apart they are, the more accurate the determination of the comet’s orbit will be. It is not something that is done on a blackboard, we mentioned it at the beginning, you need a computer (not very powerful, it is “easy”) that determines the orbit with increasing precision as more data is taken until the method is says that it converges, the change in the solution is small when adding new observations.
After all this description of the physical problem, what is the most important lesson we should draw? Well, I would say two. The first: the universe is wonderful, how beautiful the comets are! The second: the universe is precious and implacable, physics rules and the orbit of comets has intercepted and will intercept ours at some point. The sooner we discover a potentially dangerous comet, with enough time to, on the one hand, determine its orbit precisely, and on the other hand, to prepare ourselves better. Both are within our reach, but we need to take them seriously.
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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