October 4, 1957 could mark the beginning of everything in the space age. A date that is hardly remembered, except in the limited circles of geeks either nerds (nerds, for the non millennials). The date is fundamental for the development of our daily life, since it was the moment in which the exit door from the terrestrial surface was opened. That Friday, the then Soviet Union successfully launched the sputnik 1, our first artificial satellite. Sputnik 1 was the size of a beach ball and took just over 98 minutes on average to go around the Earth. Its transmitters provided information on the density and propagation of radio waves in the upper layers of the atmosphere. 92 days after its release, it burned in that same atmosphere that it had helped study.
Losing altitude over time is the fate of all artificial satellites that are placed in low orbits. The way to do it depends on several factors and among them is one that is little known, it has to do with our star and is not related to its gravity.
Let’s start by clarifying what is considered a low orbit: they are those that are less than 2,000 km away from the Earth’s surface. That means that in a day, these satellites can go around the Earth several times (about 16 times at most) and that the data they collect can be quickly transmitted to the surface. They are therefore especially suitable for observing the Earth with high resolution. How do they do it, for example? Copernicus satellites. These orbits, although very common due to their close proximity, can cover a very small area. This is why they are often released together in groups known as satellite constellations They form a kind of network that surrounds the Earth. This allows them to cover large areas working simultaneously.
But these satellites, sooner or later, end up coming home. In a low orbit, a satellite experiences the gravitational effect of the three nearest large bodies: the Earth, the Moon and the Sun. It also suffers friction and a third variable effect that has to do with the solar magnetic field. The gravitational effect is obvious; the effect of solar activity and the force of friction are not so much.
The effect of friction is easy to understand: in low orbits, in the path that the satellite travels, there is still some material from the Earth’s atmosphere that opposes its movement and causes it to lose altitude over time. the space telescope hubble and the International Space Station are two examples of this type of satellite. It could be said that, over time, they slowly fall towards us. Most of these satellites have propulsion systems to modify their height. Others don’t. Some simply use up the fuel that they had destined to carry out this type of maneuver over time.
To estimate the lifetime of one of these satellites, it is essential to estimate the force of friction. This decreases exponentially with height, especially since as we move away from the surface there is less and less material from the atmosphere. There is a critical height, about 1,000 km above the Earth’s surface, where the braking produced by the atmosphere acts on scales of between one thousand and ten thousand years.
And above a certain height, it is the solar wind that dominates the evolution of the trajectory. Solar activity is the unknown variable in estimating the lifetime of satellites in low orbits that do not have internal propulsion. It works like this: the Sun provides extra energy to the atmosphere in its most active periods, causing the low-density layers to move up and be replaced by those that are lower down, which are denser. Satellites in low orbits need to be given several nudges a year to keep them in orbit. In the case of being at the maximum of the solar cycle, to which we are now moving and will be reached in 2025, we must give them more pushes than normal.
The frictional force increases when the Sun is more active simply because the density of the environment in which the satellite moves increases. This is a long-term effect, an effect erosive. And the Sun will affect, being now in one of its maximum activity, the fall of the orbit of the hubble, for example, although we still don’t know exactly how much. In addition, sometimes there is also a sudden effect related to geomagnetic storms, where the solar wind, when interacting with the Earth’s magnetic field, can again cause the effects described in the previous paragraph, changing the orbits of the satellites.
That is why we must always be attentive to the Sun, lest, like in the old Gala village and by tutatis!, the sky end up falling on our heads.
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. Perez Gonzalezresearcher at the Center for Astrobiology, and Eva Villaverresearch professor at the Instituto de Astrofísica de Canarias.
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