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The Sun, the unknown factor that explains why satellites fall

2023-06-10T05:03:49.265Z

Highlights: All artificial satellites placed in low orbits without propulsion will eventually fall to Earth. Calculating the moment depends on the activity of our star. Solar activity is the unknown variable in estimating the lifetime of such satellites. The effect of friction is simple to understand: in the path traveled by the satellite, there is still some material in the Earth's atmosphere that opposes its movement and causes it to lose height over time. There is a critical height, about 1,000 km above Earth's surface, where the braking produced by the atmosphere acts on scales of between one thousand and ten thousand years.


All artifacts and stations placed in low orbits without propulsion will eventually fall to Earth, and calculating the moment depends on the activity of our star.


October 4, 1957 could be marked as the beginning of everything in the space age. A date that is barely remembered, except in the limited circles of geeks or nerds (nerds, for non-millennials). The date is fundamental for the development of our daily life, since it was the moment in which the exit door of the earth's surface was opened. That Friday, the then Soviet Union successfully launched Sputnik 1, our first artificial satellite. Sputnik 1 was the size of a beach ball and took just over 98 minutes on average to circle the Earth. Its transmitters provided information about the density and propagation of radio waves in the upper layers of the atmosphere. 92 days after its launch, it burned in the very atmosphere 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 very little known, has to do with our star and is not related to its gravity.

Let's start by clarifying that it 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 circle the Earth several times (around 16 times at most) and that the data they obtain can be quickly transmitted to the surface. They are therefore particularly suitable for observing the Earth with high resolution. As do the satellites of the Copernicus program, for example. These orbits, although very common due to their proximity, can cover a very small area. That is why they are usually launched together in groups known as constellations of satellites that form a kind of network that surrounds the Earth. This allows them to cover large areas working simultaneously.

More informationThe 'Hubble' falls: the study of SpaceX and NASA that could give you a little push and a new life

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 frictional force are not so much.

The Hubble telescope on April 25, 1995.NASA

The effect of friction is simple to understand: in low orbits, in the path traveled by the satellite, there is still some material in the Earth's atmosphere that opposes its movement and causes it to lose height over time. The Hubble Space Telescope and the International Space Station are two examples of such satellites. You could say that, over time, they slowly fall towards us. Most of these satellites have propulsion systems to modify their height. Others do not. Some simply exhaust over time the fuel they had intended to exercise this type of maneuver.

To estimate the lifetime of one of these satellites, it is essential to estimate the frictional force. This decreases exponentially with height, especially because as we move away from the surface there is less and less material in 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 have no internal propulsion. It works like this: the Sun brings extra energy to the atmosphere in its most active periods, causing the low-density layers to move upwards and be replaced by those below, which are denser. Satellites in low orbits must be given several nudges a year to maintain 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 erosive effect. And the Sun will affect, being now at one of its maximum of activity, the fall of Hubble's orbit, for example, although we still do not 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 as in the old Gallic village and for tutatis!, the sky ends up falling on our heads.

Cosmic Vacuum 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 the scientific point of view but also philosophical, social and economic. 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 quintillion atoms per cubic meter, which invites a reflection 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 Institute of Astrophysics of the Canary Islands.

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Source: elparis

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