Stardust and eternity – 3.3.4

Artificial satellites

Since 1957, when the first “artificial satellite”, Sputnik 1, was launched to orbit the Earth as part of the Soviet Union’s space programme, several thousands of artificial objects have been launched into space with the aid of rockets; over a thousand of them are active and orbit the Earth. Man-made satellites are called “artificial” to distinguish them from “natural satellites” orbiting planets – such as the Moon. Their size and altitude vary depending on their use – such as weather monitoring, broadcasting of television programs, and military purposes. Their use in astronomy is also very important. Indeed, while the Earth’s atmosphere protects us from the harmful effect of cosmic rays, energetic gamma rays, X-rays, and extreme ultraviolet radiation from the Sun, it also blocks a large part of the electromagnetic spectrum useful for astronomical observations (low ultraviolet and infrared radiation), so that on the surface we can only see specific portions of the spectra of celestial objects – forming the so-called “atmospheric window”. Orbiting space observatories, such as the Hubble Space Telescope, thus allow us to observe the universe in a range of electromagnetic bands otherwise blocked by the Earth’s atmosphere, not to mention the advantage of avoiding the blurring effects of the atmosphere to obtain clearer images.

Many artificial satellites, although no longer active, still orbit the Earth like space junk, reflecting sunrays and effectively contributing to light pollution. Indeed, they appear in astronomical images as faint background stars. This interference also occurs at other wavelengths – especially at radio frequencies – interfering with the terrestrial radio antennas that radio astronomers use to receive the natural (or even, potentially, alien!) radio signals from space. In this respect, the brightest and therefore most polluting artificial satellites are the large space observatories, such as NASA’s International Space Station and the Hubble Space Telescope, as well as the mega-constellations of thousands of global communication satellites orbiting the Earth, used for television, radio, internet, telephone and military applications. Fortunately, their trajectories can often be monitored and predicted using dedicated internet sites and software.

Most artificial satellites of the Earth are in geostationary or low-Earth orbits. “Geostationary satellites” revolve above the equator from West to Wast and travel at the exact same rate speed as Earth’s rotation. Thus they appear to be stationary on a fixed position of the Earth. This is particularly necessary for telecommunications and weather forecasting of specific areas. “Low-Earth satellites” orbit are relatively close to the Earth’s surface, usually at an altitude of less than 1000 km, and are commonly used for higher resolution photopgraphy. Satellites in this orbit – including the International Space Station – take about a hundred minutes to circle the Earth. Space telescopes are also placed in orbit around specific points in space, the so-called “Lagrange points”, where the gravitation of the Earth and the Sun combine in a way that keeps the satellite’s orbit stable. This is the case of the James Webb Space Telescope: it orbits Sun at the speed of the Earth’s revolution – 1.5 million kilometers away from the Earth – at the second Lagrange point (or L2). This particular location, with the Earth between the telescope and the Sun, effectively shields direct light and the heat of the Sun – thus allowing a less polluted space environment for in-depth astronomical observations.

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The International Space Station – Credits: NASA/Roscosmos

Starlink satellites over Tubingen – Image by W ikiImages – Pixabay

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Images

October 1957 – Sputnik, the first artificial satellite, was launched into space (Thuy Mai, NASA)