How do satellites work? Why do they fly without fuel and not fall?

When the first ideas about how to create Science Driven began to emerge, one of the first articles I wanted to write was this one, since on the one hand, I am fascinated by the science behind satellites and on the other hand, I think it is something that the general public does not know, so I found it very interesting to explain it.

For many decades we have had satellites flying around the earth, supporting us in communications, weather control, land and sea navigation (GPS) and many other uses.

Satellites are launched into space and remain orbiting the Earth, working tirelessly, until they break down or something destroys them.

But how do they work tirelessly? How do they fly? Where do they get all that energy?

Sputnik 1

The first satellite to be launched into space was Sputnik 1, a historic technical achievement launched by the Soviet Union on 4 October 1957. This event marked the beginning of the Space Age and the Space Race between the Soviet Union and the United States and had a profound political and cultural impact during the Cold War. This satellite, the first to orbit the Earth, was developed by Soviet engineer Sergei Korolev, who led the design and construction of the craft and its launch vehicle.

This event prompted the United States to increase its efforts in the space race, eventually leading to the development of its own space program and the launching of satellites, leading to the creation of NASA and a significant increase in investment in science and technology education in the United States.

Sputnik 1 was a polished aluminum metal sphere with a diameter of 58 centimeters and a weight of about 83 kg. It was equipped with four long antennas and two transmitters to transmit radio signals (beeps) to Earth, which served to confirm its presence in space for scientists and radio amateurs. In addition, they helped collect important data on the density of the ionosphere and the conditions in outer space.

The satellite completed one orbit around the Earth approximately every 96 minutes. Sputnik 1 only operated for 21 days before its batteries ran out and it fell out of orbit in January 1958, its legacy living on as the catalyst and driver of the modern space age.

Photo: Sputnik 1

Vanguard 1

Vanguard 1 corrected the major problem that Sputnik 1 had: the batteries.

Launched by the United States on March 17, 1958, this satellite was part of Project Vanguard and marked a significant advance in space technology, being the first satellite to use solar energy, rather than batteries, to power its instruments.

Vanguard 1 was a small spherical satellite, with a diameter of just 16.5 cm and a weight of about 1.5 kg. Solar panels were mounted on the body of the satellite and powered a radio transmitter, allowing it to continue sending information long after previous battery-powered satellites had ceased to function. Although the power generated was very small, it was enough to keep the transmitter operational.

Vanguard 1 remains one of the oldest satellites still orbiting Earth, and its success with solar power was a milestone that still endures to this day.

Photo: Vanguard 1

Currently, all satellites generate their own energy for the operation of their sensors, antennas, microprocessors, lenses and all the technology they use for their daily operations.

Satellites do not have any type of motor or element that moves them or keeps them at a constant height (although they do have some thrusters to correct occasional deviations caused by external agents). They also do not consume any type of energy for their movement, except during the launch phase.

Its operation is much simpler and at the same time more fascinating than it seems.

How do satellites fly?

This is possible thanks to a delicate balance of forces and three main factors: The centrifugal force produced by its circular movement around the Earth, the gravitational force, caused by the gravitational attraction of the Earth and the absence of friction in space, which allows it to not be slowed down once it is put into orbit and to spin eternally.

• Centrifugal force: The same force that clothes have when they spin in the washing machine or that makes a car skid around a curve. Centrifugal force is what propels any object that spins in a circle towards the outside of the circle. This is the force that a satellite feels moving in a circular path, pointing outwards, away from Earth.

  The formula for centrifugal force is

  \( F = m \cdot r \cdot \omega^2 \)

  Which is equal to the mass of the satellite (m), multiplied by the distance of the satellite from the center of the earth (r), multiplied by the square of the angular velocity of the satellite (w)

• Gravitational force: It is the force with which the Earth's gravitational field "pulls" the satellite towards the center of the Earth, that is, towards the ground.

  Its formula is:

  \( F = G \frac{m_1 m_2}{r^2} \)

  Which is equal to the gravitational constant G (\(6.674 \times 10^{-11} \, \text{m}^3 \text{kg}^{-1} \text{s}^{-2} ,\)) multiplied by the masses of the Earth and the satellite and divided by the square of the distance between the satellite and the center of the Earth.

And this is where the magic of science comes in, as the centrifugal force pushes the satellite into outer space and the gravitational force pulls the satellite towards the center of the earth, the point at which the satellite remains stable is when the gravitational force is equal to the centrifugal force, then they cancel each other out and the satellite can continue to spin forever.

If we analyze both formulas, we have the following factors: Mass of the satellite, distance between the satellite and the earth, speed of the satellite, the constant G and the mass of the earth.

Since the masses of the Earth and the satellite do not vary and neither does the constant G, we are left with the fact that the balance of both forces depends only on 2 values, the height of the satellite above the Earth and its speed, so if we need a certain speed, we only have to adjust the height and if we want it to fly at a certain height, we only have to adjust its speed.

This balance of forces, together with the absence of friction, allows a satellite to rotate indefinitely around the Earth without the need for engines or any type of propulsion, only physics.

How is a satellite put into orbit?

First of all, as discussed so far, it is necessary to calculate the height and speed at which we need the satellite to orbit around the Earth, depending on the use that is to be made of it, whether it is geostationary or not, etc., etc.

Once the altitude and speed at which the satellite is to be placed are defined, it is mounted on a space rocket that is responsible for traveling to the indicated height and when it reaches the necessary speed, it releases the satellite so that it can begin its journey and carry out its tasks normally. The satellite may need to perform maneuvers using its own thrusters to adjust to its final orbit accurately. This may include changes in altitude, inclination or position within an orbit.

Geostationary vs non-geostationary satellites

Geostationary satellites are those that remain fixed over a specific point on the Earth, so that they always see the same part of the Earth. This is achieved by placing the satellite in a geostationary orbit, that is, placing the satellite at a speed exactly equal to the speed at which the Earth rotates on its own axis, in such a way that, by always going at the same speed, the relative position of the satellite over the Earth is always the same. As we saw previously, the balance depends only on the speed and the height, so knowing that the speed must be the same as that of the Earth, it is only necessary to calculate the height at which to place the satellite.

Since the speed of the Earth is constant, this implies (after doing the calculation) that the height of all geostationary satellites is 35,786 km above the center of the Earth, thus creating the so-called geostationary orbit, in which any satellite that is placed there will always remain over the same point on the Earth, so they orbit the Earth once every 24 hours.

This way of placing satellites is common in climate control and telecommunications satellites, since they always have to operate over the same areas.

Non-geostationary satellites are those that are placed in any other orbit, adjusting their speed based on the height at which they are placed. They are usually placed in low Earth orbit (LEO), medium Earth orbit (MEO), and polar orbits, among others.

Their altitude can vary from just a few hundred kilometers above the Earth to a few thousand. They usually operate in much lower orbits than geostationary orbitals, so their speed is higher.

Non-geostationary satellites in lower orbits (such as LEO and MEO) must move faster than geostationary ones to maintain their orbit due to the greater gravitational force at lower altitudes.

Non-geostationary satellites are often used for terrestrial photography, cartography, GPS and advanced communications, such as the satellites of Elon Musk's Starlink network, which provide high-performance internet to the entire planet, thanks to their minimal distance from the Earth, which means that connections have very low latency.

Curiosities:

• The “Sprites” or “ChipSats” are the smallest satellites, with a size of approximately 3.5 square cm. It is a low-cost satellite for educational and research uses developed by Cornell University (USA).
• Satellites must perform periodic orbital corrections due to various external interferences. The gravity of the Moon and the Sun, as well as irregularities in the shape of the Earth (such as flattening at the poles and bulging at the equator), can alter the satellites' intended trajectories.
• Vanguard 1, launched in 1958, is one of the oldest satellites still in orbit, although it is no longer operational.
• Some communications satellites are as large as a school bus, such as ViaSat and Intelsat.
• There are thousands of fragments of inactive satellites and other debris in space, collectively known as space junk.
• Spy satellites have been in use since the 1960s to gather intelligence by observing activities in other countries.
• Satellites in low Earth orbit can travel at speeds of up to 28,000 kilometers per hour.
• SpaceX's Starlink, founded by Elon Musk, provides global internet coverage through constellations of thousands of ultra-low-altitude satellites.
• The Global Positioning System (GPS) satellite network consists of only 24 satellites, which allow precise location anywhere in the world.
• The first satellite to take complete images of the Earth was ATS-3 in 1967.
• Satellites in polar orbits pass over Earth's poles and can see every part of the planet as it rotates.
• Some decommissioned satellites are repurposed and reused for new missions rather than being discarded.
• Japan is investigating the use of wood to build satellites to reduce space junk.
• Geostationary satellites have an average lifespan of 10 to 15 years, after which they move to a graveyard orbit.
• The Moon is Earth's largest natural satellite and has been key in understanding how satellites can be used for space exploration.
• Although not frequent, collisions between satellites have occurred. A notable example is the 2009 collision between the US Iridium 33 satellite and Russia's defunct Cosmos-2251 satellite. This event generated thousands of pieces of space debris, increasing the risk of future collisions.
• The United States leads the way in the number of satellites in orbit (about 6,000), followed by China and Russia. These countries have advanced space programs and launch satellites for a variety of purposes, including communications, scientific research and military applications.
• The concept of space warfare has been explored in terms of the ability of some countries to intercept and destroy satellites. For example, in 2007, China conducted an anti-satellite test, where a Chinese missile destroyed its own weather satellite, generating international criticism and a debris field in orbit.
• Although not common, some satellites have been recovered back to Earth after completing their mission. NASA's Space Shuttle was notably used on several missions to recover satellites, such as the Long Duration Exposure Facility (LDEF) and communications satellites that were then repaired or upgraded and relaunched.
• In a comical twist, in 1978 the Soviet Consulate in San Francisco issued a “parking ticket” to the American spy satellite KH-11 for “parking” over Soviet territory. Although it was a joke, this event highlights Cold War tensions related to satellite surveillance.

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