Artificial Satellites

Artificial satellites are human-built objects orbiting the Earth and other planets in the Solar System. This is different from the natural satellites, or moons, that orbit planets, dwarf planets and even asteroids. Artificial satellites are used to study the Earth, other planets, to help us communicate, and even to observe the distant Universe. Satellites can even have people in them, like the International Space Station and the Space Shuttle.[1]

Background

The first artificial satellite was the Soviet Sputnik 1 mission, launched in 1957. Since then, dozens of countries have launched satellites, with more than 3,000 currently operating spacecraft going around the Earth. There are estimated to be more than 8,000 pieces of space junk; dead satellites or pieces of debris going around the Earth as well.

Science Background Information

A satellite is an object that revolves around another object in a path called an orbit. Satellites can be either natural or artificial (man-made). Our moon is an example of a natural satellite as are all the moons of the other planets. The earth can also be considered a satellite of the sun. Artificial satellites are put into orbit by man. Some examples are weather satellites (GOES), communication satellites (ANIK), navigation satellites (GPS), scientific satellites (TERRIERS), and military satellites (MILSTAR). The first artificial satellite, Sputnik 1, was launched by the Soviet Union on October 4, 1957 - the US launched Explorer-1 in January 1958. Congress established National Air and Space Administration (NASA) on October 1, 1958 - this was the start of the "Space Race" (add these to the time line). Prior to the creation of NASA, the US space effort was a military controlled enterprise (reference to the Cold War, nuclear scare, post-WWII missile development).[2]

How they work

Satellites are launched into different orbits depending on their mission. One of the most common ones is geosynchronous orbit. This is where a satellite takes 24 hours to orbit the Earth; the same amount of time it takes the Earth to rotate once on its axis. This keeps the satellite in the same spot over the Earth, allowing for communications and television broadcasts.

Another orbit is low-Earth orbit, where a satellite might only be a few hundred kilometers above the planet. This puts the satellite outside the Earth’s atmosphere, but still close enough that it can image the planet’s surface from space or facilitate communications. This is the altitude that the space shuttle flies at, as well as the Hubble Space Telescope.

Artificial satellites can have a range of missions, including scientific research, weather observation, military support, navigation, Earth imaging, and communications. Some satellites fulfill a single purpose, while others are designed to perform several functions at the same time. Equipment on a satellite is hardened to survive in the radiation and vacuum of space.

Satellites are built by various aerospace companies, like Boeing or Lockheed, and then delivered to a launch facility, such as Cape Canaveral. Launch facilities are located as close as possible to the Earth’s equator, to give an extra velocity kick into space. This allows rockets to use less fuel or launch heavier payloads.

The altitude of a satellite’s orbit defines how long it will stay in orbit. Low orbiting satellites are mostly above the Earth’s atmosphere, but they’re still buffeted by the atmosphere and their orbit eventually decays and they crash back into the atmosphere. Other satellites orbiting in high orbits will likely be there for millions of years.

Types of Satellites

Navigation satellites

The GPS (global positioning system) is made up of 24 satellites that orbit at an altitude of 20,000 km above the surface of the Earth. The difference in time for signals received from four satellites is used to calculate the exact location of a GPS receiver on Earth.

Communication satellites

These are used for television, phone or internet transmissions, for example, the Optus D1 satellite is in a geostationary orbit above the equator and has a coverage footprint to provide signals to all of Australia and New Zealand.

Weather satellites

These are used to image clouds and measure temperature and rainfall. Both geostationary and low Earth orbits are used depending on the type of weather satellite. Weather satellites are used to help with more accurate weather forecasting. Weather satellites operate in two orbit types. The GEO mode allows the same geographic area to be viewed continuously from a very high altitude. With the LEO polar orbit, complete Earth coverage is possible.

Earth observation satellites

These are used to photograph and image the Earth. Low Earth orbits are mainly used so that a more detailed image can be produced. The International Space Station (ISS) is a habitable artificial satellite that has been placed in a low Earth orbit. It completes 15.7 orbits per day and is maintained at an orbital altitude of between 330 km and 410 km. This is a habitable space laboratory. At an altitude of 400 km, the ISS travels at a speed of 28,000 km/h and orbits the Earth once every 92 minutes. Scientists inside the ISS are able to perform many valuable experiments in a microgravity environment.

Astronomical satellites

These are used to monitor and image space. A satellite such as the Hubble Space Telescope orbits at an altitude of 600 km and provides very sharp images of stars and distant galaxies. Other space telescopes include Spitzer and Chandra.

Satellite design

Every satellite has some of the same basic parts:

  • The bus – this is the frame and structure of the satellite to which all the other parts are attached.
  • A power source – most satellites have solar panels to generate electricity. Batteries store some of this energy for times that the satellite is in the shadow of the Earth.
  • Heat control system – satellites are exposed to extremely high temperatures due to exposure to the Sun. There needs to be a way to reflect and reradiate heat. Electrical components of the satellite can also produce a lot of heat.
  • Computer system – satellites need computers to control how they operate and also to monitor things like altitude, orientation and temperature.
  • Communication system – all satellites need to be able to send and receive data to ground stations on Earth or to other satellites. Curved satellite dishes are used as antennae
  • Attitude control system – this is the system that keeps a satellite pointed in the right direction. Gyroscopes and rocket thrusters are commonly used to change orientation. Light sensors are commonly used to determine what direction a satellite is pointing.
  • A propulsion system – a rocket engine on the satellite may be used to help place the satellite into the correct orbit. Once in orbit, satellites do not need any rockets to keep them moving. However, small rockets called thrusters are used if a satellite needs to change orbit slightly.[3]

Why Are Satellites Important?

The bird's-eye view that satellites have allows them to see large areas of Earth at one time. This ability means satellites can collect more data, more quickly, than instruments on the ground. Satellites also can see into space better than telescopes at Earth's surface. That's because satellites fly above the clouds, dust and molecules in the atmosphere that can block the view from ground level.

Before satellites, TV signals didn't go very far. TV signals only travel in straight lines. So they would quickly trail off into space instead of following Earth's curve. Sometimes mountains or tall buildings would block them. Phone calls to faraway places were also a problem. Setting up telephone wires over long distances or underwater is difficult and costs a lot. With satellites, TV signals and phone calls are sent upward to a satellite. Then, almost instantly, the satellite can send them back down to different locations on Earth.[4]

International Space Station Flew over Zimbabwe

The International Space Station flew over Zimbabwe on 10 January 2020 and could be viewed with the naked eye in areas where it was not cloudy. It flew in a north-easterly direction and was seen at 1912hrs Zim time.[5]

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References

  1. Fraser Cain, [1], Universe Today, Published: 2 December, 2019, Accessed: 18 March, 2021
  2. [2], Centre for Space Physics - Boston University, Published: 7 April, 1999, Accessed: 18 March, 2021
  3. [3], Science Learning Hub, Accessed: 18 March, 2021
  4. [4], NASA, Published: 12 February, 2014, Accessed: 18 March, 2021
  5. Ignazio Magnani, [5], Twitter, Published: 10 January, 2020, Accessed: 18 March, 2021