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Above: A Tracking Data and Relay Satellite (TDRS), used by NASA to communicate with spacecraft in orbit. Source: NASA.
This backgrounder will provide you with a broad description of satellites or spacecraft in space – what they are, how they get there, who puts them there, what they do in space, how they are built, and why they are there in the first place.
A satellite is any artificial (human-made) object that moves through space in orbit around another object. An orbit is the path, often circular, that a satellite follows as it moves around a much larger object under the influence of that object’s gravity.
A satellite orbiting the Earth at an altitude above the ground of 800 kilometres will need to travel at a speed of ~7.5 km per second – that’s nearly 27 000 km per hour! – to stay up in its orbit and not come back down to Earth.
How do satellites get into space?
To get a satellite into space and give it all that speed to stay up there, we need the most powerful engines on the planet. We need rockets. Rockets burn thousands of kilograms of fuel per second through massive and complicated engines, to provide the thrust or force needed to lift the rocket and the satellites inside it, off the ground and into orbit.
Figure 2: Launcher fairing containing multiple satellites. Image source: ISRO.
Figure 1: A rocket launches carrying satellites. Image source: Indian Space Research Organization (ISRO).
The satellite(s) (it makes more sense to launch several together at once, as long as they are going into similar orbits) are nestled within the protective fairing or nose-cone at the tip of a rocket. The fairing protects the satellites inside, which are usually folded up to fit into the tight space. During launch, a satellite will experience an extreme environment of intense vibration, large shocks, acceleration forces much stronger than standard gravity, and atmospheric pressure drops. Every satellite must be designed to survive the harsh stresses of launch. Once in the vacuum of space, the fairing splits apart and the satellites can be released into their orbit.
What types of satellites are up there?
Satellites range in size from tiny microsatellites and picosatellites (about 10 kg and 1 kg in mass respectively, so the size of a small football) to large multi-ton scientific satellites about the size of a bus! The largest current satellite is the International Space Station, housing a crew of up to seven astronauts – providing space for their living quarters, experiment laboratories, life support systems, supply and transport vehicle docking stations, and large solar panels providing all the power it needs.
Satellites come in all shapes and sizes, depending on the job they are designed to do. Even the smallest satellites contain scientific experiments, sometimes built by university or even high-school students. They use a simple payload or instrument to perform basic science, for example taking pictures of the Earth or objects in space, using an electronic camera not very different from today’s digital cameras. Small satellites are quicker to build and cheaper to launch, but are not designed to survive or even stay up in space for more than a few years. Nevertheless, some great science has been performed with these small satellites, and nowadays, it is even possible to order your own satellite assembly kit and build a 10cm-cubed cubesat – then all you have to do is hitch a ride on someone else’s launcher!
Figure 3: Hubble Space Telescope. Image source: NASA.
Some of the largest satellites are those used to peer deep into space – like the Hubble Space Telescope – and are shaped like a large camera complete with long telescope tube. Many different observatory satellites cover the whole electromagnetic spectrum, from the gamma rays emitted by the highest-energy black holes and supernovas, to the faint glow of heat or infrared from dying stars and distant planets, and of course visible light. Observatory missions such as NASA’s Kepler Mission are even now discovering new planets around other stars throughout our galaxy – will we one day travel to these distant worlds?
The most common type of satellite is also the largest. Telecommunication satellites bounce radio signals from one place on Earth to another far away, much like the dishes we see on microwave towers. These satellites need a lot of power to receive a faint signal from Earth, boost its strength, and beam it back down to another area. These satellites are usually placed in a special orbit called a geostationary orbit, where the orbital speed of the satellite is exactly the same rate at which the Earth rotates in a day, so the satellites stay above a fixed point on the Earth.
Earth Observation satellites are found at between 600 – 800 km altitude, placed in orbits such that they cover as much of the Earth’s surface as possible in a short space of time, usually moving from one polar region to the other, and as the Earth rotates underneath them, seeing a new area of the Earth’s surface every orbit. From this lower vantage point, these satellites can use powerful optical or infrared telescopes, or even bounce a radar signal off the Earth’s surface, to see incredible detail (some optical satellites are able to see individual objects down to ~30 cm across) and capture a vast amount of information. Earth Observation satellite size is typically driven by the size of its primary instrument. Those sensing optical (visible & near-infrared light) radiation or radio signals are usually small, around 1 metre cubed and a few hundred kilograms in mass. Radar satellites and meteorology (weather and climate) satellites have bigger and/or more instruments, and need more power, so tend to be larger – think the size of a small car.
Figure 4: Canada’s RADARSAT-2 satellite. Image source: Canadian Space Agency.
Navigation satellites play an important role in our everyday lives. A constellation of dozens of satellites are placed in multiple orbits, so that from any point on the Earth’s surface, there will always be at least three satellites overhead in the sky. Positioning devices using GPS (Global Positioning System) receivers pick up signals with very precise timing information from these as they pass overhead. Using this timing information, the receiver is able to very accurately determine its position in three dimensions (latitude, longitude and height above nominal sea level) to within a few centimetres.
Interplanetary satellites – those that are designed to travel and sometimes even land on other worlds – come in a variety of shapes, sizes and configurations, depending on where they are going and what they will do there – both on the way and when they reach their final destination. Most famous currently is the Curiosity rover, exploring the surface of Mars, equipped with many tools a typical geologist would take along with them or have back in their lab. Deep-space probes have visited all the planets in our Solar System, and a few are travelling beyond. Such probes have also visited asteroids and comets. The next challenge for these probes will be to capture a sample – of an asteroid, comet or from the surface of another planet – and bring it back safely to Earth to be studied. There have been two successful deep-space sample return missions so far: NASA’s Stardust mission returned a tiny amount of dust from the comet Wild-2 in 2006, and Japan’s Hayabusa mission returned a bit of dust from the asteroid 25143 Iyokawa in 2010. Missions to return larger samples, such as rocks from Mars, are still in the planning stages, and won’t be launched for years. Until then, we’ve got some Moon rock brought back by Apollo astronauts, and the occasional meteorite, along with those dust samples, as our only extraterrestrial (from outside Earth) material to study.
How do we build satellites?
Scientists and engineers are always thinking of new things to use satellites for, and new ways to build them. Satellites are definitely one of the most complex and unique pieces of machinery we build today, and use the most cutting-edge technology.
Satellites are built to be very strong and stiff, yet as light as possible. They must survive launch, then years in the vacuum of space, with its high radiation environment, being hit by tiny pieces of dust called micrometeorites, and the large temperature changes from sunlight to the deep cold of space.
Low mass and high strength are achieved by using a stiff central structure with aluminium honeycomb sheets for the outsides of the ‘box.’ If the satellite has fuel tanks, those will usually be in the centre. The sensitive electronics and instruments found in satellites are protected deep inside the satellite, some even in their own casings made of tough but light metals like titanium. Carbon-fibre reinforced plastic (CFRP) is also used to add extra stiffness that does not change its shape when experiencing large changes in temperature, as satellites do in space. Satellites are built with lots of electronic components, particularly sensors used to determine the pointing of the satellite, by sensing the position of the Sun, stars, and the direction of the Earth’s magnetic field. Actuators are used to point the satellite, using reaction wheels, control thrusters and magnetic rods.
The outside of satellites are covered with blankets of thermal insulation, helping avoid heat build-up, and also offering some protection against micrometeorites. Some surfaces are covered in thin gold foil, to prevent electrical charge build-up, and reduce the radiation getting through.
When we design and build satellites, we must test them thoroughly on the ground to make sure they will survive in space, before we launch them. With only a few rare exceptions, once a satellite is in space, there is no way to repair it! Satellites are tested again and again on ground, to make sure they will survive the launch and the many years they will spend in space. We use large thermal vacuum chambers to simulate the intense cold, heat and vacuum of space, to measure how the satellite performs under these conditions. We put components, instruments and whole satellites on vibration tables to simulate the forces and shocks of launch, to make sure nothing breaks. We test how a satellite will perform when exposed to radiation, and make sure none of the equipment interferes with each other. We also run detailed tests of all the on-board software (computer programs) to make sure it will behave as it should, under all mission conditions.
Finally, because we can never be absolutely certain that something will not fail after many years in space, we build satellites with lots of internal redundancy. This means for every critical component, circuit or instrument, we make sure that there are two of them, one used as a spare if the primary one fails. Sometimes satellites can switch to their spare components automatically, and sometimes we have to tell them to. This all makes satellites very expensive to build, and the process takes many years, with lots of people involved. Yet by the time we are finished designing, building and testing satellites on the ground, we can be pretty sure it will provide years of service in space.
How do satellites work?
Satellites all have some simple features in common. There is a payload – the scientific or technical instrument(s) that are the reason the satellite needs to be in space in the first place. To control this payload, we need an on-board computer, to tell the payload what to do, to get information back from it, and control the rest of the satellite.
All of these on-board computers and instruments need electrical power. Satellites commonly use solar arrays – lots and lots of solar cells that convert sunlight into electricity – either mounted on the satellite’s body or on large rotatable wings. Batteries store extra electrical power and provide it when there is no sunlight. Attitude control systems keep the satellite pointing where we want it to point (perhaps pointing its instrument at Earth, held steady on a star it is studying, or making sure a sensitive instrument is not pointing directly at the Sun). Larger satellites, and ones we want to stay in space longer, need a propulsion system to boost the satellite in its orbit. These are small rocket nozzles squirting out fuel or propellant in a specific direction, and they stop satellites from spiralling back down to Earth when their speed in their orbit slows. Satellites above the Earth experience atmospheric drag – where even the faint wisps of atmosphere high up above the Earth will eventually slow the satellite down.
There are thermal control systems that keep the temperature under control; also mechanical systems, used when the satellite is first released from the launch vehicle and unfolds itself. The telecommunications system receives satellite commands from mission control, and responds with satellite telemetry (information about satellite state of health) and the science data generated by the payload. The on-board computer and data processing system handles the data flow from electronic units throughout the spacecraft, and to and from the ground. All of these systems are housed what is known as the bus – the core of the satellite that connects and supports the payload and power systems.
How do we control satellites?
Figure 5: Satellite Ground Station. Image source: Mark Seymour.
Satellites do not fly themselves. There is a limited amount of autonomy, that is, where a satellite can make its own decisions and protect itself if something goes wrong. Most of the time, the satellite is waiting for commands from the ground, instructions that tell it what to do. We send those instructions up from the ground in advance, with each instruction given a precise time for the satellite to carry it out. While the satellites are executing their instructions (for example, switch on, point in this direction, take a picture, send the picture back to Earth) they are also storing information on-board, and when in view of a Ground Station, send that stored information to it.
Figure 6:Main Control Room at the European Space Operations Centre. Image source: ESA.
The instructions to the satellite are prepared in computer systems we call Planning Systems – where we plan what the satellite has to do. Control Systems build the commands and send them to the Ground Station – then check the telemetry when it comes down from the satellite. If the telemetry shows us that something might be wrong with the satellite, the Control System can send an alert to engineers, who can look into the problem and try and fix it.
Who builds satellites?
Figure 7: Sputnik 1, the world’s first satellite. Image source: NASA.
Countries all over the world recognize how important satellites are, many building and even launching their own. There are hundreds of working satellites in orbit around the Earth today, and many thousand more ‘deactivated’ or non-working ones that have yet to drift back down to Earth. These ‘dead’ satellites, and spent rocket bodies, are in danger of smashing into the working satellites – we call this space debris.
Major satellite-building nations include the United States, Russia, China, Japan, India, Brazil, the group of European countries making up the European Space Agency, and of course Canada. Canada was the third country to have a satellite in space, after the former Soviet Union (now Russia) then the USA launched the very first satellites in 1957 and 1958 respectively. Many other countries today buy satellites from, and have them launched by, these major ‘space superpowers.’
What do satellites do for me?
Satellites enable many parts of our everyday lives without us even realising it, playing a very important part in keeping us safe, entertained, getting us to where we need to go, and providing us with information we need. For example, imagine this morning you checked the weather forecast on television – you wanted to know whether it would rain, be sunny, or even snow. A lot of the information the weather forecaster used to predict the weather for today came from a satellite high above the Earth, measuring the amount of cloud, temperatures, and other details from a perfect position. The signal you were watching on the TV was sent through a telecommunications satellite at some point on its journey. The cereal you ate for breakfast was grown in a field whose crops were monitored for disease by a satellite. The truck driver who delivered the cereal to the right stores used a navigation system to find the right route to take, relying on signals from navigation satellites. When that truck driver bought gas for his/her truck, the credit card was checked against that person’s bank account, using a satellite link.
How can I get involved?
There are so many ways satellites are important to our everyday lives, and our future. We need students to study sciences, mathematics, engineering and computing, to design, build and operate the satellites of tomorrow. You can get involved by following the progress of missions to other planets, and learn more about satellites in use right now above the Earth. Maybe the students at your school will one day build a satellite of their own. Maybe YOU will one day build satellites that touch down on far-off worlds, or find a distant planet similar to Earth where humans will one day live!