Episode 3: Up, up and Stay! - Satellites

Abigail James
Feb 17, 2020
12 min read

This blog post is a transcript of Series 1, Episode 3 of my science podcast, Don't touch my Radium! All sources, references and recommended material can be found here.

Last week we looked at how we interpret and gather light through spectroscopy and the use of telescopes in investigating the universe. The invention of the telescope and its subsequent advancements allowed us to see further and learn so much more about the space beyond our atmosphere. But as we learned last week, there are limitations to what we can see from the ground which restrict our ability to interact with different regions of the electromagnetic spectrum. If we want to see the light from other planets and try to use spectroscopy to make determinations about their composition, we need to get a clearer view. We need to get up above the atmosphere! We need, satellites!!

"Man must rise above the Earth -- to the top of the atmosphere and beyond -- for only thus will he fully understand the world in which he lives." Socrates made this observation centuries before humans successfully placed an object in Earth's orbit.

In 1945, Arthur C Clarke wrote an article for Wireless world titled: “Extra-Terrestrial Relays: Can Rocket Stations Give World-Wide Radio Coverage?” Then in 1946, an American astrophysicist named Lyman Spitzer proposed sending a telescope into orbit. 11 years later, Sputnik 1, the very first satellite was sent into orbit in 1957. It took another 11 years for the first telescope, the Orbiting Astronomical Observatory to be launched. These timescales may appear in the extreme but designing and launching a telescope into space is not as straightforward as you might think and sometimes the idea might be there but we need to wait for the technology to catch up. One of the things that had to be figured out, was how to get into orbit. So let's take a little bit of time to talk about rockets.

Rockets

The first rockets were created in China way back in the 1200’s. They were solid fuel mini rockets, they were fireworks! Less fun, armies also used these early rockets as weapons in wars. But when we think of a rocket I’m sure that most of us are picturing a big white structure standing tall in the middle of a desert, waiting to take off into space with a deafening roar and explosion. Well, a rocket is both a type of engine and a vehicle that uses this engine. The definition is ‘a vehicle, missile or aircraft which obtains thrust by the reaction to the ejection of fast moving fluid from within a rocket engine’.

Rockets are equipped with a main engine as well as small thrusters, the main engine gets the rocket into space while the thrusters are then used as a way to guide the rocket to it’s destination. A rocket generally operates like an internal combustion engine, they are chemically powered and use either solid or liquid propellant, sometimes both, to power their motion. In general the principle of an engine is that it burns fuel with oxygen to create an exhaust in the form of air or a hot gas, this is then pushed out the back with a velocity. Given Newton's 3rd law of motion, for every action there is an equal and opposite reaction, as the engine pushes on the exhaust it sends the exhaust backwards, then the exhaust pushes on the engine propelling it forwards. It’s basically exchanging momentum.

A jet engine will use the oxygen in the air to create its exhaust, but in space there is no air, and so a rocket engine needs a way to create oxygen burning fuel. This is done by adding oxidizers.

Rocket Fuel

Fuel + Oxidiser = Propellant. The fuel is burned with an oxidizer in a high-pressure combustion chamber creating a high energy exhaust that is then accelerated out through a nozzle at the back of the rocket. Gas molecules may be light in weight but when they exit the rocket at high speeds, there’s a lot of momentum generated. This provides the power that the rocket needs to push off the surface and get into space. One of the biggest challenges for space flight is of course, cost, as well as the technology and structure of the rocket, it takes a lot of fuel to propel it into space, The materials used can be extremely toxic and hazardous so a lot of care needs to be taken in their use, but they are also very explosive! And the more explosive, the less fuel you need which will reduce the cost as this will also have an effect on the rocket design.

So one of the main problems to solve, is how to choose the right kind of propellant. There are a number of things to consider such as stability, availability and toxicity. Two of the main considerations are efficiency and thrust. To look at how efficient a rocket is, we look at what its change in momentum per unit of propellant is. This is normally referred to as its specific impulse. The thrust is the force that it generates and it depends on the density of the propellant. As an object attempts to move through our atmosphere it’s met with atmospheric drag that slows it down, there’s also the gravitational pull of the Earth to fight against. The rocket needs to be able to generate enough force to lift the payload, unused fuel, and the rocket itself through the thick layers of the atmosphere fighting against these frictional and gravitational forces.

Unfortunately, the propellants that have a high specific impulse, don’t have high-thrust properties, and vice versa. So while we want to have a high efficiency, this will not provide enough thrust to get into space. A lower density propellant requires larger fuel tanks so this then results in the atmospheric drag being greater on the rocket, so it needs more thrust to get off the ground, more propellant. Solid propellants are higher in density and so have a higher thrust value, they are also reliable and have their own oxidizer. However, they have to be mounted separately as side boosters on a rocket and as they burn continuously their number of applications in one mission are limited. Liquid fuel, in particular low temperature liquids, such as liquid hydrogen and liquid oxygen have the highest specific impulse and even better, they can be stopped and started so are ideal for space travel.

A way to deal with this is to launch a rocket in two stages, the first stage to get out of the lower atmosphere and the second stage to direct to the desired location. The faster the rocket travels the more air resistance it meets but since the atmosphere thins with height, the stress on the structure reduces once it gets to an altitude of about 12km. As the first stage requires the bigger more powerful section, solid propellants are used as boosters to get the rocket off the ground, these tanks can then be dropped igniting the engine of the second lighter portion, which carries the payload, and uses thrusters to manoeuvre into its orbital position. The propulsion used in the second stage thrusters is traditionally either a single fluid or two fluids but they have also been developed in more recent times to use high energy electric propulsion. There are also ion thrusters that accelerate heave ions created in a plasma - very sci fi like.

Rocket types:

Sounding rockets: short arcs before returning/crashing to earth. Science experiments that don’t need a lot of time in space
Sub orbital: temporarily enters space - science experiments, space tourism
Orbital class: launch objects into orbit around Earth. Depending on the size of the payload, can send objects beyond Earth - probes and sports cars.

But let’s stick with Earth orbits for now, and have a look at what this means.

Orbits

As we travel away from the surface and through the atmosphere, there are a number of different orbital distances that can be used depending on the satellite's purpose.

We define space as beginning at the Karman line which is about 100km from the surface of the planet, this is due to the air density being so low at this height. There are three orbital ranges, Low, Medium, Geostationary.

Low Earth orbit is in the thermosphere layer of our atmosphere, it starts at about 90 km and extends to between 500 to 1000 km, the top varies as activity from the sun means it emits more high energy radiation at times, heating up the thermosphere and causing it to expand.

The ISS orbits at about 400 km, just below the traditional top of the thermosphere, as it’s closer to Earth its orbit is faster travelling with a speed of 28,000km/hour, this means that it orbits the planet in just 90 minutes.

Medium Earth orbit begins at 20,000km and takes around 12 hours to orbit the planet.
Geostationary orbits are at around 36, 000km and take 24 hours to orbit the planet. This is the area that is used by weather and communications satellites.

Where you send your satellite, depends on how long you want it to stay in orbit. Low Earth orbit is close enough to be ‘buffeted by the atmosphere’ causing the orbit to decay over time and eventually crash through the atmosphere, but it needs to have a minimum height of about 200km so that atmospheric drag doesn’t slow it down too soon.

Satellites at higher orbits, those in medium and geostationary orbits, will be there for millions of years.

Van Allen Radiation

There’s something that we haven’t mentioned yet, and this is the Van Allen radiation belts.

These are regions of plasma above low Earth orbit, where the magnetic field of the Earth traps high energy charged particles from solar winds and cosmic rays. These charged particles can be damaging to astronauts, depending on the density of the particles and the amount of time you spend in this region. They expand out in two belts, tracing the shape of the magnetic field. With the inner belt starting at about 1000km from the planet's surface extending up to 12000km high. While the outer belt starts at 13000km and extends to 60000km. We can mostly avoid the inner belt in low-earth orbit, however despite being relatively stable, its shape can change and so at times the ISS and other satellites will be at danger of higher radiation doses. When the ‘space weather’ is really intense, these belts can even affect equipment on the surface of the planet. In order to travel higher, such as the Apollo missions, we look for low density regions of these belts and travel through them at high speeds to limit the amount of radiation exposure time. However if you need to be in medium or geo-stationary orbit, then you need to build a satellite with materials that can withstand this environment.

The satellite is enclosed in a rocket for launch, as the rocket's payload, the equipment on a satellite, whether for a single purpose or for a number of functions, must be able to withstand high energy regions so it needs to be made of materials that can absorb and deflect the harmful radiation without affecting its ability to collect the information it is designed for. There is immense pressure on designers, engineers, technicians in building a satellite. There is no room for error, failure is not an option, as the hours and expense that go into designing it will be for nothing if it dies/fails etc. A satellite cannot be repaired once in orbit neither can it be returned to ground to be fixed and then sent back up into orbit. It has to be perfect the first time. It can cost around 150 million dollars to build and launch a satellite and it will typically need to last for up to 15 years.

Satellites

Conceptually depicted in literature and art for decades before being realised, the first artificial satellite, Sputnik 1, was launched in 1957. It was only the size of a basketball, ran on batteries and used antennae to send radio signals. Since then, nearly 9000 satellites have been sent into orbit and as of 2018, around 1900 are operational.

Main uses of satellites:

Communications: radio waves operate in straight lines so can’t maneuver the curvature of the earth, normally in geostationary orbits.
Navigation: gps is made up of 24 satellites in medium earth orbit.
Weather: image clouds and measure temp and rainfall, normally in geostationary as well as low earth orbit.
Earth observation: photograph and image earth so better in low earth orbit.

Lastly, what we are most interested in :

Astronomical: monitor and image space.

How a satellite is designed and the instruments it carries will depend in part on its intended function, but there are some components and considerations that apply to all types of satellites.

Propulsion System: we’ve gone through this, it’s the rocket engine to get into space and the thrusters to alter orbit if needed.
Computer system: control operation, monitor altitude, orientation, temperature.
Comm system: send and receive data, curved satellite dishes as antenna.
Attitude control system: gyroscopes, rocket thrusters, light sensors to control direction its pointing

The last three topics we’ll take a look at in more detail:

Bus: frame and structure
Power source: solar panels, batteries for when in shadow
Heat control system: reflect and radiate heat.

Bus

Satellites can range in size and mass from tiny 1kg picosatellites all the way up to large satellites that are more than 1000kg. Not only does the structure need to be able to withstand the launch as well as shock waves in space, it also needs to be light enough to be lifted by the rocket without adding to the fuel costs any further. The materials used need to resist expansion and contraction that would be caused by the extreme temperature changes in space that could cause the structure to bucke. There are also the effects from travelling at high speeds to consider, the satellite will undergo large g-forces and could also collide with other objects in space, crashes are rare but Earth's orbits are getting crowded. Skeletons are normally made of strong lightweight materials such as carbon fibre and then fitted with aluminum or titanium panels, aluminum is more common as its relative cheap and more readily available than titanium. Given the harsh environments and length of time in space, the internal workings also need to be made out of stable materials that don’t degrade but are also good conductors for heat and electricity, gold is a good choice for this and some satellites can contain up to 20kg of pure gold. Other materials commonly used in the design of a satellite are graphite, boron, fibreglass, teflon and kevlar. One of the benefits of teflon and graphite are that they can also be used as ‘dry lubricants’.

Power

A satellite needs to generate its power on board but with limits to the size of the payload due to fuel requirements, computer miniaturization and microelectronics have greatly contributed to advancements in satellite technology. Modern satellites will normally use both solar power and batteries to function. Solar panels are photovoltaic cells that use the photoelectric effect to convert light into electricity. They harness the light from the sun and are used to power the satellite and its instruments, while also charging back up batteries that can kick in when the satellite is not in sunlight. The panels are in an array structure that is folded up for the launch and then deployed once the satellite reaches its destination, they can be up to 20 to 30m long. Each array will be fitted with a drive mechanism so that it can move and make sure it’s always showing the maximum area to the sun as the power that is generated is directly proportional to the intensity of the light. If you remember from last week when we talked about the particle-like behaviour of light, the energy the electrons carry depends on the intensity of the light.

As seems to be the theme with the design requirements and restrictions for a satellite, the power sources will also need to be small, sturdy and be able to last for potentially 15 years in the harsh environment of space. There is ongoing research into the use of nuclear devices which could potentially outdo solar energy but for now, the best changes your satellite has, is to use the energy from the sun.

Thermal Architecture

We’ve mentioned the harsh environments and the requirements for the structure and power sources. Drastic temperature changes means that to keep the internal structure, mechanics and electronics at functional temperatures, more than one stage of insulation is required. In space the temperature ranges from -270 degrees Celsius, that’s about 3 Kelvin, in the shadow; up to 150 degrees Celsius, about 400 Kelvin, in direct sunlight. You need to be able to insulate the internal structure, dissipate internal heat created by the electronics as well as reflect high temperatures coming from the sun.

Insulation of the internal structure requires both protecting the satellite from the extreme high temperatures, but also keeping the heat in when in the Earth's shadow. Covering your devices with aluminum can provide some shielding from some forms of the radiation they may encounter, you can also consider having back up copies of critical systems in case of failure.

To dissipate the internal heat generated by the electronics, pipes are used like radiators to move the heat away from the equipment. The final layer of protection is in the external body of the satellite, materials will vary as the technology improves but a standard satellite will have thin surface mirrors that are used to reflect the sun's rays and will be only about 100 microns thick. There might also be further layers of material to increase the insulation properties such as fiberglass, spacers and beta cloth (a fireproof material used in space suits). A powder coating may be used on the exterior to further protect the satellite.

It always comes back to the payoff between what you can do in terms of what the satellite needs versus what you can do in terms of what the rocket can lift; it can cost around $10,000 per 500g of payload to launch it into space, so if you can find lighter smaller materials that are also resistant to radiation, then you’re going to be able to do a lot more. This is where the ‘waiting for technology to catch up comes in’.

So what happens when your satellite dies or reaches the end of its mission? Earlier we mentioned how a low earth orbit satellite will crash back into the atmosphere, but what about the medium and geostationary one? Well small thrusters will use the last of the fuel to send the satellite further away from our planet into a ‘graveyard’ orbit, where it will live for maybe millions of years among the space junk that the Earth has collected.

Join me next week where we’ll discuss the additional equipment and instruments required to turn a basic satellite into a space observatory and we’ll look at what it takes to get out of orbit and explore our solar system.

References and Sources

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