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  • Rights: University of Waikato
    Published 27 March 2013 Referencing Hub media
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    Dr Allan McInnes explains why satellites need large rockets to launch them into orbit1 and why many satellites are launched from near the equator.

    Point of interest:
    A satellite2 in an equatorial orbit provides repeated observations of the same area. An example of this is a weather3 satellite.

    Transcript

    DR ALLAN MCINNES
    If we want to orbit at 200 kilometres, then we need to get to that height. But that’s not enough because if we were at 200 kilometres and not moving at all, we’d just fall right back down to Earth – that would be the gravity4 pulling us back down again. So we also need to have a certain amount of speed going sideways that’s going to cause us to stay in that orbit in the same way that if you’re swinging a tennis ball on the end of a string, you need to give it some speed so it actually swings instead of just falling down. So the way we get a satellite up to both the altitude5 it needs to be at and going at the speed we need is to put it on a rocket.

    We need to use a rocket because we’re trying to go above the atmosphere6, so planes that use the atmosphere to fly aren’t going to work. If you’re flying a plane, you have oxygen7 in the atmosphere that will burn the fuel8. Once you’re above the atmosphere, you need something else, and the way a rocket works is that it takes fuel with it and also it takes something to burn that fuel. We actually carry oxygen or some other kind of oxidiser. So that’s part of the reason rockets look so big actually is because they’ve got to carry not just fuel but also oxidiser – you’ve got to carry a lot of stuff just to get a small satellite into orbit.

    A lot of satellites need to be sent into orbits that are what we call equatorial, so ones that circle the equator, and the easiest way to do that is to start from near the equator. There’s a very small advantage just from the fact that you get a little bit of extra energy from the Earth because it’s spinning. But the main advantage is just that because you’re near the equator, you can go into pretty much an equatorial orbit straight away. If you start from a higher latitude9, then you have to do a manoeuvre that changes the angle of the orbit. So you have to flip from being something that might be at 30 or 40° angle to the equator to something that’s going around the equator. And that manoeuvre – which is called a plane change manoeuvre – can be quite expensive. It takes a lot of rocket fuel to do it, which means you either need a much, much bigger rocket or you can launch a smaller satellite.

    Acknowledgements:

    Cryosat II footage courtesy of European Space Agency/ESA.

    1. orbit: The path of an object as it revolves around another object. For example, the path the Moon takes as it moves around the Earth is its orbit.
    2. satellite: Any object that orbits around another object.
    3. weather: Daily or short-term conditions like temperature, cloud cover, precipitation and wind affecting a certain area.
    4. gravity: The force attracting something towards the centre of Earth (or other large mass, like a moon or planet) – the reason that things fall to Earth.
    5. altitude: 1. The height of something, usually height above sea level. 2. In astronomy, the angular distance of a natural or artificial satellite above the horizon.
    6. atmosphere: 1. The layer of gas around the Earth. 2. (atm) A non-SI unit of pressure equivalent to 101.325 kPa.
    7. oxygen: A non-metal – symbol O, atomic number 8. Oxygen is a gas found in the air. It is needed for aerobic cellular respiration in cells.
    8. fuel: 1. A combustible substance that provides energy. 2. A body fuel such as fat, carbohydrates and protein that supplies energy for animals’ activities.
    9. latitude: A geographic co-ordinate that specifies the north-south position of a point on the Earth’s surface. Lines of constant latitude (or parallels) run east-west as circles parallel to the equator. Latitude is used with longitude to specify the precise location of features on the surface of the Earth.
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      orbit

    1. + Create new collection
    2. The path of an object as it revolves around another object. For example, the path the Moon takes as it moves around the Earth is its orbit.

      gravity

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    4. The force attracting something towards the centre of Earth (or other large mass, like a moon or planet) – the reason that things fall to Earth.

      oxygen

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    6. A non-metal – symbol O, atomic number 8. Oxygen is a gas found in the air. It is needed for aerobic cellular respiration in cells.

      satellite

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    8. Any object that orbits around another object.

      altitude

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    10. 1. The height of something, usually height above sea level. 2. In astronomy, the angular distance of a natural or artificial satellite above the horizon.

      fuel

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    12. 1. A combustible substance that provides energy. 2. A body fuel such as fat, carbohydrates and protein that supplies energy for animals’ activities.

      weather

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    14. Daily or short-term conditions like temperature, cloud cover, precipitation and wind affecting a certain area.

      atmosphere

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    16. 1. The layer of gas around the Earth.

      2. (atm) A non-SI unit of pressure equivalent to 101.325 kPa.

      latitude

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    18. A geographic co-ordinate that specifies the north-south position of a point on the Earth’s surface. Lines of constant latitude (or parallels) run east-west as circles parallel to the equator. Latitude is used with longitude to specify the precise location of features on the surface of the Earth.