Geostationary Satellite - A satellite that has a period of 24 hours and orbits the Earth’s equator at an altitude of 36 000 km.  , Gravity Turn - A spacecraft takes a slight turn when it reaches a certain altitude after a vertical launch. This minimises the effect of the gravitational pull of the body on the spacecraft, allowing it to reach a certain horizontal speed for its desired orbit.  , Orbital Period - The time taken to go around the Sun in one full revolution. The time taken by any celestial object to rotate on its axis., Orbital Radius - The distance between an object and the centre of the body it is orbiting.  , Kepler's 3rd Law - As the orbital radius increases, the orbital period also increases.  , Gravity Assist - Using the gravitational pull of a celestial body to gain or lose orbital velocity.  , Hohmann Transfer - The movement of a spacecraft from one circular orbit to another by gaining or losing orbital velocity.  , Docking - The joining together of spacecraft modules in orbit.  It requires a very precise Hohmann transfer from one orbit to the target orbit., Ion Drive - Ion thrusters accelerate ions in an electric field to generate thrust rather than burning fuel. They only require a small amount of fuel to do this.  , Re-entry - When a spacecraft re-enters through the Earth’s atmosphere. (Slowing down and converting huge kinetic energy to heat energy through friction with the atmosphere), Vacuum - A region containing no particles and therefore no atmosphere.  , Light Year - The distance that light travels in one year. 1 light year = 9.47 x 1015 m. , Age of the Universe - Time between now and the Big Bang - thought to be around 13.8 billion years. , Red Shift - The light coming from distant galaxies is ‘redder’ (of a higher wavelength) than what it should be. This tells us that these galaxies are moving away from us, which provides evidence that the universe is still expanding.  , Big Bang Theory - An effort to explain what happened at the very beginning of our universe. , Cosmic Microwave Backgroud Radiation - Radiation at a temperature of 2.7 K that is detected everywhere in space. , Hubble's Law - The observation that galaxies appear to be moving away from us at speeds proportional to their distance.  , Refracting or reflecting Telescope - An optical instrument used to observe distant objects by collecting lots of light via a large objective lens or mirror.  , Radio Telescope - A telescope with a large curved dish that picks up weak radio waves from space. The waves cross over at the receiver and combine to give a strong signal. , Continuous Spectrum - The spectrum of different colours produced (ROYGBIV) when white light is split with a triangular glass prism.  , Line Absorption Spectrum - Consists of a complete (continuous) spectrum with certain colours missing. These appear as black lines in the spectrum., Line Emmission Spectrum - Consists of lines of light that are distinct colours rather than a continuous spectrum.,

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