In the Big Bang, only hydrogen, in the form of protons, was created. Satellites are pulled by theAbout 300 million years later, the very first protostar was born. The Earth’s gravitational force of Weather and spy satellites orbit the Earth passing overtemperatures were hot enough to allow fusion to take place. Later the first gravity. This is the centripetal its two poles as the Earth spins beneath – low polar orbits.galaxies formed. force without which the The International Space Station orbits about 300km satellite would shoot off into above us and takes about 90 minutes to orbit the Earth! space. Geostationary satellites are a long way out in Space (about 36 000km) and appear to remain above the same placeThe carbon in our bodies was formed because their orbital period is 24 hours.by fusion in a first generation star.The elements were spread into spaceby the star exploding as a supernova.About 9 billion years after the BigBang, the solar system was formed The Stars and uswhen dust and gas were pulled by Satellitesgravitational attraction. We arestardust! Planets are pulled by the Sun’s gravitationalThe Moon was formed by the collision force of gravity. This is the centripetal forceof the Earth with another planet. The without which they would shoot off intoless dense material became the Moon. space. The orbits of the planets and comets are in Space fact elliptical. Asteroids and near-Earth objects can crash into the Earth. The force of gravity increases as the distanceOur Sun has been stable for about 5 between the Sun and comet decreases – sobillion years because the gravitational comets speed up as they approach the Sun.forces balance the radiation pressure. The Sun heats the comet and melts its ice toThe Sun sends out solar flares – high- Centripetal forcesspeed charged particles that disrupt our When Time and form a tail. and planetssatellites.Our Sun will become a Red Giant, then a Space beganwhite dwarf.Much larger stars will explode as asupernova to form either a very dense When an object goes round in a circle at constant speed, it is inneutron star or, if it is really big, a black We know there was a beginning to fact accelerating because its velocity is changing. This requires anhole, where the gravity is so strong not the Universe because the galaxies inward pulling force – a resultant force called the centripetal force.even light can escape. appear to be rushing away from us. It can be friction for a car going round a roundabout, or tension in aCosmic ra Their light is red shifted by the string, or gravity for a planet or moon. Doppler effect. Unmanned probes have explored the Solar System and its planets Also the radiation from the Big Bang such as Voyager and the Mars Explorer robot – they can withstand is still around us – the microwave the lethal environment of Space, such as cosmic rays. background radiation, which we can pick up with our TV aerial.