The Earth and the Universe The exploration of space is called astronomy. Most of our observations in space have come from using telescopes of different kinds. Observing the universe from the ground has problems: The atmosphere distorts the image. Stars twinkle. Clouds can prevent observation. Light pollution from cities spoils the images. The best images come from space telescopes which orbit the Earth. The pictures are transmitted back to the ground by microwave links, and are enhanced by computers. E.g. images of Hubble Space Telescope Other Kinds of Telescope Normal telescopes rely on visible light. However astronomers use radiation from other parts of the electromagnetic spectrum. Radio telescopes pick up radio frequency radiation. They have large dishes that focus radio waves onto a receiving antenna (aerial). Other telescopes pick up: Infra red; Ultra violet; X-rays; Gamma rays. Obviously we can't see these radiations so astronomers rely on computers to generate the images. Our solar system consists of an average star we call the Sun, the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. It also includes (Pluto) : the satellites of the planets and numerous comets and asteroids. The Sun is the richest source of electromagnetic energy (mostly in the form of heat and light) in the solar system. The Sun's nearest other sun is a red dwarf star called Proxima Centauri, at a distance of 4.3 light years away. The whole solar system, together with the local stars visible on a clear night, orbits the centre of our home galaxy, a spiral disk of 200 billion stars we call the Milky Way. T2011 Page 1
Day and night We can see that the Sun seems to move across the sky starting in the east in the morning, and then toward the west as the day ends. We know that the Sun appears to move across the sky because the Earth rotates on its axis. The Earth takes 1 day, (24 hours) in order to make one complete spin on its axis. The Earth The earth orbits the sun in an elliptical orbit and the moon orbits the earth with the same kind of orbit. Looking down from the North pole, the earth spins in a anti -clockwise direction once every day. The earth s axis is tilted with respect to the plane of its orbit at an angle of about 23.4 degrees. The earth orbits the sun in a anti - clockwise motion, coming to the same position among the stars every 365.26 earth days.( 1 year) We would also see that the moon also orbits the earth in an anti -clockwise motion. T2011 Page 2
Because of the tilt of the earth, not every place on earth gets light every day. Also, some places have extremely short days. As the earth revolves around the sun, the place where light shines the brightest changes. This motion gives us the different seasons. For instance, the poles receive less light than does the equator because of the angle that the land around the poles receive the sun s light. When the north pole is tilted toward the sun, the northern hemisphere is presented to the sun at a greater angle than the southern hemisphere and the northern hemisphere gets warmer. When this happens, the northern hemisphere gets summer while the southern hemisphere gets winter. When the south pole is tilted toward the sun, the two seasons reverse hemispheres. Because of centrifugal force involved when an object spins, the earth is not a perfect sphere, but is somewhat flattened at the poles and bulges out at the equator. The distance from any point on the equator to the centre of the earth is longer than the distance from either pole to the centre of the earth. T2011 Page 3
The Moon The moon is the earth s only natural satellite. Its revolution period around the earth is the same length and direction as its rotation period, which results in the moon always keeping one side turned toward the earth and the other side turned away from the earth. The side turned away from the earth is called the moon s dark side, even though it is lit half of the time. The moon s period of revolution is about 27.32 days long. The moon has a noticeable effect on the earth in the form of tides, but it also affects the motion and orbit of the earth. Phases of the the Moon As the moon circles the Earth, the shape of the moon appears to change; this is because different amounts of the illuminated part of the moon are facing us. The shape varies from a full moon (when the Earth is between the sun and the moon) to a new moon (when the moon is between the sun and the Earth). T2011 Page 4
How the Universe Began Most astronomers believe that the Universe started from a single tiny point in a titanic explosion called the Big Bang. The Big Bang is thought to have occurred about 18 000 million years ago. What is the evidence for this? Galaxies are moving away from us as shown by red-shift; The further they are away from us, the faster they are going; Cosmic background microwave radiation is described as "the echoes of the Big Bang". The background temperature of space is 2.73 K (= -270 o C), a higher temperature than expected. But still pretty cold. Present abundances of hydrogen and helium. Material was thrown out as the Universe expanded, and in places came together under the influence of gravity to form galaxies, stars, and planets. There are many thousands of millions of stars. Latest evidence suggests that many have planets. Who knows; there may be life. A star is a ball of mostly hydrogen and helium gas that shines extremely brightly. Our Sun is a star. A star is so massive that its core is extremely dense and hot. At the high core temperatures of a star, atoms move so fast that they make a fusion reaction forming more massive atoms and releasing a great amount of energy. The definition of a planet is a celestial body that: is in orbit around the sun; has a nearly round shape; has cleared the neighbourhood around its orbit. Recently Pluto was removed from being th ninth planet but was classified as a dwarf planet because it has not cleared the neighborhood around its orbit. T2011 Page 5
Stars and Galaxies The reason that we are here is that we live on a planet that is just the right distance away to give us conditions that are suitable for life. All the other planets in the solar system are too hot or too cold. Astronomers believe that there are many stars that have solar systems, if not the majority. They cannot see the planets; they are too far away, but they can detect the wobble caused by the gravitational pull of planets on a star. Distances in space are big, and are measured in light years. 1 light year is about 10 16 m. The nearest star, Proxima Centauri, is 4 light years away. That means that light, travelling at 300 million meters a second, takes 4 years to get from there to here. Flying at the speed of a supersonic plane (60 m/s) it would take 500 000 years to get there. Stars are in huge clusters called galaxies. Our galaxy is called the Milky Way and is about 100 000 light years across. It contains many millions of stars. The Sun is on one of the arms of the galaxy. Be careful not to confuse galaxies with constellations. These are patterns formed by stars. These are helpful to our observations from the Earth Birth of a Star Stars are formed from clouds of gas called nebulae. Particles of dust and molecules of gas come slowly together under the force of gravity. The process takes many millions of years to happen, although the shock waves from the explosion of a supernova may help the process. If there is enough material, it will get hot enough for fusion to occur. The temperatures involved have to be many millions of degrees Celsius. The star lights. T2011 Page 6
Life of a Star The Sun has been in a stable state for about 4500 million years. It is half way through its life now. The reaction is nuclear. The nuclear fusion reactions in the Sun are right at the very core. Here there are extreme temperatures and all elements exist in a form in which all the electrons are stripped off. This is called plasma. Old Age of a Star Helium is a waste product of fusion. Although some helium nuclei are used up in fusion to larger elements, helium starts to build up in the star. This makes the star swell into a giant star. Red giants are so called because the surface temperature is lower than yellow stars like the Sun. However they are still pretty high. Death of a Star After a while, a red giant starts to shrink under the effect of gravity and finally becomes a brown dwarf. It gradually cools and ends up as a dead star. Depending on its size, it can collapse into a very small space. It is thought that the Sun will end up as a neutron star of extremely high density but no bigger than the Earth. The Sun will go out with a whimper. By contrast, stars that are bigger than the Sun can end their lives with something of a bang. They swell to red supergiants. Suddenly the star collapses in on itself. This collapse may take only a few minutes. The result is an extreme explosion called a supernova. This is the most violent event that occurs in the Universe. The remains of a supernova form a black hole. This is a place where most of the mass of the star is concentrated into a planet sized space (or smaller). The material is incredibly dense. Gravity is so strong that even light cannot escape. T2011 Page 7
Gravity We feel the pull of gravity on the Earth but why gravity should occur at all is something of a mystery. It occurs between any objects that have mass; It is a very tiny force; It has a very long range (infinite in fact); It is always attractive. Gravity always pulls objects together. It NEVER pushes them apart, so we say that it is an attractive force. Between objects of "normal" size the force is so small that you cannot feel it. Gravity is only felt where the objects are very large. Planets are big objects. The Earth has a mass of 6 10 24 kg. Being such a big object, the Earth has a gravity of 10 N/kg. When we look at things like atoms, gravity is far too small. Gravity and the Universe Many of the processes that occur in the universe happen because of the effect of gravity. Although gravity is so tiny, its range is infinite. This means that it is possible for tiny dust particles millions of kilometers apart to come together. The forces involved may be tiny, but space is a friction-free environment. This means that dust particles will accelerate towards each other. The process will be slow, taking millions of years, but they will get there in the end. Gravity decreases with distance. If you double the distance, the force goes down by 4 times. T2011 Page 8
Satellites in Orbit Orbits occur because of gravity, not because there is no gravity. Where satellites orbit the Earth, the acceleration due to gravity is between about 2 to 9 m/s 2. So there is still an appreciable pull. An orbiting satellite is constantly falling to the Earth. However it travels at such a speed that it always misses the Earth. Since space is a vacuum, there is no friction, so the satellite does not slow down. Cosmonauts in satellites feel weightless. This is because they are falling at the same rate as the satellite, not because there is no gravity. Types of Orbit The picture shows two common kinds of orbit. Polar orbit Radio signals to ground station. Geostationary Geostationary satellites have an orbit time equal to the rotational period of the Earth. This means that relative to a point on the ground they appear stationary. The orbital speed is zero. It is about 7 km/s. T2011 Page 9
A satellite in geosynchronous equatorial orbit (GEO) is located directly above the equator, exactly 22,300 miles out in space. At that distance, it takes the satellite a full 24 hours to circle the planet. Since it takes Earth 24 hours to spin on in its axis, the satellite and Earth move together. So, a satellite in GEO always stays directly over the same spot on Earth and so we always know where GEO satellites are. If our antenna points in the right direction, we'll always have direct contact with the satellite. Many communications satellites travel in geosynchronous orbits, including those that relay TV signals into our homes. Because they're so far away, GEO satellites have a very broad view of Earth. Geostationary satellites are used for communication. Satellites in polar orbit are used for monitoring. They orbit the Earth every 90 minutes and as they make each orbit, a new section of the Earth passes underneath. This allows all the ground to be "swept". Polar orbits are used by satellites that carry out monitoring, for example for weather and spying. A Polar orbit is a particular type of Low Earth Orbit. The only difference is that a satellite in polar orbit travels a north-south direction, rather than the more common east-west direction. T2011 Page 10
Social and economic benefits of space explorations. 1. Space exploration will eventually allow us to establish a human civilization on another world (e.g., Mars) as a hedge against the type of catastrophe that wiped out the dinosaurs. 2. We explore space and create important new technologies to advance our economy. Space exploration can also serve as a stimulus for children to enter the fields of science and engineering. 3. Space exploration in an international context offers a peaceful cooperative venue that is a valuable alternative to nation state hostilities. International cooperation is also a way to reduce costs. 4. Exploration of space will provide humanity with an answer to the most fundamental questions: Are we alone? Are there other forms of life beside those on Earth? It is these last two arguments that are the most compelling.it is challenging to make the case that humans are necessary to the type of scientific exploration that may bring evidence of life on another world. There are strong arguments on both sides. Personally, I think humans will be better at unstructured environment exploration than any existing robot for a very long time. Humans will be exploring space. The challenge is to be sure that they accomplish meaningful exploration. T2011 Page 11