The Space Journey

by Christer Fuglesang

  4. Albert Einstein

  Albert Einstein was one of the very greatest scientists of the 20th century. He was born in Germany in 1879 and studied physics at a technical college in Switzerland. In 1905, he worked as a patent examiner in Bern, the capital of Switzerland, where he gained several fantastic insights. That year, 1905, Einstein wrote four scientific reports, each of which could have made him world-famous. What made him most famous, however, was what came to be called The Theory of Relativity. He realized that the speed of light in a vacuum always is the same: no matter how you move about in relation to a light source, the light will always come towards you at one and the same speed. Consequently, people that move about in relation to each other measure time and distance differently. Time and distance are relative; only the speed of light in a vacuum is absolute.

  Another of Einstein’s great insights was that light could occur as separate particles. This claim was made in contrast to what all the other scientists believed during this period; they thought it to be obvious that light was like waves. But as time went on, it was shown that Einstein was right; these particles later came to be called photons. In fact, it was for the photons that Einstein was awarded the Nobel Prize in physics 1921 – belatedly and well-deserved, according to most people. The reason for this late recognition was, among other things, that many prominent thinkers thought that the idea of relative time seemed far too strange. But nowadays you can learn the basics of the theory of relativity already in high school.

  Albert Einstein in 1921 during a lecture in Vienna.

  Einstein was soon offered a professorship in the capital of Germany, Berlin; however, when the Nazis came to power in 1933, he moved to the USA. He had Jewish ancestry and Germany became unsafe – even for one of the world’s most prominent physicists. Eventually, Einstein became an American citizen; there is a famous photo where he sits in an auditorium when he is about to receive his citizenship – he is dressed in a suit and shoes but has no socks on his feet.

  Einstein is also famous for his letter to the President of the USA at the beginning of World War II. In this letter he claimed that there was a risk that Germany might be able to construct a nuclear bomb and that it would be better if the USA hurried to construct one itself; this was also done. In addition, he was offered the presidency of Israel in 1952. He declined, however, and continued his research in the USA until his death in 1955.

  5. The force of gravity, acceleration and weightlessness

  Everybody knows that apples fall down from apple trees. But why do they fall? Isaac Newton explained this several hundred years ago in 1687, by saying that there is a force between all things that have weight; or with a more scientific expression: all things that have mass. (But you might wonder: do not all things have mass? No, as a matter of fact, they don’t. There are particles that are called massless and they don’t have any weight at all. One example is the photon, the light particle, mentioned earlier in point 4). The greater the mass things have and the closer they are to each other, the greater the force. In fact, it takes very big things for the force to be properly observable. The Earth, for example, is such an enormous thing; and therefore, it pulls other things towards it, like apples out of the apple tree. We call this force gravitation.

  Here I am hovering in weightlessness inside the International Space Station.

  In addition, everybody knows that it’s harder to move a heavy thing than a light thing. To push a pram is easy, but to push a car takes a lot of strength. And, it’s the moment when you want to get the motion started, pick up speed, which takes force. The process of speeding up is called acceleration.

  The force of gravity on the apple from the Earth is just as strong as it is on the Earth from the apple. However, the apple has an incredibly smaller amount of mass than Earth; that’s why it’s the apple that accelerates the most and why we can see the apple falling down to Earth and not the Earth going up to the apple.

  A fantastic correlation is that the mass of an object decides both the amount of gravity that affects it as well as the way this object accelerates. As a result, all things fall equally fast down to Earth (on condition that the air doesn’t slow down the fall and air resistance varies for various objects – for example a feather and a stone).

  Weight is the force that the Earth pushes down on an object that is on the ground, and which you can measure on a scale. When you are weighing yourself you are observing it as your weight, but it is also your mass. It is the ground (or the floor, or the chair in an aeroplane, or something else that keeps you in place) that makes you feel the force of gravity. If there is nothing to prevent it, you will become weightless. If you jump off a bench you are, in fact, weightless for a brief moment before you reach the ground. Weightlessness and free falling are the same thing!

  A rocket in space that has turned off its engines is in free fall. That’s why the astronauts are hovering about inside the rocket, in weightlessness. A rocket or a spacecraft, such as the International Space Station that spins in an orbit around the Earth several hundreds of kilometres above the surface, is also in free fall. But, because of its high forward velocity, it doesn’t fall down to Earth. The force of gravity from the Earth only results in a bending of the spacecraft’s direction of travel, which, in turn, keeps it in orbit around the Earth at a high altitude. It is the force of gravity that keeps the spacecraft – as well as everything inside it – in an orbit around the Earth.

  But if the rocket’s engines start it feels as if someone is pushing the rocket from the outside. The rocket’s speed increases, that is, it accelerates. The rocket, in turn, starts to push everything that is inside it; it feels like a force from the floor. (Think about how it feels to sit in car that accelerates really hard: you are pushed back against your seat.) If the acceleration is great enough and there are no windows and you don’t know where you are, the force that the acceleration causes feels just like the force of gravity you would feel if you were standing on the ground, down on Earth. And, if it is impossible to feel the difference between the force of gravity and the effect of acceleration, you could ask yourself if there really is any important difference. That was precisely what Einstein asked himself and he realized that there is no difference: gravitation and acceleration are two sides of the same coin.

  6. Jumping on Mars

  The force of gravity from Mars is only one third of the force of gravity from Earth. If you would place yourself on a scale on Mars, it would only show one third of your “true” weight. For example, if you weigh 45 kg on Earth the scale on Mars only shows 15 kg.

  The weaker force is caused by Mars being much smaller than the Earth. While the circumference of the Earth’s equator is 40 000 km, Mars’ equator is just 21 000 km. As a result, Mars is much lighter, or in other words, has much less mass. Mars’ mass is just one tenth of the Earth’s, but since it is also smaller the planet’s force of gravity can pull something on its surface with a greater force; and therefore, the force of gravity on the surface is one third of the Earth’s and not one tenth.

  This was one of the things Newton found out, as was mentioned in the first paragraph of point 5. The other thing Newton gained insight into was that an object accelerates with the same amount of force as the amount of the force that acts upon it. So if the force of gravity on Mars is just one third of that on Earth, a person that jumps accelerates only with one third of the usual acceleration. That is, the speed increases on Mars with one third of what it does on Earth. And, when you jump you hit the ground with a force equivalent to that of a jump from a third of the height. By the way, the Moon is even smaller than Mars and there you only weigh one sixth of your Earth-weight.

  7. Spirit and other robots on Mars

  The first spacecrafts built by man that landed on Mars were the Soviet Mars-2 and Mars-3, in 1972. Maybe the most famous early probes were the American (NASA’s) Viking-1 and Viking-2 that landed in 1976. They were, among other things, equipped with instruments for investigating the possibility of
there being life on Mars, but no certain signs were found. The Viking-probes could not drive around on Mars, but some newer spacecrafts that have landed on Mars have had so-called rovers or robots with them that have been able to drive around on the planet. Sojourner was maybe the first more famous robot; it drove around during three months in 1997. In 2004, NASA landed two robots on Mars: Opportunity and Spirit. In May 2009, Spirit got stuck in loose sand after a trip of about 10 km, but the robot continues to collect and send information to Earth. As I write, Opportunity is busy driving around and investigating a crater called Conception. However, on the 6th of August 2012, NASA landed their newest space probe Curiosity on Mars. Its overriding mission is to inquire into Mars’ habitability, geology and climate using a collection of very advanced instruments.

  An artist’s rendition of Spirit on the red sand of Mars.

  8. Light in many forms

  Under point 4, which was about Einstein, I wrote that light comes in the form of particles, but it appears as waves, as well. Just like the waves of the sea can have different lengths, waves of light can have different lengths, too. But, whereas sea waves grow longer and stronger when there’s a strong wind, light waves get shorter when they have more strength (or, more correctly, when the light wave’s particle, the photon, has more energy). Depending on its wavelength, we call the light – or the electromagnetic waves, which is the correct scientific term – different things. One of the most energetic and therefore short-waved kinds of light, is X-rays, that probably everyone has experienced, for instance at the dentist who readily takes X-ray pictures to make sure that there are no hidden tooth cavities. Radio waves, on the other hand, are long-waved and low in energy. Ordinary visible light lies roughly halfway in between and, for the inquisitive, it might be fun to know that the wavelength for green light is about half a thousandth of a millimetre (500 nanometres). Red light has a slightly longer wavelength and blue light a bit shorter. But no matter what wavelength light has, it moves at precisely the same speed: 299 792 km/s. And as mentioned earlier in point 4, this is totally independent of the way the light source moves in relation to those who observe or measure the light.

  9. The planets of the Sun

  Eight planets spin around our Sun. From the innermost out, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Until recently it was said that Pluto was a planet as well, farthest out, but in later years they have found similar objects even further away from the Sun and it is now understood that Pluto belongs to this new group of celestial bodies, which are called dwarf planets.

  The planets of the solar system in increasing order from the Sun.

  The four innermost planets are about the size of the Earth (but the Earth is the biggest). Venus has a dense atmosphere, (its “air”), and is very hot. Mars has a trace of atmosphere approximately one hundredth of Earth’s. The four outermost planets are completely different; they are gigantic balls of gas. Jupiter, which is the biggest, has a diameter at the equator eleven times the size of Earth’s.

  The four inner planets don’t have that many moons. The Earth has its solitary, but big, Moon; and Mars has only two small ones: Phobos and Deimos. On the other hand, a total of 67 moons have been found so far around Jupiter, of which four have the same size as our moon: Ganymede, Callisto, Io and Europa. Saturn might very well be the most spectacular planet with its fascinating rings. But actually, rings have also been spotted around the other three big gas planets, although they are not as distinct.

  The Earth is located 150 million km from the Sun. It’s so far that even light takes eight minutes to go from the Sun to the Earth. Astronomers have decided to call the distance between the Sun and Earth an Astronomical Unit (abbreviated to AU). Mars lies 1½ times further away from the Sun than the Earth does – that is a distance of 1.5 AU. So when the Earth and Mars are at their closest the distance between them is only 0.5 AU = 75 million km. But when they are on either side of the sun, the distance between them is 2.5 AU = 375 million km, and it takes 20 minutes for a light signal or radio signal to travel from one planet to the other.

  Jupiter, which is the innermost of the big gas planets, orbits the Sun at a distance of 5 AU; and Neptune is as far as 30 AU away from the sun. The two outermost planets, Uranus and Neptune, can never be seen in the sky with the naked eye, but the others are easy to spot: they are shining in the night sky like bright stars. In fact, Venus is the celestial body that shines the brightest in the sky, except for the Sun and the Moon.

  The further away a planet is located from the Sun, the longer time it needs to complete an orbit; you could say that the longer is its year. It takes the Earth exactly one year to take one lap around the Sun – this is precisely what we mean by the concept of one year – but it takes Mercury just 88 days, while it takes Mars almost two years, Jupiter 12 years and for Neptune, furthest away, an orbit takes 165 years.

  10. Hubble, the Space Telescope

  The Hubble Telescope was sent into space aboard Space Shuttle Discovery in 1990. It is named after the renowned astronomer Edwin Hubble, who discovered that the Universe is expanding. The telescope spins around the Earth at an altitude of about 600 km, which is high above the air, that is, the atmosphere. As a result, you can get clearer pictures of stars and other objects than you can from Earth. It is precisely the atmospheric disturbances that make the stars appear to twinkle sometimes. If you imagine the Hubble Telescope as a pair of binoculars they would be so powerful that you could see Max the Mouse at a distance of 200 km. Pictures taken with the Hubble Telescope have led to many scientific advances. Among other things, it has been crucial for estimating the age of the universe: less than 14 billion years.

  Photograph of the Hubble Space Telescope taken from the Space Shuttle Discovery 1997.

  11. Sirius, stars and suns; planets orbiting other stars; light-years

  Every star that we observe in the night sky is like the Sun: a gigantic burning ball. The planet Jupiter is big compared to Earth, but the Sun is ten times bigger than Jupiter. However, the stars are so incredibly far away that we are only able to observe them as tiny dots of light. The very closest star of all is called Alfa Centauri which lies four light-years away. This means that it’s so far away that it takes 4 years for light to travel from Alfa Centauri to us. (If someone remembers what I wrote on relative time under point 3, you surely have to ask: For whom does it take 4 years? The answer is: For those who are on Earth or are on Alfa Centauri and stand still. But for the light itself it takes no time at all.) A light year is 9 460 730 472 580 km (almost ten thousand billion km, also called 1 trillion). However, this figure is too large to be practical so the astronomers call the distance one light-year instead.

  Sirius is not the very closest star to us; however, it is twice the size of the Sun and it weighs twice as much. There are stars that are a hundred times larger than the Sun, but they are rare. The Sun is a very ordinary type of star, and that is a good thing. Small stars live longer than big ones, which burn up their fuel faster. An ordinary star like the Sun lives approximately ten billion years, and the Sun is about midway through its life.

  However, in comparison with space as a whole, Sirius is situated close to Earth. And since it shines twenty-five times brighter than the Sun, it is the star that is most visible from Earth. Sometimes it can be seen from Sweden, in the vicinity of the constellation Orion, but it is never far above the horizon.

  On the other hand, all the dots that shine in the night-sky are not stars, a few might be planets (see point 9 above). And just as our Sun has planets, we have discovered in recent years other stars that also have planets orbiting around them. These are called exoplanets. To this day, almost one thousand have been discovered and new ones are being discovered all the time. As yet, not one has been found that is similar to Earth; however, that is probably due to the fact that the Earth is quite small. It is of course easier to see larger planets, the ones that look like Jupiter and Saturn.

  12. Relative length – le
ngth contraction

  If you can see a rocket pass by with a velocity near the speed of light and can measure how time passes inside it and how long and how wide the rocket is, you actually discover not only that time passes slower aboard the rocket (point 3), but also that the rocket is shorter than it was when it was standing still on Earth. This is called length contraction. On the other hand, the rocket looks as broad as before. If for instance Uncle Albert’s rocket is 8 meters long and it passes by at a speed half the speed of light, and we then set about to measure it, we would see it was 6.9 meters long.

  When the little red astronaut looks at the little green astronaut’s rocket pass by at high speed, it looks shorter than when it is standing still.

  13. The Milky Way and the galaxies

  Almost all the stars in space, or the universe, which is the word we use in most cases when we talk about all of space, are gathered into large groups – enormously large groups called galaxies. Our galaxy, where the Sun is a small ordinary star midway out to the outer edge, is called the Milky Way. It is estimated that the Milky Way has about 400 billion stars. The stars are not spread out evenly but are gathered in spiral arms that meander out from the centre of the galaxy. In the centre, there is a smaller ball of stars, but the arms are all gathered on the same plane, and the stars spin around the centre point, just like the planets spin around our Sun. The distance from the Sun to the centre of the Milky Way is about 30 000 light-years. A long way! However, our closest neighbouring galaxy, Andromeda, is a total of two and a half million light-years away. If we would try to phone someone there, it would take five million years to get an answer!

  This is what our galaxy the Milky Way looks like if you look at it from the outside – many thousands of light-years above it. The location of the Sun is marked with an arrow. The spiral arms are clearly visible and hidden within the intensely glowing centre point lurks a gigantic black hole.