We could build a second moon

Why do planets have moons?

Earth has one, Mars has two, Jupiter and Saturn even over sixty each! Only two planets in the solar system have to do without moons: Mercury and Venus, all other planets have at least one moon. But why do most planets have moons? And what is a moon anyway?

For us, the moon is first and foremost the bright circle that stands in the sky at night. It looks small, but in reality it is a large rock ball 3475 km in diameter that circles the earth. And it is exactly the same with the other planets: They are also orbited by smaller or larger celestial bodies on regular orbits. Astronomers also call these celestial bodies “moons”.

To get to a moon, a planet usually has two options: Either the moon is created together with its planet, or the planet is created first and later captures a smaller celestial body.

These smaller celestial bodies are asteroids that fly ownerless through the solar system. When they get near a much larger planet, they are drawn to its gravity. This forces the asteroid into an orbit around the planet - the planet has got a moon. This “catching” of a moon works better, the heavier the planet is. This is why the large and heavy planets Jupiter and Saturn also have most of the moons in the solar system.

Other moons formed from debris left over when their planets formed: In the beginning, the solar system was nothing but a large disk of dust, gas, and ice. In the middle, the matter agglomerated particularly strongly - here the sun was created, surrounded by the remaining disk of dust, ice and gas. In this disk the same thing was repeated on a small scale: again compact lumps formed - the planets - and the remaining dust collected in a disk. And if there was enough matter in this disk, even smaller lumps formed there: moons. (Only when the gravitational pull of the planet was very strong were the lumps immediately torn apart. This was the case, for example, close to Saturn, which is still surrounded by rings of dust to this day.)

Both moons that emerged from the dust debris and the captured moons are much smaller than their planets.

The earth is the big exception: its moon is much larger than it should be compared to the earth. That is why it can neither have originated from leftover dust nor simply been captured. Instead, the earth owes its moon to a cosmic catastrophe that almost destroyed the planet:

Shortly after the earth was formed, it collided with a celestial body that was about half the size of itself. The force of this impact cannot be imagined: The explosion was so strong that most of the young earth melted again - and the other celestial body as well. Part of the molten mass was thrown away and gathered in an orbit to form a second ball. Over time, these two spheres cooled and solidified again. Today, the larger sphere orbits the sun as the earth - and the smaller orbits the earth as the moon.

What is the moon

It is the brightest celestial body in the night sky: the moon. It shines so brightly on full moon nights that some people find it difficult to sleep. It appears as big as the sun and the stars look like tiny points of light next to it.

But the impression is deceptive: In reality, the moon (diameter: 3474 km) is only about a quarter the size of the earth (12742 km) - and the sun (1.39 million km) is even four hundred times larger. The moon only seems the same size to us because it is so close to us - the sun (distance to the earth about 150 million km) is also about four hundred times further away than the moon. (384,400 km, an airplane needs 18 days for this distance!)

The bright light is also deceptive: unlike the sun, the moon does not shine by itself, but is illuminated by the sun. Some of this light is then reflected back from the surface of the moon and hits the earth. Just because the moon is so close to us, enough light arrives on earth to light up the night - at least if the moon doesn't just seem to have disappeared without a trace ...

What is a planet

Perhaps one or the other has noticed a particularly bright star in the morning or evening sky: Venus. After the sun and the moon, it is the brightest object in the sky. Because it shines so brightly, it is also called "morning star" or "evening star" - much to the annoyance of astronomers: Because Venus is not a star, but a planet!

The most important difference: a star shines by itself, a planet does not. Stars have a source of energy inside them, so they glow hot and emit light. A planet, on the other hand, is cold and does not shine by itself. We can only see it when it is illuminated by a star. Then the surface of the planet distributes the star's light in all directions.

Most planets belong to one star. Because planets do not arise alone, but together with a star. They then belong to this star and orbit it - such as Earth and Venus, which orbit the sun.

And why is Venus so easy to see even though it only transmits the light of the sun? This is due to their thick cloud cover, which reflects sunlight particularly well. In addition, after the moon, Venus is the celestial body that comes closest to earth: just 40 million kilometers - that is a stone's throw compared to the distances in space. Because it comes so close to the earth and its clouds reflect a lot of light, we can easily see it in the sky.

Of course, Venus is not the only planet. Like the earth, it is one of the eight planets in our solar system. And the sun is not the only star with planets either. Since there are an unimaginable number of stars, the universe just has to be teeming with planets.

What is our solar system and how did it come about?

The earth is not alone in space: people have been observing the sun, moon and stars in the sky for a long time. They discovered early on that some stars are moving. These wandering stars were observed and their paths followed. For a long time, however, their movements were not understood - until about five hundred years ago a man by the name of Nicolaus Copernicus solved the riddle: The earth and the "wandering stars" are actually planets, all of which orbit the sun at different distances.

Today we know eight planets. To remember their names in the correct order, the first letters of the sentence "M.a V.ater eclarifies mir jEden S.monday uurens Nachthimmel. “- or in short: M-V-E-M-J-S-U-N.

M.Erkur is the planet that orbits closest to the sun. Then come V.enus, E.rde and M.ars. These four inner planets have a solid surface made of rock and are still relatively close to the sun - only a few hundred million kilometers.

They are circling further out, at a distance of about one to 4.5 billion kilometers from the sun outer planets: Jupiter, S.aturn with his rings, Uranus and all the way outside Neptun. They are made of gas (mostly hydrogen and helium) and are much larger than the inner planets. Jupiter and Saturn are about ten times the size of the earth, that's why they are also called that Gas giants.

And finally, there are asteroids, comets and clouds of dust that also orbit the sun. The pull of the sun holds all these celestial bodies together and forces them to fly in a circle like on a long line. Everything together is called that Solar system. The moons are one of them - but they are held in place by the gravitational pull of the planets.

But why does the sun even have planets? This has to do with how the sun came into being: a cloud of gas and dust contracted by its own gravity and became a star. But not all of the material in this cloud was "built into" the star - around one percent was left over. And when the sun began to shine, the radiation pushed the remaining matter back outwards.

The light gases were pushed far outwards, the heavier dust and rocks remained close to the sun. From these clouds of dust and gas, the planets emerged over time. Therefore there are the gas planets outside in the solar system, further inside the rock planets - including our earth - and in the very center the sun. It contains 99% of the mass of the solar system and holds everything together with its gravity.

What are asteroids, meteorites and comets?

On some nights you can observe a special moment in the sky: it looks like a star is falling from the sky. Superstitious people even think that whoever sees such a shooting star could wish for something. But what is really behind it and where do the shooting stars come from?

In our solar system there are not only the sun, planets and moons. Many small pieces of rock and metal have also been discovered. They are much smaller and not as nicely round as planets, hence they are called minor planets or Asteroids. Like their big siblings, they circle the sun in regular orbits. Most asteroids can be found in the "asteroid belt" between the orbits of Mars and Jupiter.

Every now and then two of these asteroids collide. A crash like this creates a lot of debris and splinters. These fly away from the previous orbit, across the solar system. Some of them get close to the earth, are attracted to it and fall to the earth. These falling chunks are also called meteorite.

On earth they would literally fall like a stone from the sky - if it weren't for the atmosphere. Because the meteorites are so fast that the air cannot move to the side quickly enough. The air in front of the falling rock is compressed and therefore extremely hot. The air begins to glow and the meteorite begins to evaporate. We can then see that as a glowing streak that moves across the sky - a shooting star.

Most meteorites are so small that they burn up completely as they travel through the air. The trail then simply ends in the sky. Larger debris also lose mass on the way, but does not completely evaporate. They reach the ground and strike there.

What these meteorites do to Earth depends on how big they are. Small meteorites a few centimeters in diameter, for example, just leave a dent in a car roof.

The largest known meteorite hit about 65 million years ago. It was several kilometers in diameter and tore a crater 180 kilometers in diameter. The impact threw so much dust into the air that the sun was eclipsed for hundreds of years. As a result, plants and animals all over the world died out - this was the end of the dinosaurs.

Fortunately, such large meteorites are very rare so we don't have to worry. In addition, unlike the dinosaurs, we can observe the sky with telescopes and discover such large asteroids long before the impact.

While a shooting star burns up in a few seconds, another phenomenon remains visible longer: Comets with its tail there are days or weeks in the sky. In the past, people also attributed many properties to them - as divine signs, heralds of calamity or harbingers of happy events. But the truth is a little less spectacular.

Astronomers also call comets "dirty snowballs". They come from the outer solar system, far from the warming power of the sun. It's so cold there that water immediately freezes to ice. This is how lumps of ice and dust form - dirty snowballs.

Even a comet initially travels far away from the sun - until it is deflected by a collision and flies in the direction of the inner solar system. It gets closer to the sun and over time receives more and more light and warmth. This will cause the frozen surface to begin to thaw and even to evaporate. This creates an envelope of water vapor and dust around the comet.

At the same time, the comet gets to feel the “solar wind” - tiny particles that fly out of the sun at high speed. They hit the comet's vapor envelope. This will blow away the comet's vapor envelope, forming an elongated cloud that points away from the sun. When this cloud is then hit by sunlight, it appears as a glowing streak - the comet's tail.

The comet makes an arc around the sun and then moves away again. When it is far enough away from the sun, thawing and evaporation will also stop. The tail disappears and the comet moves like a dirty snowball through the vastness of the outer solar system. Depending on the comet's orbit, it will take many decades or even centuries before it comes close to the sun again.

Why does Saturn have rings?

When the telescope was invented, people were able to take a closer look at other planets for the first time. One thing caught my eye right from the start: Saturn with its rings.

From a distance, the rings look like a tight band around the planet. But with calculations and special measurements it was found that the rings consist of billions of small dust particles, ice crystals and rocks. They all orbit evenly around Saturn, in a dense, flat cloud so that they appear like a disk in the telescope.

It used to look like this around all planets. But elsewhere, the chunks in the cloud of dust contracted over time and formed moons. The only thing that wasn't possible was directly around Saturn: Here the gravitational force of the nearby giant planet disturbed - that's how the dust cloud circles to this day. A little further away from Saturn this disturbance is weaker, so that Saturn moons could form there.

The other major planets - Jupiter, Uranus, and Neptune - also have rings. However, Saturn's rings are made of a particularly light-colored material and are therefore easiest to see in a telescope.


Tense waiting in the control center. A series of warnings from the on-board computer almost led to the mission being aborted, now this: The planned landing site is littered with small craters and rocks. The commander Neil Armstrong grabs the control stick and tries to land the lunar module by hand. But the fuel is running out ...

Finally the redeeming radio message comes: “The eagle has landed.” For the first time, a spaceship with people on board touched down on the moon. A few hours of rest and preparation, then Armstrong opens the hatch and climbs down the ladder. With the words "a small step for a person, but a gigantic leap for mankind" he is the first person to set foot on the moon. Shortly afterwards his colleague Buzz Aldrin follows.

The stay is only short: in two and a half hours on the lunar surface, the astronauts set up an American flag, collect a few kilograms of lunar rock and set up various scientific experiments on the lunar surface.

After another pause, they ignite the engine and fly back into a lunar orbit. Michael Collins is waiting there in the Columbia space capsule, which is supposed to bring them back to Earth.

A tape measure to the moon

Among the devices Aldrin and Armstrong placed on the moon was a special mirror. It is constructed in such a way that it reflects every ray of light back to its starting point. With a well-aimed laser beam, scientists can now take aim at this mirror - and stop the time until the reflected laser beam arrives at them again. If the watch is accurate enough, you can measure the distance to the moon to within a few millimeters. They made a surprising discovery: the moon moves about 3.8 centimeters away from the earth every year!