On our stage, the role of main actor cannot but be conferred to the Sun, a star like many others in space, but very special for us because from the remains of its formation all the planets and the smaller bodies that rotate around it, and of which we are a part, have originated. The Sun is so big that over 100 planets as big as Earth could be placed along its diameter. Its mass alone constitutes 99% of the total mass of the Solar System and it is capable of releasing, in the form of light and heat, an amount of energy equivalent to 1,000,000,000,000,000,000,000,000 100 W light bulbs, or 10,000 billion atom bombs, per second. The main motor resides in the Sun’s core where every second hundreds of millions of tonnes of hydrogen atoms, the most abundant chemical element in the universe, fuse together producing energy.

The Sun is a gigantic sphere of gas at a very high temperature and in perfect equilibrium that does not collapse on itself and does not get dispersed in space thanks to the state of balance between the gravitational and pressure forces which are of equal intensity but act in opposite directions.

Being made of gas, our star does not have a solid surface; we can think of the Sun as an enormous onion made of concentric layers of gas: what we see from Earth is the outline of the outer shell called the photosphere. The phenomena that take place in this region can be viewed even with small instruments as long as they have adequate filters: in fact, one must remember never to stare directly at the Sun because its light is so intense that it would cause permanent eye damage.

The Sun
On observing the photosphere, one notices that it is not compact, but made up of many small cells. This structure, called granulation, is caused by convective motion: columns of hot gas coming from the centre of the Sun reach the surface and then sink towards the interior. Also in the photosphere, groups of sunspots can be observed. These areas appear darker than the surrounding area because within them the gas is cooler than average. Even though they look small on the Sun’s surface, these structures are so big that they could contain five planets the size of the Earth. 

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The planets
Though the Sun is the main actor, the planets are also protagonists, but an important fact must be highlighted: since the mass of the Sun alone makes up 99% of the mass of the entire Solar System, the planets are like crumbs respect to our star. In addition to this, these particles orbit around the Sun at enormous distances respect to their own size. A proportion could be calculated between the size of the Sun and that of Jupiter, the biggest planet in the Solar System and the distances between them could be scaled down by the same factor. If the Sun were the size of a grapefruit, then Jupiter would be the size of a grape about 100 metres away, the length of a football pitch. 

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Gravity and the planets’ orbits
Most of the bodies of the Solar System revolve around the Sun in orbits that are not circular but elliptical and in which the Sun occupies one of the two foci (Kepler’s First Law). In particular, planets move along orbits that are slightly eccentric, i.e. slightly squashed, and almost all on the same plane because of the mechanism with which they were created during the formation of our planetary system. Dwarf planets and minor bodies on the contrary are characterised by more elongated and inclined orbits.

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The shape of things
A central force is a force whose direction depends only on the distance between the point of application and a fixed point, known as central point. Hence, in a central force field, the force vector in every point is parallel to half-lines extending in every direction from the origin. For this reason, a spherically symmetric field is originated.

Gravitational attraction is a central force. Its formula is as follows:

F=G(m1*m2)/(r)²

The only vector capable of giving a direction to the force is r, the distance between the masses taken into consideration. 

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The Earth is a spinning top
The Earth is not motionless in space, but is subjected to different movements. The most well-known are the rotational movement around its own axis, which determines the alternation of day and night and the apparent movement of the sky above our heads, and the revolution around the Sun in a slightly elliptical orbit. The two main units of time, days and years, derive respectively from the rotation and revolution movements. The length of a day can be measured as the time interval between two consecutive transits of the Sun or of a given star on the same meridian. 

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The origin of the Moon
All the satellites of the Solar System are small: from 25 to thousands of times smaller than their relative planets. The only exceptions are the Earth-Moon and the Pluto-Charon systems; our Moon has a diameter that is only 1/3 the Earth’s diameter. This implies that maybe the processes that brought to the formation of the Moon were different from those of the other satellites. To date, four hypotheses have been advanced regarding the origin of the Moon: the Moon might be a fragment that separated from the Earth shortly after its formation (fission hypothesis)…

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We have already mentioned that the planets with their satellites and rings are not the only bodies that occupy the Solar System. To start with, between the orbits of Mars and Jupiter, there is the Main Asteroid Belt that is not just a flat disc with rocks of different sizes and shapes as we usually imagine it. Scientists have known for some years that it is actually a three-dimensional doughnut-shaped ring curved around our star. The vertical size of this tube is equal to the distance between the Earth and the Sun, about the length of the penalty area of a football pitch (according to the scale mentioned in the preceding paragraph). Within the belt, the asteroids are not distributed uniformly, but form ringed structures interrupted by some gaps, empty zones that Jupiter has “cleaned out” expelling the bodies that were present. The orbit of the Trojans, two groups of asteroids coming from the Main Belt, is influenced by Jupiter; in fact, they revolve around the Sun, one preceding and one following the giant planet. 

Far from the heat of our star, at the border of the Solar System, the Kuiper Belt and the Oort Cloud can be found, real reservoirs of asteroids and comets. The former, older but less famous than the Main Belt, begins right after Neptune’s orbit and extends up to 100 times the distance between the Earth and the Sun, i.e. the length of 20 football pitches. Today we are aware of about 40 bodies belonging to this belt with sizes greater than 100 km, but scientists estimate many more, about 50,000, not counting those with smaller sizes. Since August 2006, to classify the new bodies discovered beyond Pluto’s orbit, scientists have created a new category: the dwarf planets. Currently, there are three: Pluto, the progenitor, Xena or Eris, bigger and further than Pluto itself, and Ceres, the biggest asteroid of the Main Belt. Some comets too can originate in the Kuiper Belt. These bodies are subject to the gravitational pull of the giant planets and their trajectories can undergo modifications; this is the case of the family of Centaurs, bodies of different sizes that orbit between Jupiter and Neptune. 

The Oort Cloud lies even further from the Sun, a vast reservoir of comet nuclei that envelops us completely; it is an enormous shell with a diameter 1,500 times that of the Solar System it contains. The comet nuclei are smaller respect to the bodies of the Kuiper Belt and are made up of blocks of ice mixed with rock, with diameters ranging from 1 to 10 km. If they are subjected to the gravitational attraction of the giant planets for having passed close to them, these solid, opaque objects are removed from their secluded orbits in the dark recesses of the Solar System and become one of the most luminous and fascinating objects of the sky: comets. They plunge into the Solar System like bullets and as they approach the Sun, the water their nuclei are composed of starts to sublimate (i.e. it passes from the solid to the gaseous state) and creates the characteristic tail. Comets get more and more consumed with each successive passage near to our star, until they totally disintegrate, leaving a trail of rocky debris. This phenomenon is at the basis of shooting stars: when the Earth, as it revolves around the Sun, passes through the orbit of a comet, its debris comes into contact with the Earth’s atmosphere and air friction causes it to burn forming the trail of light we all know. 

The asteroid population
Asteroids, also known as minor planets, are rocky fragments ranging widely in size, which originated during the formation of our Solar System about 4.6 billion years ago. The first asteroid was discovered in 1801 by Father Giuseppe Piazzi at the Palermo Observatory. It was named Ceres and was considered the eighth planet for half a century. However, from 2006 it has been classified as a dwarf planet like Pluto, Eris, Makemake and Haumea. From that distant 1801, over 300,000 asteroids have been identified and catalogued and probably other hundreds of thousands, maybe a million, are still to be discovered.

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The shape and composition of an asteroid
Nearly all asteroids are irregular in shape, although some are almost spherical, and they often have a cratered surface. As they revolve around the Sun in elliptical orbits, asteroids also rotate, sometimes quite erratically. To date over 150 asteroids have been discovered with a small companion moon. There are also binary and triple systems made up of two or three asteroids.
Asteroids are subdivided according to their chemical composition into three major types: C, S and M. 

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NEOs (Near-Earth Objects)
The acronym NEO refers to all objects whose orbit brings them near to terrestrial planets. Asteroids can be found in different areas of the Solar System and besides revolving around the Sun, their orbits are also affected by forces that are difficult to predict. One of these is a collision with other asteroids that could result in the break up of the objects involved. The fragments produced can reassemble to form a new object or move along independent trajectories thus creating new asteroids. The collisions do not hurl the asteroid far from the impact zone, to make it become a NEO, for example. 

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Predicting asteroid impacts
Predicting the trajectory of an object forced to move under the influence of different planets is practically impossible. This is generally defined as the N-body problem, which does not have a simple solution as in the case of Kepler's laws, which apply only when there are two bodies. So let's discard the analytical interpretation of the phenomenon. There are numerical integration methods which consist in simulating the orbit of an asteroid and reconstructing, at short time intervals, the positions of all the objects involved. 

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Asteroids: a future source of energy
Although NEAs are dangerous, they undoubtedly possess some characteristics which make them particularly interesting. First of all, since they vary widely in composition and origin, they are objects that can be used to obtain a more accurate understanding of the mechanism that led to the formation of the Solar System. Secondly, they can be easily reached when they pass into the inner Solar System, since it is cheaper to send up probes and make spacecraft land on their surface, due to their low mass.

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Emergency measures
We now know that the Earth has been hit at regular intervals by space objects. Evidence of the collisions are the craters found on the surface of our planet; these craters range in diameter from a few metres up to 300 km. Since there is an impact risk, nowadays NEOs are monitored and studied through a network of American astronomical observatories. At the same time the best techniques to adopt in case of a potential Earth impact event are being evaluated. There are two basic strategies regarding this last point: diverting the object from its collision path or trying to destroy it.

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