Towards the end of the Eighteenth century, a group of German astronomers, who gave themselves the name of "celestial police", began an extraordinary search: to find the theoretical "missing" planet. All this took place because in 1772, one of their colleagues, David Titius, had expressed a mathematical "law" (developed later in 1778 by another astronomer, Johann Bode) which established the mutual positions of the planets in the solar system. The calculations were perfect: all the planets were where they should be according to the law. All of them, except one which just wasn't there. This missing planet should have been found in the vast space between the orbits of Mars and Jupiter. Finally, on January 1st, 1801, the Sicilian astronomer Giuseppe Piazzi found something that he believed to be the sought-after planet: with a diameter of one thousand kilometers, it was only a third as large as the Moon. Then, in 1802, another body was discovered in that same part of space - Pallas - half as big as Ceres. In 1804, another object was found, Juno, half as big as Pallas. Finally, in 1807, the "celestial police" observed a fourth object: this was Vesta, the most brilliant of them all and the only one that could be seen with the naked eye. The discoveries continued in succession and around 1890, more than 300 objects were found orbiting between Mars and Jupiter. If the planet wasn't there, were these the remains of a cosmic collision or the result of the "crushing" effect of Jupiter? Astronomers believed these hypotheses for quite some time, but today, even if they know the orbits of at least two thousand of these objects, scholars favor what is not such a science-fiction type of theory: the asteroids are nothing but the fragments of the nebula that created the solar system and which, due to the attraction of Jupiter, were never able to aggregate to form a planet. In any case, if all the asteroids discovered up to now were put together, they would barely form a body as large as the Moon. These tens of thousands of bodies, whose dimensions vary from a soccer ball to a mountain, are part of a belt (within which there are distinct aggregations of bodies) which begins at 102 000 000 kilometers beyond the orbit of Mars and extends for about 165 000 000 kilometers beyond the orbit of Jupiter. Consisting mainly of metal or rock, the asteroids are often dark or almost black, due to the high carbon content. Like Mercury or the Moon, they are pitted with a large number of craters, but their mainly irregular shape is caused by what must be very frequent impacts with their smaller "companions". This is demonstrated by the only detailed image that we have of one asteroid, taken on October 29, 1991, by the American probe Galileo during its voyage to Jupiter. This extraordinary picture shows Gaspra, a body with a length of 19 kilometers, depth 12 and width 11 that rotates around itself every 7 hours. Its surface, which is slightly lighter than the Moon, only has moderate sized craters. This might mean that Gaspra broke off in the recent past from a larger body following an impact that reshaped its face, "smoothing" out the oldest wrinkles.
Towards the end of the Eighteenth century, a group of German astronomers, who gave themselves the name of "celestial police", began an extraordinary search: to find the theoretical "missing" planet. All this took place because in 1772, one of their colleagues, David Titius, had expressed a mathematical "law" (developed later in 1778 by another astronomer, Johann Bode) which established the mutual positions of the planets in the solar system. The calculations were perfect: all the planets were where they should be according to the law. All of them, except one which just wasn't there. This missing planet should have been found in the vast space between the orbits of Mars and Jupiter. Finally, on January 1st, 1801, the Sicilian astronomer Giuseppe Piazzi found something that he believed to be the sought-after planet: with a diameter of one thousand kilometers, it was only a third as large as the Moon. Then, in 1802, another body was discovered in that same part of space - Pallas - half as big as Ceres. In 1804, another object was found, Juno, half as big as Pallas. Finally, in 1807, the "celestial police" observed a fourth object: this was Vesta, the most brilliant of them all and the only one that could be seen with the naked eye. The discoveries continued in succession and around 1890, more than 300 objects were found orbiting between Mars and Jupiter. If the planet wasn't there, were these the remains of a cosmic collision or the result of the "crushing" effect of Jupiter? Astronomers believed these hypotheses for quite some time, but today, even if they know the orbits of at least two thousand of these objects, scholars favor what is not such a science-fiction type of theory: the asteroids are nothing but the fragments of the nebula that created the solar system and which, due to the attraction of Jupiter, were never able to aggregate to form a planet. In any case, if all the asteroids discovered up to now were put together, they would barely form a body as large as the Moon. These tens of thousands of bodies, whose dimensions vary from a soccer ball to a mountain, are part of a belt (within which there are distinct aggregations of bodies) which begins at 102 000 000 kilometers beyond the orbit of Mars and extends for about 165 000 000 kilometers beyond the orbit of Jupiter. Consisting mainly of metal or rock, the asteroids are often dark or almost black, due to the high carbon content. Like Mercury or the Moon, they are pitted with a large number of craters, but their mainly irregular shape is caused by what must be very frequent impacts with their smaller "companions". This is demonstrated by the only detailed image that we have of one asteroid, taken on October 29, 1991, by the American probe Galileo during its voyage to Jupiter. This extraordinary picture shows Gaspra, a body with a length of 19 kilometers, depth 12 and width 11 that rotates around itself every 7 hours. Its surface, which is slightly lighter than the Moon, only has moderate sized craters. This might mean that Gaspra broke off in the recent past from a larger body following an impact that reshaped its face, "smoothing" out the oldest wrinkles.
