It is the end of March 1974 and NASA scientists are very excited. The Mariner 10 probe has been traveling in space for approximately 140 days: it has already photographed Venus, whose blanket of clouds has never been seen in such great detail, and now it is approaching Mercury, the closest planet to the Sun. What is hidden on the surface of this small body of rocks? Will the television cameras work again like before? Observing Mercury from the Earth has always been a difficult task because its orbit lies between that of our planet and the Sun. We have always been forced to observe Mercury behind the glare from the light of the Sun: through telescopes, in the best of cases, it appears to be a small white disk studded with indistinct spots. Mercury's orbital plane is slightly tilted with respect to that of the Earth, so it usually appears a little higher or a little lower on the solar disc. Occasionally, Mercury is on the same axis (approximately 14 times in a century) and we can distinguish it as a small, black spot on the Sun. Depending on the time of the year, we can see Mercury when it rises in the sky just prior to the Sun like a morning "star", or immediately after sunset like an evening "star". This is why the ancient astronomers thought that they were observing two different objects: the first they called Apollo, and the second Mercury. Before the voyage of the Mariner 10, very little was known about Mercury. Based on telescopic observations, it was discovered that the planet has a density that is much greater than Mars and the Moon, and a little less than the Earth. The periods of rotation around its axis and its revolution around the Sun were also calculated. Surprisingly, these seem to coincide, and they are equal to 88 Earth days. This implies that Mercury always faces the Sun with the same hemisphere, exactly like the Moon which always turns the same face towards the Earth. As a result, one side of the planet is "baked" under the Sun while the other is perpetually frozen. In 1965, this was disproven not with a telescope but with another instrument - radar. This was the only important discovery made prior to the advent of the space probes: in measuring the differences in the return radio frequencies generated by radar pulses sent to Mercury, two researchers at the American observatory in Arecibo determined that the planet rotates on its axis in only 59 Earth days and not 88 as previously believed. This means that Mercury completes three rotations every two revolutions around the Sun and that it is not the land of never-ending days and eternal nights.
It is the end of March 1974 and NASA scientists are very excited. The Mariner 10 probe has been traveling in space for approximately 140 days: it has already photographed Venus, whose blanket of clouds has never been seen in such great detail, and now it is approaching Mercury, the closest planet to the Sun. What is hidden on the surface of this small body of rocks? Will the television cameras work again like before? Observing Mercury from the Earth has always been a difficult task because its orbit lies between that of our planet and the Sun. We have always been forced to observe Mercury behind the glare from the light of the Sun: through telescopes, in the best of cases, it appears to be a small white disk studded with indistinct spots. Mercury's orbital plane is slightly tilted with respect to that of the Earth, so it usually appears a little higher or a little lower on the solar disc. Occasionally, Mercury is on the same axis (approximately 14 times in a century) and we can distinguish it as a small, black spot on the Sun. Depending on the time of the year, we can see Mercury when it rises in the sky just prior to the Sun like a morning "star", or immediately after sunset like an evening "star". This is why the ancient astronomers thought that they were observing two different objects: the first they called Apollo, and the second Mercury. Before the voyage of the Mariner 10, very little was known about Mercury. Based on telescopic observations, it was discovered that the planet has a density that is much greater than Mars and the Moon, and a little less than the Earth. The periods of rotation around its axis and its revolution around the Sun were also calculated. Surprisingly, these seem to coincide, and they are equal to 88 Earth days. This implies that Mercury always faces the Sun with the same hemisphere, exactly like the Moon which always turns the same face towards the Earth. As a result, one side of the planet is "baked" under the Sun while the other is perpetually frozen. In 1965, this was disproven not with a telescope but with another instrument - radar. This was the only important discovery made prior to the advent of the space probes: in measuring the differences in the return radio frequencies generated by radar pulses sent to Mercury, two researchers at the American observatory in Arecibo determined that the planet rotates on its axis in only 59 Earth days and not 88 as previously believed. This means that Mercury completes three rotations every two revolutions around the Sun and that it is not the land of never-ending days and eternal nights.
