In January 1986 and August 1989, NASA’s Voyager-2 probe flew successively – and quickly – over Uranus and Neptune, returning the first images of these other blue planets. And then, nothing. For more than thirty years, while several missions devoted to Mercury, Venus, Mars, Jupiter and Saturn have left Earth, no robotic explorer has returned to the two most distant planets of the Solar System, which, however, hold opinion of researchers, the keys to its formation.
This is why, in 2022, the American Academies of Sciences, Engineering and Medicine, in their ten-year report dedicated to planetary science, recommended as a number one priority for NASA an ambitious mission focused on Uranus, the more accessible of the two giants. icy, located 2.9 billion kilometers from the Sun (against 4.5 billion for Neptune). This is also why, in Science of February 17, Kathleen Mandt, specialist in the exploration of the Solar System at Johns-Hopkins University (Maryland), drives the point home by describing the interest of such a program, while stressing that it must be launched quickly .
In the boxes for a long time, the mission in question already has a working acronym, UOP, for “Uranus Orbiter and Probe”. The idea is to send two machines together: the first will go into orbit around Uranus to study it from every angle, and the second, a “kamikaze” probe, will sink into the planet’s atmosphere to to take measurements. These in situ readings are particularly important in that they can help to decide between the different scenarios for the formation of Uranus within the disk of gas and dust that surrounded the Sun after its birth a little more than 4 .5 billion years. In turn, they will allow researchers to understand how matter was distributed in this primordial disc.
A “lying” planet
Many other questions arise about Uranus, this planet presenting many quirks. The first, and not the least, is the extreme inclination of its axis of rotation. Uranus is, so to speak, “lying” on the plane of the ecliptic and therefore presents its poles alternately facing the Sun. Therefore, during the summer, which lasts about twenty years (the period of revolution of Uranus is eighty-four years), the southern hemisphere of the planet remains in shadow, and the situation s reverses in winter, when the Northern Hemisphere is plunged into darkness. This tilting on the side could have been caused by a collision with a massive body. As Kathleen Mandt writes, “this orientation causes extreme seasonal atmospheric variations (…) but observations of haze and clouds from Earth cannot provide enough information to explain atmospheric circulation and wind patterns”.
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