It was just after midnight at NASA's Jet Propulsion Laboratory mission control center, and Carl Sagan was exuberant. The Voyager 2 spacecraft had just completed its ten-year mission by making its closest passage to Neptune before continuing into interstellar space. It was the first spacecraft, and to this day unique, to visit the mysterious giant of the blue ice that hides on the periphery of the solar system.
"We are looking at the boundary of the solar system, the last planet," Sagan told a CNN television crew gathered for the occasion. "The level of excitement is the highest I've ever seen here."
On August 25, 1989, before Voyager 2 passed 3,000 miles above the Neptune atmosphere, scientists knew almost nothing about it. What they found is a planet covered with dense, methane-rich clouds that swirled around Neptune at a speed of over 1500 km / h, making it the most windy place of the solar system. At the time, the planet was the host of the Great Dark Spot, a storm the size of the Earth that has since disappeared. Voyager also examined Triton, Neptune's largest moon, and saw geysers emerge from its surface. This suggests that he was tectonically active and may be the host of a vast underwater ocean. In addition to Triton, Voyager 2 also found six other moons and four dented rings surrounding the planet.
Voyager's meeting with Neptune raised as many questions as she answered. But for 30 years, NASA has not come back. Kepler Space Telescope data suggest that ice giants such as Neptune and Uranus are among the most abundant planets in our galaxy, making it a solid case for a visit. Returning to Neptune could significantly improve our understanding of planetary formation and dynamics, but the window for organizing such a mission is closing rapidly.
Every 12 years or so, the planets align so that a Neptune-related spacecraft launched from Earth can obtain gravitational assistance from Jupiter, thereby reducing travel time. from about 12 years old. The window for gravimetric assistance from Jupiter only lasts a few years and the next one opens towards the end of the 2020s. The problem, says Mark Hofstadter, a specialist in planetary sciences at the Jet Propulsion Laboratory, is that the creation of a flagship mission of planetary exploration generally takes about ten years. This means that if NASA wants to reach the next window of gravitational assistance, it is necessary to plan a Neptune mission to start yesterday.
According to Hofstadter, an ideal flagship mission for Neptune would be a large spacecraft carrying at least 10 scientific instruments and an atmospheric probe. These instruments would be used to answer a number of fundamental questions about Neptune. Currently, he notes that scientists believe that the bulk of the Neptune mass is water, but they are far from certain. In addition, Neptune challenges our best models of planetary formation. On the basis of these models, which faithfully reproduce the formation of all the other planets, Neptune and Uranus should have grown like the gas giants Jupiter and Saturn. But they did not do it – and scientists have trouble explaining why.
"Currently, we are in a situation where we recognize that these ice giants are a bit odd, but we do not understand what they are made of, how they are assembled or why they even exist," says Hofstadter. . "Yet, they are everywhere we look in our galaxy, so learning some of these fundamental things will really advance our overall understanding of how planets form and evolve."
Hofstadter hopes that a mission back to Neptune will be feasible over the next decade. In 2017, he co-authored a report detailing various mission proposals to Neptune and Uranus. The report will help inform NASA's next ten-year global science survey, which sets the agency's exploration priorities for the next decade. The work on the decennial survey will start next year and will probably end in 2021 or 2022. However, even if a flagship mission to Neptune was chosen in priority and received the necessary funds, at the end of this decennial survey, a Herculean effort to gather the mission in time to reach the window of gravitational assistance.
In the face of this dilemma, some planetary scientists have already begun to discuss what a flagship mission on the outer solar system might look like, so that if the decennial survey gives the green light to a mission to a giant of the ice, she can start working immediately. According to Hofstadter, a particularly tempting plan involves a collaborative mission between NASA and the European Space Agency. In January, ESA completed a study on ways to contribute to a NASA-led mission to the ice giants, such as creating a spacecraft, a sister ship allowing for the launch of a spacecraft. Neptune exploration. and Uranus, or a lander for Triton. "We have started to get into the details," explains Hofstadter, but NASA's decision to adhere to ESA's plan will depend on the results of the decennial survey.
Hofstadter believes that, given the lack of time, it is also worth considering smaller mission profiles. Louise Prockter, director of the Lunar and Planetary Science Institute, could no longer agree. In March, Prockter and his colleagues unveiled their plans for Trident, a mission to fly over the Neptune Triton moon that would be launched in 2026 and flown over by the moon in 2038.
Prockter describes Triton as the "forgotten moon" of the solar system. It's unfortunate, she says, because Triton is unlike any other planetary body in the solar system. Many scientists believe that the moon actually comes from the Kuiper Belt, a huge field of objects of the primitive solar system located beyond Neptune and stuck in the orbit of the planet. According to the Voyager 2 data, it also appears to be geologically active, and it has been proven that it can withstand a vast ocean beneath its surface. Its ionosphere is also 10 times more intense than any other ionosphere in the solar system, which is difficult to explain because ionospheric activity is generally correlated with the planet's interaction with the solar wind, and Triton is rather far from the Sun.
Trident would spend about 10 days flying over the area around Neptune, where he would map almost all of Triton, study his geysers, determine if he was harboring an ocean, and fly within 300 kilometers of the "weird" surface of the moon to study its ionosphere. . She says the mission could be accomplished with about $ 500 million, well below the cost of the flagship missions, which tend to start around $ 1 billion. "We are trying to do something daring that no one thought they could do," says Prockter.
In July, Prockter will submit the Trident proposal for review as part of NASA's Discovery program. If approved, Trident's arrival at Triton will coincide almost perfectly with the 50th anniversary of Voyager's visit.
Justifying big planetary missions is always a daunting task and the delays imposed by the missions in the external solar system only increase the burden on the scientists defending them. The exploration of space is that the most interesting discoveries are rarely anticipated in advance. There is a lot of scientific knowledge about Neptune, but we will never know what we are lacking until we reach it.
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