Uranus is the stranger of the two fast twins
Because of their similar size and mass, the two ice giants are sometimes considered twins. But nobody knows exactly how closely they really resemble each other, how they are composed or how they were formed, said Ravit Helled, a planetary researcher at the University of Zurich, at the meeting. With current models, it is difficult to explain the internal structure of the planets or to answer the question of why the more distant Neptune seems to be warmer than Uranus. Scientists assume they are made up of exotic forms of water or perhaps ammonia ice, Helled says. “But we don’t know if that’s really true.”
A mission to the ice giants would also benefit exploration of exoplanets, says Hannah Wakeford of the University of Bristol. About 40 percent of the currently known planets outside of our solar system are ice giants. If you want to know what their size and atmosphere reveal about their formation, you have to get to know the counterparts of these planets in our immediate neighborhood better.
Uranus is stranger than Neptune
The meeting participants agreed that both planets would be an attractive travel destination, and both could provide a wealth of new knowledge. In theory, it would be feasible, if prohibitively expensive, to send a single probe to both planets. This is how the researchers have to decide. Neptune is attractive because its moon Triton appears to be geologically active and it may have an underground ocean that may even contain liquid water.
But Uranus, for example with its magnetic field, which is inclined relative to the axis of rotation of the planet, is overall the “stranger” of the two, he questions more scientific models, says Hofstadter. The later launch window also makes the planet a more realistic target than Neptune, says Fletcher.
Many experts are concerned about the tight schedule. A start in the early 2030s was basically the day after tomorrow for space planners, said Fabio Favata, Head of Strategy, Planning and Community Coordination at ESA, at the meeting. The European Space Agency is already working on two large missions for the early 2030s, so it would probably not be able to hold the launch window even if the upcoming “Voyage 2050” prioritization round recommended a visit to the ice giants.
A start in 2030 would hardly be feasible for ESA
Alternatively, ESA could contribute to a NASA-led mission, but that would require a U.S. decision, Favata says. A lighter and cheaper mission, which for example only flies past one of the two ice giants, would be feasible for both agencies. Such a slimmed-down variant would also provide valuable scientific knowledge, but would fall well short of what the scientists hope for, said Hofstadter.
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