one to by the mid-2030s. The window is “the right time to launch”, Mark Hofstadter, a planetary scientist at the Jet Propulsion Laboratory in Pasadena, California, said at the London meeting. “We don’t want to miss this one.” But the timing is tight. NASA is the most likely space agency to lead the kind of multibillion-dollar ‘flagship’ mission that scientists want. These typically take seven to ten years to prepare, and any green light from NASA would depend on the mission being prioritized in the agency’s Plan­ etary Science Decadal Survey, which reports in 2022. A mission to or Uranus would also face competition from proposals to return a sample from or explore .

L, NASA/JPL-CALTECH; R, NASA L, NASA/JPL-CALTECH; But whereas Mars and Venus scientists are Uranus (left) and Neptune (right), imaged by , the only probe to have visited them. building on decades of exploration, “Uranus and Neptune are genuinely out on their own, as we haven’t completed the very first phase of their exploration yet”, says Leigh Fletcher, RARE CHANCE TO a planetary scientist at the University of Leicester, UK, who co-organized the meeting. REACH ICE GIANTS Fletcher says that a mission to either should include going into around it and EXCITES SCIENTISTS sending a probe into its or to one of its , as the Cassini–Huygens mission A planetary alignment provides a favourable window did at . for visiting Uranus and Neptune — but time is tight. Blue mysteries Scientists think of the two as By Elizabeth Gibney to arrive within the lifetimes of its instruments because of their similar sizes and . But and power systems — usually about 15 years. no one knows how similar they are, their com­ omentum is building among It would also cut fuel , enabling the craft position or how they formed, Ravit Helled, a planet­ary scientists to send a major to carry a full suite of scientific instruments planetary scientist at the University of Zurich, mission to Uranus or Neptune — the (see ‘Journey to the ice giants’). To take advan­ Switzerland, told the meeting. Models strug­ most distant and least explored tage of the alignment, a mission to Neptune gle to explain ’ structures, and why planets in the . Huge would need to launch by around 2031 and more distant Neptune seems to be warmer Mgaps remain in scientists’ knowledge of the than Uranus. It’s assumed that they are made blueish planets, known as the ice giants, which JOURNEY TO Arrive of forms of , or maybe ice, said have been visited only once by a . Uranus Helled. “But actually we don’t really know that.” THE ICE GIANTS Year: 2043 But the is on to organize a mission in Scientists want to A major mission to the ice giants would the next decade, because scientists want to send a major mission also benefit exoplanet studies, said Hannah to Uranus or Neptune take advantage of an approaching planetary during a beneficial Example Wakeford, an exoplanet scientist at the Uni­ alignment that would cut travel time. planetary alignment mission versity of Bristol, UK. About 40% of known in the 2030s. Using trajectory Interest in the ice giants has grown existing technology, exoplanets are ice-giant-sized; understand­ exponentially, says , a planet­ a could ing what these planets’ sizes and atmos­ ary scientist at NASA’s Goddard Space slingshot around pheres reveal about their formation relies on

Jupiter to reach the 2.8 billion km Flight Center in Greenbelt, Maryland, who planets. understanding those in our own Solar System. co-organized a meeting at the Royal Society Delegates at the meeting agreed that they Planets & spacecraft in London in January, dedicated to exploring not to scale would be happy to visit either planet, because ideas for such a mission. NASA’s Voyager 2 is both would yield rich results. Studies show the only spacecraft to have visited Uranus and that it would be feasible to send probes in Neptune, in brief fly-bys in the 1980s. The ice a single mission to both planets, but this giants therefore represent fresh territory for ’s would be prohibitively expensive. Neptune a wide range of researchers — including the deflects the is appealing because its seems spacecraft study of planetary­ rings, , moons and increases to be geologically active and might host a and oceans, says Simon. its speed subsurface ocean, potentially of liquid water. But Uranus — which has a Jovian boost that is tilted relative to its axis — has The celestial alignment, between Neptune, more “odd” features than Neptune does, Uranus and Jupiter, next occurs in the early which challenge existing scientific models, 2030s, and would allow a spacecraft to slingshot said Hofstadter. The later for around Jupiter on its way to the planets. This Depart Venus fly-by Jupiter fly-by Uranus also makes the planet a more realistic

SOURCE: NASA SOURCE: would reduce the travel time, and allow the craft Year: 2031 Year: 2034 Year: 2036 target, says Fletcher.

Nature | Vol 579 | 5 March 2020 | 17 ©2020 Spri nger Nature Li mited. All ri ghts reserved.