The mystery and majesty Nearly 40 years after THE SPACE AGE BLASTED off when the Soviet Union launched the Voyager 2 visited Uranus world’s first artificial satellite in 1957. Since then, humanity has explored our cosmic and Neptune, scientists are backyard with vigor — and yet two planets have fallen to the planetary probe wayside. eager for new expeditions. In the 63 years since Sputnik, humanity has only visited Neptune and Uranus once BY JOEL DAVIS — when Voyager 2 flew past Uranus in January 1986 and Neptune in August 1989 40 ASTRONOMY • DECEMBER 2020 of the ICE GIANTS — and even that wasn’t entirely pre- interstellar mission, more than a dozen pro- In 1781, Uranus became the first planet planned. The unmitigated success of posals have been offered for return missions ever discovered using a telescope. Nearly 200 years later, Voyager 2 Voyager 1 and 2 on their original mission to one or both ice giants. So far, none have became the first spacecraft to visit to explore Jupiter and Saturn earned the made it past the proposal stage due to lack Uranus and Neptune, in 1986 and 1989 respectively. NASA/JPL twin spacecrafts further missions in our of substantial scientific interest. Effectively, solar system and beyond, with Neptune and the planetary research community has been Uranus acting as the last stops on a Grand giving the ice giants the cold shoulder. Tour of the outer solar system. But recently, exoplanet data began In the 31 years since Voyager 2 left the revealing the abundance of icy exoplanets Neptune system in 1989 and began its in our galaxy “and new questions about WWW.ASTRONOMY.COM 41 With a rotation axis tilted more than 90 degrees compared to its orbital plane, Neptune likewise has a highly tilted rotation axis and tilted magnetic axis. As a as well as a large magnetic axis tilt, Uranus also has a variable magnetic field result, Neptune has a lopsided magnetic field (traced in gold) that twists and (traced here in gold) and magnetosphere. NASA’S SCIENTIFIC VISUALIZATION STUDIO/JPL NAIF turns in complex patterns as the planet spins. NASA’S SCIENTIFIC VISUALIZATION STUDIO/JPL NAIF solar system formation are bringing returned a wealth of new knowledge seeing Neptune’s Great Dark Spot for focus back to Uranus and Neptune,” about these frigid behemoths, succeed- the first time.” says astronomer Candace Hansen. ing beyond everyone’s wildest dreams. But countless questions remain, And it just so happens to be the The spacecraft discovered new rings and such as how the planets formed around perfect time to consider a return trip. new moons around both planets, found the early Sun and the cause of their wild winds on Neptune when none were extreme axial tilts compared to the Time to return expected, and revealed that Neptune’s rest of the planets in the solar system. The decision to aim Voyager 2 at the moon Triton was truly spectacular, hint- For decades, scientists have clamored ice giants was made in 1981, and took ing at the possibility of a subsurface for a return to these majestic planets. advantage of a rare planetary alignment ocean that could potentially support And now might be the perfect time to of the outer planets. During its flyby of microbial life. plan a return visit, as key planetary Jupiter, Voyager 2 received a “kick” from Hansen, a member of the Voyager alignments approach at the end of the the planet, slingshotting it onto the right imaging team during the flybys of decade. If we can beat the clock, an ice path to Uranus and eventually Neptune. Uranus and Neptune, recently recalled giant mission could help us unravel A similar gravity assist from Jupiter will two of Voyager’s many highlights: “the the lingering mysteries of these planets be possible between 2029 and 2034. images of plumes or clouds (we don’t and provide new insight into their Voyager 2’s flyby of the ice giants know which) on Triton. And of course, chilling beauty. A new mission proposal, named Trident, has been selected as one of four semifinalists for RETURN TO NEPTUNE NASA’s Discovery Program. The proposed trajectory of the spacecraft would take advantage of a gravitational “kick” from Jupiter to reach Neptune and its moon, Triton. ASTRONOMY: ROEN KELLY Triton/Neptune flyby Distance from Earth: 29 AU Distance from Sun: 30 AU Neptune Begin global Earth mapping of Triton Triton Triton flyby = 1 year Neptune-shine imaging allows Venus = 2 hours comparable data to Voyager Neptune Jupiter Triton/Neptune flyby Jupiter flyby March 25, 2027 Feb. 7, 2031 April 15– Venus Earth May 5, 2026 Launch 2026 2027 2028 2029 2030 2031 2032 2033 2034 2035 2036 2037 2038 Oct. 28, 2026 Feb. 7, 2028 June 28, 2032 June 28, 2038 Earth Earth Jupiter Triton/Neptune flyby 42 ASTRONOMY • DECEMBER 2020 Migrating planets and screwy magnetospheres WHAT THE ICE GIANTS ARE MADE OF Uranus and Neptune are called ice Theories suggest that deep within the mantles of both Neptune and Uranus, diamonds may fall to the giants, and rightly so. The planets circle planets’ rocky cores. Besides raining diamonds, the planets have some of the most extreme orbital tilts in the solar system, with Uranus essentially spinning on its side. LUNAR AND PLANETARY INSTITUTE the Sun at such great distances, receiving so little external heat, that their average Atmosphere temperatures are hundreds of degrees Hydrogen, helium, below freezing. methane As it turns out, ice giants are some of Mantle Water, ammonia, the most prevalent planets currently methane found in the universe, too. As some of Core the largest planets in a star system, they Rock, ice tend to be easier to spot when they transit their host star. However, current models say that ice giants should be an anomaly, Orbital tilt 28.3° as the window for them to form is narrow. Orbital tilt The solar nebula — the cloud of gas and 97.8° dust left over after the formation of a star URANUS NEPTUNE from which planets are born — needs to be almost entirely dissipated for ice giants to snatch up the available gas and ice. They also first need to have substantial cores before they can accrete any that in a complex pattern as Uranus spins on Their cloud tops are among the lingering gas and ice. its axis. coldest places in the solar system, too: Figuring out exactly how and where Similarly, Neptune’s magnetic field is –371 degrees Fahrenheit (–224 degrees Neptune and Uranus formed could help tilted at 47 degrees from its axis and Celsius) for Uranus and about –361 F scientists better understand the abun- shifted away from the planet’s center by (–218 C) for Neptune. Only the surface dance of ice giants lurking in the more than half the planet’s radius. Its of Pluto is colder. universe. Computer simulations suggest magnetosphere traces a wild-looking But despite receiving so little light that the low density of planetesimals and corkscrew shape as the planet rotates. from the Sun, Neptune has weather — the weak solar gravity in the primordial Scientists still don’t entirely under- and what weather! Wispy white clouds outer solar system would have made it stand these anomalous magnetospheres. scoot above the planet, and in 1989, very difficult for the ice giants to form They know that planetary magnetic Voyager 2 clocked winds near a strange, where they are today. fields are generated by internal previously unseen dark spot on Neptune, And perhaps they didn’t. Like Jupiter and Saturn, Uranus and Neptune may have formed closer to the early Sun before, via gravitational processes, As it turns out, ice giants are some of the eventually migrating outward to their present positions. most prevalent planets found in the universe. But how they formed isn’t the only strange aspect about our ice giants. dynamos, or conductive global mantle reaching 1,000 mph (1,609 km/h) — the Uranus rolls. Really. It’s tilted at 97.8 oceans. But with magnetic poles so strongest of any in the solar system. This degrees from vertical, greater than any skewed off-center, the exact cause of spot, dubbed the Great Dark Spot, was a planet except Venus (177.4 degrees). For Uranus’ and Neptune’s screwy magneto- massive spinning storm the size of Earth. one-quarter of its 84-year orbit, each pole spheres is, like their formation, still Since its discovery, the storm has faded, on Uranus is in continuous sunlight. unknown. but new ones have appeared elsewhere Current theories suggest a large on the planet. By studying these dark planetesimal may have struck a glancing Magnificent blue marbles spots, scientist might find a window to blow, flipping the planet on its side. This Though the planet’s strange magnetic Neptune’s lower atmosphere. would also explain other mysteries, too, fields and uncertain formation may have Both ice giants have atmospheres such as its strange magnetic field. scientists scratching their heads, when made of mostly hydrogen and helium, Magnetospheres are typically in line Voyager 2 revealed the first images of with small amounts of methane. It is with a planet’s rotation, but Uranus’ is the planets’ atmospheres, it took our the methane gases, however, that give tipped at 59 degrees from the planet’s collective breath away. The valuable Uranus its beautiful aquamarine color, as rotational axis and offset from its center flyby revealed some unexpected puzzles methane absorbs red light. Neptune’s by one-third the planet’s radius. The about the atmospheres and internal color, on the other hand, is a more vivid result is a magnetosphere that wobbles mechanics of both planets.