A global color mosaic of Triton taken in 1989 by Voyager 2 during its flyby of the Neptune system. Image credit: NASA/JPL/USGS

Jul 19, 2015 Now it's time for a return visit to Uranus and Neptune

Sen—Well, wasn’t that just something! New Horizons’ flyby of dwarf planet Pluto was the story of the week, with initial results suggesting that everything went better than anyone might have dared hope. Pluto’s landscape is stunning, and the features on the surface of its largest moon Charon made the eyes pop too.

Though we have only seen a small number of highly compressed images so far, hundreds of pictures were taken and other measurements made. The data is trickling back and we will not see much of it for many months ahead. It will provide a goldmine for planetary scientists who are already scratching their heads over why Pluto appears still to be geologically active, rather than a cold dead world.

Like Rosetta before it, New Horizons has been hugely successful at grabbing the interest of the public. Introducing Friday’s media briefing, NASA’s public affairs officer for planetary exploration, Dwayne Brown, reported that billions of people were following the spacecraft’s encounter. “Social media is absolutely exploding with this mission,” he said.

Before New Horizons reached the Pluto system, only to speed promptly away again, many astronomers were speculating that we had previously viewed a close-up of another world that might be just like it. They were referring to Triton, the largest moon of eighth planet Neptune, which NASA’s Voyager 2 space probe snatched images of in August 1989, during its own tour of the Solar System. (You can see it in our main picture.) Triton is thought itself to be a body that Neptune stole from the Kuiper Belt, a zone of potentially numerous icy worlds that we are only just discovering.

We now have pretty good measures for the sizes of both Pluto and Triton, with the dwarf planet’s diameter coming in at 2,370 km (1,473 miles) and Neptune’s moon being a little larger at 2,705 km (1,680 miles). From a distance they appear similar too, though as Emily Lakdawalla pointed out in her blog for The Planetary Society, it is becoming clear that they have different geological histories.

Jeff Moore, a planetary geologist at NASA Ames Research Center, in California, told the Friday briefing: “The sad story for Triton is it didn’t have a New Horizons-type encounter. We need to see Triton better.”

Carolyn Porco begs to differ over the Triton encounter. Today she is internationally renowned as leader of the Cassini Imaging Team that is doing so much to uncover the mysteries of Saturn and its retinue of natural satellites. But back in the 1980s she was a young member of the team behind the twin Voyager probes’ missions through the outer Solar System, which took advantage of a rare and convenient alignment of the planets.

She took issue with Moore, telling me yesterday: “I think the New Horizons people are overstating a lot of things, as maybe we might expect in this first week, with all the euphoria they are feeling. The one thing that New Horizons has that is different from Voyager is the means to measure the temperature of the surface. But Voyager had the equivalent of all the other instruments.”

But Porco was in agreement about the need to return to Neptune. She told me: “What is really needed is an orbiter around Neptune and THAT would be the next most enabling thing that we could do.

“I have long lamented this neglect of Neptune. And I, too, am hoping that New Horizons’ results get people interested in re-examining Triton, for example. Remember, we only saw half of Triton. And we’ve learned in 50-plus years that it is common for bodies in the Solar System to have hemispherical differences. So we have to ‘discover’ the other half.”

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Images of Uranus in 1986, right, and Neptune in 1989, taken by Voyager 2. Image credit: NASA/JPL

She added: “Neptune itself is very complex, and similar in some respects to a particular class of stars that have magnetic fields offset and tilted from their bodies. It is also a different class of gaseous giant from Jupiter and Saturn. And of course there are the other moons of Neptune (to explore). Lots left to do at Neptune.”

Porco is not the only planetary scientist to feel that we have failed to give Neptune the attention it deserves, along with its inner neighbour Uranus. Both are considered ice giants, putting them in a different class to the larger Jupiter and Saturn. But while those behemoths have both been studied over many years by orbiting spacecraft Galileo and Cassini, no visits to Uranus or Neptune have been made since the historic Voyager flybys more than a quarter of a century ago.

Planetary scientist Leigh Fletcher, until this week with the University of Oxford, complains that exploration of Uranus and Neptune “is a gaping hole in our current exploration of the Solar System.” He argues that the two worlds might be of a type commonplace in planetary systems around stars, and a missing link between larger gaseous worlds such as Jupiter and Saturn and smaller terrestrial worlds such as our own. He is also keen to know why there are differences between the two ice giants, for while Neptune has some of the most powerful winds in the Solar System, Uranus appears to be relatively calm, and lacks any detectable heat source.

NASA has considered a mission to send an orbiter to Neptune, along with probes to fly into its atmosphere and a craft to land on Triton. It was passed over in 2009, and Uranus stated instead to be a high priority. In truth however, the costs would be judged so high that with NASA strapped for cash for the foreseeable future, and with growing clamour for missions to moons of Jupiter and Saturn with their underground oceans, there seems little prospect of the ice giants being on the U.S. shopping list for a while yet.

Over in Europe, teams of scientists have proposed missions to Uranus, notably Uranus Pathfinder, which Fletcher has championed, and a project which is led by Chris Arridge, of the UK’s University of Lancaster.

Fletcher told me: “We proposed Uranus Pathfinder again to the European Space Agency as a medium-sized mission at the beginning of this year. Sadly we didn’t get anywhere this time, but we’ll certainly try again at every opportunity. One day we simply have to go, and we need to stay there for a long period of time for a thorough exploration, which means orbiting, rather than just a flyby. We think it can be done, and we think it should be done. Personally I think it should be ESA’s next planetary destination beyond the JUICE mission to Jupiter.”

When robotic probes have already achieved so much, and with such potential for them to help us learn more about our own backyard in space, and so how we fit into the Universe, it is depressing that space exploration is stalling because of a perceived lack of money. One can only hope that the huge interest and support that the world has shown for New Horizons this week will help turn the tide and persuade politicians to grant more funding for such big and vital adventures.

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