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The Register's resident space boffin: All you need to know about the Pluto mission

Meeting a cold dwarf hasn't put off NASA one bit

By Geraint Jones, 21 Jul 2015

Dwarf planet or not, every schoolchild still learns the name "Pluto" as the ninth and final Sun-orbiting body alongside the eight undisputed planets - and now a spacecraft of the human race has finally visited the remote iceworld.

From the very first data that reached the ground, the colossal effort was clearly worthwhile, revealing fascinating landscapes on Pluto and its moons that will rewrite the textbooks on the outer Solar System, and the conditions under which all planets formed 4.5 billion years ago.

Pluto could have been explored earlier: the Voyager 1 spacecraft had an opportunity to travel there from Saturn in the 1980s, but the allure of Saturn’s large moon Titan meant it sent there instead, so the risky trip to Pluto wasn’t undertaken.

In the years that followed, a group of planetary scientists pushed hard for a Pluto mission, with a few false starts, but the New Horizons team, led by Alan Stern of the Southwest Research Institute in Boulder, Colorado, finally achieved the goal with a compact, efficient craft that makes the absolute most of the technology available.

The high quality of the data and the smoothness of the encounter made travelling to Pluto look easy. It isn’t at all; almost everything works against achieving it.

Pluto’s huge distance from the Sun makes solar power useless, so expensive plutonium is needed for the 200 watts of power the spacecraft needs. The journey also needs to be long, which also adds to the costs. Finally, communicating reliably over almost unimaginably large distances, 30 times further away than the Sun, presents a challenge in itself.

Pluto

The eighth dwarf ... the latest (and probably last) snap of Pluto from New Horizons

Finding a balance between speed and cost largely defines a planetary mission. In short, the bigger the spacecraft and the faster you want it to travel, the bigger the rocket you need and the more expensive it is to launch.

Huge, huge distances

For New Horizons, a compromise was reached: it’s had a reasonably short nine-year trip that was a direct trip to Jupiter and then on to Pluto, but to achieve that, the spacecraft is a compact, fairly lightweight one that can’t carry many instruments. In 2006, it became the fastest object on launch, at 16km/s, or about 100 times faster than a passenger jet.

The huge distances over which radio signals are sent to and from New Horizons mean that data rates have to be low to be reliable. During the 4.5 hours it takes for the signal to travel to Earth, its power decreases to a feeble whisper that only the largest radio dishes are capable of receiving.

The data rates are around one kilobit per second; frustratingly slow when waiting for spectacular new images of planetary vistas! Some techniques are used to boost this rate, though with some losses: the probe can be spun stably to point its dish at Earth and to put all its power into sending radio signals via its 2.1 metre dish, but in this mode, the instruments can’t be operated.

As well as the technological challenges, navigating in an unknown environment made things difficult. Even with our most powerful telescopes, Pluto and its large moon Charon are barely separated in images.

The Hubble Space Telescope was employed before and after the launch of New Horizons to search for dangers for the spacecraft, such as rings of dust grains encircling Pluto, as they could instantly wreck a spacecraft travelling at massively high speed. No rings were found, but four small moons were revealed by Hubble, showing that the Pluto system is far more crowded than expected.

New Horizons also took images of the system for several months on approach, in case a last-minute tweak in trajectory was needed to avoid an unknown obstacle.

During the encounter, the scientists and engineers had to endure a nervous wait: there were no communications with Earth. The priority was to gather data, pointing the cameras and other instruments to get the best results, and all the data was safely stored onboard.

Although the path by Pluto appeared clear, the riskiest time was near closest approach, when the craft crossed over the equator, where dust grains would congregate.

The New Horizons Team

The team at the Johns Hopkins University Applied Physics Lab celebrated arrival at Pluto at the time of closest approach, but with no radio confirmation, they had no idea whether the craft survived, and if it had, whether it had carried out its complex set of observations.

Packing the equipment

Planetary missions often have extremely tense periods where all the mission team’s years of dedicated work are riding on a successful launch, landing, or orbital insertion. Although, thankfully, losses are rare, there are few who’d honestly admit to some nail-biting periods despite professing full confidence in their spacecraft and mission plan.

In the early hours GMT of 15 July, 14 hours after closest approach, the good news was received: New Horizons was alive and well, and had packed its two solid state recorders with a treasure trove of data.

New Horizons’s seven instruments provide all the key measurements needed to characterize a planetary body. As well as a high resolution black and white camera, it carries a colour camera and spectrometer that extends to the infrared range.

Sorry, gotta go! Closing in on Jupiter, only to say bye bye

An ultraviolet camera and spectrometer completes the remote sensing suite. Other instruments are carried to detect the solar wind that streams from the Sun past Pluto, and a student-built device measures dust impacts. A final instrument makes use of the radio dish to probe Pluto’s atmosphere and to get an accurate measure of the dwarf planet’s mass.

To get efficient data rates, images and other results are compressed on board, and a carefully-considered data return plan means that lossy compressed images are returned to Earth first for a first look, and only later will the full dataset with lossless compression be transmitted.

The first images received after the encounter suffer from compression artifacts, as expected, but are of high enough quality for initial analysis work. The 15 Gbits of full resolution data won’t be all sent back to Earth for 16 months as the signals from the craft trickle in from 4.7 billion kilometers away.

What did New Horizons find? Surprises. Even from the first images, it’s already clear that at least part of the surface of Pluto is young. Planets and moons are continually being bombarded by debris, so the older a surface is, the more impact craters it has on its surface.

I love a planet with a little atmosphere

To almost everyone’s surprise, the first high resolution shot of Pluto revealed no impact craters at all, and a series of 3000m-tall mountains that can only be explained, for now at least, as being composed of water ice. Other images show extensive plains of ice showing polygonal patters that might be signs of convection in the surface layers.

Some leap to the conclusion that this young surface is a sign that Pluto is active now; that the surface of the planet is being renewed by a deep-freeze equivalent of volcanism. If that’s true, explaining where that heat comes from is going to be a puzzle.

Active geysers of nitrogen had been found on Neptune’s moon Triton in 1989, but the source of heat for that was thought to be tides. Significant tidal heat wasn’t expected at Pluto; our understanding of those processes might be wrong, or that the heat is coming from somewhere else, such as from the gradual cooling of underground seas of water.

Other, more complex processes might be at work however: Pluto has a thin atmosphere, much of which freezes onto the surface when far from the Sun during Pluto’s elongated orbit, and evaporates again when warmed. This movement of material between the surface and atmosphere might have big effects on Pluto’s appearance.

The dwarf planet’s atmosphere, probed by viewing the Sun in ultraviolet light as it passed behind Pluto, was found to extend to at least 1600km above the surface, and some of it joins the solar wind, forming a tail of ions downstream that New Horizons detected directly.

These moons aren't balloons, or named

Pluto might be losing 500 tons of atmosphere per day, far more than is lost at Mars.

Pluto’s large moon Charon is particularly puzzling, showing a young surface in places, deep canyons, and what appears to be a mountain that’s gradually sinking into the moon’s icy crust.

The north pole of the moon is dark, and might be where gases that have escaped from Pluto have frozen onto Charon’s surface. Images of the other four tiny moons have also been captured. As these are only a few tens of kilometres across compared with Pluto’s 2,370km diameter and Charon’s 1,200km, these are little more than pebbles, but could still provide valuable information on the Pluto system’s history.

The hard work of getting to Pluto is done, and the mission team is understandably elated. The probe is sending back its data, and plans are afoot to send New Horizons on to another target even further from the Sun.

The public had a taste of the excitement of a planetary encounter last week, but the mission’s scientists are relishing the years of hard but rewarding work ahead to understand the last data from our first survey of the Solar System. ®

Dr Geraint Jones is from UCL’s Mullard Space Science Lab, and has had stints at NASA’s Jet Propulsion Lab and the Max Plank Institute for Solar System Research. He currently lectures on planetary science and researches areas as diverse as planetary atmospheres, cometary science and the solar wind, and appeared at El Reg’s last Christmas lecture.

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