WITH Pluto rapidly shrinking from view, NASA鈥檚 New Horizons probe is slipping further into a vast unexplored wilderness known as the Kuiper belt. This great frigid expanse beyond the gas giant Neptune is home to millions of icy bodies, known as Kuiper belt objects, many of which are thought to be pristine remnants from the birth of our planetary neighbourhood some 4.6 billion years ago.
As the world continues to gawp at breathtaking images of the Kuiper belt鈥檚 most famous inhabitant, Pluto, the plucky little probe that snapped them is about to embark on one last fly-by. It won鈥檛 be easy: New Horizons鈥檚 latest target is much smaller than the dwarf planet and there is uncertainty about its exact position. But if all goes to plan, this valedictory mission could yet be the most revealing phase of an awe-inspring journey.
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鈥淭he Kuiper belt is a truly key population for understanding how the solar system evolved after the planets formed,鈥� says Michele Bannister of the University of Victoria in Canada, who works on the Outer Solar System Origins Survey 鈥� a project to map the Kuiper belt from Earth to get a handle on how it formed. By capturing the first close-up views of one of its smaller inhabitants, New Horizons鈥檚 final act before it falls silent could help rewrite the history of our cosmic neck of the woods.
This last assignment is far from an afterthought. In fact, the visit to a second Kuiper belt object was something the New Horizons team thought about from the outset, says of the Southwest Research Institute in Boulder, Colorado, who is part of the planning team for the extended mission. 鈥淏ack when the spacecraft was still being designed in 2002, we were thinking pretty seriously about what we would need to do.鈥�
Most importantly, the probe鈥檚 designers made sure it would have sufficient fuel reserves to fire up the thrusters that would steer it towards another Kuiper belt object. 鈥淚f we couldn鈥檛 change course we would have no ability to get close to anything other than by the incredibly unlikely chance that we would just happen to breeze right by something,鈥� says Spencer.
The big question, though, was exactly what to aim for. Although estimates suggest there are hundreds of thousands of icy bodies in the Kuiper belt (see 鈥�Welcome to the boonies鈥�), the challenge was to discover objects within reach of New Horizons.
With Spencer at the helm, the search team started scanning the far reaches of the solar system in 2004, using the Subaru telescope in Hawaii. They wanted to check there was nothing large and interesting enough to warrant a tweak to the Pluto fly-by schedule. There was not. So two years later, when New Horizons was launched on its epic voyage, it carried enough fuel to explore a small portion of the Kuiper belt, but its post-Pluto destination was still unknown.
By 2011, with New Horizons set on a specific path, the team had a much narrower section of sky to scour, in the constellation Sagittarius. But now there was a fresh obstacle: it would be difficult to pick out faint objects against the myriad Milky Way stars in the background. 鈥淲e were quite concerned by late 2013,鈥漵ays Spencer. 鈥淲e鈥檇 had three years of searching and we were finding it very challenging to look in the very dense star fields.鈥�
Above and below, New Horizons鈥檚 mission control
Eventually the group managed to discover some 50 Kuiper belt objects lurking in front of this sparkling backdrop. Unfortunately, all of them lay beyond the range of the outbound New Horizons. 鈥淚 think the closest one that we found would have required about 50 per cent more fuel than we actually had in the tank,鈥� says Spencer. 鈥淲e knew we were getting down to the ballpark of where we needed to be, but we certainly hadn鈥檛 got what we needed at that point. That was why we turned to Hubble.鈥�
The Hubble Space Telescope promised more detailed images. But first the team had to prove to Hubble bosses that they stood a good chance of discovering potential targets, and then they had to sift though an avalanche of data. 鈥淚t was nerve-racking,鈥� says Spencer, 鈥渂ut mostly it was just kind of exhilarating.鈥�
In 2014, the team finally struck gold: two Kuiper belt objects, 2014 PN70 and 2014 MU69, both of which would be within New Horizons鈥檚 reach. The amount of fuel required to steer a course towards the former would have left precious little for precision manoeuvres around the object, compromising the mission鈥檚 scientific objectives, says Spencer. 鈥淪o it was a fairly straightforward choice in the end.鈥� New Horizons would go to 2014 MU69.
The team is hoping that NASA will officially sign off on the extended mission to 2014 MU69, a decision that won鈥檛 be made until late next year. But Spencer and his colleagues have already commanded the four engine burns needed to send the probe in the direction of its post-Pluto target, currently roughly 1.5 billion kilometres from the dwarf planet (see diagram).
Assuming the extended mission gets the nod, when the spacecraft visits 2014 MU69 in January 2019, we can expect to see an object very different from Pluto. Measurements of its brightness suggest it is between 30 and 40 kilometres across 鈥� far smaller than the 2370-kilometre-wide dwarf planet. It is also expected to be irregularly shaped and covered with craters, says Spencer. And as a member of the 鈥渃old classicals鈥� group, which move in relatively circular orbits compared with other Kuiper belt objects, it should be reddish in hue.
Just as with Pluto, New Horizons鈥檚 encounter with 2014 MU69 will be fleeting, and its instruments will have to work flat out to gather data on their icy quarry. 鈥淲e鈥檒l be using our infrared spectrometer to map the composition of the surface and the ultraviolet spectrometer to look for any signs of an atmosphere,鈥� says Spencer. 鈥淲e don鈥檛 expect to see an atmosphere on something this small but we sure will be ready for surprises.鈥�
Atmosphere or not, getting up close and personal with 2014 MU69 should help to reveal whether Pluto was formed from such objects and could help to address fundamental questions about how planets form.
According to our best theory, planets are born out of protoplanetary discs 鈥� great whirls of dust and gas spinning around the midriff of a young star. The material comes together to form small clumps, which pull in more material thanks to their modest gravity. Soon you have the beginnings of planets, known as planetesimals, and eventually planets.
Objects like 2014 MU69, as residents of a region that has remained largely undisturbed since the early days of the solar system, are thought to be the ancient leftovers from this planet-forming process. 鈥淲e would very much like to know what processes formed the primordial planetesimals 鈥� the original building blocks of the planets 鈥� and 2014 MU69 is the closest we have yet come to flying by one of these,鈥� says planetary astronomer , also at the Southwest Research Institute, who helped discover 2014 MU69.
Its relatively undisturbed location far from the sun 鈥� what Parker calls its 鈥渄eep-freeze orbit鈥� 鈥� has helped to preserve it as a time capsule. So this mystery chunk should tell us about the processes by which it was formed and how its ilk helped to build the planets.
Scars of violence?
If New Horizons observes fresh impact craters, for example, it may reveal something about the formation of bodies like Pluto. Any newish pockmarks will have uncovered material from below the surface of 2014 MU69 鈥� stuff that hasn鈥檛 been altered by cosmic rays, say, or ultraviolet radiation. That should allow us to figure out its composition, says Spencer. 鈥淲e can compare that to what we expect objects like Pluto to have been made of.鈥�
Such observations could also help to solve another great mystery: how the solar system came to be arranged in its particular way. Current models suggest that the gas giants were once bunched up much more tightly than they are today, encircled by a substantial disc of planetesimals. Then something destabilised this cosy arrangement, hurling the planets into their present-day positions. The outer disc was shaken up too, although some of it endured to form the Kuiper belt.
杏吧原创s have run many simulations of this period, and yet precisely how this process played out remains a conundrum. Was it a violent upheaval or a more gentle migration? To answer that, we need to know exactly how massive the disc of planetesimals was before the gas giants migrated. And here is where 2014 MU69鈥檚 craters could help.
鈥淚n a way the craters actually preserve a reflection of the amount of mass that was around these objects at some point,鈥� says , UK. The idea is that if an object is riddled with scars, that would suggest there were once lots of objects around to crash into it, whereas if an object has only a few craters, it was probably surrounded by fewer objects. By studying the sizes and numbers of craters in New Horizons鈥檚 images, scientists should get a better idea of the mass of the disc. And by feeding that back into current models, they should get a clearer picture of how the solar system evolved into what we see today.
For Bannister and her colleagues at the Outer Solar System Origins Survey, the fly-by could also provide context for their studies of the orbits and surface compositions of distant Kuiper belt objects. 鈥淲e can finally match together what we see from Earth only as a tiny point of light with the actual ices and hydrocarbons that cover its surface,鈥� she says. And although 2014 MU69 will undoubtedly be the centre of attention during this post-Pluto mission, the team behind New Horizons will also study other objects the probe glimpses.
As for the little probe itself, once it has completed its work in the Kuiper belt, it will glide ever further into the outer reaches of the solar system. It will continue to send back data, including measurements of the solar wind 鈥� the stream of charged particles flowing from our star 鈥� until its radioactive power source runs out, probably in the mid-2030s.
Regardless of when we lose touch, New Horizons鈥檚 fate is set. With the Kuiper belt frontier traversed, it will drift off into the Milky Way 鈥� its precious data stored on Earth and its journey into darkness complete.
Image credits from top: NASA/Johns Hopkins University Applied Physics Laboratory; Southwest Research Institute/Steve Gribben; Michael Soluri; Scott Andrews/Getty)
Welcome to the boonies
We suspected there were frozen badlands beyond the planets long before we saw them. In 1951, astronomer Gerard Kuiper made a strong case for the existence of a vast swathe of icy bodies beyond Neptune鈥檚 orbit. It wasn鈥檛 until 1992, however, that astronomers spotted something other than Pluto out there. It was the first real evidence of an immense ring of debris at the outermost edges of the solar system 鈥� a region now known as the Kuiper belt.
Astronomers have since discovered roughly 1500 Kuiper belt objects orbiting between 4.5 and 7.5 billion kilometres from the sun. And yet we鈥檝e barely scratched the surface. We know that most of them are tens of kilometres across but there are a few larger bodies out there, including Pluto and its largest moon Charon. The dwarf planet Eris, for instance, appears to be almost the same size as Pluto, and might have similar geological activity. The region is also thought to be the source of most short-period comets, which take less than 200 years to orbit the sun.
But our best estimates suggest this planetary wasteland should contain hundreds of thousands of of icy bodies 鈥� remnants from the solar system鈥檚 early days 鈥� and we know almost nothing about all but a handful of them. 鈥淲e are still pretty ignorant about many aspects of the Kuiper belt,鈥� says Wes Fraser of Queen鈥檚 University Belfast, UK. 鈥淔or one thing, we don鈥檛 know much about what they are made of. They must have some sort of rock inside, but what is it?鈥�
The Kuiper belt holds secrets about how the planets formed and how our planetary neighbourhood evolved. It might even provide insights into why Pluto didn鈥檛 make it to planet status, says Fraser.
Trouble is, the region is so distant that even with space telescopes it has been difficult to get to know its inhabitants. No wonder planetary scientists are so excited by the prospect of a close encounter with one of them (see main story).
Where spacecraft go to die
Once New Horizons has slipped past its latest target in the Kuiper belt, its course will whisk it deep into the Milky Way. It will follow an armada of space probes that have sailed into quiet retirement or crashed into violent oblivion.
Like New Horizons, the two Voyager spacecraft 鈥� which toured the outer planets in the 1970s and 1980s 鈥� are heading for interstellar space. And data returned by Voyager 1 has led some to argue that it has already left our solar system.
Others have gone out with a bang. The Galileo probe to Jupiter was commanded to plummet into the gas giant so as not to litter the planet鈥檚 moons with earthly debris. And the Rosetta mission, currently hovering around the comet 67P Churyumov-Gerasimenko, will be deliberately crashed into its icy quarry next year. A similar fate awaits Cassini, due to plunge into Saturn in 2017.
A few spacecraft are lucky enough to retire on the planet they went to see. When the adventures of the Mars rovers Opportunity and Curiosity eventually come to an end, for instance, they will sit motionless on the Red Planet鈥檚 surface, slowly gathering ochre dust as the Martian breeze swirls around them.
Instant expert: Frontiers in astronomy
Alan Stern, principal investigator on NASA鈥檚 New Horizons mission, will be speaking at New 杏吧原创鈥�s event in London on Saturday 28 November.
Tickets available at
This article appeared in print under the headline 鈥淏eyond Pluto鈥�
