Month: July 2015

New Horizons to Pluto and The Kuiper Belt – Part 2

An Extraordinary Challenge
Unmanned missions to the planets are challenging undertakings at the best of times. As examples, until recently about half of all mission to Mars ended in failure; while the Voyager 1 & 2 Grand Tour of Jupiter, Saturn, Uranus and Neptune was only possible through the 1970’s and 80’s because of an alignment of the giant planets that only happens every 279 years, and which was needed to gravitationally assist the Voyager 1 & 2 spacecraft as they travelled from one planet to the next (called a gravitational sling-shot). We could not repeat such a grand tour of the outer planets today.

But as difficult as planetary missions can be, the challenges facing a mission to Pluto are even greater than those associated with virtually all other planetary missions.

The paramount issue is of course Pluto’s extreme remoteness, residing in the outermost regions of our planetary system. At 6 billion kilometres from the Sun, it takes Pluto 248 years to orbit the Sun just once. At it’s furthest from Earth it can be up to 7.5 billion kilometres away (we may even take license to state that distance as about 0.0008 light years – just shy of one thousandth of a light year!). A journey of such proportions presents extraordinary challenges – not least of which is getting there fast enough within a reasonable fraction of a human lifetime to make the mission practical.

An equally sever challenge is that of communicating with, and managing the spacecraft once it arrives at Pluto. While this is an issue for all missions beyond the Moon, it is particularly problematic with Pluto because of the 9-hour radio-communications round-trip time and for a close encounter lasting just 24 hours in total! In short, there can be no real-time communications with the spacecraft at closest approach, demanding a new capability in automated space exploration.

To set this in one final context: the first successful mission to Mars (Mariner 4) was 50 years ago, yet it has taken all of the intervening time for us to figure out how to devise a successful mission to Pluto. While missions have been proposed since the 1990’s, all were rejected until a proposed mission in 2001 which properly addressed the challenges and offered realistic solutions. That mission is the New Horizons Mission about to unfold – the brainchild of Principle Investigator Alan Stern of Southwest Research Institute in Texas, an astrophysicist and aeronautical engineer with experience in no less than twenty-four previous space and planetary missions.

Extraordinary Solutions
To achieve a successful flyby of Pluto, Stern and his team had to address an extensive set of challenges unique to travelling to Pluto.

Fastest launch in history
Firstly, to reach Pluto in a realistic time frame (deemed to be under ten years), the 2006 launch of New Horizons had to be so powerful as to push the space probe away from Earth faster than any other spacecraft in history – at a velocity of 60,000 kilometres per hour (kph). At that velocity, New Horizons would travel from Los Angeles to New York in just four minutes. Indeed, upon launch, it passed The Moon in only eight hours (it took Apollo 11 three days to reach the Moon), and yet, New Horizons’ journey to Pluto has still taken nine and a half years – and has travelled a journey equivalent to travelling to the Moon – and back – 8000 times.

Even the fastest launch in history was not sufficient to achieve a sub ten-year flight time to Pluto however; and so New Horizons, like the Voyagers, had to also avail of a gravitational slingshot assist around Jupiter in 2007 to boost its velocity by an extra 9000 kph. Otherwise New Horizons would still be seven hundred million kilometres from Pluto today, and would not arrive until September 2016 – fifteen months after its current July 2015 arrival. So successful was the Jupiter gravitational assist that New Horizons will indeed arrive at its closest point to Pluto at precisely 11.47 UTC on July 14th 2015 – exactly 50 years, to the day, after Mariner 4 arrived at Mars!

Efficiency in Space Probe and Science Payload Design
The only way to achieve such a high escape velocity from Earth and rapid transit to Pluto was to make the spacecraft as light and compact as possible. At 478 kilogrammes (kg), New-Horizons (see Figure 1) is one of the smallest probes ever sent to another planet – by comparison the Voyager 1 & 2 spacecraft are each over 700 kg, while the Mars Curiosity Rover has a mass of 900 kg. And while such a mass limitation would normally place severe constraints on the science payload, Stern and his team have devised a first rate science package:

LORRI – a stereo camera which will image features on the surface of Pluto as small as 70m across, and reveal the surface’s 3D topography

Ralph – an Infra-red camera that will analyse the chemical and geochemical composition of the surface of both Pluto and Charon

Alice – an Ultra-Violet camera that will analyse Pluto’s thin but intriguing atmosphere

SWAP & PEPSSI – Plasma and High Energy Particle Detectors that will measure radiation emanating from the Sun and Milky Way Galaxy, and how they affect Pluto and Charon

SDC – A Student (designed) Dust Collector – among the most important instruments because we currently have no details of dust strewn across space beyond Uranus. This instrument will provide significant new insight into the material composition of The Kuiper Belt

REX – An astounding Radio Science Experiment that will listen for radio waves sent from a radio telescope on Earth 4.5 hours before New Horizons’ closest approach to Pluto on July 14th; where upon the radio waves will bounce off Pluto’s surface and into the New Horizons REX detector at precisely the same time as it emerges from the dark side of Pluto, and where the radio ‘echoes’ from the surface will reveal details on Pluto’s surface and atmosphere.

Figure 1: New Horizons. A compact spacecraft suitable for the enormous voyage to Pluto (Click on image for larger view).
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Data Rate Constraints – imposing a New Kind of Planetary Encounter!
Despite the superlative efforts to make New Horizons a reality and a mission of scientific value, there is no getting away from the fundamental issue of how far away Pluto is and how that affects communications between Earth and the spacecraft. Even with a sizable 2.1 metre radio dish antenna, the best achievable reliable data link rate to Earth will be just 1-kilobit-per-second (1kbs) – tiny by broadband standards today, but the very best that can be achieved given the mass and size of the spacecraft, and its remoteness from Earth.

As a result, there is no way that New Horizons could transmit its new images and scientific data back to Earth in real time, and so the spacecraft has been equipped with two 8-Gigabyte solid-state recorders so that as it flies by Pluto it will record all data in real time and permanently, and subsequently transmit the data back to Earth in a steady stream over the next 16 months.

So this will not be like any previous planetary encounter. We will not experience a short burst of vast quantities of data on July 14th. Rather, New Horizons will deliver the images and data gathered in a 24-hour period centred on July 14th back to Earth in a constant stream for more than a year – imposing upon us a new kind of planetary mission – one where we will slowly learn about the planet and come to know it intimately over a prolonged period of time, as it we are there for that length of time.

Pluto Close Encounter – An Automated Event
As already indicated, as soon as New Horizon’s initiates its close encounter mission with Pluto, there is no possibility of direct intervention from Earth. Hence one of the most intriguing aspects to this mission has been the requirement for the entire 24-hour close encounter with the Pluto-Charon system to be completely automated.

The complex logistics associated with this have long since been worked out and have even been rehearsed by the New Horizons team many times in the last ten years on route to Pluto. For such a complicated set of events to be successful, there can be no unforeseen complications. As a result, The Hubble Space Telescope was called upon in the first few days of July 2015 to look ahead of New Horizons towards the Pluto-Charon system, to see if it could spot any hitherto undetected Kuiper Belt object in the flight path or within the Pluto-Charon system itself. To the resolution capabilities of Hubble, none were seen, and on July 4th the final instruction was transmitted to New Horizons to carry out a final course correction toward the heart of the Pluto,-Charon system, (Pluto has 5 moons in total: Charon and the four tiny moons Hydra, Nix, Kerberos and Styx); where the spacecraft will travel within the orbits of all the moons, between Pluto and Charon at a distance of just 12,500km from the surface of Pluto.

To demonstrate the precision to which the entire close encounter has been planned, the New Horizons team recently released for public view one of the many “Observation Playbooks” already predetermined for the LORRI optical camera; which reveal the extraordinary timing and planetary surface location details to be used by LORRI to image Pluto’s surface on closest approach (see Figure 2 for an example from the Playbook). You can download that Playbook by clicking appropriate link in the List of Resources at the end of this blog.

It is worth noting that in having designed such an automated mission, the New Horizons team have set out the first of a new kind of mission that can be used not only for Pluto, but which can serve future missions even further out into The Solar System (NASA’s new Space Launch System, due for first launch in 2018, will be capable of delivering space probes to a distance of 4 times the distance of Pluto in a 10 year time frame). Indeed, as described in more detail below below – it is hoped that New Horizons will itself travel to two other Kuiper Belt worlds in 2018 and 2019, and use its automated capabilities to explore those worlds too.

So let us look more closely at what are the specific mission goals for New Horizons at the Pluto-Charon system, and the time line of events to shortly unfold.

Figure 2: 1 page from the LORRI Camera “Playbook” indicating how New Horizons will image Pluto (Click on image for larger view).
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

The New Horizons Mission Goals

Pluto-Charon Phase
Pluto is already known to be like no other world we have visited to date (See Figure 3 for New Horizons image of Pluto on July 10th). With a surface composed of nitrogen and other volatile ices at -229 oC, and with such an elliptical orbit about the Sun, we suspect that there is dynamism on the planetary surface as nitrogen and the other volatile materials change state from solid to liquid to gas. We even expect complex migrating terrains and an atmosphere with weather that varies greatly as the planet orbits the Sun.

Charon on the other hand, is more like the water-ice moons of Jupiter and Saturn, and exhibits none of the dynamic features of Pluto. Because of such differences, it is hypothesised that Charon is the result of a collision between Pluto and another rogue world billions of years ago, leading to the creation of the Pluto-Charon system we see today.

So there’s an extraordinary amount to examine in a very short amount of time; and all of its instruments will be operational at the same time, gathering as much data as possible. Among the planned operations are to:

• Map the surface morphology of both Pluto and Charon
• Take high resolution and 3D topography images of selected locations
• Map the geology and geochemistry of the surfaces of both worlds
• Characterise the atmosphere of Pluto and search for an atmosphere of Charon
• Search for rings and other moons orbiting Pluto
• Observe the behaviour of volatile materials across the surface of Pluto
• Measure the High Energy, Plasma and Dust environment across the Pluto-Charon space environment.

Kuiper Belt Phase
As has been emphasised in of both of these blogs, while originally we set out to visit The Planet Pluto, since the launch of New Horizons, Pluto has been reclassified as a type of world called a dwarf-planet, while the Kuiper Belt has taken on new relevance in our quest to understand the origin, evolution and nature of the entire Solar System.

But with limited on-board fuel and only a maximum of a one degree of arc gravitational sling-shot assist manoeuvring available from Pluto, it has become a priority in recent years to identify candidate Kuiper Belt worlds which New Horizons might visit as it exits the Solar System along its current path.

Once again the services of The Hubble Space Telescope were called upon, and on October 15 2014 Hubble’s search uncovered three potential targets provisionally designated PT1, PT2 and PT3 by the New Horizons team (See Figure 4). All are objects estimated to have diameters around 30–55 km, and at distances from the Sun of 43–44 Astronomical Units – approximately 7 billion km distance (AU – 1AU is about 150 million kilometres – the distance from the Earth to the Sun, and a unit often used to measure distances across The Solar System). It is now intended to send New Horizons to at least one, if not two of these Kuiper Belt objects, with encounters expected to occur over 2018–2019. A decision on which world (or worlds) to visit will be taken through 2016 – 2017.

071015_Puto_Image_Annotated (2)
Figure 3: New Horizons Image of Pluto from approximately 3 million kilometres on July 10th. Complex geology is already beginning to reveal itself, indicating an active surface and climate (Click on image for larger view).
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Figure 4: Hubble Space Telescope image of a Kuiper Belt world in the line of sight of Pluto, for exploration around 2018-2019 (Click on image for larger view).

Pluto-Charon Mission Timeline
So with such an enthralling mission almost upon us, here are some of the details of the close approach of the Pluto-Charon system over July 14th (See Figure 5):

Pluto Close Encounter 12 hours before and after 11.47 UTC, July 14th 2015

• Closest Approach, 12,500 km on July 14th 11:47 UTC (12:47 BST)

• The busiest part of the Pluto system flyby will last one full Earth day, from about 12 hours before closest approach to about 12 hours after. On approach, the spacecraft will study ultraviolet emissions from Pluto’s atmosphere and make global maps of Pluto and Charon both in visible light and in infrared light sensitive to methane frost on the surface. Such infrared measurements will also reveal details about Pluto’s and Charon’s surface chemical and geochemical compositions, as well as the variation in temperature across the surface. New Horizons will sample material coming from Pluto’s atmosphere, and will image all of Pluto’s moons during this period.

• At closest approach, the spacecraft comes within 12,500 kilometres of Pluto and approximately 29,000 kilometres from Charon. During the half-hour when the spacecraft is closest to Pluto and Charon, it will take close-up pictures at both visible and near-infrared wavelengths. The best pictures of Pluto will show surface features as small about 70 metres across. The spacecraft will also obtain stereo maps that will allow for the construction of 3D topography maps of Pluto.

• Upon circling the far side of Pluto, New Horizons will observe Earth and The Sun as they emerge from behind Pluto and pass though its thin atmosphere, allowing us to determine the composition of Pluto’s atmosphere.

• At the same time, radio transmissions which were sent from Earth 4.5 hours previously, will reflect off Pluto’s surface and be picked up by New Horizons as it emerges from Pluto’s dark side; in so doing revealing the composition, structure, and thermal profile of Pluto’s atmosphere in exquisite detail. This will requires precise timing in radio transmissions. The one-way light time delay — the time for a radio signal to reach New Horizons from Earth – will be precisely 4 hours and 25 minutes at the time of closest encounter; and so the New Horizons team must transmit the signals to bounce off Pluto’s surface precisely 4 hours and 25 minutes before the anticipated moment when New Horizons emerges from behind Pluto.

• Even after the spacecraft passes Pluto, Charon and their four smaller companion moons, its work is far from over. Looking back at the dark side of Pluto or Charon is the best way to spot haze in the atmosphere, to look for rings, and to determine whether their surfaces are smooth or rough; while the spacecraft will also obtain images of Pluto’s night side illuminated by Charon, which casts about as much light onto Pluto as a quarter moon does onto Earth.

Figure 5: New Horizons Close Encounter with Pluto & Charon (Click on image for larger view).
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Post Pluto Encounter
• July – Jan 2016 Departure Phase programme: Continued imaging and scientific measurements of beyond the Pluto-Charon system.
• July 2015 – December 2016 – all data to be returned to Earth by December 2016
• November 2017 – All data primary analysis complete. A major conference about Pluto to be held to reveal the results of scientific analysis; and next phase to Kuiper Belt to be planned

And so, as enthralling as the coming week will be, it is also the case that New Horizons will be delivering continuous new images and scientific measurements until December 2016.

If you are interested in following this extensive mission as it unfolds, there is an excellent App available for both iOS and Android devices; while Facebook, Twitter and Web site updates will also be issued on a regular basis over the coming years – see the List or Resources at the end of this blog, all of which will keep you up-to-date during the entire mission.

New Horizons Mission – Context, Value and Relevance

Since its discovery in 1930, Pluto has been nothing more to our collective consciousness than an utterly remote, unfathomably cold and tiny world in the far off reaches of our Solar System. Seemingly of little value to contemplate in any way, the purpose of a mission there was questioned even after its launch.

But like on so many occasions throughout history, what we encounter by bothering to explore is altogether different, and usually unimagined, to our starting point.

In the nine and a half years since New Horizons set off on its epic voyage, our understanding of Pluto and The Kuiper Belt has radically altered. We now know that region of The Solar System to be occupied by upwards of a trillion worlds, each a remnant of the earliest formation of our system, each with a story to tell, a contribution to make, about how our system, and perhaps life itself, came to be.

Irrespective of how far we have come however, the coming weeks and months will radically alter, if not revolutionize once again our perception and understanding of Pluto and its system of moons, of that region of The Solar System and of The Solar System at large.

And so the science, the insight, to be gleaned from this mission will significantly improve our understanding of the origin and evolution of our Solar System, as well as provide valuable new contexts on the origin of life on Earth and a cosmic context for the emergence of other planetary systems and life everywhere.

New Horizons will complete the First Reconnaissance of The Solar System. We will have visited all of the important worlds of our Solar System at least once. This gives us a more comprehensive perspective of Earth as a member of this planetary system. From the frozen depths of the outer rim of the system all the way in to sweltering Mercury, we can now fully appraise Earth’s environment in light of just how different, and hostile to us, planetary environments can be – and are. Nitrogen is a gas on Earth, but it is so cold on Pluto as to be rock solid. Understanding such environments acts as a powerful comparator on how benign Earth currently is, but how drastic changes to its surface environment can be.

Having visited Pluto, we can now see ourselves in a broader context; not for some immediate purpose or requiring some profound reaction by each and every one of us – just – to take on board the full extent and extremes of our entire Solar System, and to be mindful of them when making decisions that require the best context available. Visiting Pluto expands that context, and allows us to see ourselves from a broader perspective.

And so, although all planetary missions can seem similar to one another, this mission will reveal its unique identity in the coming weeks, as a new kind of mission with a brand new story to tell. This is the last major moment of discovery regarding a traditionally regarded major world, and the dawn of a new era of space exploration involving extremely long voyages and completely automated exploration – unleashing a new capacity that will not always require direct intervention, and which in the future may span full or perhaps even multiple human lifetimes.

The engineers working at a ferocious rate right now as you read this had no guidebook, no set of instructions on how to get to, or explore, Pluto. Rather, step-by-step they had to figure it out, and are in the process of writing a new guidebook for the next phase of exploration of space. From Clyde Tombaugh’s extraordinary dedication in discovering Pluto among a myriad of background stars, to the New Horizons teams across both institutions running this project on behalf of NASA (The Southwest Research Institute in Texas and The John Hopkins University), all have contributed to bringing all of humanity perceptively to the outer edge of our stellar system. That’s what can be achieved in 85 years of space exploration.

And so New Horizons is pushing the boundary of our awareness to the edge of our Solar System. For the next generation of space explorers, Pluto will not be their goal. We have achieved that. Their goal will be beyond Pluto, deeper into space and toward interstellar space. New Horizons will have laid the foundation, and they will figure the rest out.

Indeed the very beginnings of such a future are already being contemplated, both by the likes of NASA and other independent organisations. Today, you can go to Boeing’s website and download the brochure for the extraordinary new Space Launch System being build for NASA in 2018, where approximately twenty types of space journey are proposed – including the ability to reach 30 billion kilometres in about the time it took New Horizons to reach Pluto. Meanwhile, concepts for the first interstellar space probe to the nearest stars within the next 85 years are similarly being examined in practical and costed terms.

We can be confident that coming generations will build on what New Horizons has achieved, and will push the boundary of our exploration beyond Pluto, deeper into the Kuiper Belt and eventually to the nearest stars.

New Horizons – Links to Resources




App: Pluto Safari (iOS & Android)
Pluto Safari provides interactive views of the current locations of Pluto and New Horizons, lets you explore a 3D model of the spacecraft and the five-moon Pluto system and helps you find the dwarf planet in the sky. The app also features a multimedia guide to Pluto, a timeline of New Horizons’ milestones and updated news about the mission:




New Horizons Home Web site

Public Outreach Website:

New Horizons Pluto Close Encounter Play Book:

NASA New Horizons Web site

Interview with Clyde Tombaugh

New Horizons to Pluto and The Kuiper Belt – Part 1

Introduction – A Moment of Exploration and Discovery

Over the coming week a compact unmanned space probe called New Horizons will fly by the dwarf planet Pluto, making its closest approach on July 14th. This is the first time a spacecraft will have visited this far off world. So remote is Pluto, at almost six billion kilometres from the Sun, and so small is it (just two thirds the diameter of our Moon) that since its discovery in 1930 we have been unable to determine much about the character of this little but important world. That is about to change.

It is worth contemplating that, whether intimately involved in the New Horizons mission to Pluto, or a member of the public witnessing it unfold from afar, we will all share in this imminent transition from not knowing, to coming to know, Pluto. Nobody is excluded – we will all share the sense of discovery about to unfold.

This is the last moment of significant discovery of a traditionally regarded major world of The Solar System. So the time for engagement with this mission is from now – just before the flyby – to experience the full extent of the close encounter and transition from almost total ignorance about this world, to coming to know it as we have come to know the other planets.

And there are potent ways across numerous modes of communication and connectivity to follow New Horizons (see the “List of Resources” section in the second blog in this series “New Horizons to Pluto and The Kuiper Belt – Part 2”, for pointers on how to follow the New Horizons mission via smart-phone, social-media and online over the coming weeks and months, and next four years of the extended mission beyond Pluto).

The search for The Planet Pluto – A quest to understand The Solar System

The quest to find and characterise Pluto has been a one hundred and seventy year quest to understand the nature of The Solar System at large. It is a quest still incomplete to this day, and the primary reasons we are going there. To understand the nature of this quest, we must consider why a search for a ninth planet was initiated in the first place.

In Search of Planet X
Only five other planets are visible in the sky to the unaided eye – Mercury, Venus, Mars, Jupiter and Saturn. So it wasn’t until well after the invention of the telescope around 1600 that the seventh planet, the gas giant planet Uranus was discovered, in 1781. Over time, it became clear that Uranus’ orbit did not fit with Newton’s theory of gravity used to explain how the planets orbit the Sun; and while some questioned the legitimacy of Newton’s theory, thankfully others held off from throwing out the baby with the bath water, and a search was initiated for yet another (eight) planet that might be perturbing Uranus’ orbit and explain the discrepancy.

That search lead to the discovery of the gas giant planet Neptune in 1846. Although the presence of Neptune largely explained the irregularities in Uranus’ orbit, further studies suggested that the orbit of Neptune itself might also be irregular, perhaps because of yet another hitherto undiscovered planet – a ninth planet even further out in The Solar System. The extent of irregularity in Neptune’s orbit was less than for Uranus however, and doubt was expressed even at the time as to whether there was sufficient irregularity to require the existence of an external influence.

On occasion, lingering and unresolved scientific episodes like this capture the imagination of some from outside the field, and in this case the enigmatic character of Percival Lowell entered the story in the late nineteenth century. Lowell was a wealthy American businessman, a fervent amateur astronomer and the person most responsible for perpetuating the contention (which survived in some quarters until the 1960’s) that there were canals on Mars built by a race of Martians. Lowell set up a well equipped observatory in Flagstaff Arizona in 1894 from where he observed what he claimed to be dozens of planetary-scale Martian canals, and even claimed to see new ones where before none had been seen (a process he called ‘gemination’).

True to character in engaging a potentially sensational celestial story, Lowell also took up the challenge of searching for a ninth planet, which he designated as “Planet X” (where X means unknown, and not the Roman numeral for ‘10’).

Despite extensive searches over a number of years, Lowell found no trace of a ninth planet. He passed away in 1916, but in his will he left one million dollars both to his observatory and to the search for Planet X. After a decade of legal complications regarding that contentious aspect to his will, a new telescope was commissioned at Flagstaff in 1927 specifically for the task of finding Planet X. A young and enthusiastic amateur astronomer called Clyde Tombaugh (Figure 1) was hired to conduct the new search, which he did with dedication and precision. Starting in 1929, Tombaugh systematically took thousands of photographic plates of the night sky along the Ecliptic (the path around the sky along which the Sun, Moon and planets appear to move). He used a device called a blink comparator with which he could flick back and forth between any two photographic plates taken of the same patch of sky on different nights, examining them by eye to see if any object, such as a new planet, could be identified in different locations on each plate as it traversed the sky. Within one year, on February 18th 1930, Tombaugh spotted a new world moving among the background stars across two photographic plates he had taken on January 23rd and January 29th that year (Figure 2).

Figure 1: Clyde Tombaugh (1906 – 1997)

Figure 2: The Pluto discovery photographic plates taken by Clyde Tombaugh in January 1930.(Click on image for larger view)

Calculations quickly revealed the orbit of the new world to be beyond that of Neptune, while early estimates also suggested it might be as large as Earth (best estimates ranged between 0.1 and 0.9 Earth masses). And so it seemed that Planet X had been found, and was given the name Pluto – named in a competition by an eleven year old girl from Oxford called Venetia Burnley (1918-2009); though some claim it was named Pluto because the first two letters in the name are the initials of Percival Lowell!

Tombaugh lived from 1906 to 1997, and it was a pleasure to see him being interviewed by Sir. Patrick Moore on the BBC’s The Sky at Night in the 1980’s, where he described, in the most humble of terms, the extensive effort involved in discovering Pluto.

It is surely fitting that a container attached to New Horizons spacecraft due to pass by Pluto on July 14th carries a small amount of Clyde Tombaugh’s ashes. The inscription on the container reads “Interned herein are remains of American Clyde W. Tombaugh, discoverer of Pluto and the Solar System’s ‘third zone’ Adelle and Muron’s boy, Patricia’s husband, Annette and Alden’s father, astronomer, teacher, punster, and friend: Clyde W. Tombaugh (1906-1997).”

And in that description – the mention of a “third zone” of The Solar System – lies the pointer to the continuation of the story of The Planet Pluto and why we are visiting it now. In 1978 it was discovered that Pluto had a moon, given the name Charon, the discovery of which finally allowed for an accurate determination of the size and mass of Pluto. It was found that Pluto was far smaller than previously thought, with a diameter of about 2500 kilometres and a mass far less than that of Earth (now known to be only 1/500th that of Earth). If there was a Planet X influencing the orbit of Neptune, Pluto wasn’t it. While some searches were conducted to find a tenth planet (a true “Planet X”) between 1978 and 1992, improved measurements of Neptune’s mass, by virtue of the Voyager 2 spacecraft passing Neptune in 1989, all but eliminated the need for any corrections to its orbit or for the need for a Planet X beyond Pluto.

The Discovery of The Kuiper Belt
But the more recent searches were not made in vein. Firstly, the 1978 and subsequent measurements of the Pluto-Charon system revealed Charon to be quite large, with a diameter of about 1200 km and orbiting at only a distance of about 20,000km from Pluto; making this an intriguing system of two small planet-like objects perfectly synchronized in their movements through gravitational resonance (also known as tidal locking). Both Pluto and Charon orbit about their common orbital barycentre (centre of mass, a point residing in space about one thousand kilometres beyond Pluto’s surface) every 6.4 Earth days; while a ‘day’ on each world is also 6.4 Earth-days long, with each world therefore only ever showing one face to the other, in much the same way that our Moon only shows one face to Earth.

Furthermore, visual detections began to emerge of a large number of other smaller objects beyond the orbit of Neptune. By the early 1990’s it was realised that stretching out in vast belts from just beyond Neptune at 4.5 billion kilometres, to a distance of more than 8 billion kilometres, are countless millions of minor worlds ranging in sizes from less than one kilometre to several thousand kilometres in diameter. This region is now called The Kuiper Belt, named in honour if the Dutch-American astronomer Gerard Kuiper, the foremost planetary scientist of the first half of the 20th century.

Today it is hypothesised that The Kuiper Belt contains upwards of one trillion tiny icy worlds, with a combined mass of two hundred times that of The Asteroid Belt between Mars and Jupiter. Indeed the Kuiper Belt may contain as many as one hundred thousand minor planet-like worlds with a diameter greater than 100km, and with some of them the size of Pluto. Several Kuiper Belt worlds about the same size as Pluto have been discovered in recent years, three of which are named Makemake, Haumea and Eris.

The Harmony of the Heavens

Multiple belts and types of worlds
Our recent studies of the Kuiper Belt, as incomplete as they are, have radically altered our understanding of not only the outer Solar System, but also of the origin and evolution of the entire Solar System itself.

For example, two distinct categories of objects within the Kuiper Belt have been identified – the so-called Plutinos or Hot-Population which orbit the Sun in highly elliptical orbits that on occasion come within the orbit of Neptune, and which tend to be grey in colour (and of which Pluto is the primary member); and the Cubewanos (pronounced Q-B-ones) or Cold-Population which reside in more circular orbits further out, which don’t cross the path of Neptune and which tend to be more red in colour (and of which the Kuiper Belt object Makemake is a member). We also see another, more dispersed belt of object called the Scattered Disk, made up of millions of tiny worlds scattered over a wide belt beyond Neptune but at steeply inclined orbits around the Sun, of which Eris is a member. It seems that the range of categories of worlds in our Solar System is far more diverse than ever realised.

It is also becoming increasingly evident that acquiring comprehensive details on the range, number and character of such worlds will provide important new insight into the origin, evolution and history of The Solar System. Why? – Because it appears that the worlds of the Kuiper Belt did not originate in their current locations, but were shepherded there over the ages through gravitational orbital resonance by the four giant planets Jupiter, Saturn, Uranus and Neptune. And so by understanding the full nature of the Kuiper Belt, we will uncover new details of the history and evolution of activity across entire Solar System since its birth.

We could regard the study of such worlds as a kind of “celestial taxonomy” – the classification of object types and characteristics as a way of coming to know the natural history of The Solar System. In biology, taxonomy has lead to a deep understanding of life on Earth – not just a description of life-types on Earth today, but also on why there is biodiversity in life and its relationship to its evolutionary natural history. Similarly, through the improving classification and characterization of the worlds within the Kuiper Belt, we will learn more about how they came about and evolved over time.

And so, in this recent and maturing perspective of The Solar System, Pluto has come to be regarded not as a planet like Earth or Jupiter; but as a newly identified category of world called a minor planet, and as a primary member of the Kuiper Belt. As a result, the International Astronomical Union (IAU), in 2006, officially moved Pluto into the newly designated category of minor planet called a dwarf-planet.

We might say that it was the search for Pluto that finally brought us to our first comprehensive view of true nature and history of The Solar System. It is the prime example among many newly discovered worlds that compel us to contemplate what are planets, and what aren’t planets; and in so doing push forward our understanding of the actual system we live in.

The old view of nine major planets orbiting the Sun in never-changing orbits is not correct. Rather, we live in a dynamic system containing a huge range of object types, all governed largely by planetary gravitational forces primarily from the four gas giant planets; which over the eons have interacted with one another as well as the countless smaller worlds – shepherding them into resonant orbits and belts based on close numerical or harmonic ratios (for example Pluto orbits the Sun twice for every three Neptune orbits), creating the extraordinary Harmony of the Heavens which we see today throughout the entire system. And by unravelling the precise sequencing of events on how this came about, we hope to gain a better perspective on the origin and history of all the worlds of our system.

Indeed we have made strides in that direction already. New models of The Solar System indicate that the so called Plutinos originated closer in to the Sun near Jupiter, and are therefore of totally different material makeup and origin to the Cubewanos which seem to have originated further out near Neptune. Meanwhile the orbits of all the major planets, and especially the four gas giants, have moved significantly throughout history, settling into harmonic resonances with one another (for example Jupiter orbits the Sun about twice for every one Saturn orbit); all the while the Kuiper Belt has been assembled from countless billions of world-lets from various parts of The Solar System and shepherded into regulated belts beyond Neptune.

In Search of Origins and a Cosmic Context

While such dynamism and harmonic movement is now known to have happened in the past and indeed continues today, we are far from a complete picture; and our best models contain significant inconsistencies. For example, current models predict fifty times more mass in The Kuiper Belt than we can see; while it is also unclear why many of the Plutino objects possess moons or are twinned with other objects, while none of the Cubewanos exhibit this feature.

So we are far from a complete picture. But we do have the means at our disposal to improve our understanding; from improved theoretical models to observations using the likes of The Hubble Space Telescope, and of course by travelling to The Kuiper Belt to investigate some of those far off worlds close up. This is an on-going quest.

The consequences to achieving a comprehensive understanding of origin and evolution of The Kuiper Belt are significant not only to understanding the natural history of our Solar System, but also toward a better understanding the origin of life on Earth. Not only will those ancient and far off worlds provide details on the water, other volatile materials such as methane and carbon dioxide and organic materials in the Sun’s proto-planetary disc during planet formation; but unravelling the full dynamics of our Solar System’s early history will provide powerful insights into the origin and evolution of all the worlds of our system, as well as the distribution and availability of biogenic materials.

Currently we are very much in the dark on whether life even originated on Earth, let alone on mechanism that originated life; but we know that the conditions of the early Solar System were critically important; and we now know that many of the answers we seek reside in space and in The Kuiper Belt. And, a more comprehensive understanding of our own Solar System will provide details on the formation and character of solar systems and planets everywhere, helping us to gain a better perspective on a broad context for solar systems – and life – elsewhere in the Universe.

And so it has turned out that the detailed investigation of the dwarf-planet Pluto and other Kuiper Belt objects is among the most important scientific investigations we can conduct, the answers to which will provide valuable new insights into some of our deepest questions on origins and a universal context for planetary systems and life itself.