In the original Star Wars movie, the planet Alderaan, home of Princess Leia’s adoptive parents, was blasted out of existence by the Empire’s Death Star on the orders of the villainous Grand Moff Tarkin. The demise of the planet Pluto was an altogether less spectacular affair. On 24 August 2006, the International Astronomical Union simply ruled that it was no longer a planet. Nobody much under the age of eighty could remember a time when the Solar System did not contain nine planets, but now we were being told that henceforth we would have to make do with only eight. It was the final indignity for this little world, once believed to be the Ultima Thule of the Solar System, a world whose fortunes it must be admitted had been in steady decline ever since its discovery by the American astronomer Clyde Tombaugh in 1930.
For a world much smaller than our own Moon, Pluto has punched above its weight in terms of the attention it has received over the years from astronomers and the general public alike. Its discovery made headlines, and its demotion sparked protests. It is set to make the news again when NASA’s New Horizons space probe arrives there in July 2015 after a decade-long voyage. This short (6,000 word) ‘nanobook’ examines the rise and fall of this enigmatic world; what we currently know about it; and how it turned out not to be the Solar System’s farthest place, but part of a vast realm of icy worlds otherwise unobserved prior to the 1990s.
The quest for Planet X
The story of Pluto’s discovery goes back to the beginning of the last century, when the American businessman and astronomer Percival Lowell came to the conclusion that anomalies in the orbit of Uranus could be explained by the existence of a large planet beyond the orbit of Neptune. Lowell calculated that what he termed ‘Planet X’ (X for unknown) was seven times more massive than the Earth and was in an orbit that took it round the Sun once every 282 years. Similar calculations had led to the discovery of Neptune in 1846, but subsequent observations suggested that the picture was incomplete. Astronomers began to suspect that more than one planet was affecting the orbit of Uranus.
Lowell, also known for his belief in canal-building Martians, was responsible for the construction of the Lowell Observatory at Flagstaff, Arizona in 1894. The observatory was initially set up to study Mars, but in 1905 Lowell decided to tackle the Planet X problem. Searches were carried out at the observatory between 1905 and 1907 and again in 1914, but no new planet was found, and Lowell’s death from a stroke in 1916 temporarily halted the search. Searches carried out by Milton L. Humason at the Mount Wilson Observatory in 1919 also proved fruitless.
In 1926, Vesto Melvin Slipher was appointed Director of the Lowell Observatory, and decided to resume the quest for Planet X. He engaged the services of Clyde Tombaugh, a young amateur astronomer who had impressed him with the quality of his astronomical drawings of Mars. Using a 13-inch refracting telescope built specially for the task, Tombaugh undertook one of the most meticulous searches in the history of observational astronomy. His method was to photograph the same area of the sky twice, then compare the images. If any of the innumerable objects on the plates had moved between the two exposures, it would have to be a comet, an asteroid, or the new planet. When the plates were compared using a device known as a blink-comparator, such an object would appear to jump. Tombaugh’s efforts were rapidly rewarded and plates exposed on 23 and 19 January 1930 showed a faint dot that moved between the two exposures. Seven weeks of careful checking followed to rule out the possibility of an error, but there was none. Tombaugh had found a new planet. On 13 March (the 149th anniversary of Sir William Herschel’s discovery of Uranus and what would have been Percival Lowell’s 75th birthday), the observatory went public. The new planet was only six degrees away from the position forecast by Lowell, who now seemed to have been vindicated.
The Incredible Shrinking Planet
The discovery of Planet X made newspaper headlines across the world: “See another world in sky” was one example. “Years of search add new planet to the Solar System” proclaimed another. It fell to the Lowell Observatory to choose a name for the new planet, and there were plenty of suggestions. Eventually the name Pluto was adopted, proposed by the 11 year old English schoolgirl Venetia Burney. Pluto was a later name for Hades, the Greek god of the underworld. It seemed an appropriate name for the remote world, but the first two letters (PL) also commemorated Percival Lowell. Today, the monogram ♇is the astronomical symbol used for Pluto.
Pluto soon lent its name to the Walt Disney cartoon character Pluto the Pup and to the newly-discovered element plutonium, but amid this blaze of publicity one rather important point was overlooked. Pluto seemed to be too small to be Lowell’s Planet X. Early estimates suggested that it was no bigger than Earth, and by 1949 Dutch-born astronomer Gerard Kuiper had estimated its diameter at 10,300 km (6,400 miles) or around eighty percent that of Earth. Kuiper was hampered by the fact that even the largest telescopes in the world at that time were unable to show a proper disk, and there was a large uncertainty in the figure. However, the new 200-inch Hale Telescope at Mount Palomar, California came into operation that year, and the following year Kuiper and Humason revised Pluto’s diameter downwards to 5,800 km (3,600 miles), making it smaller even than Mars.
Another problem was Pluto’s orbit. The orbital period of 248 years was not wildly different from the figure calculated by Lowell, but in every other respect the orbit was very strange indeed. The orbits of the planets are not perfectly circular, but stretched out in a slight ellipse. The extent to which an orbit depart from true circularity is known as the eccentricity, and in Pluto’s case it very high: the minimum of distance from the Sun, or perihelion is 29 astronomical units; the maximum distance from the Sun, or aphelion, is 49 astronomical units. The astronomical unit was historically defined as Earth’s average distance from the Sun, but it is now defined exactly as 149,597,870.7 km (about 93 million miles). Neptune is 30 astronomical units from the Sun, so for part of its ‘year’, Pluto is actually closer to the Sun as was the case between 1979 and 1999.
Also a peculiarity is the orbital inclination: if the plane of the Earth’s orbit around the Sun is taken as a datum, then the orbital inclination of Pluto is 17 degrees. For comparison, Mercury’s orbital inclination is just over 7 degrees and no other planet even exceeds 3.5 degrees. The high eccentricity and inclination of Pluto’s orbit had more in common with an asteroid than a planet.
An early Pluto-sceptic might have been the British classical composer Gustav Holst, best known for his seven-movement The Planets: Suite. It was composed during World War I and included all the then-known planets apart from Earth. Pluto was discovered four years before Holst’s death, but he declined to compose an additional movement.
Astronomers still lacked a definitive figure for Pluto’s diameter and looked to occultations (eclipses) of stars as Pluto passed in front of them as a means of solving the problem. By timing the moments when a star disappeared and reappeared, it would be possible to calculate the diameter of the planet. In 1978, astronomers at the US Naval Observatory at Flagstaff began a series of photographic observations with a view to predicting future occultations. They noticed that Pluto appeared dumbbell-shaped on some of the plates, suggesting the existence of a fairly large moon, and further observations at Mauna Kea, Hawaii, obtained separate images of the two bodies. The moon was named Charon after the ferryman who rowed the dead across the River Styx into the underworld. Charon and Pluto are 19,571 km (12,161 miles) apart centre to centre, but Charon does not ‘orbit’ Pluto in the strict sense of the word. Instead, rather like a pair of waltzing ice-skaters, the two revolve around a common centre of gravity once every six days and nine hours. The common centre of gravity – the barycentre to use the technical term – is located above the surface of Pluto, leading to the suggestion that the pair should be classed as a binary planet.
The two bodies periodically eclipse one another as seen from Earth, and by timing the eclipses it was possible to determine the diameter of both. Charon’s orbit is steeply inclined, so such eclipses only occur in series well over a century apart, but as good luck would have it, such a series occurred in the mid to late 1980s, enabling the diameter of both bodies to be estimated. Pluto is just 2,322 km (1,443 miles) in diameter, rather smaller than the Moon. Charon is 1207 km (750 miles) in diameter, quite large in comparison, and the two are 19,571 km (12,161 miles) apart centre to centre. Even the 1950 figure obtained by Kuiper and Humason had turned out to a gross over-estimate.
By application of Newton’s law of gravity, the combined mass of the Pluto-Charon system could be calculated, and once the location of the barycentre had been determined, the individual masses of the two bodies could also be calculated. Pluto’s mass was found to be less than a fifth that of the Moon – far too low to have any significant effects on the orbits of Uranus and Neptune – and Charon’s mass is 11.6 percent that of Pluto.
By measuring small changes in luminosity, astronomers also found that the two bodies are locked face-to-face. A ‘day’ on both lasts six days and nine hours, so a ‘month’ on Pluto lasts as long as the ‘day’. From one hemisphere of Pluto, Charon hangs motionless in the sky, but from the other it is never seen at all.
However, the bottom line was that Pluto could not be Planet X, and its discovery so close to where Lowell had predicted a planet would be found was a pure coincidence. There was a revival of interest in finding the real Planet X, but it was short-lived. In 1989, the mystery was finally cleared up when the Voyager 2 space probe made a flyby of Neptune, enabling an accurate figure to be obtained for the planet’s mass. It was found that previous estimates were incorrect: when the new figure was used it was sufficient to account for the orbit of Uranus without any need for another planet to be involved. Lowell’s planet had never existed; and soon Pluto’s very status as a planet began to be questioned.
How could an object with less than a fifth the mass of the Moon be considered a planet?
Museums and planetariums began to omit Pluto from their displays, most notoriously the Rose Center for Earth and Space at the American Museum of Natural History in New York. School children wrote to director Neil deGrasse Tyson to complain and demand Pluto’s reinstatement. The New York Times ran the headline “Pluto’s not a planet? Only in New York” leading to the misconception that the museum had acted alone and to humorous suggestions that Pluto wasn’t big enough to make it in NYC. More seriously, astronomers began to ask the question, what exactly is a planet and if Pluto isn’t one then what is it?
The Kuiper Belt
Beyond the orbit of Neptune lies a doughnut-shaped region of space known as the Kuiper Belt, which astronomers believe might contain 100,000 objects with diameters of 100 km (62 miles) or more. Much larger than the asteroid belt between Mars and Jupiter, the Kuiper Belt extends from 30 out to 50 astronomical units from the Sun. Like the asteroids, Kuiper Belt objects are believed to be left over from the formation of the Solar System, but they are composed primarily of ice rather than rock. The Kuiper Belt is named for Gerald Kuiper, who postulated its existence in 1951, although similar suggestions were made by the Irish astronomer Kenneth Edgeworth as early as 1943.
The first Kuiper Belt object was located in 1992 and given the provisional designation of 1992 QB1. The 200 km (125 mile) diameter body lies just outside the orbit of Pluto. Its discoverers wanted to name it ‘Smiley’ after John le Carré’s fictional intelligence officer, but the name had already been allocated to an asteroid to honour the American astronomer Charles Smiley. Consequently, 1992 QB1 remained unnamed, and it is now usually referred to simply as ‘QB1’. Other discoveries soon followed, and over a thousand Kuiper Belt objects are now known.
Kuiper Belt objects fall into two main classes: ‘classical’ objects with orbits of fairly low eccentricity; and ‘resonant’ objects that are in ‘orbital resonance’ with Neptune, that is to say their orbital period and that of Neptune can be expressed as a simple numerical ratio. For example, an object in a 2:3 resonance will complete two orbits of the Sun while Neptune is completing three. The resonant objects are in rather more eccentric orbits than the classical objects, and it is believed that they were perturbed into their present orbits by Neptune as it migrated outwards from the Sun during the early history of the Solar System.
Astronomers began to take the view that Pluto was nothing more than a Kuiper Belt object, specifically one of around 200 objects in a 2:3 orbital resonance with Neptune. Although it was by far the largest such object, it was felt that its size was not necessarily unprecedented. Neptune’s largest moon Triton probably started life as a Kuiper Belt object. Triton’s orbit around Neptune is retrograde, i.e. in the opposite direction to that of Neptune and indeed the majority of Solar System bodies. Retrograde satellites cannot have formed from the primordial solar nebula alongside their parent bodies and must therefore have been captured from elsewhere. Most are small, but at 2,707 km in diameter, Triton is rather larger than Pluto.
The question was does being a Kuiper Belt object automatically disqualify Pluto from being a planet? On the face of it, the answer was ‘no’, but it did raise the question as to what is and isn’t a planet. Strange as it might seem, few had considered this matter before the latter part of the last century. The reason was that there had never been a need for a formal definition of a planet – it is obvious that Venus, Earth, Mars, Jupiter, etc. are planets. Pluto, though small, was still thought to be larger than Mercury and hence pose no problem. Only when it became clear that Pluto is actually a lot smaller than the Moon did astronomers start to ask questions.
Early Kuiper Belt discoveries were all far smaller than Pluto, but then between 2000 and 2003, larger objects began to turn up. These included Varuna, Quaoar and Sedna, all of which were around 1000 km (620 miles) in diameter. These new objects were all named for mythological names associated with creation in various traditions. Though these bodies were still rather smaller than Pluto, there was no reason to suppose that even larger objects would not be found, and so it proved.
At the time of its discovery, Sedna was the most distant Solar System object ever observed, lying at a distance of three times that of Neptune from the Sun – but it is in a highly eccentric orbit and actually spends most of its 11,400 year orbital period at far greater distances. At perihelion, it is 76 astronomical units from the Sun, but the aphelion distance is a staggering 937 astronomical units. At that distance, even travelling at the speed of light, it would take more than five days to get there.
Even when close to perihelion, Sedna lies well outside the main Kuiper Belt and its motion against the starry background is very slow. On 5 January 2005 a team based at Mount Palomar discovered an object on plates taken some 15 months earlier that had been initially ignored because it had been moving too slowly against the background stars to be picked up by the team’s image-searching software. However, the discovery of Sedna had made the team recalibrate their software, and so the discovery was made. The Palomar team nicknamed the new body ‘Xena’ after the fictional TV character.
Like Sedna, ‘Xena’ was located beyond the main Kuiper Belt. The orbit was again highly eccentric, with a perihelion of 38 astronomical units and aphelion of 98 astronomical units, but unlike Sedna, ‘Xena’ was fairly close to aphelion when it was found. The orbit was steeply inclined at 44 degrees. ‘Xena’ was probably perturbed into its present orbit by Neptune, and is referred to as a ‘scattered disk object’.
An announcement was not made immediately because the team wanted to make more observations to allow more accurate determinations to be made of the object’s size. Fears that another team might beat them to it finally prompted an announcement on 29 July, by which time it was suspected that ‘Xena’ was slightly larger than Pluto. A few months after the announcement, ‘Xena’ was found to have a moon, enabling its mass to be determined. Regardless of its diameter, ‘Xena’ was 27 percent more massive than Pluto, bringing to a head the whole debate on the latter’s status as a planet.
One thing was for certain – if Pluto was a planet, than a more massive object like ‘Xena’ had to be also. Conversely, if ‘Xena’ wasn’t a planet, then neither was Pluto. NASA promptly nailed their colours to the mast, declaring that the Solar System’s tenth planet had been found. With their New Horizons space probe being readied for a ten year flight to Pluto, NASA possibly didn’t want to be seen to be spending 650 million dollars on a mission to an ex-planet.
Motivated by the discovery of Sedna and the possibility of even larger objects being found, the International Astronomical Union had by now set up a 19-member committee to consider possible definitions of a planet. On 16 August 2006, a draft proposal was published at 26th General Assembly in Prague. It stated: “A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.”
This concise and testable definition of a planet would have recognised ‘Xena’ as a planet and retained Pluto’s membership of the Solar System’s Premier League – but there was a problem and it echoed a debate that had occurred two centuries earlier. The issue of what is and isn’t a planet wasn’t new after all.
When is a planet not a planet?
In 1766, the German astronomer Johann Daniel Titius noticed that the distances from Sun of the then-known planets followed a simple numerical rule. He considered the numerical sequence 4, 7, 10, 16, 28, 52, 100, which is generated by the formula 4 + 3x, where x = 0, 1, 2, 3, etc., and he noticed that if Earth’s distance from the Sun is taken to be 10, the sequence gives a very good approximation to the distances from the Sun of Mercury (actual distance 3.9), Venus (actual distance 7.2), Mars (actual distance 15.2), Jupiter (actual distance 52.0) and Saturn (actual distance 95.4). The rule was later popularised by Johann Elert Bode, Director of the Berlin Observatory and became known as Titius-Bode Law (or less accurately Bode’s Law). Nobody really took much notice until 1781 when Uranus was discovered and found to neatly fit the rule with a predicted distance of 196, very close to the actual distance of 192. The only problem with the rule was the absence of a known planet in the orbit corresponding to 28 units from the Sun. Bode believed that such a planet might actually exist, and urged astronomers to begin a search for it.
If it existed at all, the unknown planet would have to be very small to have hitherto escaped notice, and as the eighteenth century drew to a close an international group led by astronomers Johann Hieronymus Schröter and Baron Franz Xavier von Zach met at the German town of Lilienthal and resolved to try to find it. Giving themselves the humorous appellation ‘The Celestial Police’, they began a systematic search with each astronomer being responsible for a particular region of the zodiac, that region of the sky in which all the then-known planets lie.
On 1 January 1801 – the very first day of the nineteenth century – a discovery was made by the Italian Giuseppe Piazzi. Ironically, he was not a founder member of the Celestial Police, although he was later invited to join the group. Piazzi was actually compiling a new star catalogue, but he picked up a star-like object that shifted its position from hour to hour. At first he thought he’d found a comet, but he soon begun to suspect it might be something else altogether. In the weeks that followed he observed it 24 times, but before he could complete his observations he was taken ill. By the time he had recovered, the object had disappeared into the evening twilight. However, the brilliant young German mathematician Carl Friedrich Gauss was able to calculate an orbit from Piazzi’s interrupted observations and on New Year’s Eve 1801, ‘policeman’ Heinrich Olbers recovered the object, which received the name Ceres after the Roman goddess of agriculture.
Ceres was immediately recognised as the missing planet, but within months Olbers had found a second object in the gap, which he called Pallas. Fellow ‘policeman’ Ludwig Harding found a third object two years later, which he named Juno, and Olbers discovered a fourth in 1807, which he named Vesta. No further discoveries were made in the years that followed, and the year before Schröter’s death in 1816, the Celestial Police were disbanded. All four objects are very small and even Ceres, which is by far the largest, is only 960 km (597 miles) in diameter. Nevertheless, they were for several decades considered to be planets. Then, from around the middle of the nineteenth century, further discoveries were made, and the four ‘planets’ were eventually downgraded to asteroids. However, Ceres is large enough to be spherical, and under the International Astronomical Union’s 16 August draft proposal this Rutland of the Solar System stood to regain the planetary status it had lost a century and a half earlier.
It was proposed to recognise both Ceres and ‘Xena’ as planets, and in addition Charon, by virtue of its size in relation to Pluto, would also be elevated to planetary rank. The Pluto-Charon system would be considered a binary planet, the only such entity in the Solar System. The resulting twelve-planet Solar System would not have been a problem as such, but it would almost certainly have been only the thin end of the wedge, with objects such as Varuna, Quaoar and Sedna scrambling to join the queue for planetary recognition. American astronomer Mike Brown, discoverer of both Sedna and ‘Xena’, has claimed that around fifty known Kuiper Belt objects are likely to fit the roundness criterion, that more than eighty probably do, and that the final tally could run to as much as 350.
…when it’s a dwarf planet
After much debate, the initial draft proposal was rejected by the International Astronomical Union members present at the Prague conference, and on 24 August 2006 they voted that a planet is: “... a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.”
As Kuiper Belt objects, Pluto and Eris did not meet the definition, neither did Ceres, as merely the largest member of the asteroid belt between Mars and Jupiter. However, there was considerable controversy over the meaning of ‘has cleared the neighbourhood around its orbit’ as most of the recognised planets including Earth share their orbits with large numbers of asteroids. Pluto was downgraded to the confusing rank of ‘dwarf planet’, a status which was also accorded to Ceres and ‘Xena’. It was made clear that despite containing the word ‘planet’, a ‘dwarf planet’ is not actually a planet. Soon after, ‘Xena’ was officially given the name Eris after the Greek goddess of discord – a fitting name in the circumstances.
Pluto’s demotion did not go down well, and was particularly unpopular in the United States where the American Museum of Natural History received a fresh round of protest letters from irate schoolchildren. Older Americans were also dismayed: after all, Pluto was discovered in America by an American. In Clyde Tombaugh’s home state of Illinois, the state senate passed a resolution to re-establish its planetary status and ruled that 13 March 2009 would be declared ‘Pluto Day’.
The new definition was criticised as confusing and unnecessarily complicated. There were complaints that only four percent of the International Astronomical Union’s membership had voted on the resolution, and that most of these were not planetary scientists. There is little doubt that the resolution adopted at Prague was a dog’s dinner: aside from the confusion over what constitutes neighbourhood clearing, you do not have to be Star Trek’s Mr Spock to see that it is illogical to call something a ‘dwarf planet’ and insist that it is not a planet. Proponents of the definition argue that a hundred or more Solar System planets are too many, but again the logic is flawed. The current membership of the United Nations is 193 member states, but nobody is suggesting that the likes of Monaco, Andorra and San Marino be downgraded to ‘dwarf countries’ that aren’t actually countries at all.
The late Sir Patrick Moore, long of the opinion that Pluto is not a proper planet, argued for the much simpler definition that Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune are planets and everything else isn’t. Such a definition might not be particularly scientific, but it certainly has the benefit of being something that everybody can understand, and it could easily be amended if a trans-Neptunian object significantly larger than Pluto or Eris were to turn up.
The problem really lies with the word ‘planet’ and the hidden assumption namely that it can be used to define a generic class of object, which is clearly not the case. Even the traditional division between rocky planets like Earth and gaseous planets like Jupiter is an over-simplification. Uranus and Neptune contain less hydrogen and helium than Jupiter and Saturn, and are now thought of as ‘ice giants’ rather than ‘gas giants’. Kuiper Belt objects such as Pluto and Eris form another class of object, as does the asteroid Ceres, but that actually tells us nothing about whether or not they should be classed as planets. Supposing these last three bodies were the size of Earth? Under the Prague resolution, they would still be classed as dwarf planets. Conversely, were Ceres located where Earth currently is, it would qualify as a planet. Again, this does not seem to be very logical. Another issue is that the current system makes no distinction between planet-sized moons such as Ganymede and kilometre-sized objects such as S/2003 J9. Both are simply classed as moons of Jupiter. Should being a moon necessarily disbar something from also being a planet, regardless of its size?
Astronomers could perhaps take a lesson from the ‘lumping and splitting’ debate that goes on in biology. ‘Lumpers’ try to cram as many species as possible into a single taxonomic category; ‘splitters’ do the exact opposite, requiring very little justification for erecting new species and genera. The word ‘planet’ is a classic example of ‘lumping’. To be useful, it needs to be split into distinct classes that would group together related classes of objects without being afraid to cut across traditional lines of classification. For example, the class ‘rocky planet’ or ‘terrestrial planet’ could include not just Mercury, Venus, Earth and Mars but also Ceres and the Moon. As for Pluto, what is wrong with calling it a ‘Kuiper Belt planet’?
Let’s not forget the science
The debate over Pluto’s status has unfortunately overshadowed the fact that there is a lot of very interesting science waiting to be done there. At the time of writing, NASA’s New Horizons space probe is just under a year away from Pluto; it is due to make a flyby on 14 July 2015. Provided all goes well, it will tell us more about Pluto in a few hours than we have learned in the last eighty-five years. Nevertheless, thanks to the Hubble Space Telescope and new generation ground-based telescopes, we know a lot more about Pluto than we did a few years ago.
Pluto is now known to have at least five moons including Charon – an impressive total considering Mercury, Venus, Earth and Mars can only muster three moons between them. Ahead of the launch of New Horizons, the Pluto Companion Search Team used the Hubble Space Telescope to try and identify further Plutonian moons. The team were successful and announced the discovery of two new moons in October 2005, which received the names Nix and Hydra in reference to the initials NH for New Horizons. A fourth moon, Kerberos, was announced in July 2011 and a fifth, Styx, in July 2012. All these new moons are far smaller than Charon, and are also further away from Pluto. Like Charon, they orbit the Pluto-Charon barycentre rather than Pluto itself.
Styx, the smallest and closest, is between 10 to 25 km (6 to 15 miles) in diameter, and lies 42,000 km (26,000 miles) from the barycentre, or just over twice the distance of Charon; Hydra, the largest and outermost, is114 km (70 miles) in diameter and lies 64,750 km (40,000 miles) from the barycentre, or about three and a half times the distance of Charon.
According to one theory, the moon system came into being when Pluto collided with another Kuiper Belt object. The impactor lost substantial mass, but survived to become Charon; meanwhile some of the debris flung up into space by the collision coalesced to form the four smaller moons. More recently, it has been suggested that the entire Pluto system was formed as a result of a collision between two large Kuiper Belt objects of about the same size. The cores merged to form Pluto and debris ejected from the collision formed a disc around the merged body from which Charon and the smaller moons coalesced.
The Hubble Space Telescope has enabled maps to be made of the surface of Pluto. Due to the immense distance, it can only resolve variations that are several hundred kilometres across and requires the use of complex computer techniques to achieve even that. Nevertheless, they show that Pluto’s surface is highly varied and there are considerable changes in both brightness and colour. Overall, Pluto is mottled orange and black in appearance, whereas Charon appears bluish.
Comparative spectroscopic analysis, facilitated by the series of eclipses in the 1980s, showed that the surface compositions of Pluto and Charon differ – Pluto’s surface is coated with frozen nitrogen mixed with traces of methane and carbon monoxide, whereas that of Charon is largely composed of ice. Their internal composition is unknown, but Pluto’s density suggests that it is comprised of around 70 percent rock and 30 percent water ice. Charon is less dense, and contains less rock. Pluto is thought to be a differentiated body with a rocky core and icy mantle; Charon may lack a distinct core. Pluto also has a tenuous but definite atmosphere, composed largely of nitrogen, but it is only present when Pluto is closer to the Sun; for most of the Plutonian ‘year’ the atmosphere lies frozen on the surface.
Thanks not just to Hubble but to considerable advances in ground-based astronomical techniques, we know considerably more about Pluto than we did forty years ago: but if all goes well, we will soon know incomparably more.
The New Horizons mission to Pluto was approved in 2001 after two previous proposals by NASA were cancelled. The triangular 478 kg (1,054 lb) space probe is about the size of a grand piano and is the first in a series of medium-cost ‘New Frontiers’ category missions to be launched by NASA, more expensive than ‘Discovery’ missions but cheaper than ‘Flagship’ missions. These terms are relative as at 650 million dollars, New Horizons isn’t exactly a bargain basement jaunt. The name New Horizons was meant to reflect the ambitious scope of the mission, although it is perhaps disappointing that the probe was not named for Clyde Tombaugh, who died in 1997 aged 90. However, it is a nice touch that a portion of his ashes are carried aboard the spacecraft.
The probe was launched from Cape Canaveral on 19 January 2006, a few months before Pluto was finally stripped of its planetary status. Launch was postponed several times due to technical problems and adverse weather conditions, and for a while there was concern that the window for an encounter with Jupiter was would be missed. While useful from a scientific point of view and providing a valuable opportunity to test the spacecraft’s instrumentation, the main purpose of the rendezvous was to use the giant planet for a gravity assist that would shorten the voyage to Pluto by three years. Fortunately, the issues were resolved, the skies cleared in time, and New Horizons finally lifted off at 14:00 EST on the afternoon of 19 January, watched live by millions around the world.
The spacecraft left Earth on a solar escape trajectory travelling at 16.5 km/sec (10.25 miles/sec) or 59,000 km/h (37,000 mph), faster than any previous launch. It took just nine hours to pass the orbit of the Moon and reached the orbit of Mars on 7 April 2006. The Jupiter gravity assist manoeuvre was carried out successfully on 28 February 2007, and New Horizons is now on course to reach Pluto on 14 July 2015, nine and a half years after leaving Earth.
After the encounter with Pluto, it is hoped that New Horizons can be sent on to investigate further Kuiper Belt objects. Unfortunately, the spacecraft has only a limited ability to manoeuvre, ruling out any possibility of a flyby of one of the larger bodies as none lie along its flight path. However, NASA is hopeful that a one or more objects of the order of 50 to 100 km (30 to 60 miles) in diameter may be located close enough to the current flight path for a rendezvous to be attempted. At the time of writing, the Hubble Space Telescope is being used to conduct a search for suitable targets that could be reached three to four years after the Pluto encounter.
The Space Age begun in 1957 with the launch of Sputnik I, and was less than twelve years old when Neil Armstrong and Buzz Aldrin landed on the Moon. This breath-taking progress was of course driven by Cold War tensions between the United States and the Soviet Union, and it tended to overshadow the fact that even then robot space probes were rewriting the astronomy textbooks.
It is safe to say that before the Space Age we knew less about the Moon than we shall learn about Pluto if the New Horizons mission is a success. We had absolutely no idea what the far side of the Moon looked like until the Soviet probe Lunik III returned the first blurry images in October 1959. At that time, some believed that Venus resembled Earth during the Carboniferous Period 350 million years ago, and was a swampy world with luxuriant vegetation. Mars was thought to be cold and arid, but its polar caps were thought to contain some water as well as carbon dioxide. The atmosphere, though thin, was thought to be breathable and liquid water was thought to exist. Many believed that the planet supported life in the form of simple vegetation.
Between 1962 and 1989, space probes visited all the currently recognised planets, and a multinational ‘space armada’ visited Halley’s Comet during its 1986 return. In a single generation, our view of the Solar System was transformed as questions that astronomers had pondered since the invention of the telescope were answered. Inevitably, new questions have been posed and many long-standing theories have been consigned to the history books. Since then, the surface of Venus has been mapped; probes have visited a number of comets and asteroids; the Huygens lander has soft-landed on the planet-sized moon Titan; and rovers have been operating on Mars since 2004. In 2012, the car-sized rover Curiosity was soft-landed on Mars using a ‘sky-crane’ system that would not look out of place in a science-fiction movie.
The Pluto flyby will herald the beginning of a new phase in our exploration of the Solar System. Eventually, probes will visit worlds such as Quaoar, Sedna and Eris, although the distances involved means that such missions are likely to take decades to reach their objectives with current technologies. Possibly, novel technologies such as solar sails will be used, which in theory could bring the journey time down to a few years. For now, though, the focus is on Pluto. Perhaps New Horizons was not such a bad choice of name after all for the first spacecraft to visit this supposedly ex-planet.