New Horizons to Pluto – The Final Piece of the Puzzle

For a small cold lump of ice and rock at the far reaches of the Solar System Pluto holds a remarkably prominent place in people’s hearts. This is evidenced by the controversy and indignation caused by the IAU decision in 2006 to demote it to the status of dwarf planet, or if you are feeling particularly spiteful, a trans-Neptunian object. Nobody likes to lose a member of the family and losing a planet seems a particularly bitter pill, but what about Eris (which may actually be bigger than Pluto), Makemake or Haumea? They describe their lonely orbits without anyone shedding a tear for their lack of planethood, no the issue is that since 1930 when Pluto was famously discovered by ,and for most of our lifetimes there were nine planets in the solar system and the most distant was Pluto. To add further romance the planet (for that is what it was at the time) was named by an 11 year old girl from Oxford called Venetia Burney who received £5 as a reward for her efforts and was still alive in 2006 to hear of her planet's demotion, her obituary can be found here.

pluto discovery_plates 

So we’ve established that we all love Pluto and feel sorry for it but what exactly do we know about this distant world? The answer is not very much, hardly surprising as Pluto is only two thirds the diameter of our own Moon yet describes a highly elliptical orbit with an average distance from the Sun of 5.9 billion km. Pluto was at perihelion in 1989 and in fact was closer to the Sun than Neptune until 1999, this distance steadily and slowly increasing since then, with an orbital period of 247 years, nothing happens quickly.

After its discovery in 1930 nothing much happened regarding Pluto apart from us knowing it was there and naming cartoon dogs after it until 1978 when its moon Charon was discovered by James Christy at the US Naval Observatory in Flagstaff Arizona, in 1990 Pluto and its companion were resolved into two separate disks by Hubble.

charon disc

Pluto and Charon revolve around each other every 6.4 days, the two being gravitationally locked keeping the same face pointed to each other in the same way that the Moon presents one side to the Earth, though in our case the favour is not returned. The two orbit at a distance of 19,500km and it is thought that the origin of Charon is similar to that of our Moon, it being created by a huge impact to the main body some 4.5 billion years ago. Pluto and Charon are considered a double dwarf-planet system as the location of the barycentre (centre of gravity) is not located under the surface of either body. It is interesting to note that as the moon is drifting away from Earth by just under 4cm a year in billions of years time the centre of gravity between the Earth and the Moon will drift outside the Earth’s surface and we will become a double planet system. Something to look forward to! 

In 2005 Hubble identified two further moons, Nix and Hydra and in 2011 and 2012 and additional two moons, Kerberos and Styx were discovered.


Can I see Pluto for myself I hear you ask, well theoretically yes. To have a chance it is best to wait until Pluto is at or near opposition over the summer months, a large aperture scope, a nice dark site and a very good star map will give you a fighting chance however it is extremely challenging and the fact that Pluto resides in the rich star fields of Sagittarius until 2023 piles on the difficulty. Never has the analogy of looking for a needle in a haystack been more appropriate when trying to spot a 13.65 magnitude point of light in amongst a host of much brighter stars.

If you have a Goto scope that is 8 inches or more and it has been accurately aligned etc. just plug in Pluto and let it take you there, you can at least say with complete honesty that you have seen it (all other factors considered), you don’t have to tell anyone you didn’t know exactly which of the points of light it was.

If that seems far too much of a challenge it is fun to repeat the efforts of Mr Tombaugh and image the star field in which Pluto resides, take a nice long exposure and then repeat the exercise a few weeks later. If you then open your images in layer based processing software such as Photoshop you can copy the second image as a layer on top of your fist image, align and then set the blending mode to difference Pluto should pop out as it will have moved in the time between the first and second images having been taken.

pluto in_starfield

Surprisingly my image above isn’t the best we currently have of Pluto, this having been obtained by Hubble and consisting of a very disappointing albedo map which tells us Pluto doesn’t have a uniformly dark or light surface. It seems counter-intuitive that Hubble can image the most distant objects in the Universe but can’t get a decent image of an object in or immediate neighbourhood. The problem however is one of size, distance and camera resolution which equates to how large these objects appear in our sky and how easy it is to capture a detailed image. I won’t go into the maths but consider M33 (NGC598) the Triangulum Galaxy, it lies around 3 million light years away but spans 73x45 arc minutes of sky, the full Moon occupies around 30 arc minutes and Pluto 0.06 – 0.11 arc seconds. A single Hubble pixel covers 0.04 arc seconds so with Pluto currently occupying a whole 0.09 arc seconds you can see that it is not going to bother Hubble’s cameras to much in terms of useable pixels.

best image_of_pluto

Now for the good stuff, we don’t need to be disheartened by our lack of knowledge as all this is going to change over the next twelve months with the arrival of the New Horizons spacecraft at Pluto.

On 19th January 2006 in what was the fastest space craft launch ever taking just 9 hours to reach the moons orbit New horizons began its journey. This epic speed has been maintained and the craft will reach Pluto years in advance of the time it took Voyager to reach Neptune.

New Horizons has already provided us with the best images yet taken of the Pluto Charon system, showing the two worlds orbiting each other - and this is even before the main encounter operations start (scheduled for 15th January 2015). Click on the image below to see the orbital animation:

pr e12_proper_nosat_3fps

The chief goals of the mission are to map the composition and Pluto and its largest moon Charon and analyse their atmospheres. Temperature and topography will be studied, known moons will be examined and new ones searched for, even the possibility of discovering a ring has also been factored into the project teams plans. If you want to find out more detailed information regarding the mission you can find it here.

Despite being just over half the weight of Voyager at launch New Horizons has not skimped on instruments ranging from LORRI (LOng Range Reconnaissance Imager) an 8 inch scope for delivery of high resolution images to what is bound to be a favourite of this site, RALPH, a camera/imaging spectrometer operating in the visible and infrared which is tasked with mapping Pluto’s composition and temperature as well as conducting visible imaging. Ralph’s composition mapping spectrometer consists of over 65,000 pixels, Voyager's had 1. Something tells me we are going to hear a lot more about Ralph towards the middle of the year.


The chief goal of the mission is undoubtedly the fly-by of Pluto and Charon, the spacecraft making its closest approach on July 14th 2015 at around 12:50 UT. While lots of valuable science will be conducted as the craft homes in on its prey a huge amount of activity will be compressed into the very short fly-by window owing to the fact that its speed and relatively light weight means New Horizons can’t slow down, it basically doesn’t have enough fuel on board to make any sort of dent in its velocity. When we think about the images achieved by the fast fly-by undertaken by Voyager 2 when it reached Neptune and Triton in 1989 we can be sure there will still be spectacular images to view come the end of the year especially with the meticulous planning that has gone into ensuring that no single second of the encounter is wasted.

To avoid disappointment on July 14th there is one note of caution, don’t expect to sit in front of your computer screen watching images of Pluto and Charon in ever greater detail pop up accompanied by gasps and cheers. The craft is gathering so much data during its closest encounter phase that it simply can’t transmit it back to Earth but will store it on-board for later transmission, potentially over a period of a year, talk about delayed gratification! There will of course be amazing images released and science done up to, during and post closest encounter but it is probably better to think of this as a gift that keeps on giving like Rosetta or Cassini rather than a seat of the pants fly-by image fest.

And after Pluto, what then? Like all planetary missions these days once the main goals have been achieved we then have the extended mission in which New Horizons continues on its journey exploring the Kuiper Belt (the region of the Solar System beyond the orbit of Neptune to approximately 50AU from the Sun). The Hubble Space Telescope has identified three Kuiper Belt Objects (KBOs) that New Horizons could potentially visit, pristine examples of what the Solar System would have been like at its birth 4.6 billion years ago. They reside 1 billion km beyond Pluto and range in size from 55 – 25 km across. Proposals for the extended mission to one of these targets are expected to be made by the New Horizons team in late 2016.

Whatever happens we are in for an amazing and spectacular journey which as with all discovery missions will without a shadow of a doubt deliver the unexpected. Whatever you think we are going to find, what we actually see and discover is going to be ten times more wonderful and amazing, I can’t wait for the summer.