Discovered in 1930, it was assumed that Pluto was a planet, with a mass comparable to that of the Earth. This was partially based on its brightness, along with the mistaken assumption that it was perturbing the orbits of Neptune and Uranus.
As the decades passed, and our observations of Pluto improved, its estimated mass dropped: smaller than Mars, then smaller still – smaller than our moon. Finally, with the discovery in 1978 of Pluto’s largest satellite, Charon, it became possible to precisely determine its mass – just over 0.2% of that of the Earth.
Despite its tiny mass and size (Pluto is significantly smaller than our moon), its planetary classification remained until 2006, when the International Astronomical Union created their official definition for the term “planet”.
Pluto, like Ceres before it, was relegated to minor planet status – becoming one of the founder members of the new “dwarf planet” class.
Air today, gone tomorrow
Despite its “demotion”, Pluto remains a fascinating object. Highly reflective, Pluto is far brighter than its fellow trans-Neptunian brethren. This contributed to its early discovery, resulting in the overestimates of its size and mass that led to its planetary longevity.
Pluto moves on a relatively eccentric orbit around the sun – one that brings it within the orbit of Neptune for 20 years out of every 248 year orbit. At its furthest from the sun, by contrast, Pluto lurks almost 49 times more distant than the Earth.
As a result, Pluto’s surface temperature varies dramatically – with the intensity of Solar radiation it receives varying by almost a factor of three through the course of the Plutonian year.
Herein, we think, lies the explanation for Pluto’s anomalous reflectivity. As it swung inwards for its last perihelion passage (which occurred back in 1989), the methane, nitrogen and carbon monoxide ices on its surface began to sublimate, building up a tenuous but measurable atmosphere.
As it recedes to the depths of the solar system, and the temperature falls once more, that atmosphere is predicted to freeze out, falling back to the surface as fresh snow.
A whistle-stop tour of the solar system’s freezer
The ephemeral nature of Pluto’s atmosphere was one of the driving factors behind the development and launch of the New Horizons mission.
The spacecraft, launched back in 2006, holds the record for the highest launch speed of any man-made object. And with good reason – the goal was to reach Pluto before its atmosphere collapsed back to its surface.
After nine and a half year’s travel, New Horizons will tear past Pluto on July 14 next year. Unlike Rosetta, which has the relative luxury of orbiting its target for months, collecting data all the while, Pluto and New Horizons will have the briefest of encounters.
The entire spacecraft is designed to make the most of a tiny window of opportunity – whizzing past the dwarf planet at a relative speed of almost 14km/s, it will hopefully approach to within around 10,000km of Pluto’s surface. At that speed, New Horizons will spend less than 40 hours within a million kilometres of Pluto.
Awake, Sleeping Beauty
In February 2007, New Horizons swung close to Jupiter, using the giant planet’s gravitational pull to get an extra kick, speeding it on its way to Pluto. The close encounter was also used to test the spacecraft’s instruments, and it returned spectacular images of the solar system’s biggest, baddest bully, and its satellite retinue.
After that flyby, New Horizons entered a series of lengthy hibernations in order to conserve power for the main event. Occasionally, it awoke, and let everyone back home know that it was still fully functional.
The final wake-up call came just a couple of weeks ago, December 6, and New Horizons is now gearing up for its main mission.
Early observations to check for plain sailing
Now that it is awake, the plan is for New Horizons to begin observing Pluto in earnest this coming January.
By the start of May, it will be close enough to the Plutonian system to return images of higher quality than can be achieved with the Hubble Space Telescope. This will allow the mission team to check for any unknown satellites or rings that might lie in the spacecraft’s path.
As it swings closer to Pluto, it will continue to image the dwarf planet and its satellites. The key period for that imaging will be the final few days leading up to the closest approach, as the spacecraft attempts to store as much information as possible to squirt back to Earth over the months that follow.
A celestial waltz, with a six day spin
Pluto and its largest satellite, the behemoth Charon, are tidally locked to one another. Each spins with a period of just over six days and nine hours – the same period as it takes for them to orbit around their common centre of mass.
This means that Charon keeps one face pointed towards Pluto at all times – just as our moon keeps the same face pointed towards the Earth. But unlike our Earth-moon system, Pluto repays the favour – keeping one face permanently pointed towards Charon.
As New Horizons swings past the system, it will endeavour to produce maps of Pluto – but due to the very rapid flyby, the best images will only reveal one hemisphere of the dwarf planet. Imaging before and after the encounter will help scientists to fill in, to some extent, the rest of Pluto’s surface.
Not just imaging …
New Horizons is more than simply an expensive, fast-moving camera platform. In total, it carries seven science packages – each with a different role to play in the survey of Pluto and its satellites.
That said, the images returned at the time of closest approach will be exquisite, and will most likely be what captures the imagination of the viewing public. In addition to full colour maps of Pluto as a whole, there is the potential that features as small as 50 or 100m across will show up in images taken around the time of closest approach.
While the cameras carried by New Horizons are imaging Pluto and its satellites, other instruments will take spectroscopic observations of the dwarf planet’s atmosphere and surface, allowing their chemistry to be studied in detail.
At the same time, the SWAP and PEPSSI instruments will be sniffing the outer layers of Pluto’s very diffuse atmosphere, capturing particles and determining their composition, and measuring how the solar wind interacts with the material out-gassed during Pluto’s fleeting summer.
New Horizons will even allow us to learn something of Pluto’s interior structure. By measuring the precise deflection experienced by the spacecraft as it whooshes by the dwarf planet, astronomers will be able to determine, to some degree, the distribution of mass within.
This could reveal the degree to which Pluto is differentiated – whether it has a core, mantle and crust, like the Earth, or is more homogeneous in composition, like our moon.
A wealth of data, returned in a slow trickle
Transferring data back from Pluto is a challenging business. At the time of closest encounter, New Horizons will be an incredible five billion kilometres from the Earth – or more than four and a half light-hours.
So distant, the total bandwidth available for the spacecraft to communicate with the Earth will be extremely limited – of order 1kbit/s. Such slow speeds will be familiar to anyone who remembers the era of the dial-up modem – and receiving images back from New Horizons will feel somewhat like browsing the internet in the mid-1990s!
Due to the incredibly low bandwidth, although the core of New Horizons’ Pluto mission will take just around one day to complete, the data obtained is likely to take around nine months to be transmitted back to Earth.
So, rather than there being a glut of New Horizons images at the time of closest approach, we will instead see a slow trickle of new, exciting results, spreading the joy over almost a year.
To infinity, and beyond …
Once New Horizons leaves Pluto behind, its work will be far from complete. For the past few years, astronomers have been using the world’s largest telescopes and the Hubble Space Telescope to search for other objects, out beyond the orbit of Neptune, that could be visited by New Horizons during its one-way trip to interstellar space.
The final schedule remains to be set – but thanks to observations using Hubble, the New Horizons team has already identified three potential post-Pluto targets – which go by the imaginative names of PT1, PT2 and PT3.
For now, though, the focus remains on Pluto itself, and humanity’s first encounter with an object beyond the orbit of Neptune.