The Hubble Space Telescope has found another new member of Pluto’s family of satellites, bringing its total number of known moons to five. While this may be an exciting discovery about one of the least well explored members of our Solar System, it does potentially have serious ramifications for NASA’s New Horizon mission, currently en route to the dwarf planet. 

The new satellite, provisionally designated S/2012 (134340) 1, or P5, is thought to be between 10 and 25 kilometres in diameter, and orbits Pluto from a distance of about 42,000 kilometres, taking just over 20 days to do so. P5 was first observed on a series of nine images taken of Pluto and its satellites, taken by Hubble’s Wide Field Camera 3, between 26-29 June and 7-9 July 2012.

For many years, Pluto’s only known satellite was Charon, which was discovered in 1978. While Charon remains by far the largest moon, the Plutonian system has seen somewhat of a population explosion in recent years, with small satellites Nix and Hydra being first detected in 2005, and the as yet unnamed moon P4 being observed in July 2011. Apart from Charon, all of Pluto’s satellites to date have been found by using the Hubble Space Telescope.

The favoured theory for the origins of P5, as well as the rest of the moons, is that a few billion years ago another large object in the Kuiper Belt, where Pluto resides, collided with the young dwarf planet. This violent impact blasted a lot of material into space, and the material which was retained by Pluto’s gravitational influence coalesced over time to form moons. The vast majority (well over 99 per cent) of this material went into forming Charon, while the small chunks of leftover debris formed the other four moons, as well as any other objects which may be in orbit around Pluto, waiting to be discovered.

Further credence is given to this theory by the fact that each of the smaller satellites are in a series of orbital resonances with Charon, which orbits Pluto four times for each orbit that Nix makes, five times for each time P4 circles Pluto, and six times for each orbit of Hydra. P5’s orbit may not have been pinned down as precisely yet, but early indications are that it is in a 1:3 resonance with Charon.

This theory of formation by collision is one of the questions that it is hoped may be answered by the New Horizon’s probe, as it makes a fly-by of Pluto on 14 July 2015. “We certainly will develop considerable new information on the moons of Pluto”, says Alan Stern, principal investigator of the New Horizons mission. “[We will measure] their comparative compositions, colours, orbits, sizes, albedos, and other properties that theorists will compare to models. We will also find smaller moons than we can see from Earth. All of this will help constrain origin theories.”

A newly discovered object in our Solar System is always a significant occurrence, but this particular finding may just herald a warning for the New Horizons probe. Numerous small moons would suggest that the neighbourhood around Pluto may be littered with objects much smaller than the moons that we know about. It is not out of the question that there may even be an undiscovered ring of small pieces of material around the dwarf planet.

For a spacecraft which is currently travelling through space at an astonishing 15.4 kilometres per second, encountering even a tiny piece of cosmic debris could cause major issues for the mission. Minimising the risk to the spacecraft is one of the reasons that Hubble has been studying Pluto, in the hope of finding at least some of the more significant threats.

There will always be objects that are beyond Hubble’s reach though, and New Horizons will need to rely on its own detection of smaller objects as it approaches close to Pluto, and there are contingency plans if it is found that the spacecraft is at risk. “We are planning a ‘Safe Haven Bail Out Trajectory’ (SHBOT) that we can move to if we believe the encounter is too dangerous”, Stern tells Skymania. “SHBOT will take the spacecraft to a safer place outside the orbits of the moons.”

This invariably means that New Horizons would pass further from Pluto than the originally intended 10,000 kilometres, but if it becomes necessary, at least the spacecraft will be able to continue its journey into the Kuiper Belt in the hopes of even more close encounters with the other mysterious worlds to be found there.