A New Moon for Pluto, a New Hazard for Spacecraft

There's some big news this week about the little planet that isn't a planet. Astronomers have found a new moon in orbit around Pluto.

The moon, currently (and cleverly) known as P5 because it's the fifth moon discovered in the system – was announced by the IAU’s Central Bureau for Astronomical Telegrams on Tuesday night. It was discovered by Mark Showalter, an astronomer working with

SETI

, in the latest cache of Hubble observations of the dwarf planet.

The main impetus behind further study of Pluto is the New Horizons mission and the spacecraft currently speeding towards the dwarf planet in preparation for its January 2015 encounter. New Horizons was launched in 2006 when three of Pluto's moons were known: Charon, Nix, and Hydra. But more moons have been added to the roster since, namely P4 discovered last July, and now P5.

The discovery of P4 made it abundantly clear to astronomers that we know far less about Pluto's system than we thought. With every new moon that's discovered, the likelihood of debris around Pluto increases. P5 orbits Pluto in the same plane as the other moons. This suggests that the dwarf planet might have been hit in the solar system’s history, releasing debris into an orbit around it that has coalesced into the moon we see today. This also means there might be more moons or rings or spheres of debris around the planet that we haven't been able to see yet.

Astronomers were looking for small moons and dust rings, things close to the planet that might be a concern for the spacecraft, when P5 was discovered. Showalter entered the Hubble data into a program that dimmed the glare from Pluto and other background stars, enabling him to get a clearer view of the space around the planet. The dot of a moon jumped out. When Showalter added a series of images to create a movie, P5's orbit was unmistakable.

P5 is about half as bright as P4, which is thought to be about eight to 21 miles across, and orbits about 26,000 miles from the planet. By comparison, Pluto’s largest moon (and the first one we knew about) Charon measures 648 miles across. Moons two and three, Nix and Hydra, have diameters of 20 to 70 miles respectively.

Finding the new moon increases the likelihood that there are more objects around Pluto, and that's a hazard for New Horizons. Worse would be a cloud of debris encasing the dwarf planet. But scientists behind the mission have contingency plans in place.

As one line of defense, the spacecraft was built to be fairly resilient. It's covered in kevlar, giving it a bulletproof covering that will deflect smaller pieces of debris. Another option if it turns out the system is much more crowded than we thought is to alter the spacecraft's trajectory. A good place to redirect it would be through Charon's orbit. Charon is thought to be large enough to have cleared its orbit of debris leaving a doughnut shaped ring of clear space around the planet. This would give New Horizons a safe passage through the system. Any redirection can be done as little as ten days before the spacecraft reaches Pluto.

Protecting the spacecraft is paramount. New Horizons isn't sending data to Earth constantly, its storing it on board to download to Earth later. So if the spacecraft is hit as it nears Pluto, there's a chance we'll get no data from the most exciting phase of the mission.

The data set that revealed P5 could reveal more moons or dust rings. The observations were done last month, the time of year when Hubble has the clearest view of Pluto. But it's the last data set astronomers will gather of the planet for a while. The race is on to find out what else might be out there, lurking in this latest set of observations. If a redirect is necessary for New Horizons, the new plan needs to be finalized by the end of the year.

• Pluto’s a Dwarf, Get Over It: Q+A with Star Physicist Neil deGrasse Tyson
• Enough of the Hubbub About Pluto, The Little Planet That’s Still a Little Planet
• A Mysterious, Lost Gas Giant May Have Shaped Our Solar System