Astronomers have approved official names for the newest and smallest of Pluto's five moons, discovered in Hubble images taken in 2011 and 2012. They'll be seen much better when the New Horizons spacecraft goes by in two years.
Astronomers have a long, colorful, and occasionally contentious history of naming objects in the solar system, as demonstrated by this 1930 communication concerning a newfound object beyond Neptune. But they don't have a monopoly on picking suitable monikers. So it's refreshing that the discovery team for the newest and smallest pair of Pluto's five moons have opted for names chosen by popular demand.
These two moonlets were spotted in sequences of Hubble images, one in 2011 and the other in 2012, and for a while were nicknames simply "P4" and "P5". (The International Astronomical Union, it should be noted, created the official designations S/2011 (134340) 1 and S/2012 (134340) 1 — but it's obvious why you didn't see these used much.)
With the New Horizons spacecraft already en route to Pluto, discoverer Mark Showalter (SETI Institute) and his observing team decided to give the public a say in what the final names would be. So a website offered a chance to vote on 21 candidate names drawn from mythic underworld realms.
The votes are in — nearly a half million of them, including some 30,000 write-ins — and the winners are Kerberos (for P4) and Styx (for P5). In Greek mythology, Styx is the river (and the goddess thereof) that separates Earth from the underworld. Kerberos is the three-headed guard dog who prevents the dead from escaping the underworld. The IAU picked this spelling over the variant Cerberus, which is already used for an asteroid.
Actually, the top vote-getter was Vulcan. After all, that's the Roman god of volcanoes (a little-known fact), and Vulcan was the home planet of Mr. Spock in Star Trek (a better-known fact). But it's also been used for many decades in astronomical mythology, for a planet purported to circle the Sun well inside Mercury's orbit.
Showalter tells me he tried "pretty hard" to convince the IAU to accept Vulcan, the people's choice. "There are scattered episodes in the literature where the connection can be made, such as Hades' frequent use of volcanoes as his exit point from the underworld," he says. His proposal also noted that a planet interior to Mercury doesn't actually exist, so Vulcan ought to be valid for naming something else.
But Showalter anticipated the IAU's decision (technically made jointly by its Working Groups for Planetary System Nomenclature and Small Body Nomenclature) and fully supports it.
For those of you keeping score, the discovery of Kerberos and Styx brings the grand total of known planetary satellites to 180 (including those around dwarf planets).
Frankly, the New Horizons team was less concerned about what the two moonlets are called and more about the fact that they exist at all. As I detailed last year, these little bodies have very little gravity. So when something hits them, or Nix or Hydra, the resulting debris gets scattered widely in Pluto's vicinity, creating temporary rings. These would be too faint to detect, but they could pose a serious threat to the approaching spacecraft.
As the diagram here shows, New Horizons had been targeted to fly through the system well inside Charon's orbit. But would that route be safe? Maybe not! So last year principal investigator Alan Stern (Southwest Research Institute) led an intense effort to quantify the risk of collision and to develop several alternate routings — dubbed Safe Havens By Other Trajectory (SHBOTs).
It turns out that Pluto's largest satellite has the spacecraft's back. Charon is massive enough that its gravity will perturb the orbits of any dust particles in its vicinity and quickly sweep the area clear. Computer models show that the chance of mission-ending collisional damage is just 0.3%, so recently the team decided to stick with Plan A. Stern discusses the potentially dangerous moon dust and the SHBOT effort in this blog.
So for now it's full speed ahead for the spacecraft. Hard to believe, but only two years from now — on July 14, 2015 — New Horizons will zip just 7,800 miles (12,500 km) from Pluto. In fact, even though it's still a half-billion miles from Pluto, the spacecraft captured Pluto and Charon nestled closely together in snapshots taken July 1st and 3rd. (Charon just celebrated an anniversary of sorts: it was discovered 35 years ago, in June 1978.)
It's conceivable that other moonlets will be found around Pluto between now and then, so those SHBOTs might still come in handy. In any case, New Horizon's upcoming flyby is sure to rekindle the debate about this little world's status as a "dwarf planet".