Aspiring lunar explorers, take heed — any newly discovered ridges on the moon must be named for a geoscientist. If you want to name a landform on Saturn’s satellite Titan, you’d better be a fantasy or science fiction fan: Mountains and plains on the lake-covered moon are styled after places in Tolkien’s Middle Earth and Frank Herbert’s “Dune” series. And almost everything on Io, the eruptive moon of Jupiter, must have a name associated with fire, volcanoes, or Dante’s “Inferno.”
So decrees the International Astronomical Union, the official arbiter of planetary and satellite nomenclature since 1919. As ever more powerful telescopes and ambitious new robotic missions add to the identified real estate of the solar system, the IAU’s brilliant, byzantine and sometimes marvelously nerdy naming guidelines help bring order to our crowded skies.
The IAU’s rules are in the news this month after the Carnegie Institution for Science announced it needed help naming several moons of Jupiter discovered last year. Carnegie astronomer Scott Sheppard, who spotted the new moons using a giant telescope in Chile, said suggestions should be tweeted to the handle @JupiterLunacy using the hashtag #NameJupitersMoons.
But, protocols for planetary nomenclature being what they are, Sheppard can only consider names that meet a few key criteria:
It must come from a character in Greek or Roman mythology who was either a descendant or lover of the god known as Zeus (in Greek) or Jupiter (Latin). It must be 16 characters or fewer, preferably one word. It can’t be offensive, too commercial, or closely tied to any political, military or religious activities of the past 100 years. It can’t belong to a living person and can’t be too similar to the name of any existing moons or asteroids. If the moon in question is prograde (it circles in the same direction as its planet rotates) the name must end in an “a.” If it is retrograde (circling in the opposite direction), the name must end in an “e.”
Easy peasy, right?
“Jupiter is one of the more restrictive ones,” Sheppard said. Even though the king of the gods was quite the philanderer, there is a limit to the number of mythological characters who meet the IAU criteria. “The ‘ends in e’ scenario is actually running out of names,” the astronomer said.
Gareth Williams, an astronomer at the Harvard Smithsonian Center for Astrophysics who serves on the IAU working groups for planetary system and small bodies nomenclature, said that these stringent guidelines are necessary to avoid confusion in the study of the cosmos. Before the union came along in the early 20th century, the solar system was a mess.
Political fights and international disputes occasionally broke out over the names of new planets; Uranus was very nearly called “George’s star” after England’s King George III awarded an annual stipend and plush new digs in Windsor Castle to the planet’s discoverer, William Herschel. Improvements to telescopes that made it possible to identify the inhabitants of the asteroid belt resulted in scores of new rocky bodies being discovered every year. (One scientist derisively called them “vermin of the sky.") But few researchers took the time to cross-check whether their supposed “discovery” had actually been seen before. Maps of Mars and the moon were similarly rife with conflicts. A given crater or dome could have three different names — and a given name might describe two entirely different objects.
It wasn’t until 1913 that anyone published a definitive list of every known feature on the moon — then the solar system’s most-studied object. Work by Mary Adela Blagg, an English astronomer who meticulously tracked each new discovery and mismatched name, led to the creation of the IAU’s first formal list of lunar landmarks in 1935. Two decades later, the organization published a similar guide to Martian topography, drawing mostly on maps developed by astronomers over the past century.
With the advent of the Space Age, “people were making new discoveries by the bucketload,” Williams said. It was far more than one person could keep track of, so the IAU established a Working Group for Planetary System Nomenclature (WGPSN) to oversee the naming process.
When the first images are beamed back from a mission to a new celestial body, the spacecraft team behind the encounter typically proposes categories and themes for naming the landforms they observe, said Rita Schulz, the chairwoman of the WGPSN.
After NASA’s New Horizons probe flew past Pluto in 2015, for example, scientists on the team and at the IAU devised a naming scheme focused on stories of the underworld and voyages of discovery. They agreed that mountains on the planet would be named for historic explorers while dark spots and plains on its moon Charon could commemorate the destinations of fictional space expeditions. As a result, we have the Tenzing Montes on Pluto — 20,000-foot-tall ice mountains named for Nepalese mountaineer Tenzing Norgay — and a macula, or dark spot, on Charon called “Mordor.” (Space is full of Lord of the Rings references.)
Once themes are decided, anyone can suggest a name for a new body or feature so long as he or she can demonstrate it is scientifically useful. New suggestions are reviewed by the relevant task group and the members of the overall working group and, once approved, are published in the Gazetteer of Planetary Nomenclature. Among the most recent additions are Hippocamp, a tiny moon of Neptune discovered with the Hubble Space Telescope, and Statio Tainhe, the landing site where the Chinese spacecraft Chang’e 4 touched down on the far side of the moon — a first for humanity. “Hippocampi” are the horselike sea monsters that drove Neptune’s chariot in ancient myth; “Tianhe” comes from the ancient Chinese name for the Milky Way.
Having themes “minimizes the chance that someone will want to give the same name to two different features on two different bodies because the themes will be different on each,” Williams said. “It also ties things together. If you know your mythology . . . you can immediately tell what body it’s on and what type [of feature] it is.”
Lately, space scientists have needed to learn more than just Greek and Roman myth. As secretary for the Working Group for Small Body Nomenclature, Williams said he has sought to diversify the themes asteroids and other small objects to reflect the “changing life experiences of the astronomers that are naming them.”
He is particularly proud of asteroids named for popular musicians. “Not the sort of teeny-bop noise that you get,” he said, “but people who are as worthy as the great classical composers,” such as Jimi Hendrix, Billie Holiday and David Bowie.
Minor planets — any body in orbit around the sun that is not a planet or comet — are the most freewheeling of solar system objects. With a few exceptions, most new asteroids can bear any name that is not offensive or overtly commercial or political. These space rocks carry the names of mathematicians, chemists, engineers, high school teachers, the members of the Beatles and the British comedy group Monty Python, all three Bronte sisters, runner Jesse Owens, actress Zsa Zsa Gabor and activist Malala Yousafzai.
But even the indulgent members of the minor planet community have their limits. In the 1980s, when astronomer James Gibson christened a newly discovered asteroid with the name of his cat, Mr. Spock, “certain people — I’m not going to mention any names — felt that was inappropriate,” Williams said. “So now that is strongly discouraged.”
But the IAU does not change names once they are chosen. The asteroid 2309 Mr. Spock circles the sun still.