S/1 90482 (2005) needs your help

“S/1 90482 (2005)” is really not much a name as a license plate number. As does a license plate number, it tells you pretty much everything you need to know to identify the object in question. “S” is for satellite. “/1” means it is the first discovered. The “2005” at the end tells the date of discover, and the “90482” tells whose satellite it is, but only by yet another number. This number refers to the 90482nd minor planet (in the old terminology; no one quite knows what the new terminology is, but the numbers keep coming) to be officially recorded. That object is more commonly referred to as the large Kuiper belt object Orcus. We don’t ever call the moon of Orcus by its official name of S/1 90482 (2005). Instead, around here, it is referred to mostly as “the moon of Orcus.”

It’s time to change that.

Not all of the Kuiper belt objects known and number have names, and, as I have written here earlier, I think most don’ t need them. It is OK to consign them to semi-anonymous license plate numbers if they are never really going to be thought about as more than one of the crowd. But a few special objects get studied and talked about and written about enough that need not so much just names, but also personalities. Orcus was one of those objects. Its personality was quite apparent from the beginning.

We discovered Orcus in early 2004. At the time it was the 4th largest known Kuiper belt object, though by now it has dropped to something like 8th. The most interesting thing about Orcus to me was that it appeared to be the anti-Pluto.

Pluto has what was originally thought to be a peculiar orbit. It circles the sun precisely two times for every three times that Neptune goes around the sun. Though it took astronomers a long time to realize it, this peculiarity is not a coincidence. Neptune’s gravity so dominates the region of space where Pluto is that Neptune has herded Pluto into this very special orbit. Pluto is not the only one that Neptune is pushing around. We now know of hundreds of similar objects in the Kuiper belt, including, now, Orcus.

Pluto’s orbit has a few other interesting features to it. It is so elongated that, for a brief time during its revolution about the sun, it actually comes close to the sun than does Neptune. So does Orcus. When Pluto comes close to the sun, though, it is never actually close to Neptune, partially because at that point in its orbit it is high above the disk of the planets, hitting the most extreme spot of its tilted orbit. Just like Orcus.

In fact, if you look at the orbits of Pluto and Orcus (and I encourage you to do it if you never have; check out the extremely cool orbit plotter at JPL but you'll have to zoom out to find Orcus), you will see that they are nearly identical except for 2 things. Their elongated orbits point in nearly opposite directions, and, right now, Pluto is nearly as close as it ever comes to the sun while Orcus is nearly as far away as it ever comes. In fact, because Pluto and Orcus are forced by Neptune to have precisely the same orbital period, they will always stay in opposite phases of their orbits.

Orcus is the anti-Pluto.

Several years ago, when searching for a name for what was then known only as 2004 DW, we decided to concentrate on the anti-Pluto aspect of the object’s personality, and we came up with Orcus. In the version of the Orcus myth that I like to tell, Orcus was, essentially, the early Etruscan grim reaper, collecting the dead and bringing them to the underworld where another god – Pluto – ruled. As the Etruscan mythology was incorporated into Roman mythology and blended with Greek mythology, Orcus lost his separate identity and Pluto became the master of all of the functions of the dead. Orcus became in some ways simply an alternate name for Pluto, but it also remained a slightly more evil and punishing incarnation of Pluto. In that incarnation, the Latin word Orcus was the origin of words such as ogre and orc.

In my new mythological/astronomical view, Pluto the Kuiper belt object is now named after that earlier version of Pluto, before the Romans came along and swept everything together. And Orcus is his counterpart, destined to eventually be pushed aside by the rising Pluto. Orcus seemed a very appropriate name for this new object in the Kuiper belt.


About a year later, while looking carefully at Orcus with the Hubble Space Telescope, we realized that it had a moon. In the past year we have been studying the moon of Orcus intensely and are in the final stages of writing a scientific paper on all of the interesting things about this moon. Which means it is time to stop calling it “this moon” and give it a proper name. But what?

Here’s where you come in. Send me suggestions! I’ll submit the best suggestion to the International Astronomical Union on Sunday, April 5th (about 2 weeks from now) with your name as part of the official citation (if you want it to be).

If you make a suggestion I would like to know not just what the suggestion is, but why you think its appropriate. As you can tell by now, this is the part that matters to me!

To help you out, let me tell you some of the other interesting things about the satellite. It has about a ten day orbit around Orcus, in a tight precise circle. We suspect – though can’t yet prove – that Orcus and its satellite have their same faces locked towards each other constantly, like an orbiting dumbbell. Only one other Kuiper belt object and satellite are known to do this. Who? Pluto and Charon, of course.

The origin of the satellite of Orcus is confusing. Pluto and Charon are thought to have formed in a giant collision. Haumea clearly had a shattering blow to disperse moons and other family members. But small Kuiper belt objects are thought to acquired moons by simple capture.

Orcus is right in the middle. Was the satellite from a collision or a capture? We had hoped to answer this question by observations from the Hubble Space Telescope. If the satellite had looked just like other known collisional satellite, we would have been pretty convinced. It doesn’t. Unfortunately that tells us less. We can’t rule out either. We have some ideas of new Hubble Space Telescope observations to try to tell the difference. For now, though, we’re just confused.

While the scientific paper will have more details and calculations, that will be the gist of it, and those properties are all you get to know to try to discern the personality of the moon of Orcus and to try to pull out the right name.

You can send suggestions as comments to these pages (www.mikebrowns planets.com in case you are reading this elsewhere) or simply email me (mbrown@caltech.edu ), but please put “Orcus” in the subject line so I don’t mistake you for a potentially business partner from Nigeria.


Good luck. S/1 90482 (2005) is counting on you.