Explanation: Nix and Hydra were first introduced to human eyes in Hubble Space Telescope images from May 2005, as Pluto's second and third known moons. Now Hubble images have revealed a fourth satellite for the icy, dwarf planet. Provisionally designated P4, it completes an orbit of Pluto in about 31 days. Presently Pluto's smallest and dimmest known moon, P4 is estimated to be 13 to 34 kilometers across. The newly discovered satellite was first spotted in Hubble observations from June 28, and later confirmed in a follow-up on July 3 and July 18. These two panels are composites of both the short and long exposures that include brighter Pluto itself along with Pluto's largest moon Charon. Camera noise and image artifacts also show up in the long exposure segments. The Hubble observations were made while searching for faint rings around the distant world in support of NASA's New Horizons mission, set to fly by the Pluto system in 2015.
Pluto was the god of the underworld in Roman mythology. Charon, Pluto's largest moon, is named after the ferryman who carried the souls of the newly deceased across the River Styx, which divided the world of the living from that of the dead. Nix, Pluto's second moon, was the Greek goddess of darkness and night, and Charon's mother. Hydra, the third, was a many-headed serpent that guarded a back entrance to the underworld located deep below the surface of a lake.
...
In choosing a name along these hellish lines for P4, ... One name that seems to come up most is Cerberus, the dog who guards the gates to hell. Three-headed Cerberus is Hydra's sibling.
I will never forgive them for taking away Pluto's planet status. If it has 4 moons it should qualify as a planet. Why is that not a criteria for a planet: to have
moons? Anything further out is Kuyper belt or Oort cloud, further in are only asteroids or other moons. I will always consider Pluto a planet.
George V. Edwards
Re: APOD: Pluto's P4 (2011 Jul 22)
Posted: Fri Jul 22, 2011 8:35 am
by 500pesos
Nix and Hydra look so much bigger than Charon in these photos, how come Charon is the larest moon of Pluto?
Re: APOD: Pluto's P4 (2011 Jul 22)
Posted: Fri Jul 22, 2011 10:23 am
by neufer
500pesos wrote:Nix and Hydra look so much bigger than Charon in these photos, how come Charon is the larest moon of Pluto?
Everything in the vertical middle dark strip (i.e., Pluto & Charon and their diffraction spikes) is greatly reduced in brightness in order that we may see the dim moons.
Re: APOD: Pluto's P4 (2011 Jul 22)
Posted: Fri Jul 22, 2011 12:10 pm
by biddie67
What a Mighty Mite is our Pluto!! 4 moons and counting!! You'll show them and will regain your "planet" status the old-fashioned way - by your hard work of collecting and maintaining your moons.
P.S. Once again - many thanks to the HST for another interesting find!!
Re: APOD: Pluto's P4 (2011 Jul 22)
Posted: Fri Jul 22, 2011 12:50 pm
by orin stepanek
I too; like to think of Pluto as a planet! Is Charon a moon? http://www.wrongplanet.net/postt100501.html If Charon and Pluto are deemed a double dwarf; than there are only 3 moons in the system! It makes for an interesting situation to be resolved.
Re: APOD: Pluto's P4 (2011 Jul 22)
Posted: Fri Jul 22, 2011 12:59 pm
by Planet Guy
As if we needed any more evidence that Pluto is a planet! Sheesh. This reminds me of a poem I read in the Washington Post:
Twinkle, Twinkle, Planetoid
Twinkle, twinkle, planetoid
Out so far in inky void
Rocky core with ice encloaked
Your planethood has been revoked
The I.A.U. struck a blow
To the cosmic status quo
They're not quite sure of your fate
But it leaves us only eight
Was it that your orbit's tilted
Why you were so rudely jilted?
Could it be your little tryst
With Neptune that's got them pissed?
Despite the fact you have four moons
Your reputation they impugn
But take some comfort in their crime
They'll all be dead in one year's time!*
Twinkle, twinkle, far from sun
So long for now, it's been fun
*One year on Pluto is 248 Earth years
(I took some liberty with the poem, as it originally said "a moon."
Re: APOD: Pluto's P4 (2011 Jul 22)
Posted: Fri Jul 22, 2011 1:09 pm
by Matt
For those that think Pluto should be counted as a planet - Don't forget that they once thought that Ceres was a planet too, until they found others in the same region. Pluto is the same way. There's lots of other rocky/icy bodies way out there that kind of dilute the planet status. Just sayin'...
Re: APOD: Pluto's P4 (2011 Jul 22)
Posted: Fri Jul 22, 2011 1:23 pm
by ExplorerAtHeart
Forever a dwarf planet.
Re: APOD: Pluto's P4 (2011 Jul 22)
Posted: Fri Jul 22, 2011 1:49 pm
by BMAONE23
Just a few more moons and they could be renamed
Dopey
Sneezy
Happy
Sleepy
Grumpy
Bashful &
Doc
Then it truely would be a Dwarf Planet
Click to play embedded YouTube video.
Re: APOD: Pluto's P4 (2011 Jul 22)
Posted: Fri Jul 22, 2011 2:08 pm
by hpfeil
It never ceases to amaze me that so much information is deduced from faint specks on three CCD images.
Does anyone know enough detail to tell me the angle of Pluto's moon plane relative to Earth? I wouldn't necessarily expect all Pluto's moons to line up like the Jovians appear to do from Earth, but it really looks like we've got a top-down view, which is much more than I would expect.
Re: APOD: Pluto's P4 (2011 Jul 22)
Posted: Fri Jul 22, 2011 2:28 pm
by BMAONE23
DrRocketAF wrote:Does anyone know enough detail to tell me the angle of Pluto's moon plane relative to Earth? I wouldn't necessarily expect all Pluto's moons to line up like the Jovians appear to do from Earth, but it really looks like we've got a top-down view, which is much more than I would expect.
Unlike most planets, but similar to Uranus, Pluto rotates with its poles almost in its orbital plane. Pluto's rotational axis is tipped 122 degrees.
Interestingly though, according to this same site, Pluto is currently oriented with it's equator pointed at us.
Re: APOD: Pluto's P4 (2011 Jul 22)
Posted: Fri Jul 22, 2011 2:31 pm
by ExplorerAtHeart
BMAONE23 wrote:Just a few more moons and they could be renamed
Dopey
Sneezy
Happy
Sleepy
Grumpy
Bashful &
Doc
Then it truely would be a Dwarf Planet
Click to play embedded YouTube video.
Totally
Re: APOD: Pluto's P4 (2011 Jul 22)
Posted: Fri Jul 22, 2011 2:36 pm
by Chris Peterson
DrRocketAF wrote:Does anyone know enough detail to tell me the angle of Pluto's moon plane relative to Earth? I wouldn't necessarily expect all Pluto's moons to line up like the Jovians appear to do from Earth, but it really looks like we've got a top-down view, which is much more than I would expect.
Pluto is in an orbit that is inclined at about 12° to the Sun's equator, and the planet itself is tilted about 120° with respect to its orbital plane. So depending where Pluto is in its orbit, we will see its moons (which are all on Pluto's equatorial plane) from a wide range of viewpoints.
Re: APOD: Pluto's P4 (2011 Jul 22)
Posted: Fri Jul 22, 2011 2:41 pm
by Mad Doc
Now bring back Pluto as a planet catagory that the Astronomy says it (Pluto) is not!
DrRocketAF wrote:
Does anyone know enough detail to tell me the angle of Pluto's moon plane relative to Earth? I wouldn't necessarily expect all Pluto's moons to line up like the Jovians appear to do from Earth, but it really looks like we've got a top-down view, which is much more than I would expect.
Pluto is in an orbit that is inclined at about 12° to the Sun's equator, and the planet itself is tilted about 120° with respect to its orbital plane. So depending where Pluto is in its orbit, we will see its moons (which are all on Pluto's equatorial plane) from a wide range of viewpoints.
Re: APOD: Pluto's P4 (2011 Jul 22)
Posted: Fri Jul 22, 2011 2:54 pm
by Chris Peterson
hpfeil wrote:It never ceases to amaze me that so much information is deduced from faint specks on three CCD images.
Indeed. This is one reason I consider astronomy to be such a pure and elegant science.
Of course, the deductions made from these images are not made in isolation, but depend on a good deal of well developed theory. These images provide directly the apparent brightness of each object, which can be used (with some reasonable assumptions about albedo) to estimate size, and they provide positions with respect to time, which using Keplerian orbit theory allows for the extremely precise determination of orbital elements. With a few more images, intensity versus time data is also extractable, which can produce solid information about the rotation of each body. All, as you say, from specks on a few CCD images. Beautiful.
Re: APOD: Pluto's P4 (2011 Jul 22)
Posted: Fri Jul 22, 2011 3:00 pm
by Chris Peterson
Mad Doc wrote:Now bring back Pluto as a planet catagory that the Astronomy says it (Pluto) is not!
I don't know what "Astronomy" is in this context.
Pluto is still a "planet" as most people use the word, and still a "planet" to most astronomers. The IAU (which certainly has no official standing to define any words, but merely makes recommendations for usage by professional astronomers) has produced a convoluted definition for "planet" which is illogical and ambiguous in the case of Pluto (how can a "dwarf planet" not also be a "planet"?) But there is nothing stopping you, or anybody else, from calling Pluto a planet. I certainly do, and I call it a planet when I discuss the matter with my students.
Given the inadequacies of the current IAU definition, it is nearly certain that we'll see additional, probably substantial changes in the future.
Re: APOD: Pluto's P4 (2011 Jul 22)
Posted: Fri Jul 22, 2011 3:18 pm
by bystander
BMAONE23 wrote:Just a few more moons and they could be renamed
Dopey Sneezy Happy Sleepy Grumpy Bashful & Doc
Then it truely would be a Dwarf Planet
Ahhh, but if the that were the moons' names, Pluto would become Snow White and would no longer be a dwarf.
I still like Clyde as P4's name.
Re: APOD: Pluto's P4 (2011 Jul 22)
Posted: Fri Jul 22, 2011 3:56 pm
by orin stepanek
bystander wrote:
BMAONE23 wrote:Just a few more moons and they could be renamed
Dopey Sneezy Happy Sleepy Grumpy Bashful & Doc
Then it truely would be a Dwarf Planet
Ahhh, but if the that were the moons' names, Pluto would become Snow White and would no longer be a dwarf.
I still like Clyde as P4's name.
Pluto hung around with Mickey and Donald a lot; and he had a couple of girlfriends. Dinah and Fifi! Those would be good names for P4!
Re: APOD: Pluto's P4 (2011 Jul 22)
Posted: Fri Jul 22, 2011 4:55 pm
by Leon1949Green
Wonderful pictures, and discussions. I shall defy the IAU and call Pluto a planet. And I have to think about it for a while, but changing it from a hellish name to the heavenly one of Snow White, with her dwarfs for moon names is worth considering.
Re: APOD: Pluto's P4 (2011 Jul 22)
Posted: Fri Jul 22, 2011 5:00 pm
by jerrythebiker
I am looking forward to July 2015 to see what other wonderful discoveries are made by New Horizons. Just think of the other discoveries that might be shared on APOD between now and then. Four years worth! Keep on truckin' APOD!
Leon1949Green wrote:
And I have to think about it for a while, but changing it from a hellish name to the heavenly one of Snow White, with her dwarfs for moon names is worth considering.
Pluto's P4
Is Charon a moon? 
