Artist illustration of Pluto (centre) from one of its small moons. The largest moon Charon is on the right. Image credit: NASA, ESA and G. Bacon (STScI)

May 26, 2015 New Horizons eyes Pluto's known moons, but could find more

Sen—NASA's New Horizons spacecraft is zeroing on in the Pluto system. With the dwarf planet's small gravitational pull and the spacecraft not equipped with enough fuel to enter into orbit, New Horizons will only be making a short stay, passing through with its closest approach to Pluto on July 14th. Now, as the fastest spacecraft ever launched edges closer to Pluto each day, New Horizons is taking the best family portraits we've ever had of the dwarf planet and its family of icy satellites.

There are currently five moons known to orbit Pluto. In order roughly from largest to smallest: Charon, Hydra, Nix, Styx, and Kerberos. All have been named after Greek and Roman underworld mythological references, befitting companions to the Roman deity of the Underworld.

The largest moon Charon was discovered nearly 40 years ago in 1978 by a ground-based telescope. Charon is nearly half the size of Pluto. Charon is sufficiently massive that Pluto and Charon orbit their common barycenter center outside of Pluto (you can see Pluto's wobble in the LORRI animation resolving Pluto and Charon—you see that both Pluto and Charon are locked in a dance about an invisible center point). This is why Pluto/Charon has at times been referred to as a binary system or binary planet.


New Horizons observations of Pluto and its largest moon Charon showing their orbits about an invisible mutual center of mass taken with the LORRI camera. Image credit: NASA/APL/Southwest Research Institute

As New Horizon's zooms towards Pluto, the probe's Long Range Reconnaissance Imager (LORRI) is getting sharper and sharper images of the host of moons that orbit the dwarf planet.

Last week, the team released the first image and animations of four of Pluto's smaller satellites. The rest of Pluto's moons are more like ice chips compared to Charon. Nix, Hydra, Styx, and Kerberos were found in the past with deep imaging using the spatial resolving power of the Hubble Space Telescope (HST). These smaller satellites are estimated to range in size from a few tens of kilometers to a few hundred kilometers in size.


Orignal and procssed images from New Horizons showing the orbits and locations of Pluto's Moons captured by New Horizons and the LORRI instrument. Image Credit: NASA/Johns Hopkins APL/Southwest Research Institute

Earlier on in the Solar System's history, it is thought that Pluto suffered a giant impact with Charon being formed mostly from the impactor. The smaller satellites likely coalesced from the icy debris ejected into a disk orbiting about Pluto.

Pluto isn't the only Kuiper belt object thought to have suffered such a violent fate. There is evidence that impacts were common for the largest of the bodies residing in the Kuiper belt, the sea of icy planetesimals residing in orbit beyond Neptune.

The majority of the dwarf planet-sized bodies including Haumea, Eris, Quaoar, and Orcus have tiny moons discovered by adaptive optics on the largest optical telescopes and by the HST. It is very likely that these tiny moons are chunks of material excavated off their parent bodies during an impact and are now left in orbit around them.

Dwarf planet Haumea is thought to have had a mantle shattering collision that left its rocky core spinning nearly at breakup speed with just a thin veneer of water ice covering its surface. The rest of Haumea's original icy outer shell was ejected into the Kuiper belt and formed the two small icy moons that orbit the elongated dwarf planet, as well as a host of other fragments in what makes up the first observationally detected collisional family in the Kuiper belt.

With New Horizons, the aftermath after the Pluto moon-forming collision will be the best studied in the Kuiper belt. New Horizons will help test and improve this collisional theory for the origin of Pluto's moons, by refining the orbits of the known moons and, closer to the encounter, giving better estimates of their sizes and hopefully even their composition.

The five moons are the ones we know about, but New Horizons will provide us with the best view of the Pluto system, getting beyond HST resolution over the coming months to provide the best vantage point to find any remaining bodies and debris orbiting the dwarf planet.

Starting earlier this month, each week New Horizons is now imaging to look for additional undiscovered moons and rings that might be orbiting about Pluto. To give you an idea how challenging a task this has been, you can look back to the discoveries of Nix, Hydra, Styx, and Kereberos. Nix and Hydra were discovered in HST observations a decade ago, but the smaller Kereberos and Styx were only found three and four years ago respectively.

Additional moons or rings of smaller particles could be a reality. In a paper recently submitted to Astrophysical Journal Letters, Simon Porter, a member of the New Horizons science team, and Alan Stern, Principal Investigator (PI) of the mission, used computer simulations to examine the locations where additional moons could be lurking in the Pluto system and be stable over the age of the Solar System. They found that there are dynamically stable regions where additional satellites and debris could safely reside interior to the orbit of Styx and also between the orbits of Nix and Kerberos as well as between the orbits of Styx and Nix.

There is another reason to look for these moons and ringlets. The safety of New Horizons during its voyage through the Pluto system may depend on it. Encountering dust or ice fragments a few millimeters wide could severely damage the spacecraft at the speed the spacecraft is moving (about 15 km per second). The spacecraft’s instruments and computers are actually shrouded in Kevlar (the material used to make bullet proof vests) as a layer of protection. Athough a collision with a moon is highly unlikely, the greater concern is dust and small particles hitting a critical component on the spacecraft.

More smaller moons and moonlets could be signs of lots of dust in the system produced by micrometeorite impacts on the moons. Eventually that material would land on the surfaces of Charon or Pluto, but there is still a small chance that there could be unseen moons shepherding these particles into a ring of material. It's very likely the safety of the spacecraft is not in jeopardy, but if the Pluto system is found to be too debris littered the New Horizons team has back up plans and alternate trajectories that can be used if necessary to protect the spacecraft during the encounter.

Whether additional small moons and rings exist, it's important both for the science and the mission to find them before July.