Hubble spots fourth moon of Pluto


The Hubble Space Telescope has been looking at the twin dwarf planet system of Pluto and Charon ahead of Nasa’s New Horizons mission expected arrival in the neighbourhood in 2015.  They’ve already mapped the surface of Pluto, to an extent, and now they’ve spotted a fourth moon in the system– if you count the dwarf planet Charon as a moon of Pluto, that is.  The new moon has been designated “S/2011 P1” or “P4” until a permanent name is chosen.  [All images credit: NASA, ESA, and M. Showalter (SETI Institute)]

From the Hubble site press release

The new moon is located between the orbits of Nix and Hydra, which Hubble discovered in 2005. Charon was discovered in 1978 at the U.S. Naval Observatory and first resolved using Hubble in 1990 as a separate body from Pluto.

The dwarf planet’s entire moon system is believed to have formed by a collision between Pluto and another planet-sized body early in the history of the solar system. The smashup flung material into orbit around Pluto, which then coalesced into the family of satellites now seen.

Lunar rocks returned to Earth from the Apollo missions led to the theory that our Moon was the result of a similar collision between Earth and a Mars-sized body 4.4 billion years ago. Scientists believe material blasted off Pluto’s moons by micrometeoroid impacts may form rings around the dwarf planet, but the Hubble photographs have not detected any so far.



Here’s the associated text for the above image

These two images, taken about a week apart by NASA’s Hubble Space Telescope, show four moons orbiting the distant, icy dwarf planet Pluto. The green circle in both snapshots marks the newly discovered moon, temporarily dubbed P4, found by Hubble in June.

P4 is the smallest moon yet found around Pluto, with an estimated diameter of 8 to 21 miles (13 to 34 km). By comparison, Pluto’s largest moon Charon is 746 miles (1,200 km) across. Nix and Hydra are roughly 20 to 70 miles (32 to 113 km) wide.

The new moon lies between the orbits of Nix and Hydra, two satellites discovered by Hubble in 2005. It completes an orbit around Pluto roughly every 31 days.

The moon was first seen in a photo taken with Hubble’s Wide Field Camera 3 on June 28, 2011. The sighting was confirmed in follow-up Hubble observations taken July 3 and July 18.

P4, Nix, and Hydra are so small and so faint that scientists combined short and long exposures to create this image of Pluto and its entire moon system. The speckled background is camera “noise” produced during the long exposures. The linear features are imaging artifacts.

The tiny satellite was uncovered in a Hubble survey to search for rings around the frigid dwarf planet. The observations will help NASA’s New Horizons mission, scheduled to fly through the Pluto system in 2015.

Here’s the Hubble image of the Pluto system taken on 28 June 2011


And the corresponding image from 3 July 2011

By way of comparison the proto planet, Vesta, where Dawn is orbiting, is around 530km wide.

And an illustration of the known moons’ relative orbits at Pluto [Image credit: NASA, ESA, and A. Feild (STScI)]


This illustration shows the relative orbits of Pluto’s moons: Charon, Nix, P4, and Hydra. The orbits are nearly circular and tilted to our line of sight, so they appear elongated.

As the New Horizons mission press release notes

Aside from giving the New Horizons team something to plan for and look forward to, the discovery offers a peek into Pluto’s violent past. “The discovery of this moon reinforces the idea that the Pluto system was formed during a massive collision 4.6 billion years ago,” says Weaver. “The smaller satellites, including this one, probably came together in the resulting debris disk.”

This discovery also suggests other small bodies may lurk in the Pluto system – and, if so, New Horizons should root them out when it flies by. But more “targets” might not be the best thing for the mission.“The discovery of P4 is exciting, but it also raises the possibility that our New Horizons spacecraft may enter a more hostile environment than we previously imagined,” says Glen Fountain, New Horizons project manager from APL.

“We don’t want our spacecraft running into any debris that’s still hanging around from the massive collision that spawned the formation of Pluto’s smaller satellites,” adds Stern. “For this reason we’re going to look with New Horizons, and even before we get there, we’ll be looking with Hubble and other tools, we hope.”


And who knows what the James Webb Space Telescope will see?

Assuming it eventually gets off the ground…

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