Planet or Not…

Pluto Has Moons

The Pluto system. Courtesy STScI/NASA/ESA

That distant world called Pluto has surprised astronomers again, yielding up yet another moon.  Pluto’s largest moon is Charon and was discovered in 1978.  Two more — Nix and Hydra — were found in 2005. The new one, called P4 (for now), is quite small, somewhere between 13 to 34 kilometers across, and small enough that it was probably missed in earlier images of the system taken by Hubble Space Telescope. This latest HST image was taken as part of  a search for ring material around the distant dwarf planet, in support of the New Horizons mission, which is en route to Pluto.

So, how would Pluto, itself a small world like many others in the outer solar system, get moons?  The current thinking is that a collision between Pluto and another world early in the history of the solar system would have flung material out into orbit. Eventually, the pieces and parts would have coalesced back together, forming the family of moons we see today.

When I read this story, the first things I wondered were “Why search for rings around Pluto?”  and “Where would the material for Plutonian ringlets come from?”  A long-ago collision would have provided material for rings, but by now, that material would have been cleared away or coalesced into moons, such as Nix, Hydra, P4 (and maybe even Charon?).  To maintain a ring system, you need a constant source of material being tossed out to space.  At Pluto, that source may well be material “chipped away” from the icy surface by the impacts of tiny micrometeoroids.  That would provide chips of ice to form a faint, thin ring. If it exists, it hasn’t yet been detected. But, HST would be the best instrument we have at this time to find the ring.  Once New Horizons gets there, it may well “see” the ring, if it exists.

I like it when HST finds things like this. It’s a continuing reminder that the venerable telescope has a lot of life in it yet; and will keep surprising astronomers with new finds.

Back to the Moon

In Stereo

ARTEMIS P2 Enters Lunar Orbit

Well, this is kinda cool. NASA has taken two satellites that would have been shut down in 2010 and put them in orbit around the Moon to give us a continual up-close-and-personal view of the  lunar surface from about 60 miles away. The spacecraft, called the Aceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS) probes, arrived at their lunar orbits on June 27th and July 17th, respectively. These twins were once in different areas of near-Earth space, part of a five-spacecraft system called THEMIS (Time History of Events and Macroscale Interactions during Substorms).  Along with three other probes, these two spacecraft  studied the solar wind, Earth’s outer magnetic field, and how the two interacted with each other.

Over a period of time, scientists maneuvered this pair of satellites from their original Lagrangian point orbits into places closer to the Moon. It’s a very neat re-use of space hardware that otherwise would have been shut down. The THEMIS mission itself is continuing — the other three THEMIS probes continue their original science mission, studying the substorms that are part of its names. These are atmospheric events visible  near Earth’s poles as sudden increases in the brightness of the aurorae. The findings from the mission may help protect commercial satellites and humans in space from the adverse effects of particle radiation.

So, what kind of science will the two diverted ARTEMIS babies do at the Moon?  Given that these spacecraft bear instruments that are sensitive to magnetic fields, they should be able to collect data about the very weak lunar magnetic fields that DO exist, provide information about the  lunar core (which does not appear to be generating a magnetic field), and information about any pockets of magnetism that might exist in the Moon’s crust (outer layer). Essentially, ARTEMIS will probe the Moon’s magnetic environment.  The data it gets will help scientists understand more about the interior structure of the Moon.  This is a very cost-effective way to do further lunar science, and it will be interesting to see what the next five to seven years of ARTEMIS efforts uncover.