New Horizons continues on its inexorable way to Pluto/Charon, now some 21 AU out, which places it between the orbits of Uranus and Neptune. The latest report from principal investigator Alan Stern tells us that the 2011 checkout of the spacecraft was completed on July 1, a two-month process that included a test of the REX radio occultation experiment, coordinating with the Deep Space Network as the Moon interrupted a radio signal from Earth. According to Stern, spacecraft tracking over May and June shows New Horizons on a ‘perfect course’ toward the distant world, one that will demand no course correction until, at the earliest, 2013.

I wanted to bring Stern’s report into play here because of the image below, which shows Pluto’s newly discovered moon P4 along with the other moons now known in the system. The fact that I hadn’t yet run it told me that it was time to do some catching up with this impressive mission.

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. 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. Credit: NASA, ESA, and M. Showalter (SETI Institute)

P4 (the name is temporary) was found during a search for rings around Pluto. At an estimated diameter of 13 to 34 kilometers, it’s the smallest moon yet discovered in this system. Obviously, the more we learn about Pluto/Charon, the better the New Horizons team will be able to plan for its brief period of close up observations. And it’s likely we’ll find still more tiny moons in a system that is thought to have been formed by a collision between Pluto and another large body in the early Solar System. P4 was found with Hubble’s Wide Field Camera 3 on June 28 and later confirmed through subsequent imagery. Still no signs of any Plutonian rings, however.

Meanwhile, John Spencer, a member of the New Horizons mission science team, has posted an interesting look at the effort to find a Kuiper Belt object that New Horizons will study after the encounter with Pluto/Charon. We’ve talked before about the Ice Hunters project, where volunteers can help pursue the search using the power of networked computers at home. Hunter now reports on his trip to Hawaii, which provided the chance to work with the Subaru telescope on the summit of Mauna Kea, an experience would-be astronomers can only envy. Spencer talks about recapturing ‘the romance of the old way of connecting with the universe,’ something many working astronomers seldom do these days, and describes the trip to the top:

After a night and a day of acclimatization, adjusting our bodies to the thin air, we climbed into 4WD vehicles and made the half-hour drive to the summit as sunset approached. It is always an amazing transition from the relative domesticity of Hale Pohaku and its mamane trees to the vast, alien, apparently lifeless landscape of the summit and its giant telescopes. This was the first time at Subaru for some in our group, so Josh Williams, the telescope operator, gave us a quick tour of the darkened, cathedral-like space of the dome, almost filled by the huge bulk of the telescope with its 8-meter diameter mirror. We also made quick trip around the catwalk outside the dome, to admire the fabulous view, before returning to the warmth and comfort of the control room, where we were to spend the night.

The work involved calibration observations, warm-up tests using near-Earth asteoids, and finally the acquisition of the images that might lead to KBO finds. But toward the end of the observing period, a broken coolant hose ended operations (and kept Subaru down for another three weeks). Spencer says the team left with 70 percent of what they were there for, in any case, and his laptop hard disk returned from the journey with data that will appear soon on Ice Hunters.

Image: The Subaru dome (left) is silhouetted by the Milky Way, as the telescope searches for KBOs. The search area is among the star clouds in the upper left of the image. (Credit: John Spencer).

Whether or not New Horizons gets to make that flyby of a distant Kuiper Belt object depends upon a number of things, among them NASA approval of an extended mission (it’s hard to see how this could be turned down given the rarity of our getting a spacecraft this far from the Sun), as well as the discovery of an appropriate KBO. The New Horizons team is looking for an object at least 50 kilometers across for a flyby and high resolution imagery, as well as spectroscopic investigations and study of possible moons or traces of an atmosphere. I can only echo Spencer’s invitation for those of you who haven’t yet done so to join the Ice Hunters search today. Remember, although we’ve found more than 1000 objects beyond Neptune’s orbit, it’s estimated that there may be half a million objects bigger than 30 kilometers across out there.