When I was a boy, I became fascinated early on with the outer planets. The further out, the better as far as I was concerned, and as you might imagine, I had a special fascination with Pluto. In the summer, I used to haunt the library in the nearby suburb of Kirkwood (in St. Louis, where I grew up), working my way through all the books on astronomy and space I could find. Because I was reading all of them, I would encounter older volumes, some pre-dating the discovery of Pluto, and more recent tomes with details about the planet I didn’t know. It didn’t matter; I just kept reading.
What was fun about all this was that I kept expecting to find something new each time I opened a book, and was sometimes rewarded with a fact that brought this distant realm into perspective. The news that Hubble has now found a fifth moon orbiting Pluto awakens that same sense of satisfaction, for as we keep tuning up our observing skills, we’re learning much about the outer system that surprises us. The fact that this tiny dwarf planet — I would prefer to think of it as a ‘double planet’ more than a ‘dwarf’ — has such an elaborate set of moons is unexpected.
Image: Pluto’s newly discovered moon P5 (circled). Researchers are intrigued that such a small planet can have such a complex collection of satellites. The new discovery provides additional clues for unraveling how the Pluto system formed and evolved. The favored theory is that all the moons are relics of a collision between Pluto and another large Kuiper belt object billions of years ago. Credit: NASA, ESA, and M. Showalter (SETI Institute).
The existence of P5 is another useful piece of information for the New Horizons team as their spacecraft streaks toward its 2015 encounter at Pluto/Charon. The moon, currently designated P5, looks to be between 10 and 25 kilometers across, residing in a 93000 kilometer circular orbit that is evidently co-planar with the other four satellites. With five moons, Pluto is likely home to a good deal more debris that we haven’t yet found, a factor in working out the safest trajectory for the spacecraft. We’ll be watching Pluto carefully up to and beyond the New Horizons encounter.
A Not So Quiet Hibernation
Meanwhile, New Horizons, the hero of this piece, continues its relentless journey, now almost 24 times as far from the Sun as our own planet and once again in a state of hibernation. The ‘deep cruise’ phase of the mission lasts until the encounter operations begin to accelerate in the summer of 2014, with closest approach to Pluto on July 14, 2015. Even though most of its subsystems, including science instruments and flight electronics, are turned off, the Student Dust Counter (SDC) is to be left on during the hibernation period, measuring dust impacts as the spacecraft pushes deeper into the system than any dust detector has ever been sent before.
Yes, the Voyagers can measure dust impacts, but they do so with their plasma wave instrument rather than through an actual dust detector. Both spacecraft have been detecting micron-sized impacts for years, noted because the impact of a dust particle causes it to be vaporized and heated to a plasma of electrons and ions. The resulting plasma cloud creates a voltage pulse in the plasma wave receiver. Researchers can count the impacts over a period of time and learn much about the density of the impacting particles in the outer interplanetary medium.
We need to learn about dust in the outer system, of course, because as we push deeper and move faster small impacts become much larger factors, risking the very life of the mission if we’re talking about interstellar flight speeds. Interestingly, the Voyager dust measurements show the number density of impact particles to be more or less the same for both Voyager 1 and 2, a noteworthy item given that Voyager 2 is much closer to the ecliptic than Voyager 1. That suggests the outer system dust both spacecraft are running into is likely cometary in origin, or at least that it does not originate from a planetary source.
But back to New Horizons. The latest report from the spacecraft team now tells us that the Solar Wind Around Pluto (SWAP) and Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instruments will also remain active during hibernation, representing a major upgrade from the instruments on the Pioneer and Voyager spacecraft. The report quotes Matthew Hill (JHU/APL) on their ability to measure charged particle radiation from the solar wind and elsewhere:
“It’s been more than 30 years since we’ve had a spacecraft venture beyond Saturn, and it’s the first time we’ve had observations from this region while having supporting measurements both farther out [from Voyager 1 and 2] and closer to the Sun [missions at Mercury, Earth and Saturn]. Events associated with solar flares and coronal mass ejections that propagate through the solar wind plasma can now be observed throughout the heliosphere as never before. With solar activity on the rise, the timing is great to have these state-of-the-art New Horizons instruments observing the heliosphere.”
The report is calling the new observations ‘enhanced science’ — the use of SWAP and PEPSSI during hibernation had not been part of the original mission schedule — and the hope is that it will provide a welcome additional dataset as we continue our study of the heliosphere. With the Pluto/Charon encounter ahead, we have much to look forward to, but even after the encounter, we’ll eventually be using the James Webb Space Telescope’s infrared capabilities to probe this icy realm. Who knows how many more moons of Pluto, or Kuiper Belt bodies beyond, we’ll have uncovered by then?