#postcards from pluto

Happy New Year!  Welcome to 2015, the year of Pluto!  It’s hard to believe that we’re so close to encounter time!  Things are going to be picking up quite a bit around here.

  So, today I just got back from holiday break with my family back east and found a nice holiday present from my boss, New Horizons Principal Investigator Alan Stern sitting on my desk.

  The note says that it’s a Pluto stress ball, sold at Lowell Observatory in Flagstaff, Arizona (where Pluto was discovered in 1930 and well worth a visit).  Now, I know complaining about Christmas gifts is pretty rude, but this is not a very accurate depiction of Pluto.  Let’s talk about why.

  Firstly, the stress ball is the wrong color.  While we can’t yet see Pluto clearly, indirect mapping and spectroscopy tell us that it’s reddish in color and has bright and dark patches.  This stress ball is gray and the same color and brightness all around, like, well, Charon.

  The stress ball is also covered in craters.  We don’t know if Pluto has craters or not. It might have some, or there might be frequent resurfacing, and Pluto might be full of mountains, canyons and plumes.

  There are three major types of craters: simplex, complex and multi-ring basins.  Which one you see depends on how big the thing that crashed was.  Small impactors make simple, bowl shaped craters.  If the impactor is big enough, the crater will be more pie-pan shaped and there will be a peak in the middle, and that’s a complex crater.  A humongous collision will make a multi-ring basin, like Orientale on the Moon.  All of the craters on the stress ball are simple, and some of them are, well, a bit big to be simple craters. 

  In general, the simple craters are about 5 times wider across than they are deep, and complex craters and basins are even shallower. Based on what we've seen on moons of Saturn and Uranus, we predict that Pluto's craters would be 10 times wider than they are across before they get shallower too.  Based on highly scientific measurements from sticking my pencil eraser in some crater and measuring them with the ruler on my protractor, I’ve determined that these craters are only 3-4 times wider than they are deep, in other words, too deep!  Making craters deeper requires different surface properties, and so perhaps Pluto and Charon are actually made of squishy foam?

  The craters are also very well preserved.  Alan, fellow postdoc Simon Porter and I have written a paper predicting what types of craters on Pluto and Charon we’ll see and what different scenarios will tell us about the makeup of Pluto’s surface and the population of objects available to hit them. 

We find that escape erosion (the idea that Pluto is losing its surface gradually to space) and relaxation (due to meltier nitrogen at Pluto temperatures) might cause Pluto’s craters to either shrink, flatten or disappear altogether in just a short amount of time.  This won’t happen on Charon because it’s covered in rock-hard water ice (I explain why the two surfaces are so different here).  If something hits Charon and takes a big chunk out of it, unless there’s tectonic activity on Charon or something, that big hole in the ground will be there forever.

  Actually, the idea that there’s a big chunk missing out of Charon isn’t unheard of.  In 2005, Charon passed in front of a bright star (aka a stellar occultation), and folks from the MIT-Williams occultation group caught it from several sites (I used to work for this collaboration, but this was before my time). There were some anomalies that were consistent with either a timing discrepancy between measurements from the SOAR and Gemini Telescopes (check out Figure 2), or there’s a large, disfiguring crater on Charon that makes it not otherwise round.

Personally, I think it’s a timing discrepancy, but we’ll be able to track where that spot was on Charon and learn the answer soon.

Hi ddbeck,

PT2 and PT3 are more desirable than PT1 simply because they’re brighter.  Brighter means bigger, which means more science.  Brighter means we can see the object sooner with LORRI.  Brighter means the residuals on orbit fits will be smaller, and we will waste less fuel targeting the object and correcting our course.  However, we do not have enough information yet to know if either PT2 or PT3 will be reachable.  PT3 looks promising and PT2 is still possible, but not promising, and that’s the main purpose of our follow-up observations at present.  We discovered PT2 and PT3 over a month after finding PT1, and the time between the first and last observations (“arc length”) is the major factor that dictates how well we can predict an object’s orbit.

The encounter with PT1 would also happen during the winter.  The sun will interfere with communications only for a few days in January (not an encounter killer at all, but still sub-optimal). We could avoid that problem by going to the other objects.  PT3 would have a summer encounter.

We will be following up with all our objects with Hubble, but planning for next year hasn’t happened in earnest.  This week, we just started to get our last set of observations on these objects until spring.  Next summer, when the conditions are most favorable, we plan to try to detect these objects from the ground, but it will take a lot of image stacking and we don’t know if we can do it.  We’ll probably try with some of the big telescopes used for ground-based searching, like Subaru, Gemini and Magellan.

As for physical characteristics, another binary would be great (we can use Kepler’s laws to find the mass of the primary), but none of our potential targets appear to be binary.  That’s not a surprise—these objects are at the limit of detection.  A smaller body orbiting body would be beyond that limit.  I’m told a fast rotator might make imaging more difficult, so we’d like to try to get light curves of these guys. We’re going to try to get colors on the object, but I’m not sure if we’ve decided if we would prefer to see a KBO with an unusual color or a typical one (red, for this type of object).  The team still has more to talk about here.

Even if one of the brighter potential targets turns out to be reachable, we might just go with PT1 anyway.  We’re interested in the idea of looking at some of the KBOs the search team discovered (but we can’t reach) at their closest approach to the spacecraft, and conserving fuel will allow that to happen.

Finally, I can say that the New Horizons team has found three possible post-Pluto encounter targets!  I'm working on a post with more detail, but meanwhile, Emily Lakdawalla over at Planetary Society wrote an excellent article about our new objects (I think she said half of what I was planning to say anyway).

Meanwhile, if you have any questions about the search or our new objects, I would be happy to answer them!