P–21 days: Navigating Pluto

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Continuing the countdown, focusing on optical navigation using images of Pluto and the satellites.

As of midnight this morning of June 23rd, Eastern time, we’re 25.4 million kilometers from Pluto and moving 1.2 million kilometers closer every day.

The whole thing comes in a rush; time is not a leisurely independent quantity here—it’s relentlessly urgent! Another maneuver approaches fast: TCM17B1 executes in seven days, the next-to-last opportunity to correct errors before the flyby, and Navigation needs to come up with the “correct” trajectory ahead of that to support its design and implementation, a process that will take most of that seven days.

They need answers now! Both Navigation Teams—PNav (Project Nav) and INav (Independent Nav)—are busting their butts to figure out what’s going to happen in the target plane—that big Plutonian dart board in the sky. The error ellipses shrink down slowly as we get closer, but the solutions wander around, some of them inside the previous error ellipses, some out, depending on what combinations of optical data are used and a host of other assumptions, all in the form of experiments to figure out the most realistic answer. How many toad tongues and bat wings do we need to add for a brew that is true? (With apologies to Danny Kaye and The Court Jester; I couldn’t resist.)

Ideally new solutions wander around inside all the older ellipses as the newer ones shrink down around them. Lately INav has been holding fairly steady, but PNav dithers around, sometimes in, sometimes out of the ellipses computed only a day or so earlier. The game right now is to figure out which OpNav (Optical Navigation) modeling is correct, and which misleading. Pluto itself seems to be the problem.

Images of Pluto, that is. Since we’ve never seen this Honorary Planet up close before, extracting an accurate estimate of its center from fuzzy optical data is as much art as science at the moment, and nobody will know what the real answer was until we actually get there and know all the Plutonian “blemishes” in detail; the craters and icy plains and mountains and scarps, and whatever else we find.

However, at this red-hot-moment, they are only just beginning to pop out in sharper detail, the dark regions and the light, and in order to realistically extract the center of a 2400 kilometer wide body to within a few tens of kilometers, you have to model the blemishes with an accurate “albedo map.” The problem is, the details pop out faster than you can model them, so you play a catch-up game with the Pluto images, and if the current albedo map is behind the times, so is your estimate of where we’re

Plutonian Albedo Map

Plutonian Albedo Map

going in the target plane. There are other models for center-finding too, and they’re also in the mix of experiments, all of them throws-of-the-dart to see where we come up relative to Pluto—all of them to see which ones we believe.

INav seems to have avoided the problem up to now by the simple expedient of not using Pluto images at all and relying on images of its companion, Charon. Because Charon is smaller, it has smaller center-finding errors. PNav is coming around to the same conclusion: Don’t trust Pluto!

Fortunately, the smaller satellites Nix and Hydra are beginning to pop out of the optical wood-works, and because they are little more than specs in the images, mere pin-pricks of light, they will be much better located against the starry sky. After a while tinier Styx and Kerberos should become usable too. Ironically, the best information of where we’re going relative to Pluto is going to come from those satellite pin-pricks in the images, and not from Pluto itself.

Meanwhile, I depart for Maryland Monday to become a small part of this Plutonian melee with whatever humble capacity I can bring—it’s been a long time since I’ve done this kind of interplanetary navigation, and the learning curve is extremely steep and precipitous.

And meanwhile, nobody can stop the relentless march of time as Pluto expands in our vision and the activities grow more urgent. The pressure is on.

May_12_Pluto

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