P–33 days: Navigating Pluto

Here’s the second of a series of posts I hope to write from now until P day. Ideally, you should read them chronologically, but alas, that’s not the way blog posts post. Maybe someday I’ll reorder them into an immensely long single article, if I write many more. That shouldn’t be too hard, since there can’t possibly be more than 33, right?

 

Position: About 39 million kilometers from Pluto.
Velocity: 13.8 kilometers per second toward that destination as of this red-hot moment (red-hot moment being a technical term for right now, but don’t let this engineering-speak throw you; it’s a simple concept.)

Right now is 2015/06/12 00:18 GMT, which is June 11th at 8:18 pm for those of you on the east coast, 5:18 pm for the west coasters, and sometime in between for those somewhere in the middle (unless I’m writing this tomorrow instead of today, but then … never mind). Thirty-nine million kilometers is about 24 million miles for the metrically challenged, and 13.8 kilometers/second is a zippy 8.6 miles per second using the tried and true rule-of-thumb conversion factor of 1.6. I’m too lazy and time-challenged to look up the umpteen-decimal-digit conversion, so that’ll have to do.

Whatever the numbers, the arrival time—the big moment—the instant the rubber meets the road—is at the Pluto closest approach time, July 14th, 7:50 am Eastern if all goes as planned. At that red-hot moment, New Horizons, the-little-spacecraft-that-could, will pass about 14 thousand kilometers from the center of Pluto, give or take a tad (another technical term), a lonely 4.8 billion kilometers from home.

Distances in miles and kilometers are pretty meaningless when the numbers are this big, so let’s boil them down by expressing them in terms of the distance from the Sun to the Earth. That’ll be in terms of AU, meaning Astronomical Units. At the moment, we (New Horizons) are 31.7 AU from home, and only 0.3 AU from Pluto. Only one percent left to go. There, isn’t that better?

At this distance, it takes four-and-a-half hours to send a message up to the spacecraft, and another four-and-a-half hours to get the reply back. Not a particularly robust mode of communication if you’re exchanging jokes or having an argument. Even worse if you’re trying to drive a spacecraft going 12 times faster than a speedy bullet. There’s no room in a laboratory somewhere with a big steering wheel and blinking lights in front of an Enterprise style view screen with the spacecraft spring-loaded to make instant changes, like a race car responding to the driver’s twitch of the wrist. No, it’s all done by computer and planned days, months, and sometimes even years in advance.

Yesterday we discussed a maneuver. I use the term we loosely, because basically I listened in on a mission telephone conference between mission engineers and scientists to decide whether or not to do a trajectory correction maneuver this Sunday. I say “listened” and not “participated” because I don’t know enough yet to participate without making a jackass of myself. It’s been nine years since I paid much attention to this mission, being wrapped up in another mission to Mercury—that’s a lot of miles traveled and water under the bridge. Catching up to what the Navigators are doing and how they do it is pretty much like drinking a river: there are so many technical perplexities evolved since launch, it takes weeks to get the brain around them (if ever).

It was a lively discussion on whether to correct the accumulated trajectory errors this Sunday or defer them to a later scheduled time after they’ve grown a bit more—the pros, the cons, and in-betweens—the yeas and nays and furthermores. Reminded me a little of Congress, but without the rancor. After all, we are civilized and rational.

Considering whether to do the maneuver or not, being an old airplane pilot (with de-emphasis on old) I’m philosophically inclined to use an airplane metaphor. When you’re on a long final approach to touchdown, say seven miles out, do you maintain your altitude for as long as you can and then dive for the runway? No! You enter the glide slope and make as many corrections as needed on the way down. Waiting for the inevitable inevitably means you save up all your mistakes for the end, and wind up over-correcting or under-correcting and getting into a PIO, technically known as a Pilot Induced Oscillation to us pilots, quite possibly leading to a PIC, otherwise known as a Pilot Induced Crash.

So much for metaphors. But it’s apt, I think, because I’m inclined to never give up the opportunity of correcting a small known error at this red-hot moment in exchange for a larger error later on, during such a critical final approach to a runway we’ll not have another opportunity to land on in this lifetime.

Anyhow, the maneuver, named TCM16B2 (Trajectory Correction Maneuver umpty-ump) will be performed in the wee hours of Sunday morning, just after midnight. The size is pretty small as these things go, just under two feet per second, slower than you can walk, but it makes up about 760 kilometers error at the target point, and about 84 seconds at the target time.

Beyond this maneuver, there are more opportunities to correct further developing errors at P–20, P–14, and P–10 days. After that … the end. Not enough time to plan and execute more maneuvers. The best that can be done is to continue taking tracking data right up to the last day and use the continually improving knowledge of where we are (and where we’re going) to uplink tweaks to the spacecraft: where to point the cameras and other instruments—and when to snap the pictures.

Will we be successful and take pictures of real things on and about Pluto, or will we get back the equivalent of a thumb over the camera lens: a lot of high-definition pictures of empty space? It will all become clear in 33 days.

PlutoDog2

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