P–24 days: Navigating Pluto



Tools of the Navigation Trade

… wherein we continue the countdown with talk about target-planes, TCMs, trajectory errors, cabbages, and kings.

As of the stroke of midnight early this morning beginning Saturday, June 20th (U.S. Eastern time), New Horizons was about 29 million kilometers from Pluto and closing at 13.8 kilometers per second, or 1.2 million kilometers every day. Since the entire civilized world is metric (excluding the United States, Liberia, and Myanmar), I’ll let the units stand, but if you absolutely have to, you can convert to miles using a factor of 1.6 kilometers per mile.

News Flash: TCM17, which was scheduled to execute next Wednesday, June 24th at P–20 days, is cancelled. The velocity change of 7 centimeters per second, providing competition for a fast-moving turtle, would have made too little difference in the arrival of the spacecraft compared to the current size of the known errors. That small velocity change will be deferred to TCM17B1 at P–15 days when it will have grown a little and the errors have shrunk. At least some people on the operations teams will get the weekend off, but the Navigators continue to add new data and run new solutions today.

News Flash 2: The Dreaded Target-plane Drift, bane of previous Voyager flybys of Uranus and Neptune, has not reared its ugly head as yet, and shows indications of perhaps never doing so. The trajectory solutions remain relatively steady with only a little dithering while the associated error ellipses (more on that below) shrink down around them. However, I’m not holding my breath yet.

End of News Flashes

Use your imagination to float in space beside New Horizons. The Plutonian system, encompassed by the orbit of its most distant known satellite, Hydra, spans less than a quarter of a degree in your unaided vision, less than half the size of the Moon seen from your house. In the telescopic view of the LORRI (Long Range Reconnaissance Imager) instrument—the prime camera for Navigation—it now fills most of the image and daily grows larger.

With images from LORRI, as of yesterday the Navigators predicted our arrival error in the target plane to within an ellipse of about 90 x 50 kilometers, “1-sigma”. That’ll continue to improve as we get closer. Statistically the 1-sigma means our actual arrival would be within that ellipse about 39% of the time. (For math purists the 39% is for a 2-dimensional Gaussian distribution; for a 1-dimensional distribution it’s the familiar 68%.) That size error from a distance of 29 million kilometers ain’t bad shootin’, and we owe it to the magic of OpNav (Optical Navigation) and the collective expertise of the Navigation Teams, including all ten members of the PNav Team (Project Navigation of KinetX Aerospace) and seven members of the INav Team (Independent Navigation of JPL). The large size of the Navigation effort attests to its importance to the success of the mission.

Think of the target plane as an enormous dart-board centered on Pluto, with our target-point about 12,600 kilometers down and to the left of Pluto, just outside the circular orbit of it’s biggest companion, Charon. Pluto is about 2400 kilometers across (not a big body, only two-thirds the size of the Moon), so our target-point is about five-and-a-half diameters away. You’re a giant, pitching darts from 39 million kilometers out. Some of them hit that small ellipse, others fall outside, but they’re constrained to a tight grouping that, if the ellipse were 3 times larger, would fall inside 99% of the time.

The biggest problem for the mission is not that small 90 x 50 kilometer error ellipse in the target plane; it’s the much larger error in the predicted distance to go, in the neighborhood of plus or minus 1000 kilometers uncertainty in the distance of Pluto from the Sun and Earth.

Why don’t we know it much better than that? After all, we’ve been tracking Pluto for 85 years since discovery by Clyde Tombaugh in 1930.

We don’t know it because 85 years is about one-third of the orbital period of 248 years, and in order to pin-down the heliocentric distance to a much smaller error, astronomers and the scientists/engineers at JPL who publish planetary orbits world-wide (and specifically for New Horizons) would need most of a full orbit of Pluto tracking behind them.

Unfortunately the Navigators, wizards that they are, can’t do much about this arrival time quandary at the moment. The OpNav images they use, taken against a background of stars, are very effective in telling us where we’re going in the plane of the image, which is basically parallel to the target plane, but it’s hard to squeeze information out of an image in the perpendicular direction.

This is just like in your picture of Aunt Molly about 29 feet away against a backdrop of mountains; it’s easy to measure her position in the up-down, left-right plane of the picture relative to features on the mountains (assuming you know all the camera parameters like field of view, size of the pixel array, etc., which you do), but hard to determine exactly how far away she is—the in-out direction—within a couple of feet unless you have the scale, meaning knowing exactly how wide and tall she is (which you don’t, since Aunt Molly is not a particularly cuddly person and you’ve never been up close for an opportunity to measure her).

We can’t do anything to adjust the arrival time to better than 70 seconds or so because our last scheduled maneuver, TBM17B2, is 10 days out, but the knowledge in the timing won’t improve significantly until 3 days out, much too late for a maneuver. If the project didn’t do something about that, New Horizons would fly by Pluto clicking off pictures at the wrong times, perhaps over a minute too early or late, which, at 13.8 kilometers per second, means a total error—early to late—spanning 2000 kilometers more or less.

Fortunately, the project can do something about it. Even though the trajectory can’t be easily adjusted after P–10 days without a big risk of something going wrong and totally blowing the mission, the knowledge continues to improve with continued OpNavs (because we start getting the scale by watching how the system expands in the images), and the Navigators deliver a “knowledge update” late in the game that’s considerably more accurate in the arrival time. With that, the sequence team tweaks the timing of the already-uploaded sequence of events for the cameras and other instruments, and all turns out well. We get pictures of the things we want to see.

We hope. Nothing is ever guaranteed in this world or in deep space beyond death and taxes. However, rest assured and be comforted that the Navigators “almost always get you there!”


Methodology of the Navigation Trade (Thanks to Douglas Adams)


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