Tag Archives: deep-space navigation

P–24 days: Navigating Pluto

 

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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!”

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Methodology of the Navigation Trade (Thanks to Douglas Adams)

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Mercury Minus Ten, Pluto Minus Eighty-five

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Image credit: NASA–NSSDC/MESSENGER

As of 2015/04/20 there are 10 days until the MESSENGER spacecraft goes splat somewhere in Mercury’s northern regions, and just under three months until the-little-spacecraft-that-could, New Horizons, flies by our last commonly accepted outpost, planetoid Pluto.

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Image credit: NASA–NSSDC/New Horizons

 

These two missions bookend our solar system, innermost and outermost, if you disregard the billions of tiny bodies stretching farther out in the Kuiper belt and Ort cloud, farther than the eye can see and with more Pluto-sized bodies than the schoolchildren can memorize (which is probably one practical reason Pluto was demoted from planethood in the first place).

MESSENGER has been spaceborne since 2004, New Horizons, 2006. New Horizons was pretty much a straight shot to Pluto; MESSENGER was a complicated mess, the equivalent of a 6 bank shot in billiards, flying by Earth once after launch, Venus twice, and Mercury three times before dropping into the pocket—that is, going into orbit—on the fourth Mercury encounter in 2011.

Now MESSENGER is out of hydrazine and quite literally running on fumes, squirting the helium gas that used to pressurize the fuel straight out of the tanks and through the thrusters. Not as efficient as the hydrazine, but sufficient to delay the inevitable a few maneuvers and days at a time as the orbit closest approach altitude inches (kilometers!) toward an inevitable rendezvous—a kiss of death with the Mercury surface somewhere in the North-polar region, the final splatt. One more planned maneuver on April 24th should stave it off until about the 30th of April, and then it’s bye-bye MESSENGER.

New Horizons was the fastest ever spacecraft at launch, peaking at a blistering 43 kilometers per second relative to the Sun. The inexorable hand of gravity slowed it so that—even though a distant flyby of Jupiter gave it a boost—today it glides at a more leisurely pace, 14.5 kilometers per second through the deeps and darks of space. When it gets to Pluto it’ll fly by that remote outpost—more than 30 times the distance from Sun to Earth—at a tad under 14 kilometers per second.

Interestingly, the distance New Horizons has traveled all the way out to Pluto is less than MESSENGER traveled in its pinball encounters with the inner solar system planets to lose enough energy to get into orbit at Mercury. Gee, Mr. Wizard, you have to lose a lot of energy to get to Mercury.

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Image credit: NASA–APL

Too bad MESSENGER’s demise can’t be delayed two more months. Then the Deep-Space Navigators of both spacecraft, who work for KinetX Aerospace, would be able to say they’re navigating simultaneously to the extremes of the solar system. Nevertheless, to do them both within a few months of each other is pretty noteworthy.

Important science was done at Mercury and more knowledge comes from Pluto in July, but the purpose of this blog is to celebrate the art and science of deep-space navigation—and particularly the two KinetX Navigation Teams (both of which I was once a member)—that guided us there, for truly the journey is more than half the fun …

And we almost always get you there!

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Voyager at Neptune

This OpEd ran in the Sunday L. A. Times, 1989/08/06

Neptune! Almost 3 billion miles from earth. It’s a cold, impersonal place, a speck of a planet through even the largest telescope, but an enormous gas giant full of mysterious wonders when seen up close.

In a few more weeks we’re going to experience it up close, you and I. We will see it through the eyes of a solar diplomat: a traveler, explorer, adventurer, and representative of the human race; a large metal, plastic, silicon representative named Voyager.

When we arrive, what should we expect? The unexpected! That’s the lesson learned from previous visits to unexplored planets. Are there rings at Neptune like those at Jupiter, Saturn and Uranus? Possibly, but in incomplete arcs, unlike rings anywhere else. Are there other satellites, besides the already known Triton and Nereid? Yes! One has already been discovered, and there’s tantalizing evidence of more—perhaps many more, maybe even clouds of them — lurking just beneath the fuzz in the images painting the screens of video terminals at the Jet Propulsion Laboratory. In just days, now, they could rise to visibility above the electronic background noise.

Neptune has its own internal heat source, and radiates more energy than it receives from the sun. Why? Is this what drives its turbulent atmosphere, generating the large spots already found? Soon, we may know.

And Triton? We’ll find whether it has an atmosphere, and whether we can see through to the surface. Will there be pools of liquid nitrogen, or will gasses be frozen in slabs littering a desolate landscape of craters and mountains?

Among the answers to the questions we know to ask will be more questions we’ve not imagined—the unexpected!

When Voyager arrives at Neptune—on August 25th—it will be the first time since creation that anything human-made has been to that planet. We should enjoy, appreciate, and celebrate the event, since it will also likely be the last time it will happen during our lives.

That’s because a very special arrangement of the solar system, one that occurs only about every 175 years, was required to allow Voyager to make the trip in “only” twelve years. It had to go by Jupiter first, making a hard left turn in that planet’s gravity to pick up energy in a crack-the-whip fashion to go on to the next planet. That was Saturn, which turned it left again, gave it more energy, and pointed it towards Uranus. At Uranus, in 1986, it picked up still more energy and made course for Neptune. Since then it has been “cruising” at ten miles per second toward the planet. At Neptune it will skim over the north pole three thousand miles above the atmosphere, turn downward so that it’s headed south out of the solar system, and make a final encounter with Neptune’s largest satellite, Triton, before beginning a larger interstellar voyage.

The science at Neptune is important, but think also about the voyage, the adventure. Knowledge is good for the human mind, but travel is food for the psyche. And Voyager’s travels have been and will be prodigious. It has been on its way from earth since 1977, wending a crooked path through the outer solar system. Now, in a handful of days, it makes its final rendezvous, a close brush and embrace with Neptune, before flying out of the solar system to begin an odyssey through the Milky Way galaxy; an unattended, lonely voyage that may last from millions to billions of years.

Towards the beginning of that longer journey, a mere few hundred thousand years in the future, our sun will have become a faint, uninteresting star in Voyager’s eternally night sky. But no one will be with the spacecraft to appreciate that fact, and our ambassador will slowly tumble—sightless, senseless, and alone —in an immensely empty void.

A fellow engineer on the navigation team claims that the spacecraft will be on display in the Smithsonian Museum 200 years from now. He thinks that by then we’ll have both the technology and wherewithal to go out, find and catch Voyager, and bring it back. I’d like to think we would be able to do that, but if I’m still around I’ll vote to leave it alone. There’s something wonderful about the thought that a piece of ourselves is somewhere out there on a winding journey between the stars on its way to eternity. It’s like having immortal children.

So this is an adventure, and we’re all on board. The solar system is our playground, and after that—the stars! There are hazards ahead—for example, unseen ring particles orbiting Neptune could smack into us, prematurely ending Voyager’s life—but we’ll probably make it through to see the wonders of Neptune and Triton.

Then will begin the grander voyage—the one that requires us to be romantics instead of realists; dreamers rather than schemers: Even though Voyager will go blind and deaf after a few tens of years; even though it will die an electronic death, it will still have the germ of human creativity and daring incorporated into its very structure. It carries two messages—an explicit one in the form of a golden record, and an implicit one stated by its profoundly improbable existence. And both messages will say to the finder, in essence, “I am from the planet earth. I am of the human race. We are small and insignificant, but our souls are large because we have set out on a journey to know the universe.”

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