Tag Archives: trajectory

P-1 day: Navigating Pluto


Where? 1.2 million kilometers to go at 1.2 million kilometers per day, as of 7:49 AM this morning of July 13th in Baltimore, almost exactly one day before the blessed event. Pluto is about 400 pixels across in the LORRI camera, but to the naked eye it’s still only 23 percent the size of the full moon. Over the next 24 hours that image will blow up, far overflowing the boundaries of LORRI, requiring that mosaics be done to cover more than the camera’s tiny footprint on the surface.

Where to? Pluto! At 7:48:45 in Como, North Carolina, 11:48:45 GMT, plus or minus 36 seconds or so.

Where from? The over-crowded planet that we all adore.

Today is the Navigators’ well-deserved day off while the rest of the project works its collective fingers to the bone. Our last delivery, Crit 37, is delivered. From here on, the laws of physics solely guide the spacecraft and nothing more can be done except to enjoy the ride. The pressure is off and decompression begins.

For your entertainment, here’s an excerpt from The Darkest Side of Saturn. It’s a few days before the Voyager (Nomad in the story) encounter with Neptune. Harris Mitchel does a radio interview with an obnoxious host about the upcoming event. As for the Pluto flyby, the Neptune event also has dual occultations explained in the story, with the main difference that Neptune’s gravity flings Nomad through occultations with the planet and its big moon Triton, while for the much weaker gravity of Pluto, the flinging is more like a very gentle, almost unnoticeable bending of the trajectory.


Navigating Neptune

The next morning as Harris drove to work along the surface streets between San Marino and the Lab, he tuned his radio to K-N-E-E, a low-powered downtown AM station. Reception was poor and he had to strain to hear through the static. Finally he found what he was listening for.

It was sobering to hear the host, James Conland, announce, “In the next segment, we’re talking to Harris Mitchel who wrote an article in last Sunday’s Times about spaceship Nomad’s encounter with Neptune. That’s up in fifteen minutes.”

Me! Fifteen minutes! And he was running late. He jogged left off New York Drive onto North Lake Avenue, then immediately right on Woodbury Road and drove as fast as he dared.

“. . . and the segment after that, we’ll take calls on how to bash liberals with the facts. Just remember, my friends, James Conland is your Truth Meister, believe me. The truth is putty in my hands!”

Uh-oh. A warning claxon went off in Harris’s brain. A right-wing whacko. What have I gotten into?

Woodbury Road became Oak Grove Drive, and as he sped into the final curve leading toward the entrance of ATL Harris saw, off to his right across the arroyo, the large motley collection of buildings that constituted the Lab growing out of the foothills of the San Gabriel mountains. Three minutes later he drove past the sign that announced “NASA Advanced Technology Laboratory.” He turned left just before the guard island and zipped along Mesa Road to the west parking lot.

Vacant parking spaces were filling rapidly, even at 7:20 in the morning. The impending encounter attracted a lot of visitors to the Lab—scientists, engineers, and media people—from all over the world. To the considerable irritation of the regulars who parked there, the security guards sent the overflow visitors traffic into the regular parking areas. Harris pulled into a spot near the far end of a row and alternately trotted and walked the two hundred yards to the main guard shack, attaché case swinging by his side. He noticed a large CNN TV van parked along the side of Oak Grove Drive, thick black cables spilling out of the back doors and snaking across the sidewalk through the fence to the auditorium. There’s the first one. There’ll be more in another day or two.

He flashed his ID card into the window of the guard shack as he went by and walked into the pleasant tree-shaded, grass-lined mall that served as a vestibule to the rest of the Lab. It looks more like a sleepy college campus than a laboratory, he thought to himself for the umpteenth time. An ornamental concrete pond that had lain empty for years to impress the earnest frugality of the Lab onto visiting congressmen and other federal budgeters was filled again with water, and fountains spurted merrily on both sides of the footbridge running across it. The water ran down a concrete creek bed lined on both sides by banks of luxurious grass and pine trees.

Harris paced briskly across the mall to the spacecraft operations building. He took the steps three at a time to the second floor and walked into his office at 7:27 precisely, sweating and breathing hard, but with time to spare!

None of his office mates were in yet. He scribbled a note across a piece of scratch paper, “Please do not disturb. Radio interview in progress,” and taped it to the front of the door. He sat down at his desk, pulled out the crib sheet filled with Nomad and Neptune “gee-whiz” facts that he’d constructed the night before, and waited for the phone to ring.

Neptune. A tiny blue dot almost three 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.

Next week 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 Nomad . . .

That’s the way his article had started. Apparently it had been syndicated in newspapers across the country; even though it had been out less than a week, he’d already received letters from Michigan, North Dakota, and New York.

Neptune. In seven more days, when Nomad II dove over Neptune’s north pole and intercepted Triton on the other side, the planet and its large moon would become as real to Harris as Saturn and its rings had been nine years earlier. Already the planet loomed enormously, a body whose light powder blue atmosphere streaked with white clouds and a large oval dot strained to spill over the framed confines of Nomad’s narrow angle camera into the more bountiful space of the wide-angle camera.

The phone rang at 7:31.

“Mister Mitchel?”


“Hi, I’m Polly Marx at K-N-E-E. How are you this morning?”

“Hi Polly. I need a coffee transfusion. Other than that, I’m ready.” He made his voice a lot more confident than he felt.

“Great! We’ve got two more commercials to go and then you’re on. Will you hold?”


“Ummm . . .” There was a pause.


“I read your article and really liked it.”


“I know this is going to sound awfully forward, and you don’t know me at all, so I hope you won’t misconstrue, but I wonder if you’d like to do lunch sometime?”

“Hmmm. Well, maybe.”

“Think about it, please. I’ve always been interested in the space program and I’d love to talk to somebody like you who’s actually in it. And I could give you a tour of the station, here.” She giggled. “Besides, your article gave me the goose bumps and I want to see what you look like.”

Harris laughed. He got her phone number.

“Bye, now,” she said in a soft voice. “Hope I hear from you.”

I wonder if she looks like her voice sounds.

Polly’s sexy voice was replaced by a jingle about the awesome flavor of Puddy-Do Wiener Dogs (“Pud-dy-Do, the on-ly Wien-ner Dog for you, Pud-dy-do!”), followed by a frenetic pitch by Long John’s Body Shop in Long Beach, (“Drop by and get your body adjusted.”), followed by an advertisement for Trash (“The movie that trashes your preconceptions about bad taste.”), followed by a pitch from Get Rich Investments, Inc. (“The gold market is ex-ploding today.”), followed by . . .

That’s FIVE commercials, Harris thought just as an unctuously self-assured voice interrupted and smoothly rumbled into his ear, “Hello, Mister Mitchel, how are you this morning, and how’s Uranus . . . I mean Neptune, heh-heh?”

“Oh, Neptune’s fine, and my anus is, too. How’s yours?”

“Good, good. Oh, what a relief it’s going to be to talk to a real person instead of a nerd.”

“Thank you.”

“We’re on in twenty seconds, so I’ll do a quick intro and we’ll talk, okay?”


The last commercial resumed and ended. Conland’s voice came back.

“Now for a little something about one of the only things the federal government has any business doing, and that’s the space program. Space! The final frontier! We need space to grow. We need it for our real estate developers on Mars—new lands to exploit without having to put up with the environmental freaks. In a week we’re going to look at some of that potential new real estate with our Nomad spacecraft. It’s about to send a report back on the planet Neptune and its stupendously big moon Triton. On the line we have Harris Mitchel whose poetic article on the Nomad encounter with Neptune ran in the Times editorial pages last Sunday. Believe me, I normally choke on the word poetic, but when an article is about the free human spirit and Yankee ingenuity on the new American frontier, what other word do you use? How are you, Harris, and how’s the quest for Neptune?”

“Hello, Mister Conland. I’m fine. I’m afraid we’re going to have a problem developing the real estate on Neptune though, since there’s no place to lay cornerstones. It’s full of gas and maybe liquids right down to the core, totally un-usable. Maybe you’d do a little better on Triton, but you’d better bring your parka since it gets nine hundred times less light from the Sun. We’re pretty sure we’ll find either liquid or frozen nitrogen there.”

“Then I’ll bring my fur-lined mittens and bunny slippers, too. What’s your job on the Nomad project, Mister Mitchel? Are you one of the scientists?”

“Actually no, I’m an interplanetary navigator, and we . . .”

“If somebody told me that on the street I’d call for the boys in white jackets, but in your case it’s actually true.”

“Right. I’m one of the people more concerned with the journey than the destination. We’re the ones who make Nomad go where she’s supposed to go.”

“She, huh? It’s hard to guide a woman, isn’t it? Real estate aside, why are we going to Neptune, Mister Mitchel? Why are we spending our hard-earned money on a planet three gazillion miles away? What does Mister American Taxpayer get out of this?”

“He gets the same thing that every man, woman, and child in the rest of the world gets, Mister Conland: a lift in his spirits and a feel-good feeling. He gets a bright shiny future to think about, instead of a crusty dismal past full of wars and crime and hatred and other depressing things. He gets to look outward toward a new world instead of inward at an old one. That’s what he gets out of Neptune. I think it’s worth it. Don’t you?”

. . . 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 Nomad’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, Nomad 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 billions of years . . .

“How do you navigate to a planet three billion miles away?”

Harris explained that he and his teammates collected Doppler and ranging data from the Deep Space Network stations that tracked Nomad. They collected images from Nomad’s telemetry—pictures of Triton and Nereid and the newly discovered satellites.

They dumped the tracking data into a large black cast-iron pot, metaphorically speaking—actually the pot was a large computer—along with other ingredients such as data calibrations and planet and satellite ephemerides. They stirred vigorously. They incanted techno-babble and tasted frequently, adding spices, a priori covariances, and toad tongues now and again until the brew was complete and the answer was apparent: they knew where the spacecraft was and where it was going well enough to plan the last maneuver that Nomad would ever do.

In three more days they would execute TCM20 (Trajectory Change Maneuver number 20), and four days later Nomad would go skimming—actually sizzling at twenty-seven kilometers per second— just above Neptune’s atmosphere. The giant’s gravitational hand would grab her, wind up and swing her in a tight arc and pitch her downward at a steep angle toward Triton on the far side of its tilted orbit about Neptune.

“You’re not going to hit Triton, are you? Splat! End of Nomad, end of project.”

“No, no, we’re aiming about forty thousand kilometers behind it, trying to pass through the dual occultation zone.”

“That sounds doubly occult to me, Mister Mitchel. Is this a sinister liberal plot?”

Harris laughed. He was beginning to relax. “Well, it’s true that I’m a liberal . . .”

“I’m amazed, you sound so reasonable, but I’ll try not to hold it against you.”

“. . . but there’s nothing at all sinister about me or the dual occultation. We want to make Nomad pass behind Triton as seen from both the Earth and the Sun. That way we get information about Triton’s atmosphere and surface by Earth tracking Nomad until it disappears, and Nomad watching the Sun until it disappears.”

“Is it hard to hit this zone? Is it as hard to hit as a homeless person at two hundred yards with an AK-47?”

“It’s about as hard as finding kindness in a right-wing conservative’s soul, which is harder than starting an ice-cream franchise on the sun.”

“Ouch. Heh-heh.”

“It’s hard, Mister Conland, because Triton is so far away on the other side of Neptune and the zone is so small. We have our last chance to target it with our final maneuver in a few days. After that the laws of physics take over and it’s completely out of our hands. Nomad passes over Neptune really deep in its gravitational well, and any errors left over from the maneuver get magnified on the rest of the trip to Triton. I’ll give you an analogy: imagine you’re a planet sized baseball pitcher standing on Neptune’s north pole like it was a pitcher’s mound . . .”

“And my feet are freezing because they’re in that cold atmosphere, right?”

“Yeah, and maybe your cleats are dug into methane icebergs. Anyhow, even though the zone we want to hit is fourteen hundred kilometers wide, at the scale of this planet-sized pitcher, Triton’s dual occultation zone looks like it’s about three times smaller than the strike zone at home plate. You have to throw a perfect strike over a six inch wide home plate to get a dual occultation.”

“Gee, Mister Wizard, space navigation is hard! And what if I don’t get a strike? Do I get pelted with beer bottles by the fans? What’s the consequence?”

Harris suppressed a sudden impulse to say, I have to run naked through the streets.

“You’re totally humiliated because you said you could do this and you didn’t. As a consequence, you lose all your credibility and live the rest of you life in shame and degradation and never get invited on a radio talk show again.”

“That’s the worst of all.”

“Oh yeah, and you get pelted by the scientists. They throw outdated textbooks at your head because you lost a science opportunity, probably the only opportunity in our lifetime to accurately measure the components of Triton’s atmosphere.”

“So you’re letting it all hang out here, Mister Mitchel? This is your career on the line?”

“It’s a tough, dirty job, Mister Conland, but somebody has to do it.”

Conland wound down the interview. They’d been going almost thirty minutes.

“Bring it home for me, Mister Mitchel. What’s in store for Nomad? What happens after Neptune?”

. . . Even though Nomad will go blind and deaf after a few tens of years; even though she will die an electronic death, she will still have the germ of human creativity and daring incorporated into her very structure.

She will carry two messages—an explicit one in the form of a golden record, and an implicit one stated by her profoundly improbable existence. And both messages will say to the finder, in essence, “I am from 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.

“Well said, Mister Mitchel. Not bad for a mush-brained liberal and an engineer to boot. If you hit the occultation zone, you can come back on our program. If you miss, we’re gonna tell Congress to kick your butt and cut off your funding.”

“Thanks, Mister Conland. It’s been a challenge and a pleasure educating a right-wing fruitcake, and I hope I have a chance to do it again. Meanwhile, don’t shoot too many welfare mothers.”

“You sure know how to hurt a guy, Mister Mitchel . . .”

“So do you.”

“. . . but seriously, it’s been a pleasure.”

“And seriously, Mister Conland, I’ve enjoyed it.”


P-3 days: Navigating Pluto

Where? 4.0 million kilometers out, getting 1.2 million kilometers closer each day, as of midnight this Saturday morning of July 11th in Columbia, Maryland. Pluto is about 120 pixels across in the LORRI camera, but to the naked eye it’s only 7 percent the size of the full moon.

Where to? Pluto! At 7:49:02 AM, July 14th Columbia time, plus or minus 47 seconds or so.

Where from? Earth.

Yesterday afternoon the Navigators recommended to the project that an update to the onboard sequence be made, since the current solution location in the target plane (think of it as the throw of a dart) and its corresponding cloud of uncertainty has drifted towards the top center of the 65 x 130 kilometer box that represents the acceptable region for the flyby.

It’s actually a cube rather than a box, since there’s also a limit to the acceptable arrival time of 200 seconds centered on 7:49:49 AM Columbia, Maryland time. If the trajectory goes through that box and the spacecraft closest approach is within 100 seconds of the desired time, all is well and the current design is good. If it goes outside that box/cube, then either some science data may be lost, or the sequence will need tweaking to make the observations optimal again.

The Navigation Team recommendation was to upload a tweak to the canned-in spacecraft sequence design to guard against a drifting solution, essentially re-centering the cube around the current solution. The independent Navigation Team agreed. The Science Team agreed. The Mission Operations Team agreed.

The mission leader didn’t agree. Alan Sterns’ reasoning was that there’s too much risk of missing the whole flyby if the spacecraft should go into safe mode again, like it did on July 4th, an unanticipated event. If that happened, it’s quiet possible that the spacecraft couldn’t be returned to normal operation in time for the flyby, resulting in a loss of almost all of the mission objectives. Better to risk some suboptimal science observations than the whole mission.

Nobody wants that, not the scientists, not the engineers, and not the Navigators, so there has not been any grumbling about the decision. It was justified.

Today we processed Crit 35 optical navigation data.

The good news is that the mild solution drift we’ve seen in the last few days in both the target plane and the arrival time seems to have abated, and the error bounds continue to shrink up around the solution.

There is no bad news.

My wife just landed in Baltimore.

Life is good!


P-4 days: Navigating Pluto

We are the Navigators. We say where we are and where we’re going.

Where are we? As of midnight this morning, July 10th, we were 5.2 million kilometers away, eating up those remaining kilometers at the rate of 1.2 million a day.

Where are we going? Pluto! To 12,550 kilometers above the surface, plus or minus a few, at the closest approach time of 7:48:49, July 14th, on the US east coast, plus or minus around 54 seconds or so (1-sigma). Of course we couldn’t know that for sure until the radio signal hits the ground about four-and-a-half hours later … if there were a radio signal. There won’t be because New Horizons will be looking at satellite Charon at that red-hot moment, with the antenna pointed away from Earth.

Where were we? That’s in the past, we don’t particularly care. The most important thing is not where we’ve been but where we’re headed. And that’s the future, not the past. But if you have to know, we are from Earth, we are of the human race.

Politicians should also be Navigators. They should tell us where the human race is, and where it’s going. But they don’t. Are we headed into a long-term presence in space, leading to colonies in the solar system and beyond? Or are we slouching toward overpopulation disasters like pollution, climate change, and nuclear Armageddon? They can’t tell us that because nobody knows. We are a chaotic, unorganized civilization with no Navigators to point the way, and little guidance to correct our trajectory.

Meanwhile, back here on Earth, we have Crit 34 to worry about: Critical delivery number 34 of optical navigation data has just been delivered to us, hot from the spacecraft through the DSN (Deep Space Network), through a maze of data pipelines to our very own Optical Navigators in our big room at APL (the Applied Physics Laboratory) that we call The Bullpen. Our Optical Navigators have just finished calculating the centers of 2 Hydra, 3 Pluto, 3 Charon, and 2 Nix images taken yesterday, and now we, the rest of the Navigators on the team are going to combine that information with the rest of the Doppler and ranging data from the DSN and come up with an answer: When do we get to Pluto?

The solution will shift by X seconds, with an uncertainty of Y, and we’ll report that to the project at large. Then the decision will be made: Will we tweak the already loaded sequence to accommodate the new results? The betting is that we will, since recent solutions have moved us earlier than the nominal arrival, and the error bars are shrinking down around those answers. We’ll know by the end of today.



See Voyager at Neptune for the previous deepest space adventure of the far, far past.

P–11 days: Navigating Pluto


Part of the Project Nav Team: Jeremy, Bobby, Chris, Derek, Dale, Coralie, Tony. (Photo credit: Dale Stanbridge)

As of midnight beginning the morning of July 3rd on the U. S. east coast, our intrepid New Horizons spacecraft—the long-distance eyes and ears of the human race and our ambassador to the outer reaches of the traditional solar system—cruised along at 13.5 million kilometers from Pluto, looming closer by 1.2 million kilometers every day.

The Plutonian system of satellite orbits now swells rapidly in our telescopic eye: the camera named LORRI (LOng-Range Reconnaissance Imager). If abstract orbits were visible, the outermost one—Hydra’s—is about twice as large as the one-third degree field-of-view of the camera. That means the system is only a little larger in New Horizons’ black sky than the full moon seen from your back yard.

Even the orbit of Styx, the innermost of the small satellites, overflows the image boundary by about 20 percent. Only Pluto and its large companion, Charon, still fit comfortably within a camera frame. From this point onward, the spacecraft will have to “slew” to get OpNav images of satellites from one side of the system to the other.

The day before yesterday we had a close encounter with Styx. The band! Not the satellite.

Rock stars and around a hundred-or-so space cadets mingled in close proximity, taking pictures and signing autographs. A photo-op is a photo-op, for rockers to mingle with Lords of the Solar System, and space cadets to schmooze with Lords of Rock. Who was most impressed? I’d say the space cadets.

Which raises the question: I assume the band was named for the mythological river, but was the satellite named for the river or the band? Well, ostensibly for the river of course, but was there a hard-core rock fan lurking amongst the namers of names? And I don’t mean a geologist.

The Navigators’ DTpD (Dreaded Target-plane Drift) seems to have drifted to a dead-end, coming to rest about 25 kilometers from the aim-point, that point—just outside Charon’s orbit in the big Plutonian dart board—which is most desired by scientists and engineers alike of the project. The solution and its attendant cloud of possible errors is comfortably inside the box of acceptability, a 300 x 200 kilometer region where the photo-ops are best and all the other instruments stay happy. Now, if the DTpD is really dead and stays dead, not coming zombie-like back across the river Styx, we’ll have only one more thing to worry about. (And it’s a good thing, since there are no more maneuvers planned and therefore no way to change the trajectory. We have what we have.)

That one last thing to worry about is the arrival time, which is still uncertain by a little more than a minute earlier or later. We don’t know the distance to Pluto precisely, so we’re not sure when we’ll get there. We’ll find out in the last few days by closely watching how the satellites spiral outwards as their orbits expand in LORRI’s point-of-view. When we know how far we are and when we’ll get there, we’ll tweak our already-loaded sequence of events by the right amount of seconds, and all will be well.

That will be a very intense time for the Navigators. But we’ll cross that river when we get to it.



Conspicuously missing from the KinetX Project Nav Team image are Fred, Ken and Philip. Also not shown but very appreciated are the ones who provide reality checks to keep us honest—the JPL Independent Nav Team: Paul, Shyam, Dylan, Steve, Gerhard, Bill, and Mike.

P–15 days: Navigating Pluto

As of today, Monday June 29th, I find myself airborne headed for the Applied Physics Laboratory in Maryland while 13 hours earlier, at midnight leading into this morning on the east coast, New Horizons found itself 18.5 million kilometers from Pluto and moving 1.2 million kilometers closer every day.

The Dreaded Target-plane Drift has come and gone. For about a week, the solutions drifted downward and leftward on the target plane—that big dart board in the sky—each time more tracking data was added, until finally it departed the bottom of the acceptable target box, a 300 x 200 kilometer rectangle centered about 14,000 kilometers down (Ecliptic south) and left (eastward) of Pluto.

The cause of the Dreaded Target-plane Drift is not yet completely understood, although it’s almost certainly related to earlier errors in locating the centers of Pluto and Charon, and this was most likely caused by inadequate knowledge of the albedos of those bodies, the dark and light patterns covering their surfaces. It’s hard to model the surface brightness of a body you’ve never seen up close before.

The DTpD, while it was in play, was a very worrisome thing because the Navigators didn’t know if it was real or—much worse—caused by a hidden problem, maybe serious dynamic mis-modeling that’s been overlooked to this point or even an unknown bug in the software. This caused some very restless activity and sleepless nights for several days in a row. It now appears that the drift has stopped and the current position, just outside the box, is correct and the earlier positions were not. There was probably not a dynamic mis-modeling problem or software bug. The moving finger of Navigation writes, and having writ, moves on.

In any event, the Dreaded Target-plane Drift may be a thing of the past as the Navigators rely more on the pin-prick images of Styx, Nix, Kerberos, and Hydra. Those bodies are small enough that no appreciable diameters are yet seen and the albedos of their surfaces doesn’t even enter the problem. So … while the DTpD may never rear its ugly head again, the Navigation Team will not hold its collective breath. Eternal vigilance is the norm.

At this red hot moment, if the current dart in the dart board is the correct one, surrounded by it’s little cloud of Gaussian-shaped probability, then the science observations won’t be completely optimal at closest-approach time, and the intended occultation of the Sun and Earth by Charon may not happen.

Thus in the wee hours of tomorrow morning the penultimate maneuver, TCM17B1, will execute and move the drifted solution back into the box, the happy return of the prodigal son. It will cost a few centimeters per second change in velocity, and then all will be well.

We hope.

P–21 days: Navigating Pluto


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.


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)