Tag Archives: solar system

P–30 days: Navigating Pluto

 

Goldstone_DSN_antenna

Goldstone 70 meter antenna / NASA / Wikipedia

2015/06/14. Sunday
The third of a series of posts about navigating to Pluto

Distance: About 36 million kilometers from Pluto at 5:30 am Arizona time as I write.
Velocity: A not-much-changing 13.8 kilometers per second toward Pluto and away from Home. Not-much-changing because the pull of Sun’s gravity isn’t very strong out here.

I think I’ll do nuts and bolts today: how do the Navigators do what they do? That means I need to talk about the DSN (Deep Space Network) and navigation data types and other technical what-not. Let’s try to keep it light and entertaining, shall we? If you’re not technically inclined, you may be excused from today’s session, but be warned you will be tested on the material in the final exam. Attendance at the next session is mandatory.

But first, News Flash! The maneuver this morning was successful as I see in an email from Alice, the mission operations leader: “Initial TCM assessments are showing a nominal burn within the expected parameters.” Chris, one of our Navigators at the Mission Operations Center at APL (Applied Physics Laboratory) in Maryland says “The Doppler residual came up right around 1.06 Hz … indicating very likely a nominal burn.” Woo-hoo! We’re walking a little under two feet per second slower toward Pluto so that we’ll get there about 83 seconds later, close to the intended arrival time.

Back to the Deep Space Network. What a romantic, adventurous, ambitious name! I can hardly believe it’s real; the term evokes wonder every time I give it a little thought. Especially since there wasn’t any Space when I was growing up, much less Deep Space. I mean, most people didn’t think we could ever go into Space, and if you thought differently you were a Space Cadet.

I was a Space Cadet. All the way up to Sputnik in 1957. Then Space became IN. I thought I’d become IN, too, from Space Cadet to Visionary in a single launch, but no, that didn’t happen. It’s hard to project the necessary gravitas at age fifteen. Since then, I’ve navigated spacecraft to every planet in the solar system. From zero to sixty in only seventy-two years! (And I’m still a Space Cadet. When does the gravitas kick in?)

Where was I? Oh yes, Deep Space: a term that never fails to evoke romance, mystery, and adventure. Are we earthlings actually tracking things in Deep Space? Yes, we are! And someday, if we’re good and don’t kill ourselves first, we may even go out there ourselves.

The Deep Space Network: There are three locations almost evenly spaced around the world where NASA operates antennas that track New Horizons. The locations ensure that we (New Horizons) will be in sight of at least one of them all the time. There are several antennas at Goldstone, California; several near Canberra, Australia; and about an equal number not far from Madrid, Spain. We call those sites Goldstone, Canberra, and Madrid for short, but more commonly we talk about the antennas themselves, like DSS-14 (Deep Space Station 14, a 70 meter wide antenna at Goldstone) and DSS-65 (one of the 35 meter antennas at Madrid.)

The DSN antennas collect four different tracking “data-types” from New Horizons and funnels them to the Navigators through various channels that aren’t important to this discussion. Here they are (bear with me, it gets a little thick for a few sentences here and there):

(1) Doppler data: A DSN station sends up a radio frequency signal. It travels 4.5 hours to the spacecraft. When the spacecraft gets that uplink, it sends back a downlink signal that’s in “harmony” with the uplink. That means that in the process of turning the signal around and retransmitting it, New Horizons accounts for the frequency of the uplink on a cycle-by-cycle basis. When it gets back to Earth 4.5 hours later, it’s compared to the frequency that went up. If it’s the same, then nothing has changed, and that’s evidence that we live in a static universe in which nothing moves! That doesn’t seem to be the case. We always see a shifted frequency, pretty firmly establishing that we live in a non-static universe!

eeEOoo

Doppler effect defined

The received signal is different from what went up. Why? Ta-da, the Doppler effect of course, named in honor of Christian, its discoverer. That’s the eee-ooo sound you hear when a train goes by, but radio waves behave the same. And when you analyze that shift, you get clues about how the spacecraft moves, and just as important, how the tracking station moves because of the earth’s rotation, and when you put all that information together over the course of a tracking pass that lasts several hours, you can figure out not only how fast New Horizons is moving away from the Earth, but also you can determine its location in the sky: the Right ascension and Declination (reverting to astronomy-speak for a moment). The Doppler data is incredibly powerful in navigation, at least in the radial direction, and a change in New Horizon’s radial velocity of only 1 millimeter per second will look like a big signal.

So there—we’re through the hardest part of the discussion I think.

But wait, there’s more!

(2) Ranging data: If you time the signal (and the DSN has extraordinarily good timers, accurate to a gnat’s ass (another technical term), and know the speed of light (which we do) you can get the distance to the spacecraft to an accuracy of much less than a kilometer out of 4.7 billion of them. Now there’s a truly astounding accuracy). There are a lot of complications, of course, but that pretty well sums up the big picture for ranging data.

But wait, there’s more!

(3) DDOR data: Sometimes we use two of the DSN stations simultaneously, like Goldstone and Canberra and—over the course of about an hour—alternately track the spacecraft and then a quasar near it in the sky. Quasars are conveniently loud at radio frequencies, and they’re so far away that they don’t budge over the course of many, many years,

Delta-DOR defined

Delta-DOR defined

so they provide a very nice fixed reference system (thanks, Mother Nature!) for figuring out the direction of the spacecraft to jaw-dropping accuracy, about one-millionth of a degree. This data type has the gawky-gangling name of Delta-Differential One-Way-Range, which we usually shorten by calling it Delta-DOR, or writing it “DDOR”.

But wait … (oh, never mind). There’s one more.

The three types above—Doppler, ranging, and DDOR—are so-called radiometric data types. They’re all “centered” at Earth, so-to-speak, so the farther away New Horizons gets, the less accurate they are. To add to the uncertainty, we don’t know the distance from Earth to Pluto very well yet, so even though we might know the distance from Earth to New Horizons to that gnat’s ass, we don’t know New Horizon’s distance to Pluto to better than, very roughly, 1000 kilometers. That leads us into:

(4) OpNav data: The 4th data type, optical navigation data, or OpNav in the vernacular, is based on pictures taken from New Horizons of the things out in front of it, namely Pluto and his retinue of satellites, downlinked to the DSN and thence to Navigation. The OpNav team, a subset of the larger Navigation team, is led by Coralie. She and her team pick the locations of the tiny blobs of Pluto, Charon, Nix, and Hydra out of the noise of the images (a very difficult and tedious process with all kinds of complications) and compare them to locations of stars in the same images. This “pins” the spacecraft down against the stellar background, and since the locations of the stars in the sky are well know, so is the location of New Horizons. (The remaining two known satellites, Styx and Kerberos, aren’t used for the OpNav process because they’re too small and hard to see.) The location of the stars, Pluto, and satellites in the images constitute the data passed to the Orbit Determination team led by Fred.

So, that’s the end of the data descriptions. Not so bad, eh, if you’re still with me.

Since the OpNav data is spacecraft “centered”, it gets more powerful as we get closer to Pluto, until finally there comes a time when it overwhelms the accuracy of the radiometric data and tells us where we’re located relative to Pluto rather than Earth. From then on, OpNav data is the prima donna of the navigation show except for complications (always complications!) like when the spacecraft gets really close to Pluto and the Doppler data begins to “feel” the gravity. This doesn’t happen until the last day in the flyby because Pluto is so NOT massive (at least as compared to the planets).

It’s the job of Fred and his minions (of which I may be one), to boil all of this big slurry of data down (remember that big black witches’ pot in the first post?), scads and scads of it (another technical term), until there’s nothing left but a tarry black residue. That’s what we call the solution. It tells us where we are and where we’re going.

But wait … It’s not cut and dried with just one solution. There are a lot of unknown parameters that go into the witches’ brew, and the Navigators have to make assumptions about their values and how much they trust those assumptions. There are also a host of other variables, too much to go into, so just call them toad tongues and minced-spiced bat wings. They all affect the solutions to varying degrees.

The Navigators boil down many, many pots of witches’ brew, stirring vigorously, tasting occasionally and adding different amounts of toad tongues and bat wings to suit, so there are finally a lot of those tarry residues at the end of the process of orbit determination, each with a different taste (read trajectory). It’s the Navigators’ job to assess the flavors of all these and decide which one meets the reality of a successful Plutonian encounter, then deliver that nugget of information—a predicted trajectory—to the other teams of the mission so they can act on it.

So there it is in a nutshell.

There is no more. Today.

 

P–38 days: Navigating Pluto

Pluto and Charon artists impression

Pluto and Charon: Xanthine/Wikipedia

Pluto. A cold distant place we’ve never needed to think much about. Until now. Pluto and Charon (his ferryman of the dead): bodies at the outskirts of our traditional solar system which will soon have names of dead astronomers, poets, goddesses, writers, characters from literature, and other hoi polloi plastered all over their surfaces; names for everybody and every thing, real or fictitious, except for the very thing taking us there: New Horizons.

New Horizons. The spacecraft I helped launch nine years ago. New Horizons: the mission led by Alan Stern and guided by a host of dedicated engineers and scientists, navigated by Bobby Williams and his team, augmented by an independent Navigation team at JPL (the Jet Propulsion Laboratory).

Pluto moons and orbits

Pluto Satellites, NASA / Wikipedia

Unfortunately, New Horizons will not have its name on a single feature that it discovers on the surface of Pluto, Charon, Styx, Nix, Kerberos, Hydra, or any of the likely more-to-be-discovered satellites. So says the IAU (International Astronomical Union), the self-appointed official namer of names for all things astronomical. What a shame! Let’s have a waiver. Name something big on Pluto—a major crater, chasm, scarp, plain, mountain, sea, valley, whatever—for the spacecraft what brung us. It would be an insult not to do so.

I’m proud to be associated with the mission, and honored for an invitation to help out on the KinetX Navigation team at the Applied Physics Lab in Maryland next month in whatever small way possible to usher the spacecraft through its brief fling with the Plutonian system. This post and any others following it, if there’s time and wherewithal, are dedicated to Navigation and the process of getting there, true to the spirit of “The journey is more than half the fun.”

The journey is the important thing; the destination … not so much!

Project scientists will quibble with that. Many have expended a good fraction of a career to seeing what’s at the end of this particular tunnel, and that’s a perfectly good reason for them to celebrate an arrival. It’s just not the Navigators’ thing. The Navigators get us there, so their concern is Where are we? Where are we going?

Let me rework the statement above: We almost always get you there! is an unofficial and seldom mentioned motto of this particular Navigation team, because there is an example or two in history, like the Mars Climate Observer’s unanticipated and unwelcome arrival at the surface of that planet, to remind us that there is some justification for the modifier “almost.” (An almost that was barely avoided by the JPL Navigators for the Mars Polar Lander that augered in a few months later. Through a lot of effort they barely avoided missing the Martian reentry aim point only to see an onboard software failure late in the descent smear the spacecraft over the south polar region. See Embracing the Future.)

Will New Horizons accidently smack into Pluto? No. The flyby is too far away to make that kind of error. The more likely bad karma for Navigation would be to incorrectly estimate the position near the closest approach time and cause the cameras to point the wrong direction and snap pictures of empty space. Or to miss the dual occultations of the Sun and Earth when the spacecraft passes behind Pluto and Charon. To quote a character from literature (a likely candidate for a name on Pluto) Harris Mitchel in The Darkest Side of Saturn says, of the possibility of blowing a similar encounter at Neptune, “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 your life in shame and degradation…. [And] you get pelted by the scientists. They throw outdated textbooks at your head because you lost a science opportunity, probably the only [one] in our lifetime.”

So, the pressure is on the Navigation team, as it always is for encounters like this, to get us there safely, surely, and without error. Well, at least not any major error, because there is never any surety in human activity, and the best we can do is keep the inevitable and ineradicably small errors from turning into big ones.

Here’s the Navigation process, also described in the literary masterpiece mentioned above, paraphrased to befit the occasion: Dump the tracking data into a large black cast-iron pot, metaphorically speaking—actually the pot is a computer—along with other ingredients such as data calibrations and satellite and planet ephemerides. Stir vigorously. Incant technobabble and taste frequently, adding spices, a priori covariances, and toad tongues now and again until the brew is complete and the answer apparent: we know where New Horizons is and where it’s going. The encounter will be a success.

Sorry to trouble you with the technical description above. In future posts I’ll try to put things into more down-to-earth easily-understood terms such as the interpretation of dynamic events from signatures in the Doppler tracking data, the use of Very Long Baseline Interferometry data—spacecraft versus quasars—to nail down the trajectory in heliocentric space, and the number of pixels that can dance in the frame of an Optical Navigation image to determine the solution in Plutonian-centric space. By-and-by it will all become clear as a bell.

The Navigation Team, led by Bobby Williams (the first to navigate a spacecraft, NEAR, to a landing on an asteroid), determines the spacecraft trajectory and the orbits of the satellites circling the Plutonian system, designs maneuvers to redirect the spacecraft to whatever target the mission desires, and provides the rest of the numerous New Horizons project folk—engineers and scientists—with last minute updates on positions so that the cameras and other instruments will point the right direction at closest approach, and not click pictures or take data of empty space.

One last topic to end this ever-rambling post:

Pluto the Dog

Photo: Leo Reynolds (modified)/ CC BY-NC-SA

Is Pluto a planet?

Used to be, but not any more. Why is that? Well, besides a few technical things like how it was formed, and that it doesn’t clear its orbit of debris like all the other prim and proper planets do, I think there is a much more down-to-earth reason. It resides with the schoolchildren of our planet. Since there are probably scads and scads—hundreds and thousands or more—of new Pluto-sized bodies in the Kuiper Belt and Oort Cloud far outside the traditional bounds of our solar system, don’t you think it would be cruel to commit our school kidlets to the memorization of hundreds and thousands of dubious planetary bodies when it’s already difficult enough to name the eight already out there, even using the awkwardly unusable mnemonic “Men Very Easily Make Jugs Serve Useful Needs”?

I’d propose granting Pluto, for reasons of history and tradition, the status of “Honorary Planet.” It’s the least we could do to ease the pain of Pluto-is-a-planet advocates such as Alan Stern, the New Horizons leader (who has spoken quite nicely of Navigation in one of his own blog posts.) That way, with this mission we could then say that we’ve finally explored the last planetary outpost of our traditional solar system.

Where are we going? Pluto! That faint blob in our telescopic sky, that last place of mystery and darkness, the place we go to die, crossing the river Styx, and yet also the place we go in these last 38 days to come alive and understand ourselves and our universe a bit more.

But Pluto is only another milestone in a longer trip that the human race will ultimately celebrate in the far future if we don’t kill ourselves first. Let the journey continue.

May_12_Pluto

 

Mercury Minus Ten, Pluto Minus Eighty-five

messenger

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.

new_horizons

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.

traj_helioecl_earth2moi2_10292010

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!

saturn_0.5inch