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Guest Blogger: David Seal

September 25 - October 1, 2006

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David Seal is Cassini's Mission Planner at the Jet Propulsion Laboratory and has been involved with the Cassini mission to Saturn since 1992. Dave developed JPL's Solar System Simulator and has also worked on the Shuttle Radar Topography Mission. Outside of JPL, Dave enjoys acting in Caltech theater, disc golf, and the science of soap bubbles. Dave foolishly believes the universe is closed.


The Greatest Mission You've Never Heard About

Sep. 25, 2006 | 11:17 PDT | 18:17 UTC

As Emily's intro says, I'm currently working on Cassini, and my mission has happily become ubiquitous in space news these days. So it might surprise you a little to learn that I'm going to start out my week talking about a mission you might never have heard about.

Some years ago, before I returned to Cassini to be the Mission Planner, I had the honor of working on a great project called the Shuttle Radar Topography Mission (SRTM), and it was one of the most amazing professional experiences of my career.

What? Shuttle, you say? Don't you work at the Jet Propulsion Laboratory? What does JPL have to do with the Space Shuttle? It's a reasonable question.. but while it might seem like JPL does mostly robotic solar system exploration and the Johnson Space Center focuses on manned space flight - little overlap except for mutual respect for each others' work - our two centers have had a long and very rewarding relationship. JPL's Ulysses and Galileo spacecraft were launched aboard the Space Shuttle; JPL built the Wide Field and Planetary Camera capturing pictures on board the Hubble Space Telescope, also orbited by the Shuttle; and the Shuttle carried imaging radar to study the Earth on no fewer than four missions (SRTM being the most recent). The first of these missions was actually the very first payload the Shuttle carried, aboard STS-2 in 1981. Ten additional JPL experiments of varying scale also flew on the Shuttle from 1981 to 1998.

SRTM flew on STS-99 in February of 2000. (We were nearly STS-100, which would have been a curious milestone, but they reshuffled some of the numbers for reasons I can't recall; though it's possible they wanted 100 for a space station mission. It's an arbitrary distinction so I didn't really care. 99 was pretty cool anyway.)

Shuttle Radar Topography Mission
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Shuttle Radar Topography Mission
S99-E-5476 (16 February 2000) --- Part of the Shuttle Radar Topography Mission hardware is photographed through Endeavour's aft flight deck windows about half way through the scheduled 11-day SRTM flight. The mast, only partially visible at lower right, is actually 200 feet (60 meters) in length. Credit: NASA
Our mission was to extend the capabilities of the previous flights by reusing the existing radar antennas and adding a second, outboard antenna to be deployed on a sixty-meter mast from the payload bay. The second antenna allowed us to gather interferometry data in addition to the Radar imagery itself. The capability to do interferometry - two separate antennas separated by distance bouncing Radar off the same region of the Earth at the same time - meant that we could do topography. And getting near-global topography maps of the Earth's land masses was a totally original mission, and our prime objective.

Earth, I say? The Earth?? This boring planet? How is this exciting? Well, to begin with, let's back up. Did I really say sixty-meter boom? Extending out from the Shuttle? Sticking out of the payload bay. Starts in a can, goes out 200 feet, Shuttle flies around for ten days, it doesn't break and wrap around the orbiter, then it fits nicely back in the can. Really?

Absolutely! That's the length between antennas we needed to do proper interferometry. And just think about how big sixty meters is. We were the largest fixed structure ever deployed in space. (The Patriot's offense rushed for less distance on Sunday. Sigh....) And the can it started and ended in was only 10 feet long - that's like shrinking Shaq to just over 4 inches tall. You can imagine the concern we got from some of the engineers and safety personnel at Johnson when we pitched them our crazy idea. If you or your kid has a Shuttle model, wrap a slinky around it a couple of times and you can picture some of the worst-case scenarios that went through their minds. But can you blame them, when the lives of astronauts and the future of NASA are possibly on the line? Needless to say, we were incredibly safe and successful or clearly I wouldn't be talking about SRTM to you.

Shuttle Radar Topography Mission Mast
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Shuttle Radar Topography Mission Mast
Mast extended at AEC-Able: The SRTM mast is fully extended at the contractor facility (AEC-Able, Goleta, California).
Also visible are the many cables snaking down the interior of the mast and wires that support the mast coming down from a ceiling track.Credit: AEC-Able / NASA
SRTM was really outstanding experience for me for a number of reasons. First was the strange and magical coincidences that seemed to line up just right to make the mission possible. My favorite went as follows. Figure out the maximum inclination the Shuttle can fly (so you can map as far North and South as possible) and look for orbits that repeat with even spacing, so you can cover the circumference of the Earth evenly. How many repeat in about ten days - which is as long as the Shuttle can stay up doing science? Well, a handful. How many are low enough for the Shuttle to reach and high enough above the atmosphere that you don't have to stop doing science every few hours and fire engines to boost your orbit? A couple. How many give you a ground spacing that's less than 225 km, which is the swath width of the Radar, so you can overlap each pass with no gaps? You guessed it: one. It's strange how some of this stuff works out sometimes in space exploration. Oh, and by the way, that overlap was 7 km. Out of 225 km - that's three per cent margin for error. And that was a harsh requirement for precise orbit control. (That could take another three pages - don't get me started!) But that one orbit allowed us to accomplish the mission, and map everything we flew over, day or night, clear or cloudy.

Another plus for me was the technical discussions full of passion that happened day after day during the development phase of the project. I remember fondly countless meetings and email threads spent arguing with the other key engineers over how best to do this and that. Our conversations crackled with energy. Sometimes I think the powers that be can place too much emphasis on calmness and compromise and diplomacy. Often when engineers are wrestling with an issue, were all tempted to step in and work out an even solution that makes everyone equally unhappy. But here's a secret: sometimes the crazy person tearing their hair out scribbling on the white board making their case with elevated audio is really on to something. Sometime compromise isn't the best solution. Passion is what drives exploration, and it was crucial that our managers let our kind of energetic arguments play themselves out, and they did.

I also think we benefited from flying under the Radar (pun entirely intended) to a certain extent. We weren't landing rovers on Mars or hurling schoolbus-sized behemoths through the rings of Saturn; we were just this little Earth mission that was an extension of three others that had already flown with success. Don't mind us.. just passing through.. we arent the droids youre looking for. And so we were left alone to form ourselves into a small, perfectly-staffed team of highly skilled engineers with just the right amount of highly skilled managers and just enough reviews from JPL and NASA HQ to do the impossible in a very short amount of time.

So what did SRTM get? Wrong question. The accurate question is, what did you get. Well, you got a continuous, high-resolution, self-consistent map of the entire land mass of the Earth (under 60 degrees latitude, which was as high as we could reach). And what can you do with that? Here are just a few examples:

Oh and anyone can go get the data for their own application. It's an information set for the ages.

P.S. SRTM flew aboard the Shuttle Endeavour, which was named for Captain James Cook's ship from 1768 to 1771. During this expedition, he explored the West Coast of Africa, South America, Tahiti, New Zealand, and Australia; observed the transit of Venus across the Sun; and became the first captain to calculate his longitudinal position with accuracy. Rather appropriate for us, I believe.

The BB of Death, or My Great Opportunity to Meet Congress

Sep. 26, 2006 | 09:18 PDT | 16:18 UTC

One of my favorite works of art isThe Fall of Icarus, oiled by the Flemish master Pieter Bruegel (the Elder) in 1558. It's one of the most interesting perspectives on the Daedalus and Icarus myth in that the tragedy is entirely ignored by the nearby country folk (described by Ovid), and has found mention in a number of literary works - even those of modern-era poets W. H. Auden and William Carlos Williams. The Daedalus and Icarus myth may seem common nowadays, but is still a great one for an engineer.

I have a print of Bruegel's masterpiece on my office wall, and I look at it occasionally, particularly when I'm worrying about dust and debris smashing into Cassini (my equivalent of flying too close to the Sun). Protecting Cassini from dust has been one of my major tasks and responsibilities on the project.

It should be obvious that any orbit around Saturn has to pass through the ring plane at two locations. One of these is always near Titan, since we're very interested in Titan and we use the gravity of Saturn's largest satellite to toss us around the neighborhood. The other is much closer to Saturn, in part because we need to keep the orbit period relatively short for frequent encounters and to make progress moving the orbit around to sample Saturn's environment from every viewpoint. Also, it's good to get as close as possible to Saturn for a wide variety of scientific reasons. The exact position of the inner ring plane crossing varies from orbit to orbit, but is generally within a handful of Saturn radii.

The Saturnian System
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The Saturnian System
The Saturnian system, satellites, and rings.Credit: D. Seal / JPL / NASA
Saturn's main rings, of course, are far too dangerous to pass through. (If light has a hard time of it, chances are the spacecraft would too.) It was interesting recently to reread in Cassini's very first Mission Plan (1990) that "the discrete gaps in the visible rings, although not completely devoid of material, afford regions that can be crossed with reduced risk." This is definitely NOT a statement we would have made in the year before arrival, and we wouldn't make it now. True, the "gaps" in the main rings have less risk in a relative sense to their immediate surroundings, but we've seen enough material in these regions that there's no way we're going to gamble there with a priceless national asset. The image collection below is an apt illustration. New material has been found in, or completely covers all of these "gaps". None of these regions are safe for Cassini.
New Ringlets and Ring Material
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New Ringlets and Ring Material
New ringlets and ring material discovered by Cassini. From A to D, they show the Maxwell "gap" in the A ring, the Huygens "gap" within the Cassini division, the Encke "gap" near the outer edge of the A ring, and the continuous dust sheet extending from the outer edge of the main rings all the way to the F ring.Credit: NASA/JPL/Space Science Institute
So we're left with "carefully peering over an electrified fence at Saturn". Our domain is beyond the F ring, which is host to the G ring pretty close in, the utterly unavoidable E ring which occupies the region from Mimas out past Rhea, the satellites which may or may not support material in their orbits, and the supposed "gaps" in between.

Before arrival, assessing the hazards of these regions constituted a classic and thorny mission engineering problem. We simply did not understand a lot about these faint rings - that's why we're there now! - so calculating the makeup of the dust within the rings was a huge challenge. Measurements of these rings were limited and often contradictory. The few optical images we had of the rings and some of the particles and waves observations, by their nature, only provided vague information on the total amounts of particles, but few hints as to their size distribution, with large error bars. We knew there was possibly material there somewhere, but it was difficult to say exactly how much and what the sizes of the dust particles were. There was a little less uncertainty about the sizes of particles we were sensitive to; hypervelocity theory and laboratory testing suggested that we should worry about debris about the size of a BB, about 1-2 millimeters in diameter. The driving failure scenario is one of these particles striking the electronics bus of the spacecraft and damaging our flight electronics. But even this information was limited by the impact speeds that could be tested on Earth at the time, and even now. In a nutshell, we had to make key project decisions by combining limited scientific data and engineering analysis in an intelligent fashion upon which rested (potentially) the fate of the mission.

Nevertheless, undaunted by the challenges of turning limited data into working strategies, Dr. Jeff Cuzzi, head of Cassini's Rings Discipline Group, gathered the best scientific minds in the world on rings and dust at a workshop at Ames Research Center in January 1996 (partially at my urging, I'd like to think). Out of that work came both plausible models for the regions where we had solid measurements, and upper limits on dust for the regions where we didn't. We reminded ourselves to be comfortable dealing with large uncertainties and remained open to change when new ring theories or measurements arose. And we selected our final location for Saturn Orbit Insertion.

Up to that point, the bulk of the analysis for dust hazards to Cassini had been done by a talented and pioneering engineer named Neil Divine, a fellow JPLer and MIT grad, who sadly passed away in January of 1994 and never got to see Cassini fly bravely but prudently into orbit. I inherited his task and much of his notes on the problems we faced, expanded the models to distribute material in all three dimensions (not just in the ring plane), and wrote software to simulate Cassini's potential passages through these regions to figure out what hazards we might face.

The image below shows one of the main decision-making charts I used to design our orbit insertion. This plot is a zoom-in of Saturn's rings as seen from the side; the horizontal dimension is along the bottom and the vertical component (parallel to Saturn's pole) is on the left. As you can see, both Voyager 2 and Pioneer 11 passed directly through the G ring. According to our models, if Cassini had followed Pioneer 11's path without adopting a protective attitude, our risk of hitting a deathly BB would have been a whopping 17%! However, keep in mind that Pioneer 11 did in fact discover the G ring during its visit to Saturn, so there's no faulting the decision-making at the time. And they did collect some good data to our benefit.
Cassini Orbit Insertion
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Cassini Orbit Insertion
Cassini Orbit Insertion trajectory shown with Pioneer 11 inbound and outbound and Voyager 2 ring-plane crossings at Saturn.Credit: D. Seal / JPL / NASA
Our goal for SOI was to find a crossing location outside of the F ring, and reasonably close to Saturn to conserve propellant, but without taking too much risk. In other words, "as close as can be done safely". (These kinds of expressions are annoyingly prevalent in mission engineering; they sound reasonable but really mean "balance this versus that versus the other thing, and don't screw it up.") Mimas was the fly in the ointment: if there was material sharing its orbit - plausible given its proximity to the main rings, at least - it would achieve a forced inclination (vertical extent) and eccentricity (horizontal extent) equal to that of Mimas from interactions with the satellite. And Mimas is both inclined and eccentric. There was one measurement that suggested there might be material in
Mimas' orbit, but none indicating any material between F and G, G and Mimas, or outside of Mimas, except for the E ring which was thought to be safe. So the best choice was to avoid Mimas and cross just one region instead of two - so we opted to fly between F and G (as shown; the E ring was thought to be without risk, and still is so far). Of course that didn't stop me from having the cold sweats in the days before arrival, or nightmares about testifying before Congress. (Kind of a perverse "what I did this summer" show-and-tell.)

Most of our ring models have held up well to the wealth of dust data we've collected to date. We did, in fact, detect all sorts of dust particles at SOI - but none dangerous as far as we can tell. We've confirmed Enceladus as the primary source of the E ring with dramatic images of its water geysers. I'm not aware of any Cassini measurements that would suggest there are any large particles in the E ring, which is consistent with our decisions before arrival not to avoid Enceladus. But one of the most exciting discoveries is a bright arc of material in the core of the G ring - likely the parent population of denser and larger material that supplies the whole thing. To date, we've avoided any part of the core of the ring, near the inner edge - but we grazed the outer parts of the ring in April of 2005 (flying high-gain antenna first for protection) and got close in September of 2005 (not high-gain antenna first). At this latter crossing, we lost took an impact that damaged one of our dust sensors, and later learned that we were lined up with the bright arc, but we hadn't pinned down the exact orbit of the arc at the time. We do not have any crossings near the bright arc in the future, I can assure you.
An arc in Saturn's G ring
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An arc in Saturn's G ring
These three images of the tenuous G ring were taken by Cassini about 45 minutes apart on May 24, 2005. In the first image, a bright arc is visible at the bottom edge of the ring. In the middle image, the bright arc has rotated around to the ansa (left side) of the ring. In the right image, the arc has moved up and to the right. The origin of this arc is unknown.Credit: NASA / JPL / Space Science Institute
In last week's blog, Mark Adler described another exciting discovery - and an alarming one, to me - that we've made in just the last few weeks: a new ring of material sharing the orbits of Janus and Epimetheus. I've always been suspicious of these satellites: they're the closest moons not directly associated with any material; they share orbits which makes one curious about a possibly collisional origin; and they're sizes are less than their Hill radii. The "Hill sphere" or "Roche sphere" is a measurement of how large a body can grow by accreting material. Prometheus, Pandora, and Atlas - all nearby - pretty much fill up their Hill spheres. Janus and Epimetheus do not. Now you can argue that this makes them either more or less likely to support material in their orbits. Most people might make the case for less, i.e. they've swept up all the small stuff already, and there wasn't enough of it to fill up the Hill sphere. But it's also possible, if you ask me, that whatever processes are going on there just haven't played themselves out yet. (Perhaps it's also the mythical origin of that tricky two-faced Janus... rr.) This is all easy for me to speculate about, naturally, in hindsight now that I know there's dust there.
Saturn's Main Rings
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Saturn's Newly Discovered Ring
Recent image showing Saturn's main rings including the F ring (overexposed at left); the G ring (middle with sharp inner edge); and the E ring (outermost). The new ring is marked with a "+". Earth is the bright dot at middle right. The rays extending from upper left to lower right and the black band at upper left are scattered light and overexposure artifacts. Credit: NASA / JPL / Space Science Institute

Oh, and guess what. Our current trajectory flies through the orbits of Janus and Epimetheus in June 2007. How wonderful! And of course I have a bunch of new work to do now (well a lot of it falls to the rings scientists) to derive a model for the new ring and figure out what, if anything, we need to do to the trajectory to preserve the spacecraft.

Sometimes I feel like a street cop on the beat. Constant vigilance for new troublemakers is the key. Maybe they'd let me carry a baton around lab or something.

New Horizons Spies Jupiter

Sep. 27, 2006 | 11:08 PDT | 18:08 UTC

I think it's about time to find out what's going on on some other projects in the space arena.

I can't help but be completely jazzed by the New Horizons mission. I really, really don't care whether Pluto's a planet or not, I'm still excited about the prospect (once dim) of seeing what Pluto and its satellites look like before I shuffle off this mortal coil. And today, APL released its first LORRI images of Jupiter taken as part of a test sequence today. And at an exposure time of only six milliseconds, the camera looks more than capable of doing its job when it's at Pluto getting 1/60th or so of the reflected light.

LORRI Image of Jupiter
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LORRI Image of Jupiter
Long Range Reconnaissance Imager (LORRI) image of Jupiter on NASA's New Horizons mission. Credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute

John Spencer (two bloggers ago) tells me "of course I'm thrilled to see the picture, modest though it is. It made my day when I first saw it.... It's a real psychological milestone to see Jupiter at last. LORRI is performing almost precisely as predicted, and it gives us a lot of confidence for our ability to take great pictures during the flyby. In fact this afternoon I've been using exposure levels from that first image to check our plans for close-up imaging of the new Little Red Spot next February..."

Best wishes to John and the entire New Horizons team for their future success.

Opportunity Nears Victoria Crater

On the west coast here, the MER team is getting excited about reaching the lip of Victoria crater in the next day or so. If they make it at week's end, I'll be inclined to restrain myself since Doug Ellison is next in the blogging line and he'll tell it better than I. (Plus I have some juicy bits left that'll easily fill two more days.) But I can't resist posting the latest panoramas of the crater that made their appearance today. Byron Jones, MER flight director (well that's his title on the MER web site at least) and JPL summer softball batting champ, spared just a few moments today relating their near-term plans. Opportunity should be about ten meters from the rim by the time you're reading this, and "peeking over the edge thursday or friday". Keep in mind that these panoramas are projected onto a flat screen, and make the crater look smaller than it really is - it's 800 meters wide. That's no less than ten Boeing 747s stacked nose-to-tail, plus a 737 tacked on the end. And a Honda Civic.

Black and White and Anaglyph Views of Victoria Crater
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Anaglyph Views of Victoria Crater
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Black and White and Anaglyph Views of Victoria Crater
Black and white and anaglyph views of Victoria crater, from an image sequence used to chart the final approach to the lip. The expected destination near the end of the week is marked approximately with an "X".Credit: NASA / JPl - Caltech plus D. Seal (black & white image) and Mike Howard + Midnight Mars Browser (anaglyph)

I noticed something curious when reading about Opportunity's latest, and quite possibly greatest target. They named it Victoria after Ferdinand Magellan's 16th century flagship used during his globe-circling expedition. They're also naming many features of the crater after some of his destinations. It's kind of a coincidence after composing the entry about SRTM on Monday (skip to the P.S. at the end of that entry to see why). I wonder who would win more head-to-head games of Civilization IV with ocean maps - James Cook or Magellan....

The Blog from Outer Space

Did you know that Anousheh Ansari, the intrepid space ambassador (the NASA web site employs the cleverly neutral title of "spaceflight participant") that went up with the ISS Expedition 14 crew, is keeping a blog while on the space station? It's not bad - check it out. She has some interesting things to say about the experience (if you can gloss over those smiley emoticons). It's interesting that they got a look at Atlantis' reentry. She'll be updating the blog until at least Thursday when she returns with Expedition 13.

Anousheh Ansari on Board the Space Station
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Anousheh Ansari on Board the Space Station
Anousheh Ansari on board the space station, disproving Isaac Newton and throwing high school physics into total chaos. Ansari is the first Muslim woman and Iranian in space, and the first private female space tourist.Credit: NASA / X Prize Foundation

The Winds of Change and Annoyance

Sep. 28, 2006 | 17:11 PDT | Sep. 29 00:11 UTC

Titan's atmosphere poses a similar dilemma for us as the dust hazards I talked about on Tuesday in that it's one of our prime scientific targets, it was poorly understood before our arrival, and it poses a potential hazard to the mission. In this case, however, we're not really worried about losing the spacecraft, but if we do fly a bit too low, the atmospheric drag could cause us to tumble out of control. The spacecraft would enter "safe mode", kill the active sequence of measurements, and regain control shortly after the encounter, but take no further observations until we verified the spacecraft is healthy from the ground (which usually takes a couple of days, since we tend to be careful about these sort of things).

It's worth reviewing that Titan is unique in that it's the only satellite in the Solar System with an appreciable atmosphere. It resembles the prebiotic Earth with organic molecules and is mostly nitrogen, so a large and diverse community of scientists are very interested in studying it in line with NASA's life origins vision. If Titan weren't orbiting Saturn (and "cleared the neighborhood" of its heliocentric orbit, as is now required), it would be a perfectly respectable planet at 5150 km in diameter (not counting atmosphere), which is larger than Mercury. Titan's atmosphere is surprisingly denser than Earth's at the surface by about 50% (it's got a *lot* of atmosphere). Cassini can only get about as close as 950 to 1000 km to the surface without losing attitude control - compare that to the altitude of the International Space Station, which floats happily above the Earth at a mere 390 km!

Titan Compared with other Solar System Bodies
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Titan Compared with other Solar System Bodies
Titan (lower right) illustrated along with other comparably-sized bodies in the Solar System to scale.Credit: NASA / JPL / Caltech
Atmospheric drag does slow down the spacecraft slightly; for a close flyby, the maximum force on the spacecraft provides a paltry twenty millionths of a gee, so it really has little influence on the trajectory. However, drag also places a torque on the orbiter which has to be compensated for by the thrusters. This is caused by an offset between the center of pressure - the balance point of the visible area - and the center of mass for all of the attitudes we want to fly. The center of mass is determined by the structure of the spacecraft and the propellant in the tanks, which are in the lower half of the spacecraft body as seen in the attached image, one possible attitude facing the wind direction. The center of pressure is higher and to the left due to the high-gain antenna and the magnetometer boom, so the atmosphere will push on the center of pressure and try to rotate the spacecraft around the center of mass.
Cassini Spacecraft Seen From one Possible Angle
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Cassini Spacecraft Seen From one Possible Angle
The Cassini spacecraft seen from one possible angle by the atmospheric wind during a Titan flyby. Approximate locations for the center of pressure (for this attitude) and center of mass are shown. Since the two are offset, Titan's atmosphere will induce a torque on the spacecraft since the drag will be centered on the center of pressure, attempting to rotate the spacecraft around the center of mass.Credit: NASA / JPL Digital Image Animation Laboratory / Caltech
And again, as with the dust modeling, the big problem was gathering up what observations we had - which were not always consistent - and assessing the hazards so we could fly low enough to sample the atmosphere well, but not so low that we risked tumbling. Befor arrival, all the models we had suggested that an altitude of 950 km was the right number - safe, but reasonably deep in the upper atmosphere.

However, once we got there, we got a few surprises. The image here is an illustration of the first few data points we collected on the atmospheric density. What conclusions would you draw? The density we had assumed for safety at 950 km was around where the leftmost circle is on the plot. So not only was the atmosphere more dense - possibly, in spots - but there was some indication of an atmospheric bulge at the equator (which isn't unrealistic). And since we prefer not to gamble with a priceless national asset, we updated our models and raised a number of altitudes (mainly those near the equator) by 50 km or so - enough to lower the drag effects by a factor of two or more.
Illustration of Early Atmospheric Density Data
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Illustration of early atmospheric density data (scaled to one altitude) vs. spacecraft latitude at closest approach. Credit: D. Seal / NASA / JPL / Caltech
Now that we've flown more Titan encounters low enough to enter the atmopshere - including Titan 18 last Saturday - our plot looks a lot different, like the second image here. The green line is roughly the curve fit we had some months ago, and now that we have these other data points, what conclusions would you draw now? I don't think there's an atmospheric bulge anymore, do you? It looks like a scatter plot at a fixed density, with some variations that can be modeled statistically. As our lead attitude control engineer Allan Lee says, "that's research".
Illustration of Early Atmospheric Density Data
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Illustration of updated atmospheric density data (scaled to one altitude) vs. spacecraft latitude at closest approach. The green curve is a fit to some of the early data, which now appears not to match the updated data set. Credit: D. Seal / NASA / JPL / Caltech
So if we now fit a straight line through these points, our best guess at Titan 18 was that our attitude control thrusters would have fired up to about 49% of their capacity at closest approach (which was at an altitude of 960 km). And as Allan and the attitude control team reported on Wednesday morning, they saw a 42% duty cycle (refer to the next image). So that was good news. Now the question is, should we go back and lower those altitudes again? Ha - not so fast. See the spread of the points that still remains? Is it possible that Titan's atmosphere just varies with time, and could give us grief in the form of the higher data points in the future? Certainly. Is it worth it to create a whole bunch of work for all of the science and sequencing teams to change all of the encounter designs yet again to implement slightly lower altitudes? Definitely not - at least not at this time. Even at a duty cycle of 40% or so, the Ion and Neutral Mass Spectrometer - the primary instrument that conducts in-situ atmospheric sampling - is collecting plenty of gas to satisfy the scientists. So we don't have to lower the altitudes.
Illustration of Thruster Profile
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Illustration of thruster profile during the Titan-18 flyby. The peak "duty cycle" or thrusting effort was 42% of capacity, which matched well with predictions. Credit: Allan Lee / JPL / NASA / Caltech

So, again, that's research. That's engineering. Our Titan atmospheric modeling working group will be meeting after every low Titan flyby to look at the latest data. You do the best you can with what information you have, become comfortable with uncertainty, stay on top of things when you get new information, constantly reevaluate your strategies, and make changes with conviction when needed.

And don't screw it up.

Other Titan-18 Results

The Cassini RADAR team has posted some of their SAR lake images, and I should certainly include them here. It looks like they are seeing more lakes, as with Titan 16. The excitement in the air when I talk to the RADAR folks is tangible.

Titan's 'Kissing Lakes'
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Titan's "Kissing Lakes"
This Cassini radar image shows two lakes 'kissing' each other on the surface of Saturn's moon Titan. The image from a flyby on Sept. 23, 2006, covers an area about 60 kilometers (37 miles) wide by 40 kilometers (25 miles) high. This pass was primarily dedicated to the ion and neutral mass spectrometer sensor, so the volume of radar data was small, but amazingly Earth-like lakes are seen. With Titan's colder temperatures and hydrocarbon-rich atmosphere, however, the lakes are likely to contain a combination of methane and ethane, not water. In this example, near 73 degrees north latitude, 46 degrees west longitude, two lakes are seen, each 20 to 25 kilometers (12 to 16 miles) across. They are joined by a relatively narrow channel. The lake on the right has lighter patches within it indicating that it may be slowly drying out as the northern summer approaches. Credit: NASA/JPL
Shorefront property, anyone?
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Shorefront property, anyone?
This lake is part of a larger image taken by the Cassini radar instrument during a flyby of Saturn's moon Titan on Sept. 23, 2006. It shows clear shorelines that are reminiscent of terrestrial lakes. Centered near 74 degrees north, 65 degrees west longitude, this lake is roughly 20 kilometers by 25 kilometers (12 to 16 miles) across. It features several narrow or angular bays, including a broad peninsula that on Earth would be evidence that the surrounding terrain is higher and confines the liquid. Broader bays, such as the one seen at right, might result when the terrain is gentler, as for example on a beach. Credit: NASA / JPL / Caltech

Pluto and... Voyager?

Sep. 29, 2006 | 16:48 PDT | 23:48 UTC

Are you sick of hearing about Pluto: Planet or No Planet? (Sounds like bad TV.) I am. But every other guest blogger has written a Plutonian entry since the IAU decree, and who am I to break the streak?

I was at a social function of sorts a few houses down the street last week and ran into Doug Griffith, former Voyager engineer and Magellan project manager, and he divulged rather wistfully that Pluto was a possible target of the Voyager spacecraft. Pluto? Really?? This piqued my curiosity (which is usually a bad idea). So let's take a stroll back in time and pursue this interesting subject, and let's start with the Grand Tour.

Flinging a probe into deep space to the outer planets posed unique difficulties that seemed insurmountable in the infancy of America's exploration of the Solar System. Even with the most powerful rocket in the U.S. arsenal in the 60s, the Saturn V, it would have taken thirty years to reach Neptune on a Hohmann transfer, and it was nearly impossible to conceive - at the time - of both a project team and spacecraft that would survive that long given the limitations of budgets, politics, and early spaceborne hardware.

Gravitational assists provided the answer. The concept had been around for centuries; Isaac Newton laid the foundation in 1687 in his Principia Mathematica, fully detailing the mathematics of the two-body gravitational problem. Hints of the potential of the three-body problem (e.g. the gravity assist, Sun + planet + spacecraft) was evident as early as the late 18th century, when Anders Lexell computed the orbit of a near-Earth comet discovered in 1770. He showed that the comet had made a close encounter with Jupiter in 1767, flinging it closer to the Earth where it was first discovered, and predicted it would re-encounter Jupiter again in 1779 and be expelled from the inner Solar System. Curiously, this comet which now bears his name is the closest known approach to Earth by any comet (though asteroids have come much closer) at about eight million kilometers (about eight times further than the Moon). Clearly, a trajectory had been altered significantly - twice - without rocket motors.

Interplanetary trajectory design began in earnest after the launch of the first U.S. satellite, Explorer 1 in January of 1958. Just months later, the Department of Defense in collaboration with JPL was directed to develop and launch probes to study the moon. Michael Mintovitch, who came to JPL in 1961, is credited with being the first person to thoroughly explore the concept of gravity assist transfers between multiple bodies in any order. It became clear in the early 60s that these maneuvers were a practical way to send a spacecraft of significant mass anywhere in the Solar System. As far as I can tell, this was first used on Mariner 10, which used a Venus flyby to reach Mercury, and Mercury itself to reencounter Mercury twice more; though Pioneer 11's flyby of Jupiter was around the same time.

A few years later, Caltech grad student and JPLer Gary Flandro made a careful study of Mintovitch's work and became convinced that it held the key to the outer Solar System. In July of 1965 (perhaps earlier, in his journals) he discovered a rare alignment in the 1980s of the gas giants that would enable a reasonably-timed mission exploring all four - provided a launch could be made in the late 1970s. This was a once-in-a-lifetime opportunity. He published the following year in Acta Astronautica (then called Astronautica Acta, actually) and the "Grand Tour" - and that was his own term - was born.

Voyager's Grand Tour
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Voyager's Grand Tour
The Voyagers' Grand Tour as originally conceived at JPL. Credit: Flandro, G. A., Astronautica Acta 12, No. 4, 1966.
I was pleasantly surprised at finding this article in the JPL library. The first sentence of his abstract is "Contrary to popular belief, indirect ballistic trajectories involving close approach to one or more intermediate planets need not require longer flight duration than is characteristic of direct transfer orbits." It's amazing to see a statement like that about a mission design technique that is the backbone of every interplanetary mission launched by mankind. I never did very well in history in school, probably because it was hard for me to understand how it applied to me, but this stuff is really cool.

In his paper, Flandro describes transfers to Jupiter and Saturn, Jupiter and Uranus, Jupiter and Neptune, Jupiter and Pluto (!!!), and the Voyager grand tour of Jupiter, Saturn, Uranus and Neptune (figures for the last two are attached). The available launch years vary slightly, but generally covered 1975 through 1981, with the middle years having better performance. Flandro's article caused quite a stir, and interest in a mission spread widely and rapidly, reaching the status of a media event with widespread public interest. And after a long string of problems with budgets, priorities, and politics, the Voyager program was finally approved by NASA in 1972. But the Grand Tour, or "Thermoelectric Outer Planet Spacecraft" suffered elimination by budget cuts when the perceived development cost for long-life spacecraft was deemed not affordable. In response, scientists and engineers, principally among them Roger Bourke, then supervisor for JPL's Advanced Projects Group, worked madly to propose a lest costly alternative, the Mariner Jupiter/Saturn '77, project, which met with approval and went forward. (Cassini faced a similar threat in the mid-90s, where we removed the scan platforms to save cost.)

However, there was a lot more to MJS 77 than met the eye. As Roger Bourke says, "we outfoxed them: we built the Jupiter-Saturn-Uranus-Neptune opportunity into the MJS '77 mission design. Engineers 1, Bureaucrats 0." Lou Friedman mused to us recently, "It is too bad we never got to Uranus and Neptune - probably they would have been interesting. (Smile).
Voyager is a little like the Mars Exploration Rovers -- they never die." JPLers today think
they're pretty clever sometimes, but the guys back then were pretty darn clever too. (I totally need to party with Roger some day.) Years after launch, engineers reconfigured V2's mission to extend its lifetime and capabilities towards a full Grand Tour.

Anyway, back to Pluto. There were no options for a Jupiter-Saturn-Uranus-Neptune-Pluto tour; Pluto just wasn't far enough along in its orbit (as you can see from the image) to be reachable from Neptune - it actually was behind Neptune, and closer to the Sun! But Flandro found a Jupiter-Pluto transfer with a flight time of only seven years, and later on a Jupiter-Saturn-Pluto transfer was developed as a potential option, reaching Pluto about nine years after Saturn. Compare these to a minimum energy direct transfer to Pluto of 46 years!
Jupiter-assisted Path to Pluto.
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Jupiter-assisted Path to Pluto. Credit: Flandro, G. A., Astronautica Acta 12, No. 4, 1966.

And again, we come to the "balance this versus that" problem. Even though they reconfigured Voyager 2 for a Grand Tour, the mission was sold on the exploration of Jupiter and Saturn. In order for Voyager to go on to Pluto, the encounter trajectory with Saturn would have been further constrained. No close flyby of Titan, for example, would have been possible. And even then, Titan was a key target. As Charley Kohlhase recalls, "we also wanted a second crack at Titan in case Voyager 1 failed.... We would have given up the Voyager 2 Jupiter-Saturn-Uranus-Neptune for another Jupiter-Saturn (with Titan)." Ellis Miner - who worked on Cassini with me for a number of years - also commented: "as we were approaching Saturn with Voyager 2, we could have gone directly to Pluto or we could engineer encounters of Uranus and Neptune. We didn't even know that Pluto had a moon at that time, but it wouldn't have made any difference. The combination of Uranus and Neptune were deemed far more important than a single flyby of Pluto. If we were making that choice today, I believe the choice would be the same."

What would you have done? Sacrificed key objectives of your primary mission for a chance at Pluto? Nine years later? Banking on the (then highly uncertain) possibility that your spacecraft would still be operating? Doing what they did, and leaving Pluto for a later mission seems like the right call to me. What would you do?

References and thanks: email thread and conversations with Charley Kohlhase, Lou Friedman, Roger Bourke, and Jeremy Jones;"Voyager's Grand Tour" by Henry C. Dethloff and Ronald A. Schorn; and "NASA's Voyager Missions" by Ben Evans.

Thanks to Emily for giving me a chance to try this out; it's been a lot of work, but great fun. I'm even more impressed by her blogging capabilities now that I know how hard it is!