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The Planetary Society WeblogGuest Blogger: John SpencerSeptember 11-17, 2006
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John Spencer is a staff scientist at Southwest Research Institute's Department of Space Studies in Boulder, Colorado and is a member of the New Horizons and Cassini science teams. His research interests include the moons of the outer planets, particularly the Galilean satellites of Jupiter and the icy moons of Saturn. When he's not staring at a computer screen, he loves exploring Colorado's mountains with his wife Jane and their dog Maggie. |
Pebbles and Pluto
Thanks to Emily for inviting me back to this forum! It's going to be tough to maintain the standard set by the previous guest bloggers, but I'll give it a shot. I asked to be assigned this particular week because it coincides with a meeting of the science team for the New Horizons mission to Pluto, the second since we became spaceborne last January. We'll be assembling in Boulder this time, so I can get to the meeting by bicycle rather than by plane, which will be a nice change. But New Horizons isn't the only thing on my mind, so expect some variety in the days to come.
First let me add my two cents to the burning question of our age, the status of Pluto. Like Rosaly in last week's blog, I'm fascinated by both the level of public interest, and the passions aroused in the planetary science community, by the non-scientific question of the box into which we put New Horizons' primary target. Why is it such a big deal? I keep thinking of the late, often great, Douglas Adams, author of the Hitchhiker's Guide to the Galaxy, and a phrase of his, "the furniture of the mind". To a much greater extent than the other wonderfully diverse inhabitants of our solar system, for most of us the nine planets have been our furniture since we were children. They are the setting in which we place ourselves, in which we feel comfortable. We get upset when someone rearranges the furniture without asking us.
 The Pluto system on May 15, 2005 This and another photo from the Hubble Space Telescope have revealed that Pluto has two more, small moons in addition to the previously known Charon. Credit: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team |
Still, I do have my own opinion on this question, and you get to hear it now.
There may be cultural and historical reasons to maintain Pluto's planetary status,
but from a scientific point of view, the new IAU definition of a planet, which
excludes Pluto, makes a lot of sense. We try to conform our nomenclature to nature
as best we can. Sometimes this is easy because nature presents us with tidy categories
-- there are reptiles and there are mammals, and (platypuses notwithstanding)
there are no really fuzzy boundaries between them. Sometimes nature is less cooperative
and presents a continuum -- the distinction between pebbles and rocks and boulders
is completely arbitrary, though still useful (if you don't like this blog, please
throw only pebbles in my direction).
In our solar system we see both situations. Close to the Sun, there are four medium-sized rocky worlds (plus one big moon) that are millions of times more massive than anything else in their part of the solar system. Further out, there are four big gaseous worlds that are also vastly more massive than their neighbors. Then, in regions protected in one way or another from disruption by the big guys, there are several stable swarms of smaller bodies (the asteroid belt, the Jupiter Trojans, the Kuiper Belt, and so on) which have an essentially continuous distribution of sizes. These swarms have a few large members (like Ceres and Pluto), more that are a little bit smaller, and many more that are smaller still, all the way down to dust. To my mind, it makes sense to confer by our language a special status on the eight bodies that stand so far above their neighbors.
The distinction between mammals and reptiles is so striking that it tells
us something fundamental about the different histories of the two groups,
and a definition of "mammal" that included all hairy warm-blooded creatures that secrete food for their young, plus crocodiles, would not be very satisfactory. Similarly, the distinction in our solar system between the eight big worlds that stand alone, and the swarms of smaller objects with continuous size distributions, is so striking that it's telling us something fundamental about their differing origins. The IAU's language about "clearing the neighborhood" is
an attempt, perhaps a clumsy one, to capture this difference.
Yeah, it's not perfect. The language could be improved, one can imagine
hypothetical situations where the IAU definition might itself prove fuzzy
(what if we find a Mars-sized world in the outer reaches of the Kuiper
Belt?), and the definition may be less easy to apply to other real or hypothetical
solar systems. But I think the bottom line, that there are eight "planets" in
our solar system, plus a lot of other fascinating worlds, is a better way
to look at our neighbors, at least from a scientific point of view. The
question of how this definition applies to extrasolar planetary systems,
which was not addressed by the IAU resolution, doesn't bother me too much
-- it will be a while before we probe down to continuums of small bodies
in other solar systems, and we can revisit the question then. In the meantime,
we'll happily welcome all those alien super-Jupiters and super-Neptunes
and super-Earths into the planetary club.
Boy, I seem to have got quite excited by this issue myself. Next time I'll find something more real to talk about.
New Horizons at Jupiter: Dreams and Reality
Phew! We're three days into our four-day series of team meetings for the New Horizons Pluto mission, and I'm already verging on exhaustion. There's been a lot of information to absorb, but it's mostly been great news. Our spacecraft is healthy and is proving to be a fast learner as we teach it all the skills it needs to carry out its mission -- for instance it already seems to be pointing at targets more accurately than we'd hoped. As Alan Stern, our fearless leader, pointed out, it's amazing to realize that only a year ago we were still tinkering with the hardware as the spacecraft sat in a clean room at the Goddard Spaceflight Center in Maryland. Now it's more than halfway to Jupiter.
 New Horizons' Alan Stern New Horizons Principle Investigator Alan Stern summarizes the mission status at the science team meeting in Boulder on September 11th. Credit: Courtesy John Spencer |
We have our final meeting tomorrow morning for the Jupiter planning team
-- it's only half a day, but my friend Jeff Moore and I are in charge of this
part, and we have some tough work to do.
We've been working for several years on a plan for making the most of New Horizons' flight past Jupiter. For our prime mission, Jupiter's really just a stepping stone, a gravitational convenience, giving us a boost that shaves a few precious years off our flight time to Pluto. But it's also one of the most amazing neighborhoods in the solar system, a place full of wonders, and although we're the eighth spacecraft to Jupiter, the payload that we're carrying to Pluto is well suited for some unique Jupiter science, too.
Since we started work on this mission in earnest in 2001, we've been aware of how much we stand to learn from a Jupiter flyby, and we've been developing a wish list of observations of the Jupiter system that we'd like to make. Some of these use New Horizons' unique capabilities to fill gaps in our knowledge -- for instance we don't yet have a good handle on the atmospheres of Jupiter's four big "Galilean" moons, and our ultraviolet instrument, Alice, can help with that by watching stars as they pass behind the moons. Other observations are similar to what's been done before, but are worth repeating because of the dynamism of the Jupiter system. Jupiter's weather changes from year to year, and Io's frantically volcanic surface, described by Rosaly last week, will likely look quite different from the face it presented to the Galileo spacecraft during that mission's final observations in 2001. To come up with our list, we've supplemented the patchy Jovian expertise of the science team with invaluable help from Jupiter experts outside the team.
Once we launched last January and knew for sure that we would have a Jupiter flyby (if we had launched too late, we'd have had to skip Jupiter and take the long road to Pluto), our plans began to solidify. We soon figured out exactly where Jupiter's moons would be as we flew past, so we could plan exactly when to observe them (to our mild annoyance, they tend to cluster shyly on the far side of Jupiter near closest approach). We worked with the mission operations team to determine when we needed to interrupt our observations to downlink data to Earth, how much of our precious fuel it would take for all the fancy maneuvers we wanted to make, how much data we could afford to store on board, and so on. We worked with the instrument teams to determine exposure times, which way we need to tilt the spacecraft to get the best data and so on. For the last eight months we've debated and compromised and e-mailed and teleconferenced and simulated and drawn up our spreadsheets and polished them till they are things of beauty, and now we have a wonderfully detailed blow-by-blow account of exactly how we'd like to spend those precious weeks when Jupiter glides magnificently past our sensors. I've posted a description of our plan, mostly for the benefit of astronomers who'd like to help us out with supporting telescopic observations.
 New Horizons Slide One of the diagrams we've been using in the last few days to help us think about how to point the spacecraft during our observations of Jupiter. Credit: Courtesy John Spencer |
But there's a catch. New Horizons doesn't understand our spreadsheets. Everything
we've done has to be translated into commands written in the spacecraft's exacting
language, and then it has to be tested on our simulators, which include a rack
of electronics at the Applied Physics Laboratory that reproduces the electronics
on board the spacecraft, and speaks the same language. If our commands upset
the spacecraft in some way, it's a lot easier to fix the code and try again
on the simulator than it would be on the spacecraft. There are innumerable "gotchas" that
must be avoided, some already discovered the hard way on the real spacecraft.
Voltages must be set correctly, commands must be timed so they don't overlap,
memory must not be overloaded, temperatures must remain within prescribed limits,
and so on.
All this work must be done by a handful of people on the spacecraft operations team who have been incredibly busy up till now just learning how to fly the spacecraft, and planning the vitally important initial calibration observations as the various instruments are switched on and tested for the first time. Only now, with that work largely behind them, can they turn their attention to Jupiter, and we have only a few months left as Jupiter bears inexorably down upon us.
So tomorrow's meeting is all about how we reconcile our desires with the engineering realities. It already looks like we won't get everything we want -- there just isn't enough development and testing time. How much we *will* get remains to be seen.
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Postscript.
The Jupiter planning meeting just ended, and I feel both more optimistic about how much of our plan we might be able to accomplish, and less optimistic about the amount of free time I'll have in the next few weeks. The long mountain hike I'd planned for tomorrow, as reward for surviving the last four days, ain't gonna happen.
We spent a lot of time discussing ways to simplify the implementation process by standardizing and repeating sequences. We'd like to make each observation perfect, for instance by increasing the exposure times as we swing from the day side to the night side of the moons. But if we keep exposures the same, and accept that some images will be slightly underexposed, we'll be better off than if we lost the images entirely. We can shorten some observations, and eliminate some low-priority ones, to give us some breathing space in the timeline. We'll make it work. We'll still have a great encounter.
 Jupiter system montage Jupiter and its four planet-size moons -- Io (upper left), Europa (center), Ganymede (lower center), and Callisto (lower right) -- were photographed in early March 1979 by Voyager 1 and assembled into this collage. The moons and planet are not shown to scale. Credit: NASA / JPL |
Lunch, Jupiter, and Saturn
I'm writing over lunch, at my favorite coffee shop, The
Bookend on Pearl Street Mall in Boulder, just a couple of blocks from
our office. I don't have a specific topic in mind for today, so I'll just
describe what I've been up to since my last entry on Wednesday.
As I said on Wednesday, I had a lot of work to do after the New Horizons science team meeting, trying to simplify our Jupiter encounter plan to make it easier for the spacecraft team to implement. So yesterday afternoon I set myself up in an empty conference room, spread out the relevant charts on the big table (until they invent a lightweight 3-foot-square super-high-resolution computer display, some things are best appreciated on paper), and sat down with the master spreadsheet and a laptop full of e-mails from other team members. I worked steadily through the timeline, making lots of small adjustments to the times and durations of planned events to give us more breathing room. Sadly, I've already had to throw out a couple of lower-priority observations to make more room for more important stuff- we could *probably* have made time for them, but we wouldn't have known for sure till late in the planning process, and would then have wasted time, which we don't have, in backtracking and fixing the problem. The job involved a little weather-forecasting, with help from amateur astronomers John Rogers and Hans-Joerg Mettig in Europe, among others, who have been tracking the position of Jupiter's Great Red Spot and its new Little Red Spot. They were able to tell us where these storms will be next February, so I could check that we'll be observing them at the correct times. To appreciate how different Jovian weather is from terrestrial weather, imagine deciding today where to point a camera at the Earth to photograph a specific hurricane five months from now!
Finishing this job, and double-checking the results, took me till nearly 10pm, so it was dark when I finally cycled home to Jane, my very understanding wife.
This morning, with a crashing of mental gears, I turned my attention to Saturn, the next planet down the line, and the Cassini mission. This is a very different mission than New Horizons, much bigger and more complicated- in terms of data volume and complexity Cassini has already completed the equivalent of 28 New Horizons Jupiter encounters, one per Saturn orbit. Planning the observations takes several different teams who have been meeting regularly, usually by bi-weekly teleconference, for the last five or six years. By now we know each other's voices very well, and I look forward to these regular visits with my Cassini buddies.
A major topic for today was trying to shoehorn an important new observation into a timeline of observations to be taken next April, that was decided on several years ago. There's a process for making these types of changes in response to new discoveries, and this particular change is in response to one of Cassini's most spectacular discoveries, that the tiny ice moon Enceladus is geologically active. We discovered in July 2005 that Enceladus is emitting heat, gas, and dust from south polar fractures that we're calling "tiger stripes", and in November 2005 Cassini took a remarkable picture and some very cool infrared spectra of these geyser-like plumes, when the spacecraft and Enceladus lined up so that the sun lit up the plume from behind. Unfortunately we haven't had a good look at the south pole of Enceladus, or a close up view of its plume, since then.
This coming November we finally fly under the south pole again (though not as close as we'd like), so we'll get another view of the tiger stripes and the heat radiating from them. But we don't get another close view of the plume, which we can only see when it's backlit by the sun, till next April 24th. Current plans, made before we knew about the plume, completely ignore Enceladus during the critical period. The spacecraft is instead expecting to be busy with important measurements of the dust (ultimately derived from Enceladus) that clusters near Saturn's ring plane, and with a fairly close look at terra incognita on Dione, another interesting moon. We can't add Enceladus to the agenda without cutting into these other observations. This is where it gets interesting. The dust detector team, and the fans of Dione on the camera team, will lose valuable data if we turn the spacecraft to Enceladus, but they appreciate that Enceladus is important too. And the Enceladus fans don't want to trample roughshod on everyone else's science to get what they want. So there's a bit of a polite dance as the different interest groups feel out each other's levels of pain to work towards a compromise. How bad would it be to lose the last 30 minutes of dust data? How about the last 45 minutes? How much more could we learn about Enceladus if we observed it for those extra 15 minutes? Mostly I think the process works well, largely because we've worked together so long that we appreciate each other's science, and appreciate that Saturn is an integrated system- we need each other's help to get the big picture. Anyway, we don't have a final plan yet, but we'll be able to squeeze Enceladus into the timeline somehow, so look out for more great plume pictures next April.
After the Cassini telecon it was back to the real world with a visit from a representative from the local United Way, and we talked about fund-raising for local charities for a while. It's always good to remember that our wonderful space ships aren't the only communities that we belong to.
Wow! Thanks to the coffee shop's wireless Internet, the first Jupiter image from New Horizons just landed on my desktop! A deliberately smeared-out streak, of no use for anything but camera calibration, but it's a thrill to have Jupiter in our sights at last. The first drop of water trickles out of the firehose.
[ what is this? ]