Friday, September 01, 2017

Pioneer-11 at Saturn - 38 Years Later

Today in 1979, Pioneer 11 made its historic flyby of Saturn, humanity's first.  While it got the first closeup images of the Saturnian system, it's images, such as this one showing the rings and a transiting Mimas, while interesting, were no match for the more spectacular images that the Voyagers returned of the Jovian system that year, and were quickly supplanted by the Voyager visits to Saturn in the coming years. 

A close-up of Saturn, rings, and moons on encounter day

Due to trajectory constrains, Pioneer approached and receded from Saturn looking at the backlit side of the rings, something never seen from earth except briefly when the rings are nearly edge on during ring plane crossings.  Thus its view was interesting but also unfamiliar. 
Approaching Saturn (with Titan)
Receding from Saturn.  Due to data rate constraints, only every other pixel was returned, hence the reduced quality.

Best view of the unlit side of the rings during approach

During closest approach, Pioneer briefly passed through the ring plane and back, in theory giving it a view of the sunlit side of the rings.  Given the speed at which it was traveling and that it's scanning imager, which built up images one pixel at a time using the spin of the spacecraft, took a long time to assemble pictures, not much could be done with this.  The instrument did have, however, a lower resolution mode that was for the purpose of polarization studies, which could only take very crude images (if they could be called that).  It could, however, work fast enough to capture the fleeting view.  Here is an example of one. 
The daylit side of the rings in front of the panet, with the limb visible behind them.

These images can be built up into a pretty nifty model of the rings. 

 The day side of Saturn's rings using data from Pioneer 11 in polarization mode

 While it is neither the view normally associated with Pioneer nor should it be, it brings us full circle, seeing a world and its rings in a more familiar way as it looked 38 years ago, the day we first stopped by its doorstep while, at the time of writing, we sit 14 days before our first long-term visitor becomes one with Saturn.  After 1981, which marked three consecutive years with an encounter, it would be another 23 years before humanity returned with Cassini.  Let's hope that our imminent absence is brief.

Friday, February 03, 2017

Pre-Discovery Images of the Plumes of Enceladus

LPSC 2017 Abstract 1603 Pre-Discovery Detection of the Plumes of Enceladus

From the moment I first found out about the discovery of geysers erupting into space by NASA's Cassini mission, I began to wonder whether they had been detected before.  Pioneer data is of insufficient quality to detect them, but Voyager definitely could have.  I carefully examined all of the images targeting Enceladus, but the illumination angle wasn't ideal (very high phase, or backlit, images are best for this, but none of the available images were taken at such an angle).  I came up empty handed and dropped project.

Flash forward to the New Horizons Pluto encounter in the summer of 2015.  I was sitting in a plenary meeting and the high phase images of Pluto's atmosphere were being discussed for the first time.  Later in the meeting, someone asked a question about serendipitous images that might contain Pluto's small moons.  The next talk was particle and fields stuff that I didn't really understand, so my mind wandered, and it clicked - maybe Enceladus made a serendipitous appearance in some high phase Saturn images.

In the fall of 2016, I finally found time to work on this.  While Voyager 2 had the better camera, due to a spacecraft anomaly, it was not taking pictures during the first few days after closest approach, so after a check to see if there was anything from right after it was recovered, I moved on to Voyager 1.  I experimented with some imagery from a few days out to get familiar with its cameras under these conditions before beginning in earnest. This image of Saturn came from this effort.

Saturn from the Receding Voyager 1
As I searched through the dataset, a group of images stood out.  They were taken on November 13, 1980, a day after closest approach.  None of the Narrow Angle Camera images contained Enceladus, but eight of the Wide Angle Camera images did.  They showed the small crescent and night time hemisphere illuminated in Saturnshine as only a few pixels across.

Cleaned raw image from the dataset used

I thoroughly calibrated, deconvolved, and stacked the images.  After combining different subsets as well as the full set, I am confident in the detection of the plumes, just where they should be.

Enceladus from Voyager 1 on November 13, 1980.  The right side is illuminated by the sun, the left by reflected light from Saturn, and the plume can be seen at the bottom

In my excitement at having at last had some success with this project,I quickly wrote an Lunar and Planetary Science Conference abstract, which can be read here.  It gives more detail about how this dataset was handled.

Erupting Enceladus over Saturn on November 13, 1980
I plan to continue sifting through images to see if there are other datasets worth processing in that way.   I plan to do similar work with other worlds as well.  At the very least, this indicates Enceladus is indeed constantly erupting, extending the baseline back more than two decades. 

Monday, July 13, 2015

Reflections on a flyby: My personal journey to Pluto

As a ten year old, I would take daily breaks from a long summer of yard baseball and building trails through the woods to go to the public library and that of a local college, hoping to spot the latest Voyager image of Neptune and Triton, its planet-sized moon.  I would scour every newspaper, magazine, CD-ROM – anything that came in, hoping to see a slightly sharper view of these mysterious worlds.  

Of course many days ended in disappointment.  When that happened, or when I had some extra time after seeing the latest from Neptune, I would seek out what else I could find.  Via NASA publications, books, and especially back issues of periodicals, I was able to relive a whole history that I had missed.  I rode around the far side of the moon with Luna 3.  I saw our view of Mars evolve from Christian Huygens first spotting of dark markings and polar caps to our first glimpse of Martian craters from Mariner 4 (incidentally, 50 years to the day before the New Horizons Pluto flyby).  I got to ride along with Mariner 6 and 7 as they zoomed in on Mars and Mariner 9 as it watched dust clouds abate and the surface reveal itself on a global scale

I would set out book and compare how worlds looked as different spacecraft approached.
Mariner-10 brought us the cloud covers of Venus and the cratered surface of Mercury, and the Venera and Viking landers took us to the surfaces of Venus and Mars.  The Pioneers gave us glimpses that were enough to tell us that the Jovian and Saturnian systems were far more complex and interesting than we thought from our earthbound view, but while they were very good at studying other things such as magnetic fields, they were small spacecraft designed to pave the way for the follow on, Voyager.  

While the previous missions revealed a world (or, in the case of Mariner 10, two worlds but separated by years), the Voyagers visited planets with planet-sized moons as well as a retinue of smaller companions.  From the volcanoes of Io to Titan’s thick atmosphere, from Europa the billiard ball with a hint of a global ocean to Miranda with its jigsaw puzzle surface, each encounter was a grand tour of a mini solar system. 

August 25 - encounter day - finally came and I was glued to the coverage on PBS.  Neptune and its great dark spot created a lot of buzz, and Triton looked like, well, a cantaloupe (at least in places).  I didn’t get to watch all the coverage – my mother made me go to bed, telling me I’d thank her for it someday (still hasn’t happened).  I remember after much arguing, responding, “OK, but I’ll be there next time.”

Next time.  There was much to do about Voyager 2 reaching the edge of the solar system, exploring its most distant planet and its moons.  That was technically true at the time – from 1979 to 1989, Pluto was inside Neptune’s orbit.   But the proclamation that we had completed the initial reconnaissance of the solar system felt hollow, because there was one major world that hadn’t received a visit – Pluto.  But the Voyagers, speeding out of the solar system, could not help us anymore.  

It would take great effort by people such as Principle Investigator Alan Stern, to finally get humanity on its way to filling this gap with the launch of New Horizons in 2006, 17 years after Voyager visited Neptune.  In the intervening years, three things changed our view of Pluto.  From ground based and Hubble Space Telescope observations, we produced our first crude maps, revealing Pluto to be one of the most high contrast objects in the solar system.  This indicated that it isn’t simply a carbon-copy of Triton, which is thought to be a captured satellite and is similar in size and mass to Pluto.  

Second, the Kuiper Belt of distant worlds beyond Neptune, predicted a half a century earlier, became real to us, transforming Pluto from an oddball to the largest of these outer worlds that we can only faintly see from earth.  Pluto remains the largest, and is the only one to have a moon so big that it could be a planet in its own right (technically, Pluto and Charon form a double planet).  Two more small moons were found, a number which has since risen to four.   

The Plutonian system is the last menagerie of worlds on the scale reminiscent of the ones surrounding the giant planets that we have yet to visit, albeit without the giant planet.  It is the only unexplored household name in the solar system.  It is the only remaining world that humanity had in mind when it reached out into the solar system with the Mariners, Lunas, and Pioneers.  

The experience of seeing the new images as they come down and the reactions of scientists in real time has been a priceless experience – much different from scrounging around at the local library – oh, and Mom, told you I’d be here.  There are plenty of small worlds left to explore and our studies of even much-visited Mars are far from complete – this is the end of the beginning, by no means the actual end, if one even exists.  But will be no single unexplored target that stands tall above the others.  There will be no great region of the solar system taunting us.  The initial reconnaissance will be complete.  Some people have gotten to live the whole journey - 66 years since Luna 3.  The rest of us have joined it in progress.  July 14, 2015 will be the last opportunity to live it as it happens.  It is truly a special time to be alive.  

Tuesday, May 26, 2015

Kanehekili Patera from Galileo

Due to the nature of its orbit, although Galileo made several close approaches to Io, some parts of its surface were never seen except from a great distance.  Some of these areas were covered by Voyager 1's close flyby, but others were not.  The are centered around Kanehekili, the volcano which can be seen erupting above the limb at about four o'clock, is one such place. A major eruption was seen here earlier this year.  Hopefully a new mission will be able to fill in this gap.

This image was taken on May 6, 1997, during Galileo's eighth orbit around Jupiter.

Processed image Copyright Ted Stryk, Raw Data Courtesy NASA/JPL

Sunday, May 10, 2015

Nereid from Voyager

When planning began for Voyager 2's 1989 encounter with Neptune, the planet had two known moons - Triton and little Nereid, a small, distant moon that, like Triton, is likely captured.  The flyby didn't go well for little Nereid.  First, it takes nearly 360 days to orbit Neptune - almost a year! - and was not in a convenient place for Voyager to meet it (unless Neptune itself and Triton were sacrificed).  Second, it would be dethroned as Neptune's second largest moon by Proteus, a newly discovered moon that would receive much better coverage. 

Voyager 2 came no closer than 4.7 million kilometers from Nereid.  All that could be made out is that it is a somewhat spherical (if a bit lumpy), 340-km in diameter little world with a relatively low albedo -15%.  It rotates in 11 hours, as determined by light curve data, further supporting the idea that it is captured (not being tidally locked is unusual for a moon). 

Here are Voyager 2's two best views.  The first shows the lumpy little world in a gibbous phase as Voyager approached on August 21, 1989, and the second shows a crescent on August 24 - Nereid was "in front" of Neptune, so closest approach was earlier than the August 25 closest approach of Neptune.

Yet another world cries out for exploration!

Processed images Copyright Ted Stryk, Raw Data Courtesy NASA/JPL

Tuesday, April 28, 2015

Looking Down On Jupiter's North Pole.

In December 1974, Pioneer 11 became the second emissary of humanity to visit Jupiter.   Being a spin stabilized spacecraft with essentially no onboard memory and only able to transmit at 2048 bits per second, it carried no real camera, but it did carry the Imaging Photopolarimeter, basically a scanning one-pixel photometer which scanned along with the spacecraft's spin.  By doing this, it could, over time, develop crude pictures.  In high resolution mode, the images were at most 14 degrees wide and 466 pixels across (although in practice most were much smaller.  And given that the spacecraft was moving quickly and these scans took around half an hour, the ones near closest approach are, in addition to being frustratingly small, badly distorted.  Although attempts were made to correct for this, they were made with 1970s computing power, so they are limited, and since the digital data is missing (perhaps forever), all we have to work with are scans of these attempts. 

Pioneer 11's dataset is unique in that after passing Jupiter, it looked down on its north pole as it headed off across the solar system toward Saturn.  No other spacecraft has gotten such a direct angle on a Jovian pole (although Juno will do so next year), but the global images are frustratingly small.  In this view, I have combined the best six pictures (12 if you count the color pairs as individual pictures) to make a mosaic of Jupiter looking down on the pole soon after closest approach.  I think I can improve on it, but having tinkered with it a year, I am ready to share the image as it stands now.

Processed Image Copyright Ted Stryk, Data Courtesy NASA/Ames Research Center.  A special thanks to the Ames Research Center history office for helping me find the data used in making this.