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The Planetary Society Blog
By Emily Lakdawalla
John Spencer: Io!
Mar. 1, 2007 | 17:42 PST | Mar. 2 01:42 UTC
by John Spencer
 It's a good thing I wasn't in charge of any critical spacecraft operations yesterday (actually, it's a good thing I'm never in charge of any critical spacecraft operations, at least not directly, but that's another story). One thing about planetary encounters is that you never get enough sleep, and by last night I was running on about 1.5 cylinders. I gave an evening public talk at the Applied Physics Laboratory (APL), describing some of the early highlights of the New Horizons Jupiter encounter, and when I finally got back to the hotel I realized I'd left my laptop power supply AND my camera in the auditorium. Now I'm at the airport about to head home, a little bit better rested, and hopefully I have all my stuff with me…
This has been an amazing week so far. We now have Jupiter behind us, though still very much in our sights, as we speed away to Pluto. Today the spacecraft is looking back at Jupiter's moons, and after completing another communications session with Earth, we'll be beginning what might be one of our most interesting observations, eleven hours spent mostly in taking multiple image mosaics of Jupiter's ring system, as the spacecraft climbs up through the ring plane.
I wrote last week about how Kandis Lea Jessup and I spotted a giant volcanic plume on Io with the Hubble Space Telescope, in pictures taken on February 14th. We suspected that the plume was being ejected by the volcano Tvashtar, one of Io's most active volcanos in recent years, and we hoped that the eruption might last long enough to be visible by New Horizons as we flew past Jupiter. But even if the plume was active, we knew it wasn't a sure thing that New Horizons would see it. Hubble could only see the plume in ultraviolet light, and New Horizons doesn't have an ultraviolet camera.
If Io cooperated, and the plume was bright enough in visible light, we would have a chance to see it very soon. We had always wanted to see our data as soon as possible, of course, but our communications with Earth during encounter week are consumed with the vital business of checking the spacecraft health and uplinking the instructions for the next set of observations. When the flight team finally completed planning that monumental task, however, there were a few gaps in the schedule where it seemed possible to squeeze a handful of images from LORRI, our long-range camera, into the downlink. So a couple of weeks ago I went through the timeline and chose a few pictures for this preview, and Debi Rose, our Science Operations leader, was able to fit them in. By chance, the two sets of Io images in my list had Tvashtar on the edge of Io's disk, perfect for viewing any plume against the blackness of space.
The first pair of Io pictures came down on Monday morning. I was busy with other things, trying to observe Io's volcanos from the NASA Infrared Telescope in Hawaii, with my friend Julie Rathbun at the University of Redlands and telescope operator Bill Golisch. Julie and I weren't actually in Hawaii, we were sitting in our offices with an Internet and phone connection to the summit. Good thing we hadn't flown all that way, because the mountain was socked in. Waiting for the clouds to clear, I phoned New Horizons Project Scientist Hal Weaver at APL about something else, and found him in the middle of looking at those first pictures, with LORRI Principal Investigator Andy Cheng, and a film crew to boot. Yes, they saw a plume! They e-mailed the picture to me, and there was the Tvashtar plume- a 300-kilometer-high umbrella sitting on top of Io like a beanie cap, in a picture twelve times the resolution of the Hubble image. A plume on IoThe Tvashtar plume (right image, 11 o'clock) as seen from New Horizons on February 26th, in the first picture returned from the spacecraft during the close approach phase. The left image is a shorter exposure designed to look at Io's surface. Credit: NASA / JHUAPL / SwRI |
Then I was off to APL for the close encounter. The two days at APL are a bit of a blur already, and too much happened for a blow-by-blow account here (but do check out our pictures of Europa, Ganymede, and Jupiter's Little Red Spot). All through Tuesday and most of Wednesday, though, I was impatient to see our second set of Io images. Longer exposures, closer range, and better lighting were going to make for an even better view of Tvashtar. Finally the pictures came down around 3 o'clock on Wednesday afternoon, and Hal Weaver brought them up on the projector screen in our science team meeting for all to see. He brightened the long-exposure image, and wham! the plume leapt spectacularly into view. The best picture ever taken of a big volcanic plume on Io! (Except, that is, for all the others still on the spacecraft.) There was amazing structure in the plume canopy, all sorts of knots and filaments. Sharper but much fainter Voyager 1 images of the Pele plume had hinted at similar structures in 1979, but Voyager exposures were too short to show that plume as clearly as we were seeing Tvashtar now (no shame on the Voyager team for that; they didn't even know that Io was volcanically active when they planned their pictures).Multiple plumes on IoIo, now a crescent as New Horizons swings around to the night side, sporting three plumes: the enormous Tvashtar plume at 11 o'clock, Prometheus at nine-o'clock, and Masubi against the night side of Io, just beyond the southern terminator. The night side of Io is illuminated by Jupiter-shine. Credit: NASA / JHUAPL / SwRI |
That's it for the close-encounter downlink. Now we have to be a little patient and wait for the real data stream, which begins in about a week and will last through April. There will be lots more Tvashtar plume pictures, because it's near the north pole and so big that it rises above the pole itself, so every Io image we take will have that plume in it! We'll have color data and maybe even infrared pictures too, though detecting the plume in the infrared will be tough. The flyby is over, but the fun is just beginning.
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