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The Planetary Society BlogBy Emily LakdawallaPlanning for PlutoJun. 12, 2009 | 10:39 PDT | 17:39 UTC
John Spencer
As you can see, the timeline is pretty packed, and we're doing lots of different things- the few gaps between observations are used to re-point the spacecraft or for essential on-board housekeeping activities. Here's one of the observations we'll be particularly looking forward to- a high-resolution mosaic of Pluto with LORRI (the Long-Range Reconnaissance Imager, our workhorse telephoto camera) which will give us one of our best (400 meters per pixel) regional views of Pluto's geology, about 90 minutes before closest approach. The observation is called P_LORRI_STEREO_MOSAIC because one of its main purposes will be to give us a stereo view of the landscape when combined with other views of the surface taken from different angles.
This image illustrates one of the challenges that has occupied much of our thinking for the past 18 months- we don't know exactly where Pluto is. We'll be taking images as we approach Pluto to refine our knowledge of its position, of course, but because we're heading pretty much straight for it, our knowledge of its location will be much more accurate in the directions perpendicular to our approach than in the direction of flight. When we fly past, we won't know Pluto's position along our trajectory to better than plus or minus 2500 kilometers, or approximately the diameter of Pluto itself. In the graphic above, this uncertainty, combined with other smaller uncertainties, is represented by the gray oval- some part of Pluto could be anywhere inside that oval. So it's possible that part of this mosaic will fall on dark sky, but it's designed so that we can be pretty sure that most of it will hit Pluto, wherever Pluto is. All our observations are designed with these uncertainties in mind. We now know when these observations will be taken, the exposure times we'll use, and how they will be stored on the spacecraft. What we don't know is what the images will actually show- it boggles the mind to imagine that. The P_LORRI_STEREO_MOSAIC images will be about as good as the best images Voyager 2 obtained of Triton 20 years ago in 1989, and those images were completely bizarre. I'm sure Pluto will be equally bizarre, but in its own unique way.
Here's an example of one of the trickier details that we've been finalizing in the past week. When we get to Pluto, the sun will be shining on the south pole (a.k.a. the north pole, according to some astronomers- the controversy over what to call the two poles of Pluto, which spins on its side, is second only in vigor and meaninglessness to the controversy over whether it's a planet). The north pole (I'm using the official International Astronomical Union definition here) will be in darkness during our brief flyby. That's a shame- we'd really like to see what's going on up there. We expect a lot of nitrogen snowfall during the long polar winter as nitrogen frost burns off the summer hemisphere and transfers to the north, but so far that's just an idea, and we'd love to see whether those winter snowfields really exist. Fortunately the sun isn't the only light source at our disposal- we also have Pluto's moon Charon, which is big and close and thus looms large in Pluto's sky. So we can observe Pluto's north pole by moonlight. In fact one of the deciding factors in choosing just when to make our Pluto flyby was the availability of Charon as a light source- we are flying past at a time when Charon lights Pluto's night side about as brightly as possible. The trouble is that Charon's light is never very bright at all. In fact it's ridiculously faint- if you tried to walk around in Charon-light, you'd keep bumping into things. At the most, we expect Charon's light to be more than 10,000 times fainter than sunlight, and even sunlight, at Pluto's distance from the sun, is only about as bright as the tasteful lighting you might use in your living room in the evening. So to see those moonlit snowfields we need long exposures: 3 minutes is about the minimum that's useful. If we tried to take a single 3-minute exposure, though, two bad things would happen. First, the photo would suffer horrible camera-shake, because we can't hold the spacecraft steady enough for a sharp photo for more than half a second or so. Second, it would be completely overexposed, because the best time to look at the night side is when we are looking back at Pluto after our flyby, and at that time Pluto is only about 13 degrees from the Sun, less than the width of an outstretched hand. Sunlight will be flooding the inside of the camera, and everything we look at will be seen through a bright wash of background light. This stray light alone will be 1000 times brighter than Pluto's night side, and will overexpose the camera in a few seconds. We know just how bright the stray sunlight will be, because we've already taken test exposures on the spacecraft, in the same orientation. So instead of one 3-minute exposure we will take about 360 half-second exposures, which we will add together once they've been sent back to Earth. We will also need to take 360 exposures of blank sky, with the sun shining into the camera at exactly the same angle as for the Pluto images, so we can subtract away all that stray light. There are even more complications. Not only the sun, but the bright sunlit crescent of Pluto itself, will be scattering light inside the camera. By changing the spacecraft pointing slightly, we can change the position of that scattered light pattern relative to Pluto, and, we hope, subtract away that too. There are lots of details like this, but we're down to checking off the last few. We're having a meeting in Boulder this week in which we are putting the final seal of approval on many of our observation designs, and by late summer we expect to have the entire close approach period finalized. There will still be a lot of testing to do, and lots more work for the flight team, but we scientists will have completed the bulk of our work. That is, until we finally have Pluto in our sights, and all these plans start turning into reality. Plug of the Day It's amazing what amateurs are doing with planetary images these days. Using calibrated data archived in the Planetary Data System, or even the preliminary raw images posted on the Cassini or MER web sites, they are generating color images and mosaics that eclipse in detail and sheer beauty the best that professionals could have produced a decade ago. One of the finest practitioners of planetary image digital magic at the moment is Croatia's Gordan Ugarkovic. Check out his Flickr page and be amazed.
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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. His favorite moons are the active ones, Io and Enceladus. When he's not staring at a computer screen, he enjoys Colorado with his wife Jane and their dog Maggie, and loves tooling around Boulder on his bicycle.