Nasa Picks "juno" As Next New Frontiers Mission |
Nasa Picks "juno" As Next New Frontiers Mission |
Jun 15 2005, 11:27 PM
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#61
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Senior Member Group: Members Posts: 1870 Joined: 20-February 05 Member No.: 174 |
Why is it spinning?
Field and Particles instruments and gravity measurements really like spinning spacecraft. You cross-calibrate magnetometer data perpendicular to the spin axis, and get "whole sky" coverage with the particles instruments you simply can't get with 3-axis controlled spacecraft. Cassini (before program de-scoping) was going to have two scan platforms: One for the imaging and remote sensing spectrometers, One (or was it two?) for scanning the charged and neutral particle spectrometers across the sky. Voyagers and the gutted Cassini had to do special maneuvers to orient particle detectors in desired look-directions. With a scan platform, Voyager still could do remote sensing. Cassini is so crippled by the descoping that it has to constantly trade-off between science investigations as there is no scan platfor at all, and it has to stop all remote sensing to replay data back to Earth, then not transit 2/3 or so of the time while it's taking data. We'd have far more pictures from the mission (better mapping coverage, etc) if they hadn't lost the scan platform to bad management and radical surgery mandated by bean counters. Oh.. Gravity studies are much helped by the stable attitude and no thruster firing of spin stabilized spacecraft. Note that the "Pioneer Effect" (teeny sunward directed apparent acceleration of the Pioneers in deep space) is utterly undetectible on the Voyagers, due to trajectory peturbations by attitude control thruster firing. |
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Guest_BruceMoomaw_* |
Jun 16 2005, 02:26 AM
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#62
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Fear not! We WILL see excellent images of Jupiter again in our lifetime, and in fact very soon. Not only will we have Juno's photos of Jupiter -- and the photos New Horizons will take during its Jupiter gravity-assist flyby in mid-2007 (which will take it just a short distance outside Callisto's orbit, much closer than Cassini came) -- but the Europa Orbiter will spend about 2 years in Jovian orbit setting itself up to enter an orbit almost parallel to Europa's, so that it can then brake into orbit around Europa itself with the absolute minimum of fuel required (which will still be a lot). In order to set up that orbit, it will make about half a dozen very close flybys of Ganymede and two or three of Callisto (along with a dozen of Europa itself) -- and they have always intended to take full advantage of its instrument payload during that long prelude to observe those two moons and Jupiter itself, using all the orbiter's Europa instruments (except maybe its subsurface radar sounder, whose antennas may not be deployed until it's entered Europa orbit). And it WILL have a high data-return rate.
The one part of the Jovian system that we may not see well again for a disappointingly long time is Io. However, serious consideration is being given to making one of the second-priority batch of New Frontiers missions an "Io Observer", which would enter a Jovian polar orbit to minimize its radiation dose, and then make a whole series of repeated close Io flybys to observe that world. (While the radiation environment at Io's orbit is so savage that it would knock out even the radiation-hardened Europa Orbiter in a few days if we tried to put it into orbit around Io instead, the radiation dose that a Jupiter polar orbiter would get during repeated close Io flybys is tremendously smaller -- such a craft could make fully 50 Io flybys before getting the same total radiation dose that the Europa Orbiter mission will get.) There's a good chance that we'll see this launched some time in the early 2020s. |
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Guest_BruceMoomaw_* |
Jun 16 2005, 02:38 AM
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#63
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Footnote: the reason that the radiation dose for a Jovian polar orbiter that makes 50 Io flybys would be so much smaller than that for an actual Io Orbiter is because Jupiter's trapped radiation is concentrated not only close to the planet (except for its very closest region), but also around its equatorial plane, where the Galilean satellites orbit. This is also what allows Juno to get away with orbiting so close to Jupiter (with solar panels, yet) without getting quickly fried; during the part of each orbit when it's near Jupiter's equator, it's also very close to the planet and thus closer than the range of Jupiter's most intense radiation belt.
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Jun 16 2005, 05:43 AM
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#64
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Senior Member Group: Members Posts: 1870 Joined: 20-February 05 Member No.: 174 |
And.... It's moving perpendicular to the belts.... like the proverbial bat-out-of-hell!
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Guest_BruceMoomaw_* |
Jun 16 2005, 07:44 AM
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#65
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It would be more accurate to say that they intend to repeatedly thread the hole in the "doughnut" of high-intensity radiation around Jupiter.
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Guest_Analyst_* |
Jun 16 2005, 12:34 PM
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#66
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Bruce, I want your optimism when it comes to future space missions. Your are talking about proposed missions in the 2015 to 2030 timeframe as if it's just waiting and there they are. They will not, at least not all. Roadmaps get changed, very quickly, new ones emerge and disappear.
Look at the Voyager odyssey in the late 1960ies and early 1970ies (TOPS, Grand Tour and so on), look at Galileo and Cassini (Cassini could have done several asteroid flybys, but they saved some dollars in cruise mode). I'm talking only about the cornerstone missions what left the pad. Or see Alan's fight for a (small) Pluto mission. Analyst, pessimist, realist? |
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Jun 16 2005, 01:53 PM
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#67
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Senior Member Group: Members Posts: 2530 Joined: 20-April 05 Member No.: 321 |
QUOTE (Analyst @ Jun 16 2005, 05:34 AM) Bruce, I want your optimism when it comes to future space missions. Your are talking about proposed missions in the 2015 to 2030 timeframe as if it's just waiting and there they are. They will not, at least not all. Roadmaps get changed, very quickly, new ones emerge and disappear. Analyst, pessimist, realist? Perhaps in order to please Congress, NASA issues a new, sweeping mission document, revolutionary in its statement of mission goals, that promises a Whole New Plan about every 18 months, and it's like the last New Plan never existed (although the new new one often includes many of the elements of the old new one). Of course, with O'Keefe's (mercifully) short tenure, we also have had high turnover at the top, not to mention a change in the US Presidency, in the last few years. Griffin may be less faddish than his predecessors, but we'll have to see about that. |
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Jun 16 2005, 05:20 PM
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#68
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Junior Member Group: Members Posts: 81 Joined: 25-February 05 From: New Jersey Member No.: 177 |
QUOTE (BruceMoomaw @ Jun 16 2005, 02:38 AM) Footnote: the reason that the radiation dose for a Jovian polar orbiter that makes 50 Io flybys would be so much smaller than that for an actual Io Orbiter is because Jupiter's trapped radiation is concentrated not only close to the planet (except for its very closest region), but also around its equatorial plane, where the Galilean satellites orbit. This is also what allows Juno to get away with orbiting so close to Jupiter (with solar panels, yet) without getting quickly fried; during the part of each orbit when it's near Jupiter's equator, it's also very close to the planet and thus closer than the range of Jupiter's most intense radiation belt. Huh? SOLAR PANELS? When will they learn... no chance of RTGs at all? How the heck do you any real data rate from Jupiter with solar? |
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Jun 16 2005, 06:25 PM
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#69
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Founder Group: Chairman Posts: 14433 Joined: 8-February 04 Member No.: 1 |
QUOTE (MiniTES @ Jun 16 2005, 05:20 PM) Well - it's solar arrays, or no mission at all given the mass and financial constraints. Consider Rosetta and Stardust - both will/have gone out way beyond martian orbit relying on solar power alone Doug |
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Guest_BruceMoomaw_* |
Jun 17 2005, 12:08 AM
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#70
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Yup -- they've had solar panels planned for a Jupiter Polar Orbiter from the very start, a decade or more ago. They also intend them for the Jupiter Multiprobe Flyby mission, whenever that flies -- and one study for NASA's 2001 Outer Planets Exploration Workshop concluded that they were also entirely practical for a SATURN Multiprobe Flyby mission: http://www.lpi.usra.edu/meetings/outerplan...01/pdf/4113.pdf
The reasons are simple: (1) Much lower cost. (2) No safety concerns. (3) They're not so heavy that you can't easily make them big enough for a Jupiter mission to have a data-return rate of several hundred bps or more -- which is all Juno or those other missions require. (4) The orbits planned for these missions involve much lower radiation exposure -- one of the major vulnerabilities of solar cells -- than a Jupiter orbiter in the equatorial plane. |
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Guest_BruceMoomaw_* |
Jun 17 2005, 12:21 AM
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#71
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Guests |
QUOTE (Analyst @ Jun 16 2005, 12:34 PM) Bruce, I want your optimism when it comes to future space missions. Your are talking about proposed missions in the 2015 to 2030 timeframe as if it's just waiting and there they are. They will not, at least not all. Roadmaps get changed, very quickly, new ones emerge and disappear. Look at the Voyager odyssey in the late 1960ies and early 1970ies (TOPS, Grand Tour and so on), look at Galileo and Cassini (Cassini could have done several asteroid flybys, but they saved some dollars in cruise mode). I'm talking only about the cornerstone missions what left the pad. Or see Alan's fight for a (small) Pluto mission. Analyst, pessimist, realist? You're right -- but the plans for a Europa orbiter seem, at long long last, to be firm. Mike Griffin was absolutely explicit on the subject in his Congressional testimony; the Decadal Survey and the new Solar System Roadmap have both unambiguously declared it to be the most important non-Mars Solar System mission for the future (as the Decadal Survey did for the Pluto probe just before Congress finally ordered Bush and O'Keefe, over their dead bodies, to fly it); and see the new word from NASA to the Outer Planets Assessment Group at its meeting last week: http://www.lpi.usra.edu/opag/jun_05_meetin...jun05report.pdf Apparently Griffin will recommend it for a start in 2007 and flight around 2014 -- just as the S.S. Roadmap recommends. As for those other missions: the report also says, on the negative side, that the third New Frontiers mission's AO and launch will be delayed until 5 years after Juno, instead of 3 as had been planned --although they hope to speed the schedule back up again after that. You're certainly right in saying that the current schedule in the S.S. Roadmap is over-optimistic -- such plans always are. But after the remaining three top-priority NF missions are also flown (the Moon, Venus, a comet), the Io Obsever was ranked high among the second-priority NF missions by the Decadal Survey -- apparently about first or second-place among them. So, notwithstannding NASA's inevitable delays, there is still a real chance that the Io mission will get flown some time before the end of the 2020s. |
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Guest_BruceMoomaw_* |
Jun 17 2005, 12:25 AM
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#72
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You'll notice that I HAVE backtracked from the Io Observer having "a good chance of being flown in the early 2020s" to "a real chance of being flown before the end of the 2020s" -- having now considered the implications of the newly announced stretchout in the New Frontiers schedule. But beyond that, I refuse to retreat.
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Guest_vjkane2000_* |
Jun 17 2005, 02:55 AM
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#73
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QUOTE (BruceMoomaw @ Jun 16 2005, 05:25 PM) You'll notice that I HAVE backtracked from the Io Observer having "a good chance of being flown in the early 2020s" to "a real chance of being flown before the end of the 2020s" -- having now considered the implications of the newly announced stretchout in the New Frontiers schedule. But beyond that, I refuse to retreat. By the end of the 2020s, I'll be 75. I'd rather not wait that long, so I'm hoping for a Discovery-class mission to be proposed and accepted. One, called Volcan, was written up several years ago. One can imagine what such a mission might look like: Solar powered. Number of encounters would be limited by cumulative radiation dose. Galileo experienced ~40Krad per encounter. The solar panels will likely be the limiting factor in the number of enounters. I'm not sure how quickly they degrade. The Volcan proposal include something like 3-5 enounters. Perhaps that can be stretched a bit, and/or the radiation per orbit lessened by choosing an encounter point or orbit inclination to minimize radiation. The instrument compliment could be pretty simple: The Deep Impact Hi-Res and medium-res camera with visible imagers and near-IR spectrometers. A radiometer would need to also use the Hi-Res optics (possibly replacing the near-IR spectrometer). If the encounters will be very close, then a magnetometer would be nice. The mission would include as many close encounters as the radiation limit will allow (probably at least 5, maybe 10?) spaced 1 or more months apart to allow time series studies of changes. After that limit is reached, perijove would be raised to a safe distance to allow continued observations from a distance (which is why I'd like to see something with the optical capabilities of the Deep Impact Hi-Res on the craft). Long distance time studies would continue as long as funding would allow. During this phase, observations of Jupiter or encounters with Ganymede or Callisto (radiation at Europa is too high) could be possible, but add to mission complexity and cost. |
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Guest_BruceMoomaw_* |
Jun 17 2005, 07:17 AM
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#74
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It's a possibility -- but I suspect you're talking about something that would still hit the NH cost band (which may be why Volcan was rejected). One other Io Discovery mission was proposed: William Smythe's "Firebird" -- a solar-powered craft which would make just a single flyby of Io, but would take thousands of photos at extremely high speed during that one flyby, plus a comparable number of near-IR mineral maps and IR thermal maps, and release two little subprobes to make very low-altitude flights through volcanic plumes carrying mass spectrometers. It would then play back its huge store of recorded data at liesure over several months after its Jupiter flyby. This, of course, would lack any ability to observe changes on Io -- as well as covering much less of its surface -- but it would also cost a lot less. It was rejected, too; but in this case we really are talking about something with a good chance of hitting the Discovery cost band if properly adjusted.
This, of course, raises another, cheaper possibility -- the fact that any craft that makes a Jupiter gravity-assist flyby en route to a more distant destination will also have a chance at one close Io flyby. New Horizons could have made one if that idiot Goldin hadn't delayed it from Nov. 2003 to Jan. 2006 for no reason -- in fact, during the 1990s, when the Pluto Express flyby was being planned for that launch window, Germany expressed interest in adding an Io volcanic-plume probe with a mass spectrometer itself. In any case, the white paper on Io missions presented to the Decadal Survey -- which is pretty much the latest thinking on the subject -- can be found at http://www.lpi.usra.edu/opag/io.pdf . |
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Jun 17 2005, 07:22 AM
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#75
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Senior Member Group: Members Posts: 1870 Joined: 20-February 05 Member No.: 174 |
You really *do* want a very high power telescopic system on such a mission. A high inclination orbit will give close encounters with Io over a very limited range of orbital longitudes and thus illumination conditions. Even if you get 10 flyby's over 1 year, with Jupiter's 12 year orbit around the sun, the sun-angle at each flyby changes by 1/12 of 360 degrees.
The higher resolution the telescope, the better your monitoring coverage, and the better the detailed geologic mapping coverage from say 10,000 km. You haver longer "dwell" time at a given resolution if you aren't in a close periapsis flyby when you're getting that resolution. An all-reflecting optics telescope can feed anything from ultraviolet to middle infrared with different instruments arrayed around a "pickoff mirror" at the focal point. That gives lower resolution imaging spectrometers more time at a given resolution as well. Imagine global VIMS coverage of Titan at 1 km/pixel instead of spot coverage at 2 km/pixel which Cassini is getting. |
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