OPAG Reports, Formal proposals/evaluations of future outer SS missions |
OPAG Reports, Formal proposals/evaluations of future outer SS missions |
Nov 9 2007, 08:28 PM
Post
#1
|
|
Senior Member Group: Members Posts: 2530 Joined: 20-April 05 Member No.: 321 |
http://www.lpi.usra.edu/opag/announcements.html
That's one little URL with a lifetime's worth of reading material. Three detailed studies are available in PDF format. The missing body is Titan, which will be the subject of a forthcoming report. The three focus missions are: Europa Explorer: Fairly detailed description of a mission that is pretty much what Europa Orbiter would have been. Jupiter System Observer: Basically, Galileo 2 (without the antenna mishap!). The craft would start with a 3-year tour of all the Galileans, then spend 1 year in an elliptical Ganymede orbit, then the rest of the mission in a tight, polar Ganymede orbit (like MGS at Mars). That would map the heck out of Ganymede, but also be close enough to the rest of the system to make long-range observations for years. Note that Ganymede would thereby provide a lot of radiation shielding. Enceladus: where three profiles are examined in depth: Enceladus Orbiter only; Enceladus Orbiter with soft lander; Saturn orbiter with Enceladus soft lander. There's more to chew on here than I have had (or may ever have) time for, but I'll throw in my two cents' worth: Seems like a Europa-only mission would only benefit from coming after a JSO. EE would explore Europa much better than JSO would; why even have JSO observations at Europa if EE came first? In many ways, these two missions are competitive. EE would have the big payoff, but JSO seems like basic recon that would prime EE, especially giving specs on radar performance. But if we waited til JSO was 4 years into its mission before completing design of EE, then put EE sometime mid-century. If an Enceladus mission included a Saturn orbiter, then maybe the same orbiter could provide data relay for separate Titan elements. However, a lot of the Enceladus science goals would require an Enceladus orbiter, so I don't think a Saturn orbiter for Enceladus/Titan will win out. Note that Enceladus orbital velocity is low enough that the craft could manage to take lots of hits from ice pellets and survive. Put a bulletproof vest on the craft and let it soar through the plumes endlessly. |
|
|
Nov 13 2007, 05:14 PM
Post
#2
|
|
Member Group: Members Posts: 706 Joined: 22-April 05 Member No.: 351 |
Some more details comparing the Europa explorer and JSO options.
There's a very good detailed description of Europa Explorer's strawman instruments and the way they would be used -- as of last May -- at http://www.lpi.usra.edu/opag/may_07_meetin...gship_study.pdf . You'll notice that they are very similar to those in the JSO Flagship final report, with the following differences: (1) The final version of JSO calls for a half-meter aperture telescope with the Narrow-Angle Camera and Vis-NIR Spectrometer hooked up to it, for a resolution of 0.4 meters/pixel from 200 km range for the Camera. (This is actully a change from the design of JSO's strawman payload last May, which called for separate optics for the camer and spectrometer -- the result being that they both had considerably lower resolution while weighing only 5 kg less total than the new setup.) Europa Explorer as of last may called, by contrast, for a Narrow-Angle Camera with a resolution 5 times poorer, but weighing only 15 kg -- plus a separate Vis-NIR spectrometer weighing 30 kg. I rather expect to see a similar combined-optics system now adopted for Europa Explorer (whether it features a comparably big telescope or not). (2) JSO contains a big hulking Thermal-IR Spectrometer weighing fully 43 kg -- whose separate IR optics, in fact, are as huge (0.5 meters) as those for JSO's NAC-VNIS combination! (This is also a new change; last May, the TIR Spectrometer was to weigh only 20 kg but was to be accompanied by a separate 15-kg Thermal Imager.) Clearly good high-res thermal-IR spectra are considered very important on this mission, in order to study Jupiter's atmosphere and Io. Europa Explorer, as of my latest info, carried only an 8-kg Thermal Imager. So presumably a full-fledged Thermal-IR spectrometer is the most likely instrument to be listed for JSO but rejected from Europa Explorer -- since the latter is supposed to focus more on Europa, with general Jupiter-system science being a lower priority than it is for JSO. (3) The Medium-Res Stereo Camera on JSO, again, has a resolution 5 times better than the stereo camera on Europa Explorer (plus a pixel swath twice as wide) -- but weighs twice as much. (Europa Explorer would also carry a 3-kg Wide-Angle Camera.) (4) The Plasma and Energetic Particle Spectrometer on JSO weighs twice as much as the 12-kg one on the Europa Explorer (although it uses only a little more power) -- but Europa Explorer would also carry a separate 15-kg Ion & Neutral Mass Spectrometer, which is not on JSO, to analyze substance sputtered off Europa's surface by Jupiter's radiation. (This is the lowest-priority instrument on Europa Explorer.) I don't know what to make of this difference -- since the Europa Explorer plasma instrument, unless it's descoped, would also have ability to make time-of-flight analyses of the composition of plasma. Presumably the JSO version is more sophisticated in its sensitivity or resolution. So, really, the instrument payloads for the two missions are strikingly similar. The main differences seem to be higher optical resolution for the cameras and near-IR spectrometer on JSO, and the absence of a thermal-IR spectrometer on Europa Explorer. All of which is to some extent changeable; the final version of Euripa explorer might easily end up carrying a thermal-IR spectrometer and combined optics for its cameras and spectrometers, although I imagine these would all be lighter-weight and lower-capability than their equivalents on JSO. One more important note: it was stated at the Icy Moons Workshop that JSO would carry just as much radiation shielding as Europa Explorer, in order both to allow those Io flybys (reduced from four to three in JSO's latest version) and to prolong its lifetime in the Jupiter system for as long as possible. The overall features of the final version of the JSO spacecraft and that of May's version of Europa Explorer are very similar: Europa Explorer: 7225 kg total mass, 2608 m/sec delta-V JSO: 7262 kg; 2705 m/sec The main differences are that JSO would carry a lot more total instrument mass (310 versus 212 kg), an 8th MMRTG to power the craft, and of course more area of radiation shielding to cover the extra instruments (although, since JSO would only be designed to endure 1.8 Mrad versus Europa Explorer's 2.3 Mrad, the shielding would be thinner and so its total mass would be virtually identical -- 165 versus 162 kg). The buses for the May version of Europa Explorer and that of the current JSO -- minus their science instruments and radiation shielding -- have almost the same mass: 1889 kg versus 1934. But the new JSO has not only more science instrument mass (310 vs. 212 kg), but considerably more radiation shielding (243 kg vs. 165) -- so that its total dry mass is 220 kg higher than that of Europa Explorer. Also, JSO carries more propellant for the additional 97 meters/sec of delta-V it required for its mission. All this is apparently possible because the 2017 launch opportunity listed for JSO is a good deal better than the 2015 window listed for Europa Explorer: 7810 kg capability versus 7225 (using a Delta IV-heavy and a VEEGA trajectory in both cases). So: delay Europa Explorer by a couple of years, and you could put a lot more on it, too. One other difference: JSO's final version allows mission data rates of fully 600-1600 kbps, whereas the May version of Europa Explorer provided only 200-300 kbps -- but this seems due to the fact that JSO would have a 50-watt Ka-band transmitter with a low-powered X-band backup, whereas the May version of Explorer used a 50-watt X-band transmitter and only a 3.5-watt Ka-band one. The power requirement of JSO's version is only modestly higher, and its mass is actually slightly less than for Explorer's version -- so I imagine the final version of Explorer will be changed to the JSO setup. We seem to be looking at only minor differences between Europa Explorer and JSO: the latter would trade off somewhat thinner radiation shielding to allow more instruments an 8th MMRTG, and maybe a somewhat higher com rate. And, even then, the only significant instrument differences are (as mentioned above) just smaller optics for the cameras and VNIR spectrometer (which Europa Explorer would partially compensate for by lowering itself into a 100-kg attitude Europa orbit, versus JSO's 200-km Ganymede orbit), and the lack of a full-fledged thermal-IR spectrometer. -------------------- |
|
|
Lo-Fi Version | Time is now: 28th March 2024 - 07:05 PM |
RULES AND GUIDELINES Please read the Forum Rules and Guidelines before posting. IMAGE COPYRIGHT |
OPINIONS AND MODERATION Opinions expressed on UnmannedSpaceflight.com are those of the individual posters and do not necessarily reflect the opinions of UnmannedSpaceflight.com or The Planetary Society. The all-volunteer UnmannedSpaceflight.com moderation team is wholly independent of The Planetary Society. The Planetary Society has no influence over decisions made by the UnmannedSpaceflight.com moderators. |
SUPPORT THE FORUM Unmannedspaceflight.com is funded by the Planetary Society. Please consider supporting our work and many other projects by donating to the Society or becoming a member. |