New Frontiers 4: Argo? |
New Frontiers 4: Argo? |
Aug 21 2008, 01:36 AM
Post
#1
|
|
Member Group: Members Posts: 723 Joined: 13-June 04 Member No.: 82 |
There does not appear to be a thread about this proposed New Frontiers mission, so I am starting this one. If there is an existing thread that I am unaware of, then by all means merge them.
Argo is a proposed outer solar system multiple flyby mission, rather like an updated Voyager mission, but based on the New Horizons bus, and using a similar instrument suite. It would launch between 2017 and 2019, with either a Jupiter / Neptune / KBO or a Trojan / Saturn / Neptune / KBO trajectory. Even a combined Jupiter / Saturn / Neptune / KBO trajectory is possible. Neptune would present a much different system than in 1989, as telescopic observations show a more dynamic Neptune atmosphere, due to the change in season, and much more of Triton and the other Neptune satellites would be visible (most of Triton's northern hemisphere was in darkness in 1989, but will be well lit in 2030). The second big payoff would be the vastly greater access to KBOs (~4000 times the accessable volume of New Horizons), with several already-known large KBOs (400km diameter or larger) within reach. The objects reachable with Argo are expected to include: 18 cold classical KBOs (interesting because they apparently formed in situ beyond Neptune's orbit, rather than further inward) 40 KBOs with diameters between 200km and 400km 9 KBOs with diameters greater than 400km several binary KBOs plus the possible Jupiter Trojan early in the mission. The wide expected range of choices allows for the selected KBO to be of very high scientific interest (and naturally, follow-on KBO targets could be selected after the primary KBO target has been selected). Typical flight times from launch to the Neptune flyby are about 10 years (Jupiter gravity assist) or 13 years (Jupiter Trojan flyby), with the large KBO flyby 2 or 3 years later. Expected cost including launch vehicle (according to the linked pdf): under $800M with the following strawman instrument package: High resolution visible camera: New Horizons (NH) or reduced Cassini heritage Near-IR spectrometer: NH heritage UV solar & stellar occ. spectrometer: reduced Cassini heritage Far-IR linear radiometer: Diviner heritage Magnetometer: replaces NH dust instrument Charged particle spectrometer: NH heritage Gimballed high-gain antenna: heritage radio science instrument The big uncertainty at this point seems to be the availability of plutonium-powered RTGs by the time of launch. |
|
|
Nov 6 2008, 05:00 AM
Post
#2
|
|
Member Group: Members Posts: 599 Joined: 26-August 05 Member No.: 476 |
Weapons grade plutonium and uranium can not be used in RTG's as those compounds are to "hot" and the radiation would interfere with spacecraft science instruments. The thought that occurred to me is could an RTG have a say 10 to 15 percent mix of the weapons grade material with the existing supply and/or could the weapons grade material pellets be in the interior of the RTG canister as to be shielded by the surrounding material. Pu-239 and U-235 (whether weapons-grade or not) has far less heat output per unit mass than Pu-238. Pu-238 has a relatively high heat output due to relatively short half-life (80-something years) vs Pu-239 or U-235 (24K or 700M years). In this sense, Pu-238 is "hotter" than Pu-239 or U-235. A piece of Pu-238 will probably be glowing hot depending on what it is alloyed with while you can hold a piece of Pu-239 in your hands without discomfort (it will still feel warm from alpha decay of other isotopes mixed in). In the sense of penetrating radiation, the main decay mechanism of Pu-238 is spontaneous alpha decay which is stopped by several feet of air at standard atmospheric pressure. This vs the highly penetrating gamma rays from the fission reactions which are the main decay mechanism for Pu-239 or U-235, so Pu-238 may be thought of as less "hot" with less penetrating radiation. In order to get a high heat output from Pu-239 or U-235, you'd need a near-critical mass and some moderating material to produced a sustained chain reaction, i.e., a nuclear reactor. Then you'd have to worry about shielding against the gamma radiation and neutrons from the fission reactions. In short, Pu-238 has an entirely different means of heat production and shielding requirements than Pu-239 or U-235. One does not mix with the other. The "weapons-grade" part comes into play in reducing the overall mass of plutonium or uranium required to sustain a chain reaction, and allows for much more compact reactor designs. Reactors on naval ships and for space applications use "weapons-grade" material. NASA has to get its plutonium from the Department of Energy, which maintains the only facility(ies) for processing the proper isotope of plutonium. For a while, NASA also purchased Pu-238 from Russia, but even that source is dwindling. As pointed out, the facilities for Pu-238 production were shutdown. It is expensive to produce Pu-238. |
|
|
Nov 12 2008, 10:10 PM
Post
#3
|
|
Junior Member Group: Members Posts: 47 Joined: 16-July 05 Member No.: 435 |
For a while, NASA also purchased Pu-238 from Russia, but even that source is dwindling. As pointed out, the facilities for Pu-238 production were shutdown. It is expensive to produce Pu-238. Which makes me wonder about this passage in The Planetary Society Blog: QUOTE (Emily Lakdawalla) [Argo] can't happen in the next New Frontiers opportunity because the U.S. doesn't have enough plutonium available for the next New Frontiers to be nuclear-powered. So it has to be New Frontiers 4, which implies a launch date in 2019 or 2020, with the Neptune flyby happening around 2027. Emily, what makes you think any more Pu-238 will be available in 2018? I do not know of ANY plans to restart its production. |
|
|
Lo-Fi Version | Time is now: 24th September 2024 - 12:52 AM |
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. |