Countdown to Jupiter: Juno just seven days from orbit Juno is on the seven-day countdown to entering Jovian orbit, and it's going to be a wild ride. On May 31, the probe crossed the boundary between solar gravity and Jupiter's. That also marked the start of manoeuvering towards an orbit that's going to take it within 5,000 km of the planet's cloud tops for 37 flybys. As space …
If you want pedantry, the crafts total original weight was 3625kg, that includes the craft, fuel and oxidizer. The craft itself is 1593kg, so the electronics vault shielding, which is 200kg comes in at 12.5% of the crafts weight. This excludes the shielding for cabling and instruments outside the vault.
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Juno was the wife of Jupiter, so anything it well named as something keeping a close eye on Jupiter.
The radiation belts around Jupiter are so intense because Jupiter has an ENORMOUS magnetic field. The magnetic field strength at the cloud tops is ten times that of Earth and the magnetosphere also much lager that that of earth.
I understand that Juno uses a titanium vault for the computer - well-described here - but don't quite follow the material selection. The article says lead is too soft, so presumably the vault is structural; spacecraft designers are efficient like that. But then it says, "anything else is too difficult to work with." Well, no, titanium is not fun, either. It's not as easy to machine and shape compared to aluminum and steel.
There are other issues with titanium. It's not particularly dense, and it doesn't have a particularly high atomic number, so it doesn't seem like a good choice of radiation shielding. High strength steels would be stronger, stiffer, denser, have a slightly higher atomic number, and be easier to work. Tungsten, which would be more difficult to work with than titanium, would be much denser and could be stronger. There's a whole list of alloys that seem like they'd be better choices than titanium.
But, it wouldn't be chosen at a whim. LM Space materials engineers would have done trade studies on the material selection and they'd know what a pain titanium is.
So, does anyone have a reference explaining the radiation environment or how titanium excels as shielding?
One can only guess. There are certainly more parameters, such as the thermal expansion coefficient. Other points to consider would be how brittle a material is and how well-behaved for machining (like making threads). Still another point might be that the parameters are better studied/researched than for other choices of material.
There are certainly more parameters, such as the thermal expansion coefficient.
Titanium's is fairly low. I end up using it because it's close to sapphire's, but that's a very different situation than Juno.
Other points to consider would be how brittle a material is
Titanium isn't a winner in fracture toughness, but it's better than some. It keeps good properties down to extremely low temperatures, but that's not unique and Juno should never let its core get cryogenically cold - space probes aim to keep their guts at about room temperature at all times.
and how well-behaved for machining (like making threads).
Titanium is annoying to machine. It's gummy, conducts heat poorly, and doesn't generate chips cleanly. It's not a good choice for threads both because of the machining issue and its willingness to gall if you so much as wave a screw within a couple inches of it. (Easy fix: use a threaded insert.)
Still another point might be that the parameters are better studied/researched than for other choices of material.
I have selected less optimal materials because I knew them better than new unobtainiums. :) However, aerospace aluminum and steel alloys should be pretty well known by LM Space.
I wonder if it's an activation-related thing. As I recall, aluminum and some steel alloying elements may be activated by radiation bombardment (usually neutrons, but...) into radioactive isotopes, which would be awkward for radiation shielding. Maybe titanium doesn't activate and the radiation environment is such that activation is a risk.
I'd like to know too.
Titanium is 56% as dense as steel, while retaining a similar stiffness-to-weight ratio. Titanium has a high elongation number, between 20 to 30%, while typical steels are up to 15%. The lower the number the more brittle. I am wondering if the gravitational differences it might experience could have required a less brittle material. I'd love to know if this guess gets anywhere near the truth.
May well destroy a book series I recall reading a *long* time ago - its somewhat vague, but involved someone becoming a benevolent dictator ruling from a moon around jupiter. It was a fun series. Can't for the life of me recall the title.
In any case, Cheers, but no beer till the orbits done. Want steady hands on those keyboards. Good luck guys and gals.
@ Alistair, Answer: The Sirens of Titan by Kurt Vonnegut Jr.
A good book with the introduction of a chrono-synclastic infundibulum which meant Roomford the dictator existed simultaneously at all points along the spiral that was the infundibulum.
More at https://en.wikipedia.org/wiki/The_Sirens_of_Titan.
I originally read this in the '60s.
Looking forward to viewing the data that comes back from Juno.
A late thought: why is it no one has sent something to probe Uranus?
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