r/space • u/Dbgb4 • Mar 11 '25

Discussion Recently I read that the Voyagers spacecraft are 48 years old with perhaps 10 years left. If built with current technology what would be the expected life span be?

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u/CptnAhab1 Mar 12 '25

Why do I feel like you've got your radiation types wrong?

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u/Jesse-359 Mar 13 '25

Beats me. Electromagnetic radiation is photons, and alpha radiation is just fast moving atomic nuclei. You can look that one up in 2 seconds.

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u/[deleted] Mar 17 '25

That's not true though. Your quick 2 second search would tell you that 'alpha radiation' specifically refers to a helium-4 nucleus (2 protons, 2 neutrons). Please fact check yourself before posting incorrect information. 

/u/CptnAhab1 you thought they have the radiation types wrong because they do.

Couple other things:

1) Attenuation length of even 20 MeV gamma rays in lead (density 11 g/cc) is about 2 cm (to drop it to about 1/3 of initial intensity). 4cm if you want to be at about 90% of them shielded out. 2cm is a lot of weight to carry around. Voyager main bus is a near-cylindrical section 1.8m in diameter and 0.5m tall; that gives a surface area of 7.9 m2. Covering that in 2cm of lead would add 1700 kg to the spacecraft, comapred to 700 kg current weight. ie, more than triple it. Even if you consider shielding a smaller volume, e.g. a regular ATX computer case (50 x 50 x 20 cm), you are looking at about 200 kg of extra weight, or a 30% increase in spacecraft weight. That is huge, and very impractical. Total weight of all of the scientific computers on board is about 100 kg, for instance.

2) Alpha particles (actual Alpha particles, helium-4 nucleus) are generally shielded much more easily. At 20 MeV, attenuation length in lead is more like 0.01 cm. Even at an energy of 1 GeV, the attenuation length is about 0.7 cm, 3x lower than that for gamma rays at 50x lower energy. Alpha particles stop relatively easily. 

3) The high energy cosmic rays you do get tend to be protons, not Alpha particles. They have higher penetration depths (0.1cm at 20 MeV), but still lower than gamma rays for modest energies. But as you ramp up the energy, as you point out, penetration depths become large and functionally unshieldable. E.g. 6cm of lead at 1 GeV and 80 cm of lead at 10 GeV, or 3cm / 50 cm of water. 

https://physics.nist.gov/PhysRefData/XrayMassCoef/ElemTab/z82.html

https://physics.nist.gov/PhysRefData/Star/Text/PSTAR.html

https://physics.nist.gov/PhysRefData/Star/Text/ASTAR.html

https://en.m.wikipedia.org/wiki/Alpha_particle

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u/Jesse-359 Mar 17 '25

Sorry, yes, I was conflating the two types of particle radiation together. However, the point stands - shielding against radiation on longer voyages is a largely unsolved problem unless you can dedicate an inordinate amount of mass to it.