In space, a thin sheet of lead is not radiation shielding but a radiation amplifier.
The problem being that high-energy cosmic rays are unlikely to interact with the lightly built spacecraft, going right through it. But if you add a thin layer of a good radiation shielding material, then there is substantially increased chance that they will interact with that material, and produce a very large spray of secondary particles. And those secondary particles will also be going fast enough that when they hit more shielding material, they will also result in more particles.
Then some of those secondary particles will be neutrons, which will easily penetrate the thin shielding (lead half thickness for 4MeV neutrons is 68mm), and irradiate the surroundings.
This has been very clearly demonstrated on the ISS, any metal tool has substantially higher radiation levels around it.
Thank you for this post. I was wondering if a thin lead sheet would be beneficial for the cockpit ceiling and maybe aisles of jetliners to protect the crew from the prolonged exposure to increased radiation. Do you think this is a bad idea for the same reasons as the spacecraft? (Of course there are other materials besides lead, that was what first came to mind because I incorrectly thought it was a panacea for all radiation types).
Air pressure at airliner altitudes is still about 20-30% of the sea level value. That means 20-30% of the atmosphere is above that—a column of mass equal to 2-3 meters of liquid water.
A thin lead sheet would be a rounding error next to that.
This is an oversimplification that's rather wrong, but: a decrease in altitude of just 300 meters, at airliner levels, puts an additional atmospheric mass equal to ~1 cm of lead (Pb) above your head.
Have you seen the explanations of radiation where they say flying (as a passenger) is about equal to the dosage of a dental X-ray (or something similar)? Someone who spends their career getting exposed at that rate might be worth making them a shield.
It's *not possible* to make a shield. The cosmic radiation that reaches that altitude is highly energetic and highly penetrating—enough so to go through 3 meters of water—and would be completely unaffected by lead sheets. Any easily-shielded types of cosmic radiation have already been blocked by the atmosphere.
It's not worth it. Like, you could mandate planes fly 100 meters lower, but a lead coating would be so heavy. Let's say a 737 with a max weight of 70 tons. Covering the top half with lead would mean a 200 square meter sheet, and at 1cm thick it would weigh more than 20 tons.
The rad vault on Clipper is an aluminum-zinc alloy, not lead. There are different kinds of radiation to worry about (alpha, beta, gamma, neutron, protons, heavy ions), and I think certain shielding approaches good for one aren't always good for the others.
Different sources of radiation interact with electrons or nuclei (1:1 with number of atoms) or nucleons (individual protons/neutrons, 1:1 with the mass). For instance, neutrons bounce off nuclei in nuclear reactors, and the lighter they are, the more energy the bounce can siphon off from the neutron. So having more, lighter (low-Z) nuclei (hydrogen in water and carbon in graphite are commonly used) provides better slowing of the neutrons vs. heavier (high-Z) elements, like lead.
> Is more shielding not the obvious answer? A thin sheet of lead around the sensitive parts should do the trick
Lead "is effective at stopping gamma rays and x-rays" [1]. Jupiter's radiation comes from "trapped particles [that] are about ten times more energetic than the ones from the equivalent radiation belts of Earth" and "several orders of magnitude more abundant" [2]. When those encounter lead they cause bremsstrahlung radiation [3], a sort of subatomic shrapnel that can be more dangerous than the original radiation.
Lead is also heavy, which means not only increasing the mass of the spacecraft, but its balance and thus propulsion profile. That might mean upgrading and moving thrusters and propellant tanks--in effect, a complete redesign.
(It's a good question that doesn't deserve to be downvoted.)
Could they find some margin to make it a bit thicker? I know this would increase the weight but if my image of how big this electronics vault must be I'd imagine they could find something less critical to shave off to offset it.
Unless the launch is postponed 2 years, I think any redesign of the vault at this point is unlikely. Clipper was originally designed to be launched on an SLS rocket and that was swapped out for a less powerful Falcon Heavy* so there isn’t going to be much room for extra mass. Additional mass may require more planetary "slingshots" and add more years before Jupiter arrival.
Hopefully SpaceX is able to resolve its Falcon second stage problems before Clipper is scheduled to launch.
* There were some discussions about adding a Thiokol Star 37 or Star 48 apogee kick motor to the Falcon Heavy stack for Clipper but for various reasons this didn’t happen.
Went searching for the "various reasons". Found this:
> Falcon Heavy rocket, having three launches under its belt, has proven more powerful than originally anticipated. Previously, it was thought that launching Europa Clipper on a Falcon Heavy would require a “kick” stage — essentially a small booster attached to the top of the rocket. The Falcon Heavy’s impressive performance has made that unnecessary. Moreover, mission designers at Jet Propulsion Laboratory have found a path to Jupiter called a MEGA trajectory: after launch on a Falcon Heavy, Europa Clipper would fly to Mars for a gravity assist, and then return to Earth for another, and then on to the Jovian system. (The mission previously believed that the rocket would necessitate a Venus gravity assist, which would require special thermal protection for the spacecraft.)
> The window for a MEGA launch opens in 2024 and would take only three years longer than an SLS flight. A Falcon Heavy expendable launch is about $150 million. A single SLS launch is now estimated to cost $2 billion.
> I'd imagine they could find something less critical to shave off to offset it
You’re still changing the spacecraft’s balance. Imagine moving one of an airliner’s engines a foot to the left. It can be done. But it’s a big change.
Now consider that “modern jet airliners have…useful load fractions, on the order of 45–55%,” while orbital rockets’ payload fractions are “between 1% and 5%” [1]. Deep space craft are another order of magnitude more sensitive.
Adding a little shielding here and there is the aeronautical equivalent of hanging a bag of bar bells off the tips of one of the wings.
If you are going to the trouble to take apart and redesign the system, it would be far easier and less dramatic to just replace the possibly out of spec transistors.
Yes, and if they had larger mass budgets they could over-engineer things like shield thickness to have wider safety margins, and mitigate unexpected problems like this one. One can speculate future space probes generally will become more more reliable, as the the cost of mass-to-orbit goes down, and engineering constraints become looser.
(I wonder if Starship is useful for this type of problem: if you could adapt the orbital-refueling method to serve as radiation shielding, and put an electronics vault in the middle of the propellant tank? Could you adapt Starship into a spacecraft bus in this way?)
Water tanks are the most likely source of radiation shielding: propellant tanks get used up and go empty, while for any lengthy mission, water is either going to be recycled back into the tanks or you will have to take blue water tanks and over time turn them into grey water tanks, either way you will have those tanks much more filled than the propellant.
These spacecraft always have tons of instruments for measurings along loads of different axes. For example, a magnetometer specifically designed for testing a hypothesis about Europa's magnetosphere. Looking at the wikipedia page it seems like there are about a dozen of these. Perhaps worst case scenario they could determine which one was least critical relative to its weight and eliminate it to increase the mass budget.
1. You’d need more than a thin sheet of lead. The radiation in space can be very energetic. It can easily penetrate several cm of shielding and if it is absorbed, you get secondary radiation.
(Note: I'm not a physicist and have no idea what I'm talking about in this domain)