Neutrino detection generally requires large volumes and large masses (for very large values of "large"), neither of which are as yet feasible for space based systems. The IceCube neutrino observatory, for example, makes use of a volume of ice that is on the order of a cubic kilometer (over 900 million tonnes). Suffice it to say, building cubic kilometer solid structures in space that weigh hundreds of millions of tonnes is outside of our engineering capabilities at present. However, within the next several decades this might be feasible.

That's also understating the size. While the final detection is happening in the ice, the column of air above it is also important.

Could we not just land a detector on the far side of the moon. Thats pretty large no?

It is but you need both a big hunk of mass and a way to detect the neutrino interactions inside it. Ice Cube is shot through with detectors. One of the OG detectors measured the particular isotope created during interactions.

https://en.wikipedia.org/wiki/Homestake_experiment#Methodolo...

The Super-Kamiokande has photomultipliers staring at a bunch of water looking for Cherenkov radiation resulting from neutrino interactions

https://en.wikipedia.org/wiki/Super-Kamiokande

None of these are much of a practical option with the moon.

You need to have the reaction mass to make use of. Most neutrino detectors use special chemicals in custom tanks, IceCube uses antarctic ice. You would need not just mass but ice or water for a neutrino detector, ideally highly compressed optically clear ice. The only possible candidate in the Solar System that fits that bill at present is Europa. Though it's possible that similar conditions might also exist on Mars (we know there are sub-surface glaciers, but we don't know how deep they go or how pure the ice is. And potentially some of the large sub-surface oceans (on Ganymede, Enceladus, etc.) might work too but we know even less about their properties.

Not all detectors were optical so if you found a moon made of, say, dry cleaning fluid, you'd still be good.

Each detector has a limited range. And only works well in certain mediums, generally water or water ice. Even if the Moon was solid ice, we would need to surround it with detectors, as well as drill a huge number of holes all the way through it and place detectors (probably hundreds of thousands or millions) in each hole.

I had this idea back in my undergrad when my professor first told me about neutrinos (which was his forte). The harder I looked the more infeasible it seemed. The big problem, is creating a detector. The reason this is a big problem is because neutrinos are so small and so neutral. How small? An electron is 511keV. A neutrino is <= 0.21eV. That's 7 orders of magnitude! And on top of that, they have a neutral charge. There is no electric dipole moment either. So how does it interact? Basically by striking another particle. And remember that atoms are basically empty space. So chances of that are slim.

So to place a detector in orbit we'd need one of two things. Space travel to become cheap enough where putting a massive body up is cost feasible, or a radically different understanding of how to detect neutrinos. (First is more likely)

The former also comes with some problems. You'll find more about this by asking why detectors are so far underground. And there's a big connection there to generating the resolution you'd need to map the interior of the Earth.

An in space detector is (at least to my understanding) not that easy, as you need the surface of the Earth to interact with the neutrinos. These interactions result in muons, which allow to detect that there was a neutrino in the first place. Also, there is a pretty big source for neutrinos pretty close to Earth, but Earth is in its orbit, as opposed to the other way around.

The IceCube neutrino detector consists of about a cubic kilometer of ice. You're not going to put one of those in orbit.

But it would make a cool joint project for NASA and the NSF to fund. Capture a comet big enough that you could mine enough water to make a cubic kilometer sized detector in Earth orbit.

That could probably also wipe out half a continent if you mis-aim it...

Then again, it would look spectacular in the sky!

There are other types of neutrino detectors. Not that it really makes the problem any easier to solve, but this isn't the only way.

That's what I was thinking. They have sent neutrinos from one lab to another with a man-made source and a detector smaller than 1km^3. The idea is to put a strong source in orbit and aim it at an orbiting detector. Over time this configuration would be used to scan the entire planet. Still a huge challenge.

That's essentially the main idea most people come to. Using a source so you can concentrate on specific ones and not get distracted by solar, cosmic, or terrestrial neutrinos. Or you could just use terrestrial ones. But still, that detector is the hardest part to figure out.

Exactly

You might be able to tow it in from elsewhere using a light sail, however. Beyond our current technology, but might be doable by 2100, or sooner with a fortuitous comet trajectory.

And once it is in orbit, how do you propose stationkeeping against exospheric drag? Preferably a solution that will remain reliable for 10^n (n>>3) years.

A cubic kilometer of ice is 9.167e11 kg.

Also, just keep the orbit above 800km, and the light sail will still be able to counteract its own drag.

If the orbit gets too low, turn the ice into rocket fuel and use it to refuel ships boosting into higher orbits.

I'm pretty sure that a cubic kilometer of ice in orbit would have more value in other purposes than as a neutrino detector!

Sure, but as long as it's up there, might as well use it in every way possible.

The muons are produced in the upper atmosphere (from cosmic rays interacting with the atmosphere). You can't do this in space. Not too mention the size of a neutrino detector, although you can use the e.g. regolith of the moon as a neutrino detector (via the Askaryan effect). In fact there are experiments pointing radio telescopes at the moon to look for ultra-high-energy neutrino interactions. My not-so-polite term for them is lunatics :).

> lunatics

Oh you :)