# Detecting a Nuclear powered vehicle from outer space (at least 8 AU or further out) feasible?

I sometime see this quote in forum arguments against "Klingon/Romulan-style" cloaking in video games that tries to be close as possible to real-life. It is quite difficult to ascertain where the actual quote originally came from.

It goes something like this "You can detect a nuclear powered submarine on earth from as far as Saturn orbit".

However Saturn's own magnetic/lightning storm and occultation (from ocean water and earth and various moon thereof) make me seriously question if this is even remotely viable.

Assuming worst scenario case, the submarine is on Earth, the opposite side from Saturn, and Saturn itself is not occultating Earth but causing interference to some degree. What would it take to detect such a vehicle (assuming it is the only nuclear powered present on Earth)?

Is the detection based on electric activities as opposite to the nuclear power source? I am curious how much the detection is from the nuclear power itself vs EM fields.

If the nuclear powered submarine had the nuclear system taken out and used something else that is based on chemical batteries or some non-radiation-emitting based power source does it make harder to detect?

• We have trouble finding a submarine in the ocean from that ocean's surface. Any long-range detection would require the submarine to be at the surface. – Hobbes Mar 1 '16 at 7:51
• You cannot detect a submarine using nuclear emissions, the sources that say that are just plain wrong. Submarine detection primarily uses sound, either passive (ie listening) or active (pinging). Other detection methods are by using a Magnetic Anomaly Detector to find subs closer to the surface or radar if the submarine is at periscope depth and has a mast up. – GdD Mar 1 '16 at 9:07
• There ain't no stealth in space. That said, your question seems to be about stealth on Earth (where the sub is located), not in space. – Ajedi32 Mar 1 '16 at 17:04
• Why was science-fiction tag added? There is no "fiction" component to my question? – Vyndicu Mar 2 '16 at 21:43
• @Vyndicu, I think asking about a submarine isn't what you want. If you're trying to understand hiding in space, ask about something that isn't blanketed by the ocean and the atmosphere. – Joe Mar 2 '16 at 22:00

I sometime see this quote in forum arguments against "Klingon/Romulan-style" cloaking in video games that tries to be close as possible to real-life

I gather the "World Building" Stack exchange might be a place for this, if you want that sort of speculative answer.

If you want an engineering answer: first of all nuclear subs are very hard to detect... this is the whole point of them in fact. They would be useless if the enemy could easily track them.

Secondly, if the Nuclear plant (of any vehicle) put out enough radiation to be easily detectable at distance, the crew would have received a very high dose and be dead. Also not very useful in a combat situation :)

If you want a way to detect "Cloaked" sci-fi vessels, look for their engine emissions. A (real world) rocket engine would put out streams of hot gas or plasma, and even a sci-fi "drive" of some sort would obviously have some sort of measurable effect, otherwise it wouldn't impart any measurable speed on the craft. You canna' change the laws of physics!

• Less of a speculative answer as cloak is not part of the question. I was just putting the "quote" into context. – Vyndicu Mar 1 '16 at 15:16

Romulan warbirds and klingon bird of preys (as well as federation ships) are made of materials absolutely unknown today and we don't know the properties of that materials. Furthermore, we don't know much about their engines and what kind of radiation they would emit. And all we know about their sensor technology is that it's really fancy. So, let's stay in our century.

In principle, a submarine does not emit anything that allow to locate it from a larger distance. The only methods so far are sonar, and listening to noise from the submarine, and measuring the distortion of the magnetic field by this large piece of metal in the ocean. The first two options work under water and close proximity only, the third can be used by deep flying aircraft, but still works for low distances only. :-(

You mentioned radioactive stuff. Most people know α, β⁠ and γ radiation. It is also known that the first two can be shielded easily (and completely) by a thin layer of metal, while γ radiation needs more care. Typically, thick shields of lead or similar heavy materials are used to drop the radiation level down to a value comparable to the level of the environment. (But this is to protect the staff!) Furthermore, water isn't the best shielding material, but the thickness of the water layer above the submarine compensates this. So, no chance to detect this. :-(

Well, α, β⁠ and γ radiation aren't the only types, there are more. Neutrons play a key role in nuclear reactors, but can easily be shielded - and it turns out water is the best material. :-(

When a β-decay occurrs, not only a β-particle (an electron) escapes from the nucleus, but also an anti-neutrino . The interesting fact is that (anti-)neutrinos can not be shielded: You would need a stack of lead as thick as the diameter of earths orbit around the sun to catch just 50% of the neutrinos...
A 1GW nuclear power plant releases about $10^{21}$ anti-neutrinos per second. It will be less for a submarine, but this is a number I found, and it can easily be scaled down. This number seems to be large, but in a distance of 1km, this leads to a flux of $8\cdot10^9$ anti-neutrinos per cm² and second. For comparison: The sun emits large amounts of neutrinos, the flux here is $6\cdot10^{10}$ per cm² and s on earth. (Keep in mind: sun: neutrinos. Reactor: anti-neutrinos)

On Saturn, the flux from our reactor is just about $3\cdot10^{-9}$ per cm² and s (or 1 anti-neutrino per squaremeter and 10 hours), while the flux from the sun is just 1/100 of the value on earth, i.e. $6\cdot10^8$ per cm² and s.

An observer on Saturn needs a large detector and a lot of time to get enough neutrinos. AND he somehow hast to distinguish all the different submarines and power plants, as well as the natural radiation of the earth. :-/

Now, the initial advantage of neutrinos of extreme rare interaction with matter becomes a disadvantage. These interactions are also what a sensor measures. This means you need a really really vast sensor volume and a really really long observation time to observe just a really really tiny fraction of all the anti-neutrinos flowing through this volume. :-(

Next, the path (origin) of low energy anti-neutrinos as emitted by radioactive processes can not be determined, so distinguishing between several submarines is not possible. :-(

By the way: There have been ideas to use large supertankers filled with a liquid detector material to discover nuclear power plants. Those would be sent to the coast of a suspicious country to check if they have nuclear power plants. But that's just an idea to my knowledge, and I don't know how they should work. I guess, they would measure the rate only.

Finally, it is not possible to detect a nuclear driven submarine from saturn today. Even if we assume that in the future we will be able to detect EACH anti-neutrino as well as its origin, the flow from earth on saturn is so low that we still need much observation time and vast detectors. At least, it might be possible to detect that there is an anti-neutrino source somewhere in space, which would be enough to detect a cloaked submarine in space...