[ghoomketu]

It's fascinating how Voyager 1, despite my lack of space knowledge, utilizes a nuclear power source for 40+ years, offering steady and reliable power without any moving parts that could degrade over time.

In contrast, India's decision to rely on solar panels led vikram lander to be dead in just 14 days due to lack of sunlight (afaik).

I'm curious about the rationale behind this choice when nuclear power seems like a far superior option. Can someone shed light on this decision?

[freeone3000]

Sure!

First, the nuclear power source is a giant hunk of plutonium. It is expensive to get, dangerous to use, and due to concerns about further refinement, is restricted internationally.

Second, it is toxic inherently — the source is continuously radioactive at a hazardous level to humans, plutonium itself has acute and long-term toxic effects aside from the radioactivity, and if a launch fails, the rtg will disintegrate and poison hundreds of miles (see Kosmos 954, which disintegrated over Canada)

Third, it is HEAVY. They produce 40W per kilogram. Solar panels produce three times that much on Mars, and can be folded compact for launch.

Voyager used an RTG because its planned mission took it far beyond where sunlight can generate power, and it could do so because it had the budget of NASA and plutonium from the Department of Energy.

Solar panels are way cheaper, lighter, easier to procure, easier to launch, and tend not to cause international incidents.

[jojobas]

Kosmos-954 didn't poison hundreds of miles, square or otherwise.

They could only find a dozen of radioactive bits, each only dangerous within a very small area around it, and not really leaching anything due to its ceramic nature. Most of the fuel dispersed and became harmless by dilution, probably never even reached the surface.

[Salgat]

I wonder if you could do a hybrid approach, where the nuclear device is very small, but able to charge the battery over a longer duration to the point where the solar panels can be repositioned and utilized again.

[mlyle]

Lots of missions use radioisotopic heaters, where you don't bother with the thermocouples and just have the material get warm and protect components which are vulnerable to low temperatures.

That's the main reason why spacecraft don't survive a temporary power outage: terrible environmentals.

But at this point, we don't have a lot of Pu-238, which is one of the only decent candidates.

[augusto-moura]

I don't know the exact reasons why Vikram didn't get a fission reactor. But I can assume from similar missions:

1. Solar is pretty good as far as Mars and it gets worse as it travel further from the Sun. This is why most probes that travel past Mars need a nuclear reactor (Voyager, Pioneer, Cassini, etc). Going closer to the sun they get even better

2. Sending radioactive materials on rockets presents a risk and it is avoided if possible, lunar probes are usually cheaper and can still benefit from solar, so no need for nuclear. Imagine throwing plutonium in the atmosphere in the case of an accident

3. Nuclear reactors in probes are small and rely on decay radiation, they _usually_ have pretty small powet output, solar has a lot

4. And last but not least, price, solar is much cheaper than nuclear

[mmooss]

> fission reactor

Am I wrong that the plutonium in the Voyagers is not in a fission reactor but in an RTG (Radioisotope Thermoelectric Generator), which converts the heat from the plutonium into electricity. ?

I suppose the heat is result of fission, but I don't think an RTG is what is meant by a fission reactor. ??

[Vecr]

Voyager uses a RTG. The USSR used some full-on nuclear reactors in space, but as far as I know no one else publicly did.

Edit: Yep, not sure how I forgot about that.

[joeyo]

The US put a fission reactor in space too. They did it first, in fact: https://en.wikipedia.org/wiki/SNAP-10A

Edit: it’s apparently still there and will be for a long time (!) albeit non-functional:

  > Decay date: April 3, 5966 (planned)

[wrycoder]

Fission reactor without a moderator!

[selectodude]

Using plutonium works great but there are two issues. 1) they don’t output that much power. Few hundred watts at most, and they decay at a fixed rate. 2) you need to get your hands on a decent amount of plutonium. Great for dirty bombs, hard to source.

[TkTech]

Both Canada and the US have restarted production specifically to produce RTGs for NASA, but the process takes time to scale up and automate. It's gone up 4x in 4 years and continues to increase, so this is a problem that will eventually be "fixed".

[floatrock]

Isn't it something like space-reactor plutonium is a waste product from nuclear weapons production, and since we don't really make nuclear weapons at scale anymore, we aren't really making (refining?) plutonium anymore. And NASA has some amount on reserve, but they're rationing it out carefully. So the Clipper probe had to go with a massive solar array (100ft, the length of a basketball court) because they would rather save their plutonium for some future rover mission.

[nick__m]

it don't make sense to stock hot plutonium to use it later, it loose power and decays overtime wherever you use it or not!

[jordanb]

A good reason is the lack of availability of the needed isotope (Pu238).

The Europa Clipper has a huge array of solar panels instead of an RTG due to the last of the available supply going into the New Horizons spacecraft.

Pu238 was a cast-off isotope from nuclear weapons development so it was more readily available during the cold war. We should be happy that it's scarce now.

Also solar panels have gotten a lot better than they were when Voyager was launched, but even today anything going out past Saturn is not going to be able to use solar energy.

[wffurr]

There was just an article about a whole array of “nuclear batteries” using all sorts of decent chemistry: https://news.ycombinator.com/item?id=42118306

Are any of those candidates or are they just too small or with too poor a mass ratio compared to plutonium RTGs?

[jordanb]

Maybe. I don't know. I'd point out that tritium is a gas, so it may be hard to contain for long periods of time especially in space.

[pkaye]

RTGs need plutonium 238. I've read even US doesn't have a lot available. The Europa Clipper will be using solar panel for example. India could also use batteries and a standby mode during the 14 days without sunlight. But any extra weight would add to the launch cost. Maybe in future missions as they get confident with successful landing, they will have bigger lander that can survive the lunar night. Even the early Mars rovers from NASA were tiny and solar powered (ie Sojourner in 1997.)

[YakBizzarro]

Availability :) a RTG requires Plutonium 238, which needs to ne created almost on purpose in a nucleare reactor. Not all nations have this ability or they are running such expensive programs. Also in the USA they are reserved for programs where there is very little light available

[elzbardico]

India is a nuclear armed-nation with some long-standing border tensions with some of their also nuclear-armed neighbors.

The military probably have priority on the decisions about the allocations of their plutonium stocks.

[krger]

> I'm curious about the rationale behind this choice when nuclear power seems like a far superior option. Can someone shed light on this decision?

India’s plutonium has already been spoken for.

[datameta]

Dead in 14 days? Was there a miscalculation that lead them to believe they could make one full night?