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Comment by miracle | Hacker News Reader

gives 100 times more thrust, but needs 1000 times more power...

Yep,you need to go for nuclear on this one. Not that we have never done just that ( the US has flown one reactor in space while the Russians have flown up to thirty) rather, I don't see this option being exercised anytime soon as part of any space exploration programs.

Igor.

I don't see why this is such a big deal outside of the wacky environmentalists. Nuclear power is a viable and powerful way to power spacecraft, especially if we want to do more than robotic space exploration.

My naive concern has always been about the fallout of a spectacular launch fail of the sort that involves fuel rods spread over a large area.

If this is a solved problem, then my fears are unfounded. Let me know.

You realize, of course, that not too long ago we were launching actual nuclear bombs into the upper atmosphere and detonating them on purpose. And it didn't really mess things up that bad.

In that context, the remote possibility that a rocket might accidentally fail in a way that somehow managed to crack open its little reactor, spilling some radioactive material in a way that somehow harms anything whatsoever, would seem quite trivial. I'd be more worried about airliners crashing into me on the freeway if I were you.

In other words, I'll bet you five bucks that nothing radioactive will drop onto your house the next time they launch a satellite with a nuclear reactor onboard.

You should really check out the safety features of RTGs. As one example, in 1970 the lunar module from the failed Apollo 13 mission (which had an RTG with a plutonium dioxide core) was burnt up over Fiji and dropped into the Tonga trench in the Pacific. The heat and stress of this event was far beyond anything that would happen during the launch of a spacecraft. The area where the RTG ended up is still tested for an increase in background radiation and to date nothing has shown up that would indicate that the containment vessel has leaked.

But we are not talking about RTG:s here, we are talking about real reactors.

... Which are potentially even safer, because the fuel is not terribly radioactive until it has actually been in the reactor. Ship it up in parts, assemble when safely in orbit.

There are a bunch of radioisotope thermoelectric generators in space right now, cassini for instance.

Those don't put out enough energy for this. You'd need an actual reactor.

And I remember reading that we are a little short on the right fuel for those right now, since it has not been manufactured in years.

While for the most part I agree, I don't see why being concerned about the well-documented hazards of nuclear fission equates to "wacky", or why being flippant about these risks should equate to "non-wacky" for that matter.

True, I should have toned down the rhetoric. I just get frustrated with how a small minority of people can delay progress in something that, to me, is so fundamental to the advancement of our civilization.

I don't see this frequently talked about, but this is one of the big reasons to hope that one of the alternative (non-ITER) fusion proposals succeed, because most or all of them are much smaller than a tokamak, and easy to launch into space, at which point they could power an ion drive for a long time. This is true even after ITER succeeds brilliantly and solves all our energy problems.

Ha ha, I told a funny!

A working fusion device of almost any kind would open up the solar system, without the dangers of lofting fission reactors. (Also, they get better energy density, and while fission would probably cut the mustard, in space you'll take every bit of energy density you can.)

For a fusion device to work you need energy in first. In other words, you need a fission device first.

Igor.

I'm pretty sure that's "common wisdom" from the ITER project, not a true statement about all the alternate fusion proposals. Starting up a Bussard reaction does require a certain amount of energy, but not to a "requires a fission reactor" extreme.

(Besides, it might be possible to launch a running Bussard reactor, jumpstarted on the ground. Given how it works, 8 or 10G vs the 1G it already has to work in may not be a significant difference. It almost certainly would be possible to launch a running focus fusion device, though I'm a lot more skeptical about that one. Well, I'm skeptical about the whole field, but some are more plausible than others.)

You don't need to sign your comments.

Both Voyagers have nuclear reactors (of sorts) in them.

Nuclear batteries, powered by thermocouples that transform the heat of decay into a small amount of electricity. For long-running batteries, they've got pretty good energy density, although even the most advanced designs struggle to hit 15% efficiency.

The nice thing about these is that there's only a few kg of radioactive material, and an accident high enough to generate fallout will also burn up both the fuel and the shielding.

It will still be radioactive even if you burn it.

Instead they are actually designed NOT to burn up, but rather fall to the ground in one piece.

I'd love to get one of those for a laptop. Does the power, size, weight work out? Imagine running your laptop for 20 years without ever plugging it in!

(Consensus on HN is not to sign messages)

Launch infrastructure into orbit so that you can beam power to it. This way, the heavy powerplant can stay on the ground. Have an onboard RTG for the control systems and the science package. Then you don't even have to send power to a probe full-time. Even an intermittent boost with the available ISP will be an improvement.

The orbital infrastructure could consist of mirrors, which would be relatively lightweight.

Spacecraft solar panels: ~170 W/kg and 200kw = 1lb of thrust. ~55 million miles to Mars.

Let's say 3000 lb of weight for 1 lb of constant acceleration - time to break at other end of trip. Add in a pay load we are talking ~1/5000g which still get's you 2km/s after 12 days. This would take ~300 days to get to mars. But, you could still accelerate on the trip. You would need to adjust for the loss of sunlight as you get further from the sun, but I think something like 120 day's might be possible with solar power.

Granted all of this is orbiting the sun, and you can get a little boost from a close approach to the moon etc.

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