Actually, refined or enriched uranium still isn't very radioactive, you can handle fuel pellets for a nuclear reactor with your hands without much of a problem (other than issues for how you would handle lead, since it is still a heavy metal for toxicity purposes).

It's after they've been in the reactor, and have had some fission going on, that you don't want to get anywhere near them. It's the fission fragments, the pieces that are left over after uranium is split that are far more radioactive. Heavy elements need a higher neutron to proton ratio to be stable than stable lighter elements do. (band of stability: https://en.wikipedia.org/wiki/... http://en.wikipedia.org/wiki/N... ). So when you end up splitting a heavy element, you end up with lighter elements that have a lot more neutrons than what would be stable for them, and they decay from there.

a 1.3km sphere of iron would mass over 9 billion tons, nickel would be even heavier. It would be easier to bring the foundry to it, rather than the other way around.

And a US Citizen as well. (And also Canadian)

Simple solution for the Van Allen belts: remove them.

https://en.wikipedia.org/wiki/...

If you're going to be building a space elevator, getting rid of the Van Allen belts is a relatively easy task in comparison.

A major flaw in your argument here: Uranium isn't very radioactive. Fuel pellets for nuclear reactors, that have not yet been used, you can handle with your hands without much in the way of problems (heavy metal poisoning, like with a block of lead, would be far more significant than any radiation here). On the other hand, once they've been through a reactor, you don't want to be anywhere near them.

The major source of radioactivity comes from the fission products. Heavy atoms need to be more neutron-rich to be stable, and lighter atoms are stable with a more even ratio of protons and neutrons. So when you split a heavy atom, you generally end up with a couple of lighter atoms which are far too neutron-rich to be stable, and which fairly rapidly decay into other things, producing and radiating away energy in the process (i.e. radiation).

That's where all of the lasting radioactivity comes from, after the reactor has been turned off. Uranium has far too long a half-life, billions of years, or hundreds of millions of years, for the main couple of isotopes, to have much effect on the amount of radioactivity. (generally, the longer the half-life, the less radiation, since a long half-life means less decay events per period of time. The candle that burns half as long burns twice as bright, or something like that. Isotopes that decay over a period of years for example, would produce approximately 8-9 orders of magnitude more radiation.)

If you want a small nuclear power source, you can go with a polonium-210 RTG. polonium-210 generates about 140KW/Kg, and has a half-life of around 138 days. And it decays by alpha radiation, so it doesn't need much radiation shielding. It's been used before, such as in soviet lunar rovers, Lunokhod, although as a heat source, rather than a power source.

Although, given that you have to create all of that polonium, transmuting it in tiny quantities, it's going to be a bit expensive. All of the logistics to keep it fueled with gasoline would likely still be cheaper, and so you could produce a lot more of them. And that's not even mentioning the obvious concerns about a machine running around full of polonium.

"almost nobody" seems to include the US legal system: Dowling v. United States (1985) http://en.wikipedia.org/wiki/Dowling_v._United_States_(1985)

http://en.wikipedia.org/wiki/Copyright_infringement#.22Theft.22

In moral terms, theft and copying are not the same thing. Taking a loaf of bread from you, or watching how you made a loaf of bread, and then going home and doing the same thing myself, are not morally equivalent. Even if you say two different things are both wrong, it's incorrect to assume from that, that they're necessarily the same wrong, in type or degree.

It looks like that's what they're doing though in some capacity, basically running most of it during the day on solar power, and then just using a small RTG to keep it warm enough that it doesn't freeze to death during the night, and possibly keep communications and stuff like that running.

Just because it's a machine doesn't necessarily mean that all of its components can survive -170C temperatures.

And even Curiosity doesn't do work at night, it uses a smaller RTG than needed to power all it's components, and charges up batteries at night for operations during the day. Pu-238 is a bit hard to come by (and expensive) in large quantities, so you do what you can to limit the size of an RTG.

Yeah, it's a bit blatant on that point, but saving the lives of 100,000 people every day wouldn't be helping people? A cure for aging would save more lives than a cure for everything else combined.

Ouch, yeah, I know ion drives are low thrust, but that takes quite a bit longer than I was thinking, and I didn't calculate exactly how long that would end up taking. (Although I get about half that time with those figures, 11 months for low Earth orbit->mars transfer. But slowing down at the far end even would be tricky and time consuming as well.)

Something that might be useful for moving cargo around, but yeah, not quite that suitable for manned missions unless you can bump up the thrust by a huge amount, which would require a lot of efficiency.

Well let's see, It's about 3.8 km/s from LEO to mars transfer orbit. and 6.1 km/s from low Earth orbit to low Mars orbit, if you're not areobreaking. For low earth -> low mars orbit, at 20000s (196 km/s exhaust velocity), that would take a mass ratio of around 1.0316, or around 2.37 tons of propellent for a 75 ton dry mass rocket. At 800s (7.84 km/s ), that would take a mass ratio of around 2.177, or around 88.3 tons of propellent for a 75 ton dry mass rocket. If you can areobrake at the mars end, and only need the low earth -> mars transfer costs, those numbers would drop to 1.47t and 46.8t.

So, you'd more than a 25t difference to account for propellent costs. If you're starting with a base 75t ship in both cases, If you can airbrake at the far end, it's at least 45t extra for NERVA, bringing it up to around 120t. If you have to slow down with rockets, it's more like an 86t difference, bringing it up to around 161t. And those are just for hoemann transfer orbits, minimum delta-v, maximum time.

So there's some potential there, if you can toss on an extra 30t of power production and engines (over and above the NERVA reactor mass, since I assumed a 75t dry mass for each so that would already be included for both), and get the thrust output up an extra order of magnitude. With something like http://en.wikipedia.org/wiki/Dual-Stage_4-Grid (only one I could find near 20k isp), 100N would take about 10MW (Ouch). According to http://en.wikipedia.org/wiki/Solar_panels_on_spacecraft You can get 300W/kg solar arrays, meaning you could get 10MW using about 33.3t (10MW at around Earth orbit side at least). That's almost practically viable (There would probably be extra mass in support structures and such, but in space, and under mm/s^2 scale acceleration, it wouldn't be under much stress) And you could save a bit of travel time in the middle part by adding extra propellent to do something a bit faster than a strict hoemann transfer orbit, with that much power, a bit of extra propellent can go a long way.

Nuclear Thermal Rockets can have a higher efficiency than than conventional chemical rockets, but it's not as much as you might think. There's a limitation that to have a higher exhaust velocity in a thermal rocket, the exhaust needs to be hotter. And it can only be so much hotter before your reactor starts becoming molten rather than a solid. Which means that efficiency tops out at a bit less than double the exhaust velocity of conventional rockets.

Now, that's still useful, if you can get enough thrust to get up off of the planet (and to overcome the weight of the reactor in the process), then you might be able to lift quite a bit more into orbit. Except the petition is for an NTR that would only operate in space. And in space, where you don't really have to worry about the amount of thrust, and your speed is limited by your fuel and your exhaust velocity, things like ion drives can reach efficiencies an order of magnitude higher, or more. Which means, an NTR in space only wouldn't be as useful, compared to nuclear-electric or solar-electric propulsion.

I suppose an NTR not used for Earth surface to orbit might still be useful in landing or taking off from other objects. Really, that's where its strength would be, if you can get it to have high enough thrust, then it would be useful for getting things into orbit and back, as a surface-to-orbit ship. But as far as orbit-to-orbit ships go, ion drives and other electric propulsion can get a lot more speed out of the same tank of propellant.

http://en.wikipedia.org/wiki/Endogenous_retrovirus
http://en.wikipedia.org/wiki/Horizontal_gene_transfer

Virus infects salmon, new virus being produced ends up incorporating part of salmon DNA, virus gets passed to cucumber, virus inserts salmon DNA into cucumber and it ends up incorporated into it's genome, new offspring has genetic material from both cucumber and salmon. In practice, there may have to be a number of intermediaries there, but that's the idea, and it's 100% natural, and has happened numerous times before, and the results of such can be seen in the DNA of a number of living things.

It's just a matter of how high your standards are for something to be essentially equivalent. I mean, Alpha Centauri system is about 4.37 light years away, Alpha Centauri A is about 10% more massive than the Sun, Alpha Centauri B is about 10% less massive than the Sun. I say "Eh, close enough".

Bah, you call Hoover Dam a giant hydroelectric project? Now this: http://en.wikipedia.org/wiki/Atlantropa is a giant hydroelectric project, building a dam across the Strait of Gibraltar, and lowering the Mediterranean sea by 200m in the process. :)

Wouldn't a significant percentage of the mass of the surface of the Earth also have come from the impactor, and therefore also have had it's isotope balance altered in the same way?