Then you might find this interesting: https://www.jpl.nasa.gov/images/pia03480-estimated-radiation-dosage-on-mars

The main impact of a magnetosphere is that it protects atmospheric water from having its hydrogen split off by the solar wind, and then escaping, which hydrogen is far more prone to doing due to its lower molecular mass. Hence why Venus is bone dry but has nearly a hundred times as much atmosphere as Earth despite getting twice as much solar radiation. This is far too slow to be of significance to human activities, though. Terraforming will involve undoing billions of years of losses in just centuries, if you can terraform a planet then maintaining its environment is trivial.

There isn't a "point" to a magnetosphere, we just happen to have one. Its importance as a radiation shield is wildly exaggerated, and Earth regularly goes through periods with no strong, organized global field. The existing atmosphere of Mars provides more surface protection than Earth's magnetosphere provides in LEO, and Earth's atmosphere provides most of its protection from cosmic rays. A terraformed Mars would have an atmosphere with nearly 3 times the column mass due to its lower gravity, and even without an magnetosphere would have far better protection than Earth.

As for orbit, only LEO is protected. Satellites and probes are better off outside the magnetosphere than they are in medium Earth orbit where the belts are, and missions with humans have to plan trajectories that take them around the belts. And geomagnetic storms only pose a problem on Earth because we have a large magnetic field to get buffeted around by changes in the solar wind that we otherwise wouldn't notice.

> This would help a a lot for future missions and future terraforming.

It would mostly mean that satellites and spacecraft would have radiation belts to deal with, and Mars itself would be subject to geomagnetic storms. Neither of these is very "helpful".

> piping oxygen from the south polar mines to bases where humans will live

Regolith is roughly 50% oxygen by mass, it will be far more practical to just crack it out of minerals than pipe it across the moon from polar craters.

Water can be cracked into hydrolox propellant, but Starship doesn't use hydrolox, avoiding it due to the difficulty of handling and storage and its low density. At any rate, the great majority of the propellant mass is actually oxygen, which...again...can be obtained anywhere on the surface. Starship could take on lunar oxygen after landing, getting 78% of its return propellant from lunar sources, without any dependence on polar ice.

Which is good, because it's not certain there's enough easily-accessible ice at the poles to burn as rocket propellant. It may be better to conserve it for uses on the moon itself.

Helion doesn't need He-3, their reactors are supposed to produce their own by D-D fusion. And at any rate, the glassy beads of the regolith apparently contain up to about 2 parts per thousand of water. The He-3 content is more like 15 parts per billion.

I'm not the one saying it's feasable, reasonable, or even desirable to 3D print 100% of a rocket, that's Relativity.

Uh...yes? If it's such a superior way to manufacture things that it's automatically the right choice for the rocket to such an extent that they're trying for a 100% 3D-printed rocket, why wouldn't they?

Marketing is really all it is. Notice how, as much as they talk up the rocket being 85% 3D printed, they aren't using 3D printing for much else. The rocket's much flashier than, say, the strongback.

> and SpaceX move any impending RTLS flights to ships.

RTLS missions only exist in the first place because it's cheaper and faster, and avoids contention for the ASDS ships, which are unavailable for significant periods of time as they transport cores back and move out to support the next landing. Moving an RTLS landing to an ASDS has a substantial cost and schedule impact, and isn't something SpaceX is going to want to do regularly.

> Sometimes a chunk of metal is easier to get

Sometimes it's the only way to get a particular material. You're not getting single-crystal structures out of a powder bed, for example. And processes such as forging bulk materials can have desirable effects on its microscopic structure.

And yes, there are also things you can do with additive methods that you can't with casting and machining. Different manufacturing approaches have different tradeoffs. Ultimately, you'll make most effective use of these techniques by applying them where they're most effective, rather than, oh, trying to print an entire rocket or something.

This would probably be closer to what the depot itself will be, as the tankers will need heat shields and flaps. This might also be some hybrid, maybe just a cheaper way to test propellant transfer when they're still working on recovery.

There's already ~4 billion metric tons of uranium in seawater. Dissolving the entire reactor and dumping it into the ocean would have no measurable effect. RTGs are actually more dangerous, as they require isotopes with high enough levels of radioactivity to generate useful amounts of heat, and those materials are at their highest levels of activity the moment they are produced...you can't just delay turning them on until after they've safely launched.

Thorium is just another possible fission fuel. It's often proposed to be used in a molten salt reactor, but molten salt reactors are not all thorium reactors and thorium reactors are not all molten salt reactors. There's no shortage of safe uranium reactor designs, it's just been impossible to get them implemented because of the anti-nuclear groups.

Yeah, even ignoring the politics, Starship should be able to get launch cost <$200/kg, lower than just the energy costs for the space elevator in Edwards' study. You're not going to get launching something with a nuclear fission bomb and a massively hardened nuclear space gun to be lower than that, just due to the costs of the bomb itself, never the massive propellant costs and complications of snagging it from a suborbital trajectory with a Starship.

Just put the payload on a Starship.

Oh, the shcramjet (not a typo). Yeah, I'd forgotten about those.

This will give you 10% or so better performance. That's not nothing, but it's not the difference between asteroid mining being viable or not.

Air-breathing engines aren't comparable to rocket engines. They have big specific impulse numbers because the specific impulse is no longer the impulse available from the propellant, but what's available from just the fuel after that fuel's been combined with the air. And since it's wildly variable with airspeed, it only makes sense for craft that cruise in a given range of airspeeds...in this case, Mach 5 and less.

The idea of using detonation to improve efficiency is quite old. The V1 "buzz bomb" used a pulsejet engine in 1944-1945, and experimental versions of pulsejets using detonations are about as old. The vibrations inherent in pulsejets of any sort have prevented them from being used much.

The basic concept of using continuous detonation waves in some form has probably been around for just as long, but has been more difficult to implement.

I didn't say HEU, I said enriched uranium. HALEU is enriched to a U-235 content of 5-20%, natural uranium is only 0.72% U-235.

This isn't a discovery, it's not existing technology, nobody assumed it wouldn't work, and it's not clear yet how well it actually will work. It's an early test of a new technology that has been in the conceptual stage for decades, has only recently been gotten to work, but which has been expected to improve performance.

Realistically, anything Earth-like would likely be uninhabitable. Aside from the forward and backward contamination issues, an entirely alien biosphere wouldn't contain any diseases or poisons adapted to us, but would be saturated with things that are moderately to severely toxic or just noxious, and complex organic substances that our immune systems have never encountered before, some of which would be likely to cause severe allergic reactions.

In short, it'd probably stink horribly and send you into anaphylactic shock, and if it didn't, it'd probably have environmental toxins that would kill you slowly. Habitable environments are those with the natural resources needed to support habitats where we can support Earth life, not those already filled with alien life.

Mars? Oh no, perchlorates! Yeah, about 0.5-1% of the regolith consists of salts twice as toxic as table salt that are unstable, easily washed out with water or decomposed by heat or reducing agents, and which do not bioaccumulate. Worry more about heavy metals and long-lived organic compounds leaching out of plastics and such.

And you can use heavier things as propellant, like ammonia (water and methane are both bad choices for various reasons), but anything but LH2 gives you only slightly more performance than chemical engines.

Meanwhile, instead of a pile of steel, copper, and nickel alloys carefully arranged to burn stuff really well, you need enriched uranium arranged to sustain a nuclear fission chain reaction. That's a huge step up in cost and regulatory complications, and nobody's going to do it for something barely better than a chemical engine, so LH2 it is.

Yes, that part is completely wrong. Nuclear rockets still use propellant. Nuclear thermal rockets use about half as much by mass as the best chemical rockets, but they only get their peak performance with LH2, which takes up about 5 times as much volume for the same amount of mass. A nuclear thermal spacecraft will be a big pile of propellant tanks (likely drop tanks so you don't have to carry empty tank mass around) strapped together with a nuclear rocket engine at the back and a small payload tacked onto the front.

The "45 days" claim appears to be in reference to the "wave rotor" stuff that's been getting massively overhyped. Basically, as described, they propose sticking a widget between the nuclear reactor and the nozzle that somehow doubles the specific impulse while halving the flow rate.

This means doubling the power output of the reactor. Since the power output of the reactor is already limited by the need to keep it from melting, and the reactor is cooled by the propellant flow which you've just cut in half, it's not clear how this doesn't result in the reactor, well, melting. Also, even if it worked, doubling the specific impulse isn't nearly enough of a gain to allow a 45 day trip to Mars.

They then throw in nuclear-electric propulsion, which requires heat exchange loops, many megawatts of electrical generation capacity, giant radiator arrays, and arrays of ion thrusters. They assume all this can be done "with minimal addition of dry mass", and this is how they double the performance again to get their 4000 s number. However, it doesn't actually appear to have anything to do with the wave rotor.

NASA's giving one guy $12500 to look at it. It's not taking anyone to Mars any time soon.

As close as you can safely locate the telescope to the nearest magnetic monopole. You can do what you like with the telescope afterwards, I just want an answer to whether monopoles exist.

Yeah, the issue isn't accuracy. It wouldn't be that difficult to hit the ISS. The solution space for a rendezvous with near-zero relative velocity is rather more restrictive.

For Earth, there's vast areas suitable as landing locations, where it doesn't really matter what direction we approach them from. We just need atmospheric entry to happen at a reasonable angle and velocity.

It's not a matter of it being hard to calculate, it's a matter of the solar system not being physically arranged to conveniently allow it.

A minimum-energy transit will come at Earth roughly aligned with its orbital motion. To match planes with the ISS, the return must happen at one of the two times a year where the ISS orbit is also aligned with that motion, which means the trip must have started on the opposite side of the sun from that point, half a transfer orbit earlier. But we don't control where other solar system objects are or what their motions are. Windows to/from Mars occur every 26 months. If by chance things are properly aligned one year, it will be 60 degrees off the next time, and most Earth-Mars windows will be unusable: it will be 3 launch windows, 6.5 years, before they align again. And they in reality don't line up in such nice whole numbers, so in reality you're going to have a substantial plane correction to make on arrival, even with such limited windows.