sevenperforce

Kerbiloid's explanation is not considered a likely one by the scientific community. Earth has an oversized core and will remain geologically active for a long time thanks to subsuming Theia's core during the impact. The moon has an extremely small and inactive core because the collision ejecta which ultimately coalesced into the moon was mostly material from the crusts/mantles of Theia and proto-Earth rather than heavier, denser core debris. As others have said, Theia likely formed from the planetary disc in a similar fashion to proto-Earth, which is why their compositions were already fairly similar. I wonder what the gravity of proto-Earth would have been. Off-topic, but given the mechanics of impact formation, perhaps angular momentum could serve as a defining line between what constitutes a planet-moon system and what constitutes a binary planet system. If the combined mass and angular momentum of the system would make it impossible for a single body to remain stable, it's a binary system; if not, it's a planet+moon system.

Yet once you've got landing legs and engines capable of landing on Ceres, the only thing preventing you from doing the same thing with Luna is a bit more fuel.

Iirc, the only reason that they build the Space Shuttle in the first place was to smuggle one particularly sensitive spy satellite into orbit using the very, very large cargo bay.

I think it all comes down to the mission profile. The thing is, if we're going to have a really wide range of possible missions (which is part of the point of having a single interplanetary transfer ship), then the hab is going to need to have a long-lasting power supply and autonomous maneuvering capabilities. And so if you're thinking of something like a landing on Luna or Ceres, does it really make sense to lug along the dead weight of a separate descent/ascent engine, separate fuel tanks, and a separate hab with separate life support systems...when you have a perfectly good hab with high-thrust engines and autonomous capabilities? I would be inclined to argue that it only makes sense to use a separate hab and landing craft and ascent craft if you've already established a semipermanent presence, but don't yet have access to fuel on the surface.

Lugging along a lander with its engine, plus some sort of ascent vehicle, plus a smaller hab feels like a waste to me when you're dealing with airless worlds. The space-hab that would descend is only going to be the hab itself, the selectable-thrust/isp engines, and descent/ascent fuel; the main drive and aux fuel tanks would stay in orbit. It only makes sense to use a separate lander if you can aerobrake with a capsule and land near an existing permanent hab. Sadly, this is true. Even though an air-augmented LOX-afterburning selectable propellant nuclear thermal turborocket is probably the most realizeable SSTO we're going to see

Why would you want the ship to reenter at Earth? Something like the HERMES wouldn't reenter, it would stay in orbit when not transiting. Or are you referring to something like the Orion capsule here? As I mentioned before, trying to build an interplanetary transfer spacecraft capable of many missions to multiple worlds would probably be best realized if the hab was built as an autonomous craft on its own, docking with a larger frame to carry auxiliary fuel, cargo, and the ion engines. While propulsively-landed aerobraked capsules a la Dragon V2 would always remain the primary way to land on worlds with atmospheres, building a hab that is sturdy enough to break away from the rest of the spacecraft and make independent landings on worlds without atmospheres (Ceres, Luna, etc.) makes sense. Otherwise, you will be designing a different lander and ascent vehicle for every mission to a new world, which defeats the whole purpose. And if it's that sturdy and carries its own high-thrust engines, then the possibility of occasional Martian landings emerges. Definitely worth considering, especially if the engines are nuclear thermal rockets with selectable propellant to allow multiple thrust/Isp configurations; that makes refuel and SSTO a breeze. Of course, if we've already gone to those lengths, we might as well go a bit further and allow for propulsive Earth re-entries...certainly not routine, but for periodic repairs or in emergency situations. Drop tanks and high-lifetime NTRs with selectable propellant (and optional air-augmentation) make even Earth SSTO routine. Probably couldn't carry much payload if you had to turn around and come back under power, but we've got to assume that fuel is going to be in orbit (since that would be required for an Earth Orbit Rendezvous assembly of the Hermes anyway). Spin'er up!

A few points... Reactors need not necessarily be heavy nor dangerous. They're not featherweight, granted, but we've come a long way since the late 70s. And Russia's strained history with them had more to do with putting them on satellites in unstable orbits with unreliable RCS that resulted in uncontrolled re-entry. As I mentioned above, using a bimodal NTR that can provide both thrust and electrical power allows you to use a minute propellant flow as an active, expendable coolant which also provides a bit of additional impulse. Again, this also means you don't have to shut off the reactor at all, just decrease the reaction rate. The primary shielding can also be the neutron reflector, in order to improve the overall power-to-weight ratio, and the secondary shielding can do dual duty as a heat radiator. Recall that the ship needs a source of heat as well, so that's an additional sink. I'm all about augmenting with solar power where possible, though scaling is an issue. Solar panels are very lightweight, but they occupy a great deal of space and must be orientable toward the sun, which increases weight as you scale up. The thrust of ion thrusters is limited by two things: power and surface area. So if you can manage a lot of surface area with high power, you can get decent thrust. I'd love to see 3D-printed panels with solar cells on one side and very small ion thrusters on the other, that could be folded up, launched all at once, and then folded out:

The "return to Earth" issue isn't that big of a deal, given that these are intended as long-duration reusable spaceships which would definitely need periodic resupply launches. In any case, some experimental ion thrusters can use hydrogen or even water as propellant, so that's promising. We've had working nuclear fission reactors in space for quite a while now. Seebeck-effect thermocouples on radioisotope thermoelectric generators are terribly inefficient, but thermionic generators approach efficiencies of 20% when operated at a high enough temperature, while a good alkali-metal thermoelectric converter can reach efficiencies of 41% in operation. So waste heat, while challenging, won't be a serious problem. For our extended-persistence interplanetary transfer spaceship, it would probably make sense to give it a pair of fission reactors which could be operated either as nuclear thermal rockets (using inert propellant) or as power generators (to power the craft and its ion engines). Such a design has the advantage of allowing active cooling of the reactors by a slow trickle of propellant if you need to run at high power, and it prevents startup/shutdown cycles on the reactor. This is called a bimodal NTR. Another really nice thing about a nuclear thermal rocket is that you can have essentially selectable thrust and Isp by varying your propellant type. For example, a spacecraft powered by a liquid-hydrogen NTR may not have enough thrust to get off the ground...but if you add external drop tanks, you can inject higher-density propellant to boost your thrust tremendously, allowing for highly efficient staging with just a single engine cluster. Engines are expensive; tanks are cheap. When the higher-density propellant is LOX, this is called a LOX-augmented NTR, or LANTR; when combined with a power generation mode as described above, it becomes a trimodal NTR. With the right form factor you could even leave the door open for my favorite tech, atmospheric thrust augmentation. Ion engines have such a low T/W ratio that I'm not sure it makes sense to put them on the primary ship. Probably better to depend on the hydrogen cycle NTR for moving between orbits and dock with the external frame at EML2 or wherever else you want.

It really wouldn't make sense for insurance companies to consider the entire launch provider as a single entity. When you're insuring really small things in bulk (like people, for example), it makes sense...but at the corporate level, insurance companies leverage ALL the information they have. SpaceX has made an established practice of selling launches with the express intended purpose of using the launch process as a testbed for future development. That's their whole schtick. If they feature their first re-used boosters with a reduced launch price, then it's understood at the start that the launch is experimental, which reduces the fallout and loss of reputation if there's a failure. If they manage three or four of those successfully, then they'll probably go through the insurance process to integrate reused boosters into their ordinarily lineup.

Update, given the pros and cons of various form factors.... I'm not sure how ambitious we want to be. If you've got access to cheap propellant in orbit, then it becomes possible to re-enter Earth's atmosphere without lots of heavy, failure-prone heat shielding, simply by burning off a large proportion of your dV and then aerobraking at a more modest speed down to terminal velocity, and using a final burst to land propulsively. So while this certainly wouldn't be the typical landing mode, something built to last as long as we're thinking could probably be built to survive an emergency Earth return. A massive internal cargo bay isn't too terribly important, because the interplanetary transfer frame can hold cargo with relative simplicity, but you need a little pressurizeable cargo space. With the right form factor, you could even allow for centrifugal gravity if you absolutely HAD to have it for a particular mission. I confess I really like a slimmed-down Firefly form factor. The thrust of ion engines depends primarily on surface area, so if you have two high-thrust low-Isp engines on winglets for maneuvering and a large rear region with your high-Isp engines, that allows for a bit more thrust from them. Can ion engines be 3D-printed?

Oh, I'm quite certain that the customer will be quite aware of whether the booster is brand new or reused. I'm saying that if customers balk about a reused booster, a one-time discount might allow for proof-of-concept, whereafter reuse will become routine.

Which is why you'd probably need a bigger rocket. Just in the general lift range of FH.

Depends entirely on where the orbit is. Try this: Draw a scale graphic of the Earth. Now, draw a circle around it that is 3" larger than the Earth. This is one possible orbit. Now, draw another circle around it that is 6" larger. This is another possible orbit. The first circle bends much more tightly than the second circle, but the gravity of Earth tugs on objects at the first circle with approximately the same force as it does objects on the second circle. Thus, objects on the first circle must be "falling" much faster than objects on the second circle in order to avoid hitting the Earth. If you start out without any speed, you'll start falling vertically quite fast, but that's another matter entirely. For Earth, an object just outside of the atmosphere needs to be "falling" sideways at around 7,800 meters per second.

No fairing required; the hab wouldn't be able to survive Earth re-entry but it ought to be sturdy enough for naked MaxQ. Falcon Heavy might not be the specific launch; just saying that's a good preliminary mass range. It would be cool to see some possible mockups... EDIT: I can see the appeal of a Firefly-style vehicle: command center and living space above a cargo bay, with fuel tanks and high-Isp engines in the rear and two winglets with high-thrust rotatable engines.

Related note... Would it make sense to tow a bunch of smallish asteroids into really distant Earth orbits, to be pushed into the path of any oncoming planet-killers?

Many years, and a lot of equipment. What are we looking at? One tonne of equipment per person? Ten tonnes of equipment per person? One hundred tonnes of equipment per colony, plus twenty tons per person?

Oh, it should definitely be overengineered, but just not so much that it would be cost-prohibitive to build another one, or another two, or another five. Balance between a sturdy, adaptable design and extremely high costs.... Engines are heavy, so you can save more in the long run. Plus, if you're really going to be able to keep this thing running for a long time, you'd want it to be able to run on whatever fuel you can find. Nuclear thermal propulsion with options for various propellant types is probably ideal...maybe with ion engines as well? Design factor should probably be something like "build the largest ship you can launch on a Falcon Heavy that would be capable of making a propulsive landing on the Moon with nothing but water in its fuel tanks".

Yeah, a floating colony on Venus would be really easy to build, but it would be hard to sustain. There's no source of water or raw materials since you're nowhere near the surface. Hence the question of whether there would be scientific advantages to putting an unmanned floating probe on Venus. Maybe a gigantic inflatable radiotelescope? I doubt that. Permanent, ongoing access to space beyond LEO will become inevitable just as soon as asteroid mining becomes profitable.

With an unjustifiable optimism in which a location can be found that allows Martian colonists nearly-unlimited access to water and a large range of minerals, what is the minimum they'd need to become self-sustaining for at least a couple of years? The sort of nuts who would colonize Mars in the name of freedom from oppressive Obamacare tyranny would probably justify government-sponsored resupply somehow...they're not known for being conscientious or consistent.

Slightly off-topic, but what are some of the science opportunities of having a floating probe in the Venusian atmosphere? Anything that could be done there that couldn't be done elsewhere?

Unfortunately that would tend to stock the Mars colony with a lot of nuts...

While I admit that I'm not sure why you'd want to do this... ...one possibility would be to build a central collection chamber that feeds into a large reflective balloon. As the balloon is inflated by the accumulating hydrogen, it exposes more and more reflective surface to the sun, causing it to spiral to a higher orbit. Then it can dump its fuel at a depot and repeat the process.

While SpaceX certainly won't be doing thirty launches this year, I don't see why they won't continue to recover stages. Not to fanboy, but landing a rocket on its tail is an engineering/guidance challenge, not a physics challenge, and SpaceX has hit all the high points there. The Orbcomm landing couldn't have been more perfect, and they've gotten the kinks out of water landings as well; the only reason Jason-3 failed was because it was using the v1.1 booster with the older, weaker landing legs. There's no reason to think that the next Falcon 9 booster to launch will also be the first orbital-class booster to be reused. All SpaceX has to do is offer reused boosters at a vastly discounted price (say, $40 million) and there will be numerous companies eager to jump on it; I'll predict we see a booster relaunch before July. Either SES-9 (if they can stick the water landing) or CRS-8 would be refurbished and ready for reuse by the time JCSAT-14 or Amos-6 launch, and both JSAT and Spacecom would probably be willing to volunteer. As far as the rest of the list, the political climate is really going to have to cool off before we get any kind of meaningful cooperation between the superpowers.

It probably will, unless you do an Apollo-style thing with only a few landings, which is sadly the most likely scenario. Then it only makes sense to reuse the propulsion system, maybe for a reusable GEO satellite tug. If the political climate improves by the time we're setting out to do this, then building an overengineered interplanetary transport capable of going to Mars or to Ceres or to Venus would probably be the best way of ensuring that we actually go to Mars, to Ceres, and to Venus. Once they've invested heavily it doesn't make sense not to keep going. You don't want to over-overengineer, though, because then your cost goes too high and they're loath to build a second or a third or a fourth one. You kinda want a fleet of these things if you're going to be able to sustain a manned Martian base. So you want to design a craft which is robust enough to last for a long time and be adapted for a lot of different missions (i.e., more spaceship than spacecraft), but is still cheap enough that we could conceivably build several of them. To that end, I'd suggest a combined hab/command center with some autonomous function, probably assembled on Earth and lifted to orbit by a single super-heavy launch. Lots of open space to keep your crew from going stir-crazy and to allow for expansion/adaptation. It would need to have internal airlocks, of course, to prevent catastrophic depressurization in the event of a hull breach, but you'd still want to try and manage a fairly open design. It would need its own engines (ideally VASMIR, but alternatively some sort of hybrid engine that can use either ion prop or chemical prop through the same nozzle) and power supply (fold-out or body-fixed solar panels with a decent battery system) so that it could still limp along if a meteoroid impact or other problem took out the main propulsive engines. Probably want to throw on an electrolysis system for converting water to fuel using solar power if you needed to. For the sake of expanded options in future missions, you'd want it to be able to be used as an emergency lander with low-gravity worlds without atmospheres, since those are the worlds where an aerobraking capsule is useless. So...Luna, Ceres, etc. ought to be realizable. This spaceship would mount to a sturdy frame which would be used for interplanetary transfers. The frame would include a spartan pressurized command capsule and feature attachment points for engines, cargo, fuel, and power supplies. The frame ought to be strong enough that it could be used for brachistrochrone transfers once our engine technology advances. This allows for modular replacement/upgrading without modification to the main spaceship, and the frame could be left in orbit if an emergency landing was needed.

How about a cage match in a hemispherical pressurized chamber on the surface of an asteroid, so there's gravity, but it's very, very low gravity and you can jump to the top of the chamber easily?