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Is it possible to use Ceres is a generational garden ship? The idea would be to tunnel out a Stanford torus like set-up under the regolith in the ice layer around the equator: underground but relatively close to the surface. The inhabitants would stand with feet pointing towards the equator and head towards the core. Hope that makes sense. The rotation of Ceres would have to be sped up or slowed down to match that of Earth. Since Ceres has a lot of ice-water in its make up I thought that the hydrogen could be liberated from that to use as fuel in ion propulsion. Obviously there would be plenty of shielding. The idea would be to take it out of orbit around the Sun and travel to another star system. It will obviously take centuries. It's a one way trip and no slowing down needed at the other end. what sort of speed is realistic?

Is it too big, is this a silly idea? I'm clearly not a physicist, thanks in advance to anyone willing to take a look at this.

CERES: 615 km/s escape velocity of the sun 17.9 km/s orbital speed of Ceres around the sun 413,937,308 km distance from the sun 1,083,206,916,846km3 volume

Accelerating one ton to one-tenth of the speed of light requires at least 450 petajoules or 4.50×1017 joules or 125 terawatt-hours[8] (world energy consumption

Recent tests demonstrated that the X3 thruster can operate at over 100kW of power, generating 5.4 Newtons of thrust — the highest of any ionic plasma thruster to date.

How many of these would one need? Can they be scaled up?

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  • $\begingroup$ Welcome SimonStory, +1 for Ceres the Roman godess of agriculture.. very appropriate for a garden ship. Not sure though if this is going to actually move large distances, in a convenient way.. it's a dwarf-planet. That's a lot of thrusters you'll have to mount ! $\endgroup$
    – Goodies
    Apr 16 at 2:07
  • $\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$
    – Community Bot
    Apr 16 at 3:38
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    $\begingroup$ All that's holding Ceres together is its own gravity. If you try to spin up Ceres, it will tear apart long before you can get 1 gee of spin gravity at any point on its surface or interior. $\endgroup$
    – notovny
    Apr 16 at 3:55
  • $\begingroup$ Thank you for taking the time to reply, I really appreciate it. $\endgroup$ Apr 17 at 8:23

3 Answers 3

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Ease up on speed.. and travel to Earth first

You calculated the amount of energy to accelerate 1 ton to the speed of light not quite accurate. In order to accelerate a 1 ton object to the speed of light, you'd need infinite energy.

Go slower

What about going easy ? it is a generation ship so suppose you'd want to travel at 1% of the speed of light, that is about 3000 km/s, the required energy and 10% propulsion efficiency will result in about 1 Hiroshima bomb per kg Ceres (see this quora link)

Ceres weight is 10^18 kg so I don't think you'll find enough Ceres surface to put your thrusters.

even slower

To acellerate a dwarf planet to 1% speed of light is still madness.

Also, take into account you'll have activate your thrusters twice: one time to accelerate and one time when you decelerate. So either you move the thrusters during the travel, turn the ship, or to prepare deceleration stage, or you'll need thrusters on either side of Ceres, leaving little room for a garden.

What about 100 m/s

With thrusters 100 kW each you won't get far.. now suppose you'd go further down in speed, say calculate again for say, 100 m/s. Nice thing about kinetic energy it goes down with square speed, so you'll get sqrt(100/3000000)=10^-9 the amount, solving a relevant part of your 10^18 problem. But still, you'll need a billion Hiroshima bombs to get Ceres to 100 m/s. But if you can manage that, Ceres will arrive near Earth in about 150 years. For a generation ship, that will do fine, won't it ?

Escape from orbit?

Considerable energy must be spent to get Ceres out of its orbit, imagine a heavy object traveling at 17.9 km/s, you'll need to bend that orbit inward, to move Ceres toward Earth.

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The entire concept is something I have toyed with, and it is theoretically possible within the confines of our current thick 2021 physics textbook (far thicker than even the 2015 edition, and huge compared to the 1999 edition used most commonly in this forum). Only a few chapters need to be made thicker.

But first, I would suggest, instead of spinning the entire dwarf planet, build a spinning habitat around the equator, deep within to give a protective layer from radiation. There is no need to spin the entire planetoid. Just excavate out a circular tunnel and construct a huge habitat ring. Makes taking off and landing supplies from the surface while building it and 'landing' it at the destination much easier.

What you suggest is entirely feasible in the near-future, but forget the reaction mass for propulsion. I posit that the solution will involve the manipulation of the Higgs Boson and Field. It is this reaction that is responsible for the 'm' in "F=ma". The concept is to duplicate the idea of the Alcubierre Drive, only forget the idea of using it for 'faster-than-light', just use the concept for propulsion. Imagine a scenario where the Higgs Field is distorted in front of and behind the planetoid, and the unbalance creates forward momentum. Instead of resisting momentum, the Higgs Field is 'squeezed' to create momentum. It will still involve huge amounts of energy, but you have a entire planetoid to build a fusion reactor, with lots of hydrogen fuel available.

You might even consider a particle accelerator around the perimeter to create particles as needed.

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    $\begingroup$ Common misconception, but the Higgs field is NOT actually responsible for most of the mass of ordinary matter. It gives mass to fundamental particles like electrons and quarks, but most of the mass of ordinary matter is actually from the kinetic energy of quarks and gluons within protons and neutrons. Fiddling with the Higgs field, even if possible, would be just as likely to make matter dissolve in a nuclear explosion as change its mass. $\endgroup$ Apr 16 at 14:21
  • $\begingroup$ @Matt Dickau It all depends on which physics book you choose, the 1999 one or the 2021 book. $\endgroup$ Apr 16 at 20:11
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  • I made a mistake in my calculations, but noticed it too late. I tried to make some clarifications, but... I'll fix or delete it later. I hope someone writes a better answer and makes this one obsolete. It still has some info for the OP (otherwise I wouldn't post it) so... until a better answer posted.

  • I can't comment because of browser javascript compatibility issues, so won't be able to answer to comments. Feel free edit this answer as much as you like - english, numbers, whatever - improvements are welcome.

Spinning Ceres isn't such great idea, as @notovny pointed in his comment. It's also easy to see by such an example - let's assume that your people are 10km below the ice, shielded well, with their feet pressed to the ceiling with 1g towards equator. But all the ice, which is under their feet, wants to fly to the equator. It can be visualized (not an exact match) as an ice sky scraper standing 10km height in the earth's gravity. Even if both cases are not equal, the ice will get what it wants: To fly out in one case and crumble/fall down in another.

This is not a huge problem, just makes your happy version of an ice express. A torus and maglev will fix the problem.

  • In that sense, why not to begin with building that torus next to Ceres, using Ceres' materials? Okay, there are cons and pros, and in my opinion the pros of carving out an asteroid doesn't outweigh the cons, but at least it has a couple positive places.

As for the energy required, our Sun has more than enough energy for that, so in theory it shouldn't be a problem. If efficiency is high, it will probably be capable of accelerating at 10 m/sec2 (1g) or something like that, minus a mile (to start).

  • If they have access to even one percent of the Sun's solar energy output, then it can be done quite easy energywise, but such a choice doesn't have much justification. Ceres travelling in this way is only a result of some story, as without some reason, it doesn't look like a good choice. But sure, the story can fix that (It's a bit of a lazy move, but people eat it).

Using electric propulsion and current engines specs, you will use about 36% of mass of Ceres to reach 0.1c (with an exhaust velocity of 0.1c, which is the fusion engine's speed).

  • WARNING I made a mistake here with exhaust velocity, which makes up the rest of the answer. More importantly, the conclusion is invalid. In the case of ION engine, things are grim, but if it upgraded to the level of fusion engine then things look a bit better, except for the energy consumption and the time required.

  • Wrong numbers, Wrong conclusions

  • The correct end mass for Ceres is 30km/s exhaust velocity. There is no way to have a 0.1c end velocity, as it is 21x of the exhaust velocity, and then you will be left with about a million tons (10^9 kg) of ex-Ceres. (The rest is spewed as reactive mass.) So really you barely get to 600km/s, and starting from Ceres orbit, it's way more than one needs to escape. (600km/s escape velocity is next to the solar surface, and at Ceres, the orbit's main job is done. It is like the energy was never spend/lost.)

In engines, their TWR is an important number. It produces 5N, but how much mass it has is on its own. It is about 230kg, so max acceleration is 1m/s in about 46 seconds. This means the best time it can possibly be in this case is about 43 years to reach the speed of 0.1c. In reality, it may be a couple of times longer, depending on how heavier Ceres is than all those engines. For every 10 times larger, the trip is about 400 years later. That is probably a reasonable number.

  • I have reasons to believe that scaling it up will improve specs in the TWR aspect, but you would need energy sources, places to store the energy capturing devices, etc, so the estimate above may be a reasonable number in scaled-up version as well.

All in all, things aren't that bad with fusion, (like ION engines). It may need a few tweaks here and there, but it can become a reasonable generation ship. However, with the ION engines, which have 1000 times worse ISP(exhaust velocity), it is quite bad. Nevertheless, energy source is biggest issue here in both cases.

So one needs about 470 trillion(4.7x10^14) of such engines, and power consumption/generation 47'000'000'000 GW.

  • Post-error detected explanation: As we will be left with nothing, for a mass as large as that of Ceres, especially with those low ION engine ISP's, we have to increase ISP. We'll have the same improvement basket, but it also means that if ISP is increased 1000 times to the level of fusion engine, energy consumption also will be increased, but at a million times increase. (Energy is the square of velocity.) Sooooo... That means that it is 1'000'000x larger than of those 47 billion GW. Sooooo... Ring diameter will have to be 1000 times bigger. OR it has to be a million times slower. OR a combination of those two.

  • 47 billion GW is an energy-capture sail/system of 200'000 km in diameter at earth orbit (1300W/m2). Not impossible as well.

  • You can, however, use oxygen as reactive mass as well. You'd have to repair those engines and replace them along the way, which means factories to make all that, all the technology for all that, and many, many more other things. Oxygen can be ionized in the same way as hydrogen, keeping the high ISP. Take it from the improvement bucket, where all that scaling up lives, and it is quite a minor upgrade (and there are ion engines which work on air/oxygen containing mixture.)

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  • $\begingroup$ Thank you for taking the time to reply, I really appreciate it. $\endgroup$ Apr 17 at 8:19

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