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If you have engines that powerful, you'd be better off boosting a much smaller ship to a relativistic velocity, and getting to the target stars within a few decades.
Use what is abundant and build to last
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Void wrote:Returning to the original topic "Terraforming small icy moons and comets":
I would think that a layering of devices should make sense. A rotating metallic device in the center, of significant size with synthetic gravity. That encased in a sphere of metal. Surrounding that additional shells. Over that layers of ices.
Using slow means of propulsion perhaps solar driven, perhaps fusion driven, moving to a next new object, and then as desired, either expanding the world, or cloning it.
Using this then after a very long time entering another star system, where worlds similar to Mars, Ceres, and Titan could be prime targets.
Tell me does Pluto count as a comet? Could we terraform that? How about the Dwarf planets further out?
Sedna is way out there, about 518.57 AU from the Sun with an orbital period of 11,400 years a mass estimated to be on the order of 1 * 10^21 kg About 1.4% of the mass of the Moon, the average orbital speed is 1.04 km/sec, the escape velocity at that distance is 1.83972 km/sec.
http://www.calctool.org/CALC/phys/astro … e_velocity
So the question is, could we accelerate Sedna to the Solar Escape Velocity from this orbit? The speed of light is 300,000 km/s so to start out at 1 km/sec, we need to accelerate to 2.83972 km/sec this will put Sedna on a departure velocity of 1 km/sec after it escapes the Sun, it will take 300,000 seconds to cross the same distance as a photon does in 1 second. If we can get it up to 300 km/sec it will take 4,400 years to reach Alpha Centauri. So basically what we have to do is make Sedna habitable for 4,400 years with the mass left over from accelerating it to 300 km/sec, as for slowing down, we don't have to slow down Sedna. Sedna can pass through the star system at 300 km/sec while the colonists in smaller spaceships will slow down to match the target system's velocity.
Propulsion is expensive. In most of these interstellar travel scenarios we generally assume that human beings have mastered thermo-nuclear propulsion before we even attempt interstellar travel, but even so, propulsion is still relatively expensive.
For an interstellar craft we must balance a number of conflicting trade-offs. Going faster improves mission reliability up to a point, because systems do not need to be designed for longer operating lives. A reactor core will wear out due to neutron irradiation damage after a century at most and pressure vessels begin to creep over a similar time. So going faster may reduce the cost of some mission elements and is always more desirable all other things being equal.
On the other hand, a slower ship can be larger for the same propulsive effort, so may afford more redundancy in critical systems. A large enough ship may be genuinely self-repairing on an indefinite basis. If the ship is inevitably slow (and it seems almost certain that a trip to the stars would take more than a human lifetime with any foreseeable technology) then it makes sense to make the ship large enough to allow for the comfort of its occupants. Up to a point, if the length of mission greatly exceeds individual lifespans, then speed becomes less and less important. The success of the colonisation mission is also improved if more people and more equipment are available at the destination. However, the longer the trip, the higher the chance that something or other will go wrong; an accident, a lunatic, some weird social or religious movement, or a gamma ray burst…or whatever. As we noted before, we have to trade off many factors.
The cost of the propulsion system can be thought of as being loosely proportional to the amount of kinetic energy it imparts in the ship. It is therefore proportional to the mass of the ship and the square of velocity. So you can reduce propulsive costs by increasing the mass of the ship and reducing velocity, using the extra mass to increase ship endurance for a longer journey time. You can reduce engineering complexity by doing that as well. But these cost savings only work up to a point. If your ship gets enormously large, the propulsion system is still going to be expensive even if delta-V is small and although you save money using simpler and more redundant systems, eventually, cost will start to increase again as the ship gets bigger. To produce reasonable acceleration levels, the engine thrust (and therefore size) must be scaled up as the spacecraft scales up. Even for very slow vessels, propulsion cost still increases with size, though not necessarily linearly, as there will be scale economies. An engine capable of moving a small world to solar escape velocity must be the size of a small world and will cost as much.
In most situations, it will be advantageous to ‘engineer’ the ship for the journey. That is to say we build a crew habitat with life support systems, power supplies, navigation and manoeuvring, etc. By engineering the ship in this way, we are effectively reducing its mass, making it easier to build a propulsion system that produces the delta-v we are looking for, for the lowest total energy investment. Dead weight is never going to be free and the faster you travel the more expensive it gets.
The exception to this is where nature foots the bill. If gravitational assist is used to boost the vehicle to escape velocity, then all that is needed from the propulsion system is the initial delta-v required to put the ship on the correct initial trajectory for the gravity assist. For an Oort cloud object this might be only a few metres per second. As we are not paying for the propulsion, our vessel does not need to be particularly weight-optimised. It clearly must have a very long endurance, as it will take many thousands of years to cross the gulf of interstellar space. So we at least need the material resources that will sustain us over that period. If propulsion is cheap, then we can choose a cheap vessel. A natural body with the correct balance of raw materials is about as cheap as it gets. In doing so, we are basically relying upon human reproductive capability and growth potential, to turn a small initial investment into a big return.
But there is still likely to be a trade off in terms of size. A 1000km diameter body weighs a million times as much as a 10km diameter body and requires a propulsion system a million times larger to generate the required initial delta-v. So it is unlikely that our future travellers would use a big comet, when a smaller one would do. I used a 200-km body as a baseline, but that may turn out to be optimistic as the energy needed to move such a body even 10m/s is still immense.
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If you have engines that powerful, you'd be better off boosting a much smaller ship to a relativistic velocity, and getting to the target stars within a few decades.
All other things being equal, it is always better to travel faster. But the longer it inevitably takes, the less important speed becomes. As journey times move beyond individual lifetimes, speed would seem to be less important than overall mission reliability.
To illustrate the point: If it takes 20 years for a fast ship and 200 years for a slow one and the cost is the same, then its a no-brainer. But what if the fastest ship takes 200 years and the slow ship takes 2000? If you are a passenger on that ship, it is suddenly a lot less important how long the journey takes. You will never see the destination and neither will your grandchildren. But the comfort and reliability of the ship will be a big issue for you and your grandchildren.
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Tom Kalbfus wrote:Void wrote:Returning to the original topic "Terraforming small icy moons and comets":
I would think that a layering of devices should make sense. A rotating metallic device in the center, of significant size with synthetic gravity. That encased in a sphere of metal. Surrounding that additional shells. Over that layers of ices.
Using slow means of propulsion perhaps solar driven, perhaps fusion driven, moving to a next new object, and then as desired, either expanding the world, or cloning it.
Using this then after a very long time entering another star system, where worlds similar to Mars, Ceres, and Titan could be prime targets.
Tell me does Pluto count as a comet? Could we terraform that? How about the Dwarf planets further out?
Sedna is way out there, about 518.57 AU from the Sun with an orbital period of 11,400 years a mass estimated to be on the order of 1 * 10^21 kg About 1.4% of the mass of the Moon, the average orbital speed is 1.04 km/sec, the escape velocity at that distance is 1.83972 km/sec.
http://www.calctool.org/CALC/phys/astro … e_velocity
So the question is, could we accelerate Sedna to the Solar Escape Velocity from this orbit? The speed of light is 300,000 km/s so to start out at 1 km/sec, we need to accelerate to 2.83972 km/sec this will put Sedna on a departure velocity of 1 km/sec after it escapes the Sun, it will take 300,000 seconds to cross the same distance as a photon does in 1 second. If we can get it up to 300 km/sec it will take 4,400 years to reach Alpha Centauri. So basically what we have to do is make Sedna habitable for 4,400 years with the mass left over from accelerating it to 300 km/sec, as for slowing down, we don't have to slow down Sedna. Sedna can pass through the star system at 300 km/sec while the colonists in smaller spaceships will slow down to match the target system's velocity.Propulsion is expensive. In most of these interstellar travel scenarios we generally assume that human beings have mastered thermo-nuclear propulsion before we even attempt interstellar travel, but even so, propulsion is still relatively expensive.
For an interstellar craft we must balance a number of conflicting trade-offs. Going faster improves mission reliability up to a point, because systems do not need to be designed for longer operating lives. A reactor core will wear out due to neutron irradiation damage after a century at most and pressure vessels begin to creep over a similar time. So going faster may reduce the cost of some mission elements and is always more desirable all other things being equal.
On the other hand, a slower ship can be larger for the same propulsive effort, so may afford more redundancy in critical systems. A large enough ship may be genuinely self-repairing on an indefinite basis. If the ship is inevitably slow (and it seems almost certain that a trip to the stars would take more than a human lifetime with any foreseeable technology) then it makes sense to make the ship large enough to allow for the comfort of its occupants. Up to a point, if the length of mission greatly exceeds individual lifespans, then speed becomes less and less important. The success of the colonisation mission is also improved if more people and more equipment are available at the destination. However, the longer the trip, the higher the chance that something or other will go wrong; an accident, a lunatic, some weird social or religious movement, or a gamma ray burst…or whatever. As we noted before, we have to trade off many factors.
The cost of the propulsion system can be thought of as being loosely proportional to the amount of kinetic energy it imparts in the ship. It is therefore proportional to the mass of the ship and the square of velocity. So you can reduce propulsive costs by increasing the mass of the ship and reducing velocity, using the extra mass to increase ship endurance for a longer journey time. You can reduce engineering complexity by doing that as well. But these cost savings only work up to a point. If your ship gets enormously large, the propulsion system is still going to be expensive even if delta-V is small and although you save money using simpler and more redundant systems, eventually, cost will start to increase again as the ship gets bigger. To produce reasonable acceleration levels, the engine thrust (and therefore size) must be scaled up as the spacecraft scales up. Even for very slow vessels, propulsion cost still increases with size, though not necessarily linearly, as there will be scale economies. An engine capable of moving a small world to solar escape velocity must be the size of a small world and will cost as much.
In most situations, it will be advantageous to ‘engineer’ the ship for the journey. That is to say we build a crew habitat with life support systems, power supplies, navigation and manoeuvring, etc. By engineering the ship in this way, we are effectively reducing its mass, making it easier to build a propulsion system that produces the delta-v we are looking for, for the lowest total energy investment. Dead weight is never going to be free and the faster you travel the more expensive it gets.
The exception to this is where nature foots the bill. If gravitational assist is used to boost the vehicle to escape velocity, then all that is needed from the propulsion system is the initial delta-v required to put the ship on the correct initial trajectory for the gravity assist. For an Oort cloud object this might be only a few metres per second. As we are not paying for the propulsion, our vessel does not need to be particularly weight-optimised. It clearly must have a very long endurance, as it will take many thousands of years to cross the gulf of interstellar space. So we at least need the material resources that will sustain us over that period. If propulsion is cheap, then we can choose a cheap vessel. A natural body with the correct balance of raw materials is about as cheap as it gets. In doing so, we are basically relying upon human reproductive capability and growth potential, to turn a small initial investment into a big return.
But there is still likely to be a trade off in terms of size. A 1000km diameter body weighs a million times as much as a 10km diameter body and requires a propulsion system a million times larger to generate the required initial delta-v. So it is unlikely that our future travellers would use a big comet, when a smaller one would do. I used a 200-km body as a baseline, but that may turn out to be optimistic as the energy needed to move such a body even 10m/s is still immense.
It might make sense to eliminate Sedna's orbital velocity of 1 km/second so it falls towards the Sun, adjust its orbit so it makes a close flyby of Jupiter, and that way you can depart the Solar System at Voyager like speeds. Voyager 2 is departing the Solar System at around 17 km/sec. For an initial investment in propulsion of 1 km/sec, one can parlay that with a close Jupiter encounter to maximize the departure velocity of Sedna, maybe get from 11 to 15 km/sec. A good target for the mission would be Tau Ceti since that is located near the Solar ecliptic and is known to have several planets. Tau Ceti is 11.9 light years away, and 15 km/sec is about 1/20,000th the speed of light, the mission duration would be 240,000 years on the "starship Sedna" During that time of course their would be plenty of materials for building a larger more powerful engine while enroute, and perhaps boost the velocity up to 1/2000th of the speed of light and get there is 24,000 years. There are a lot of materials to work with as Sedna is quite large, who know what could be built over centuries of time? A good amount of time would be waiting for the planets to line up correctly and falling sunward, which I think would take a long time from Sedna's distance from the Sun.
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