Spaceships--how much IS a power point

[AlexanderHowl]

If you are on a frozen world with a vacuum atmosphere, it is a Hadean world, meaning that you are probably on surface of ammonia or methane ice (with islands of water ice poking through). Anything hotter than 130 C is probably inadvisable as, even in the case of a vertical heat radiator, you will have a lot of melting around the vehicle. A smaller hotter radiator causes the melting to be more severe close to the vehicle, so a 1,000 C radiator would likely cause the vehicle to sink through the ice within moments as the ammonia and methane just evaporate.

A stationary facility can spread the heat through subterranean radiators that benefit from convection and conduction (especially since they can find the islands of water ice that are capable of tolerating higher temperatures). If they are lucky, they might actually find a rock or two to anchor the facility to, giving more security. Alternatively, they can burrow into the water ice mountains, gaining protection from meteorites and radiation and benefitting from improved cooling. With smaller walkers doing mining and exploration, they could be a nice little home at the edge of a star system.

[YankeeGamer]

The craft is on Earth--a tribute to A Pail of Air. The massive machine is leaving the United States--a prosperous civilization under the Rockies and beyond--to explore the world. Are there other surviving civilizations? What lost knowledge can be found. Is the Svalbard Seed Bank still intact? Anything far enough above sea level is mostly exposed rock; as the air condensed, it flowed down to the nearest basin. Get low enough, and you're on frozen atmospheric gasses.

Currently, I'm not planning on running a game, but writing a tale. It's my goal to keep everything self consistent and semi plausible. Earth and its moon are out on their own. I'm planning this as an almost Star Trek flavor--in part to try writing with less exposition and more showing.

Per the Pyramid article, Enterprise needs only a fraction of a power point for its 5 MPH cruise, but she has surplus power--both for the big laser and the full sized fabricator. There's enough power to move a good million tons--a good thing, because the Enterprise will be taking some fairly steep inclines.

A SM +13 crawler is pushing the envelope, but it's fairly low, but very wide and long. For a rough concept, think of the NASA crawler with an empty launch platform on it, but MUCH bigger. A Nimitz class carrier is SM +12.

EDIT: There's always either hard rock, ruins, or water not too far down...not sure how thick Earth's atmosphere would be when it froze out.

[whswhs]

Originally Posted by YankeeGamer

EDIT: There's always either hard rock, ruins, or water not too far down...not sure how thick Earth's atmosphere would be when it froze out.

The mass of the atmosphere is 5.5 x 10^18 kilograms. The surface area of the Earth is 5.1 x 10^14 square meters. So there's about 10,780 kilograms per square meter.

The atmosphere is predominantly nitrogen. Solid nitrogen apparently has approximately the same density as water, which is 1000 kilograms per cubic meter. Dividing, we get 10,780/1000 = 10.78. Call it 11 meters deep, or 433 inches, or 36 feet.

The Earth is about 70% oceans, and the oceans are at sea level. If we assume that all the frozen air pools on the oceans, then we get 433/0.7 = 619 inches, or 52 feet.

The actual figure will be somewhere in between. Doing some googling, I see an estimate that the typical elevation of the land is 800 meters; if that's accurate (it sounds high!), I would guess that nearly all the frozen air is on the frozen oceans, and you can figure around 50 feet deep. If you're on land that is higher than 50 feet, then you're sitting on nearly bare rock, possibly with a thin layer of water, carbon dioxide, oxygen, nitrogen, and argon frost.

Hope that helps! You can probably find more precise figures but the method should work.

(Hey, that's my first Fermi problem for 2020!)

[AlexanderHowl]

Did the Sun die? Because there is no other way that it gets cold enough for nitrogen to freeze (the temperature must be below -210 C). With the insulation effects of nitrogen and the geothermal energy of the Earth (and the tidal energy from the Moon), I am not even sure that the temperature can get cold enough to cause nitrogen rain, much less nitrogen ice. Look at Titan for an example of a cold world with a nitrogen atmosphere.

[whswhs]

Originally Posted by AlexanderHowl

Did the Sun die? Because there is no other way that it gets cold enough for nitrogen to freeze (the temperature must be below -210 C). With the insulation effects of nitrogen and the geothermal energy of the Earth (and the tidal energy from the Moon), I am not even sure that the temperature can get cold enough to cause nitrogen rain, much less nitrogen ice. Look at Titan for an example of a cold world with a nitrogen atmosphere.

The reference to "A Pail of Air" is the clue. That's a short story by Fritz Leiber, originally published in Galaxy in 1951 and serving as the lead and title story of a collection of his short fiction in 1964. Its premise is that a body of stellar mass passed through the solar system and disrupted the Earth's orbit, sending it out into interstellar space. It was sort of like a Heinlein juvenile in tone and viewpoint, and closer to hard SF than most of what Leiber wrote. Presumably the described campaign shares Leiber's premise.

[Agemegos]

Originally Posted by whswhs

Hope that helps! You can probably find more precise figures but the method should work.

(Hey, that's my first Fermi problem for 2020!)

http://forums.sjgames.com/showthread.php?p=2303051

[Agemegos]

Originally Posted by whswhs

The reference to "A Pail of Air" is the clue. That's a short story by Fritz Leiber, originally published in Galaxy in 1951 and serving as the lead and title story of a collection of his short fiction in 1964.

Is that the one in which a nuclear family is living in 0.05 bar of pure oxygen, which they maintain by continually boiling liquid oxygen, thus replacing what they lose to diffusion through a non-airtight enclosure of, IIRC, blankets? Some rescuers show up?

If so, I had misremembered it as a John Beynon story.

[Anthony]

Hm. The earth's core produces about 0.087W/m^2, which for a perfect blackbody would place a lower limit on temperature at 35K. It would take a very long time to do so, however.

[Agemegos]

Originally Posted by Anthony

Hm. The earth's core produces about 0.087W/m^2

Are you including the whole geothermal flux? If so, a significant part of that comes from K-40 decay in the mantle.

which for a perfect blackbody would place a lower limit on temperature at 35K.

Nitrogen melts at 65K and oxygen at 54K, so that's okay for YankeeGamer's premise. Oxygen will rain out first, but then the nitrogen will freeze before the oxygen does. As best I can figure out, frozen nitrogen would sink in liquid nitrogen but float in liquid oxygen, so at some stage in the freezing of the atmosphere nitrogen ice will rise to the surface and form a nitrogen ice pack over the liquid oxygen. When freezing is complete you'll have up to about eight metres of solid nitrogen over a basal layer of solid eutectic oxygen-nitrogen, with water ice under that.

But you suggest that it would take a long time for Earth's surface to cool to 54K?

[johndallman]

Originally Posted by Agemegos

Is that the one in which a nuclear family is living in 0.05 bar of pure oxygen, which they maintain by continually boiling liquid oxygen, thus replacing what they lose to diffusion through a non-airtight enclosure of, IIRC, blankets? Some rescuers show up?

That's the one.