Tag Archives: space

That’s no Moon

It’s a gas dwarf.

I never made any secrets about how much I love the worldbuilding of Morrowind. (It’s gameplay is a different matter.) And I never let an opportunity pass by to tell everyone how much I love Star Wars. I also liked the world of the old videogame Albion and the whole old Planetary Romance genre in general. When I wrote down my Project Forest Moon concept paper to spice up the Ancient Lands with more mythic and puply atmosphere, that title was just a name referencing the visual style of Endor in The Return of the Jedi. But that phrase stuck with me until I recently decided to have the Ancient Lands be set on an actual moon. I know a fair bit about astronomy and while I think scientific accuracy is vastly overrated in fantasy worldbuilding, I think no creator likes to create stuff they know to be wrong within the rules of their fictional world. So I sat down to figure out a configuration that is at least somewhat plausible if you’re not getting too specific about the exact numbers involved. Or in other words, I feel pretty confident that planets like this can exist if you just find the right numbers for masses and distances to keep everything in semi-stable balance.

Having an Earth-sized moon orbiting a gas giant (like the Rebel base on Yavin 4 in Star Wars) would have all kinds of “interesting” effects that would make any kind of Earth-like environment on it vastly implausible. And you’d also end up with all kinds of funkiness regarding day length and daily solar eclipses lasting for hours. To keep things much simpler and more familiar, I chose to make the big ball in the sky a gas dwarf instead.

So what is a gas dwarf?

Gas dwarves are the most recently discovered type of planet that exists in other star system, which look very much like gas giants but are much smaller than those. In their center is a solid rocky core like a common terrestial planet which is then surrounded by a massive atmosphere of hydrogen and helium. Planets like Earth or Venus have not enough gravity to hold on to these very light gases in significant quantities, but if you go just a little bigger in size gravity is strong enough to keep these huge balls of gas together. The total mass of gas dwarves is between 1.7 and 4 times the mass of the Earth and it appears that they are one of the most common types of planets in the universe. It’s just a random oddity of the solar system that we ended up not having any of these. Being so much smaller than a gas giant the gravitational effects and its magnetic field would be much smaller than what you have in a behemoth like Jupiter or even Neptune.

I recently got myself Universe Sandbox 2, which I’ve been fascinated about for a very long time, and made a quick simulation of what it might look like if you take Earth and switch the Moon for smallish gas dwarf. I started by taking Neptune and changing its mass to 2 Earth masses. The program then did the recalculation of it’s actual size automatically. As expected, two bodies of such similar size would actually form a binary planet, both orbiting about a point between them instead of one going around the other, with the world if the Ancient Lands not being actually a moon. But it’s close enough. The screenshot at the top of this post is taken directly from the simulation I made with everything being at actual scale, with the gas giant being the same distance away from Earth as the Moon. But it’s a lot bigger and the little black dot next to the bigger blue ball is what the Moon would look like from this perspective. At 8.5 times the radius of the moon the gas dwarf would take up an area in the sky 72 times bigger. Hydrogen clouds would also reflect light much better than moon rocks, so the light of a full moon would likely be hundreds of times brighter than what we get here on Earth. However, human eyes are actually really amazing at automatically adjusting to light levels to give the brain the appearance that everything is normally lit. We did measurements of light levels in greenhouses in school and rooms that seem to be evenly lit actually get several times the amount of light close the sun facing windows than at the opposite side. Sunlight is obviously brighter than the light of a full moon, but human eyes adjust so well that you probably wouldn not have suspected that it is actually 400.000 times brighter. So even with a full moon being 400 times brighter than on Earth, the nights wouldn’t actually look much brighter to the eyes of people.

This is the Earth and the gas dwarf seen side on at actual scale. This shows the actual relative sizes and distances of the two bodies.

Tidal effects would obviously be much more severe as those caused by the Moon. However in practice, the actual rise and fall of the water is influenced much more significantly by the shape of coastlines than the gravitational pull of the moon. While there would be some bays experiencing absolutely astonishing tides, it should not be too dramatic for most coasts to completely change life near the sea. The time between high tide and low tide remains roughly 6 hours since the day is 24 hours in length. The orbital speed of the gas dwarf is marginal compared to the rotation of the forest planet.

Sadly, one thing that Universe Sandbox can not simulate is tidal locking. Tidal locking is when a smaller body slows down its rotation to the point where it matches its orbit around the larger body, causing it to always show the same side to the larger body, while the larger body would remain stationary in the sky of the smaller one. I think this is boring and want my wandering gas moon, which is why I gave it such a low mass to reduce this effect. In reality, the effects that cause tidal locking are working on every smaller body orbiting a larger one. The only question is how long it will take for the rotation to slow down before a true lock is reached. For the Earth and the Sun, tidal locking actually takes longer to reach than the Sun is going to live. One number I’ve found is that the Earth actually had days of only 6 hours when it first formed. So the fate of my world is sealed and it will eventually tidally lock to the gas dwarf. But the gas dwarf has only twice the mass of the forest planet while in comparison the Earth has 80 times the mass of the Moon. So I see it as completely plausible that a after three billion years the forest planet still has a nice 24 hour day and is a very far way from getting locked and the gas giant keeps moving in the sky.

Another interesting number is the length of a month. That is time from one full moon to the next full moon. In this particular configuration of masses and distances that I uses this turned out to be almost exatly 16 days. That would be 4 days from new moon to half moon and from half moon to full moon, and the same back of course. 16 is a very attractive number, being a square of an even number, so I keep that for the days in a month. For the number of months in a year, 24 would also be a very attractive number, being a multiple of 12. If a month where exactly 16.0 days and a year exactly 24.0 months, it would lead to a year of 384 days. Very close to what we think of when we are talking about “a year” as a unit of time. But such a perfect synchronisation would seem vastly implausible to me, so in the tradition of the Japanese aesthetic of wabi-sabi I am setting the length of the year at roughly 381 days, with the occasional leap day now and then. And sometimes a year happens to have only 15 months. Since I am lazy with such things and calendars show up rarely in practice in campaign, I’m not making any names for months or days of the week. It’s simply the first day of the eleventh month. With each month beginning at the new moon.

Another cool subject is solar eclipses. Because with a diameter 8 times larger than the Moon, the gas dwarf has a really easy time completely covering up the sun. In reality the Moon passes between the Earth and the Sun once every month during the new moon. However in most months it will pass actually above or below the sun in the sky since all orbits are not perfectly flat. How often you get solar eclipses depends on the tilt of the orbit, the size of the moon compared to the sun, and the length of a month, but they will be most common during spring and fall. There are 16 opportunities for an eclipse every year and a 50% chance for any place on the planet to be on the sun facing side when it happens, resulting in a total maximum of 8 if the orbits where perfectly flat. I really don’t want to worry about the exact math of this, so I am just arbitrarily setting the number of total eclipses a place experiences in a year at 1 or 2. However, I am pretty sure there is an orbital tilt that would lead to this result. I just don’t want to calculate that number as it will never come up in a game. On Earth a solar eclipse can last up to eight minutes. With the gas giant being eight times wider but it going around the planet at double the speed, this gives us eclipses of up to 30 minutes. So to streamline the numbers for practical use, a total eclipse lasts for 10 to 30 minutes.

So that’s the sky and the resulting calendar in the Ancient Lands. I actually tried to simulate each of the two planets having a small moon of their own, adding Deimos and Phobos to the system. When I ran the simulation, the Earth immediately flung its moon on a course to the sun while the gas dwarf threw its moon straight at Earth, leaving a huge crater lake in Morocco. I am pretty sure it should be possible to have two minor moons in the sky as well, but I am not going to include these into the simulation. They are just there in the sky looking pretty and not having any noticable effect on the planet below.

Why Venus is the coolest place to go to

Everyone is still excited about Mars. And I can remember how 20 years ago it really was the big thing to do in space. Everyone was wondering if there was or is water, and perhaps there are some tiny bugs or mosses living in cracks in the ground and things like that. But now we had a lot of probes on Mars that did really well and even had a close up shot of a bit of ice under the red dust. Reaction was “huh, that’s pretty neat”, but didn’t really seem like such a great discovery anymore, with much more interesting stuff going on the Jupiter and Saturn moons and planets around other stars. And even after a decade of combing the desert of Mars, other than ice we ain’t found shit!

Hunting for life on Mars has for a very long time been more about searching for traces of microbial life on Mars from billions of years ago. With some hope to find tiny fish on Europa or Enceladus and methane rivers on Titan, Mars really looks pretty boring now. No real need to get any scientists on the ground there to research microbes anymore. So getting people on other places than Earth and the Moon is really mostly (or entirely) because we could and it would be cool. So let’s scrap going to Mars, that place sucks. Venus would be so much cooler to go to.

venusNow the big problem with Venus is that the surface is about 470°C hot and air pressure more than 90 times higher than on Earth and enough to crush almost all submarines. Also, the atmosphere consists mostly of Carbon Dioxide and sulfur dioxide, which likes to form sulfuric acid. Which makes Venus the worst place to land on in the entire solar system, other than the surface of the sun. However, you don’t have to land on it when you could also cruise around in an airship high up in the clouds. And then it’s actually probably the nicest place for humans in the solar system other thab Earth.

  • At a height of about 50 km above the surface, the air pressure is similar to that found on Earth at sea level, with temperatures around pleasant 20 to 30°C.
  • The atmosphere consists mostly of carbon dioxide and nitrogen, which are both not harmful to humans. You would suffocate because there is no oxygen to breath, but that luxury is found nowhere except on Earth.
  • This only leaves the sulfur dioxide as the remaining nasty environmental factor, but when sulfuric acid reacts with organic matter, it leaves behind pure carbon. Carbon fibre, carbon nanotubes, and graphite are all modern materials that people get quite excited about and they all happen to be really just pure carbon. (Carbon fibre is mixed with fibres of other materials, but there are surely options that also don’t react with the acid. Some metals, like titanium, will react with the acid, but then form a thin layer of oxide on the surface which seals the metal and keeps the acid away.
  • While there are almost no oxygen molecules in the atmosphere of Venus, most of it is carbon dioxide. If you can split the carbon dioxide, you get oxygen to breath and carbon to make replacement parts for your carbon airships.
  • Even 50 km above the surface, there is still so much atmosphere above you that it helps block radiation from the Sun, which on the Moon and Mars would be a lot nastier even though they are much farther away.
  • The atmosphere also makes the whole landing part a lot easier. Landing on the Moon is easy because it has very little gravity. Landing on Earth is also easy, because you can use parachutes and wings to gently float down to the surface. Mars has lowe gravity than Earth, but still quite a lot, but also barely any atmosphere, which makes landing pretty rough. Robots can handle it, but with current technology any astronaut would feel like being in a plane crash. Or not, since he’d probably dead. On Venus, you have all the nice atmosphere so you can use parachutes to slow you down. And best thing, since you don’t actually go to the surface, you don’t have to land at all. If your airship goes down a bit faster and ends up lower than you aimed for, it will just bob back up to the altitude it was meant for. (Unless you get too deep and crushed and baked.)
  • One of the best things about Venus is that it has a similar size and mass to Earth, which means it has also a very similar gravity. Even 50 km above the surface, the pull of gravity is almost the same as on the ground, which for Venus is about 90% of the gravity we experience on Earth. Floating around for a few minutes is fun, but low gravity does all kinds of unpleasant things to muscles, bones, and circulation over time. In an airship on Venus that would not be an issue.
  • A trip to Venus is also shorter than a trip to Mars. Not hugely, but cutting a 550 day trip down to 450 is still 100 days not hanging around in space doing nothing. Carrying around 20% less food will also make the engineers very happy.

Really, the only annoyance about Venus is the sulfuric acid, which really is only a big problem when you get it on your skin and in your lungs. Which is sad, because otherwise you could go paragliding on another planet with nothing but a breathing mask. If weather is good and there are no acid clouds nearby, you probably still could get out on the roof without a full suit, enjoying the sun for a few minutes. Having irritated skin for a week and a minor sunburn would be totally worth it.

So yeah, forget about Mars, Venus is so much more fun.