What If… the Moon was another Earth?
We’ve had articles before on Universe Factory that discuss binary star systems and what possible orbits and extrastellar bodies they might support. Now it’s time to look at binary planet systems.
Binary planets are more common than one might think in space-themed sci-fi. It’s the situation where two planets orbit each other, which is often dramatised by having them close enough to share an atmosphere. In reality, two planets that close would just rip one another apart (more information on that here), but the general principle of binary planet systems is sound enough.
For the sake of (relative) simplicity, let’s assume we’ve got two Earths orbiting one another with a separation of 1000km (see the post I linked just above to explain why I picked that).
Infrastructure
So what are the effects of this on the planet? On its people and infrastructure? Well let’s do infrastructure first, because I’m seeing some interesting things there. Infrastructure is going to need to be designed to cope with being both 25% and 110% its normal Earth weight. That’s not so easy to explain, so here’s a diagram.
The two sets of arrows down the bottom are force diagrams. Someone standing at point A on planet X would experience a force pulling them towards planet X due to its gravitational field. They’d also experience a similar gravitational field from planet Y, pulling them in the opposite direction. However, since there’s an extra 1000km involved there, the force is less, so the total force is still directed towards X. This person feels 25% of what their weight would be if they were standing on Earth.
The opposite situation applies when you’re standing at point B. Instead of opposite forces, both gravitational forces pull in the same direction. The force from Y is even weaker now, because there’s an extra ~13000km involved, but adds 10% to a normal Earth weight.
This results in a big design consideration for infrastructure. You now have to cope with 10% extra weight, but only for a short period — assuming the Earth still rotates, you’d only have 110% weight for about 5 minutes. You also have to deal with fluctuations in weight and therefore fluctuations in the compression of the materials you’re using to build with. Now you have a whole new branch of materials science.
Eclipses
Eclipses are currently pretty rare events. However, assuming that these two Earths are orbiting each other on the same plane they orbit the Sun at, they’ll provide total solar eclipses for each other pretty regularly. It depends on their orbital period, but there could potentially be an eclipse every day. I’m not sure this has much of an effect on the people or the planets, but it’s pretty cool.
Travel & Communications
These two issues are pretty similar, so we may as well do one section on them. The two planets are pretty close together. Assuming they’re both inhabited, people are going to want to travel between them for a variety of reasons (mostly similar to our current reasons for travel — business, leisure, etc).
From this we create a whole new sector of the aviation industry, dedicated to space travel between the two planets. Virgin Galactic would have a whale of a time here, they’ve already got a head start.
We also create a number of issues with space travel. You’ve got to be careful when you’re launching spacecraft that they don’t orbit above 500km, and that if you’re sending something into the outer Solar system or beyond, you don’t launch it into the other planet. Something tells me Russia II wouldn’t be very happy with America I launching a Falcon 9 at St Petersburg.
In terms of communications, there’s now potential for interplanetary Internet. You need a pretty comprehensive network of low-orbit satellites — there’s another job for the space agencies — but you could in theory use the same methods of wireless communication as we do now — simple electromagnetic radiation.
The satellites are a problem because most communications satellites are in geostationary orbits. Those require an orbital radius of about 35,800km. That’s a problem for this planetary configuration because while it won’t crash into the other planet straight away, its orbit around the secondary planet will be lower and will essentially slingshot the satellite into a lower orbit around its primary, until it does crash.
Instead, you have to use low Earth orbit (LEO) satellites. This is in the same range as the ISS orbits (400km). There are some major disadvantages here already:
- you need to boost the satellite regularly to avoid it falling out of orbit
- it’s not always in the same place so can’t be relied upon to provide essential communication routes
I won’t elaborate into this further because that requires a greater depth of knowledge of orbital mechanics than I have, but it’s an engineering challenge that would need to be solved before interplanetary communications are really sorted.
It’s no wonder that this is a common thing in sci-fi. There seems to be plenty you can do with a binary planet system — even in this short article, there are things I could develop into a built world.
