Understanding the Gravity of the Situation

Dwarf Planets and Gas Giant Moons

2015 will be an exciting year for dwarf planet exploration. A few days ago, the NASA spacecraft Dawn entered orbit around the dwarf planet Ceres. Ceres is the largest object in the asteroid belt, the rocky region found between the orbits of Jupiter and Mars. The spacecraft is using an ion engine, which itself is an interesting topic (read more here)

Meanwhile, the NASA spacecraft New Horizons is closing in on the dwarf planet Pluto. New Horizons is expected to have its closest approach to Pluto on July 14. Starting in May the spacecraft will start to provide higher resolution photos than those from the Hubble Telescope. New Horizons is currently about 1 AU from Pluto and 32 AU from Earth.

You can see in the diagram to the right the approach Dawn took to get to Ceres. It was a remarkable journey and the first of its kind using new technologies and visiting Vesta along the way. As the news of Dawn's arrival at Ceres broke, I couldn't help but wonder what these Dwarf Planets are like. Our solar system is filled with rocky worlds. Some orbit the Sun while others orbit the gas giants of the outer solar system. They come in all colors and sizes, but they all do have one thing in common, they are all smaller than the Earth.

Earth: The King of the Rocky Worlds

Given the massive size of Jupiter and the Sun, we are often reminded how small the Earth is. There are tons of infographics detailing how many Earths can fit in Jupiter (1,321 Earths) or in the Sun (1.3 million Earths). Videos that show how small our planet is compared to the Sun and then how small our Sun is compared to other stars. The implication clearly being that we live on a tiny blue dot in a vast universe. Undeniably true. Still, it's sort of an unfair comparison. The Earth is a fundamentally different type of object than a gas giant or star. It turns out, compared to other objects similar to itself, the Earth is pretty big.

You can get an idea of the scale to the left. Aside from Venus, which is just slightly smaller than the Earth, you can see that all of the other rocky planets, dwarf planets, and moons in our Solar System are considerably smaller than Earth. For instance, you can see that Mars has a little more than half the radius of Earth, which means it has about 1/7 of the volume. When you consider Mars is the third largest rocky object in the solar system, it starts to put things in perspective.

The diagram is hard to read unfortunately, so I'll tell you what the next few are. After Mars comes Jupiter's moon Ganymede, followed by Saturn's moon Titan, and then Mercury (1/18th Earth's volume). Next comes two Jupiter Moons, Callisto and Io, then our moon (1/50th Earth's volume), then Jupiter's moon Europa, followed by Neptune's moon Triton and finally Pluto (1/150th Earth's volume). You see that row above the row that ended with Pluto? Somewhere near the end of that row would be Ceres. Ceres has a radius of 487 km. You could easily fit over 2000 Ceres inside the Earth. So there you have it, the Earth is huge compared to these other worlds. They are dwarfed by its size, and we're about to see, even more dwarfed by its gravity.

Earth's Super Strong Gravity

The Earth is pretty big for a rocky planet or moon in our solar system. It's also pretty dense. Mercury and Venus have similar densities compared to Earth, but Mars is about 72% as dense as the Earth. Once you get out past Mars the density of the objects drop considerably. For instance the density of Saturn's moon Titan is 34% as dense as the Earth. This makes sense, since during the formation of the solar system, you would expect most of the heavier materials to be closer to the Sun. The consequence is that Earth's gravity is super strong compared to the other rocky worlds in our solar system.

Periodically there has been talk about setting up colonies on the Moon or Mars or Ceres or even Callisto in the future, but rarely does gravity come up. That's a mistake, because all of the possible locations I just mentioned have gravity that is much lower than Earth's surface gravity.

Take Mars for example. The gravity at the surface of Mars is about .38 g. That's just a little more than 1/3 of our gravity. The moon has less than half the gravity of Mars, 0.165 g (1/6 of Earth's gravity). Jupiter's moon Callisto has gravity of 0.126 g (1/8 the gravity of Earth). Note that although Callisto is larger than our moon, it is less dense and thus has weaker gravity. Finally Ceres, with surface gravity of .028 g, has incredibly small gravity compared to the Earth. It's gravity is roughly 1/35 of our gravity.

So Mars is pretty much as good as it gets, unless we set up a colony on the surface of Venus where the gravity is about 0.9 g (9/10 Earth's). Of course the 800 F degree surface temperature and 90 atm surface pressure may not be worth the familiar gravity. Here is the gravity of some other moons and planets.

Mercury's surface gravity is around 1/3 of Earth's, close to Mars; Pluto has 1/15 of Earth's surface gravity. The Jupiter moons Io, Europa, and Ganymede have gravitys that are 1/5, 1/7, 1/7 of Earth's surface gravity respectively. Saturn's moon Titan is around 1/7 as well. As you can see their gravity simply isn't anywhere close to what we're used to.

Conclusions

So there you have it, gravity will be an issue for future space colonies, wherever they are located. Still, it will have to be something we figure out if we want the colonies to succeed. It's easy to believe our planet is tiny and inconsequential, especially when compared to the more massive objects in our solar system, but the truth is we live on a big dense planet with large gravity and that is something we will have to compensate for when we start to expand into our solar system.