Speaking of which, now that Pluto is exiled to the land of dwarf planets, the Earth-Moon system is the closest we have to a twin planet in our solar system. The diameter of the Moon (~3,400 kilometers) is ~25 percent that of Earth’s (~12,700 kilometers) — a remarkably large ratio. Ganymede is only 6 percent the size of Jupiter.
The story of the Moon’s formation can also be seen by comparing the different densities of Earth and its partner. Grind up the Earth and measure the density of the material and you get ~5.5 grams per cubic centimeter. Do the same to the Moon and it’s only ~3.3 grams per cubic centimeter … which is actually pretty close to the density of the upper layers of Earth. This likely means that the Moon was made up of the outer parts of proto-Earth (crust and mantle) after the collision and not the heavy metallic innards.
Some of this turbulent history is visible on the surface of the Moon as well. Unlike the other big moons like Ganymede, Titan, Europa, Callisto and Triton, our Moon lacks any real atmosphere and isn’t covered in ice. Sure, it is a lot closer to the Sun, so maybe it lost much of its ice (there is some), but it is a much more desolate locale compared to the icy moons of the solar system.
That being said, there are some definite advantages to going back to our planetary companion.v The Moon is only around 240,000 miles (348,000 kilometers) from Earth (on average), yet we’ve only been there 6 times. If we need a springboard to get out to the rest of the solar system, it is likely going to take getting back to the Moon and getting comfortable living down our celestial street, so to speak. There is a lot we don’t know about the Moon and its history, so let’s use this anniversary to get back into the mindset of voyage and discovery across the solar system.
[Correction 6/18/19: A previous version of this article incorrectly stated the diameter of the Earth and the ratio of its diameter to the Moon’s. We regret the error. Apparently I can’t remember to convert miles to kilometers before I do my math.]