Spec Tech: Jumping on the Moon
Spec Tech is the new Thursday series at the Clarion Foundation blog. This “college of science fiction and fantasy” will explore the uses of physics, astronomy, biology, sociology and more, as they apply to the writing of speculative fiction. Whether you’re in the midst of intricate world-building or just want a juicy fact to bolster a short story, we hope Spec Tech will be useful to you. If you’re interested in writing for Spec Tech, please email firstname.lastname@example.org with information about your area of expertise. Currently seeking historians!
Also, between now and June 30, we’re holding a design contest. Submit the winning design for a “Spec Tech” college crest, and you’ll receive your pick of a current signed first-edition speculative fiction title from Mysterious Galaxy Books. You may send your designs (or questions) to the email address above.
Writing a story that uses outer space as a location has its own set of research challenges. So many things we take for granted on the Earth—like gravity—are different on other planets. Part of the fun, of course, is imagining what things might be like. But in some cases we (read: science) already know what things would be like. For instance: if you were standing on the Moon right now, how high could you jump? When the astronauts walked on the Moon in the 1960’s and 70’s they were wearing bulky, restrictive, 200 pound suits. Any jumping they did was limited at best. Instead, lets imagine being on a moon-base with a large pressurized gymnasium wearing normal clothing. How does the lower gravity change things? This is to some degree an answerable question.
This question of jumping is less straightforward to answer than it would seem at first. The Moon has about one sixth the gravity of Earth, so the naïve guess would be you could jump six times higher than on Earth. Things are somewhat more complicated than that. When you jump on Earth you have to expend a certain amount of energy to overcome gravity as you are jumping. Once your feet leave the ground you are in free-fall until you land again. The height of the jump depends on gravity and the speed at which you can propel yourself upwards (because of the slow speeds involved you can safely ignore air resistance). However, calculating the speed at which you leave the ground depends on many things: how strong your legs are, how much mass you have and how strong gravity is. When you move from the Earth to the Moon only the gravity changes. Your legs are the same and your mass is the same. What you have to do is calculate how much faster you leave the ground to find out how high you will go.
How High You Can Jump on the Moon
Figuring out your jump speed is a little complex1 and beyond the scope of this post, so I have simplified it by taking some average weights and jump forces and working out the answer for a typical human. Here I have graphed how high an average person should be able to jump as a function of surface gravity, with annotations for some common places around the Solar System.
You can see on the Moon you can jump about 22 feet! That’s about 11 times higher than on Earth. I for one can’t wait for the Moon Olympics.
Falling on the Moon
Another interesting thing to consider is what the highest thing you could jump off of without getting hurt would be. On Earth I can easily jump off an eight foot obstacle and not be seriously injured. The falling relationship is similar to jumping. Falling from eight feet on Earth means I will be moving about fifteen miles per hour when I hit the ground. Because of the lessened gravity I could probably survive a faster impact on the moon. If I can jump eleven times higher, I can probably jump off something 11 times higher as well. But I am trying not to get hurt, so let’s be conservative and say that I can survive a fall ten times higher. So my eight foot dive becomes eighty feet! I could essentially jump off an eight story building and not break anything.
It’s hard to walk in low gravity. I don’t see this discussed too often, but it’s really hard to walk on the moon. One of the effects of low gravity is the friction between your feet and the ground is greatly reduced. The characteristic skipping motion the astronauts used to hop around the surface of the moon was a necessity, not just for show. I can attest personally to this effect, as I once had the pleasure of being strapped into a spring loaded machine designed to simulate Moon gravity. Sure enough, as I try to walk forward my feet just slip out from under me. And once I finally do get going it’s just as hard to stop. I makes you wonder if humans do end up colonizing the Moon what sort of architectural choices we will make. Why bother with stairs? Just jump. Do we adopt extra sticky floors? High ceilings will be a necessity.
If your writing takes place anywhere that has gravity different than Earth, keep in mind a lot of things will be different. If you’re going for realism, start by looking at old Moon-walk footage. Notice how the astronauts move, or how the moon-buggy they drive bounces. It looks very different than it would on Earth. Do some quick calculations; ask yourself what would change if everything was suddenly drastically lighter.
 YouTube: Bunny Hop on the Moon – http://www.youtube.com/watch?v=HKdwcLytloU