Tag: gravity

Mercury A to Z: Jumping on Mercury

~ J.S. Pailly ~ 6 Comments

Hello, friends!  It always seems like Mercury doesn’t get the same love and attention as the other planets, which is why I chose Mercury as my theme for this year’s A to Z Challenge.  In today’s post, J is for:

JUMPING ON MERCURY

If you’re anything like me, you probably lie awake at night wondering what it would feel like to walk on another world.  With each step, what would feel different, and what would feel the same?  It’s the kind of thing you can read about, or you can watch videos from the Apollo era to see what walking on another world looks like.  But to get the actual sensory experience of moving about in low gravity?  I doubt I’ll ever get to experience that for myself.

But while I may never have the first hand physical experience of walking in low gravity, a few years back I read a paper that clarified some things for me, at least intellectually.  The key thing to understand is that gravity helps you walk, more so than you probably realize.

When you take a step, you first lift one foot off the ground.  This requires your muscles to do work.  This takes energy.  But when you put your foot down again, gravity helps you get your foot back down to the ground.  Gravity makes it so your muscles don’t have to do quite as much work during your foot’s downward motion.  Gravity saves you from expending just a little bit of extra energy as you finish taking a step.  But if you’re on the Moon or Mars (or Mercury), there’s less gravity, and so your muscles get less help.  It takes a little more energy than you might expect to put your foot back down to the ground.

This is why the Apollo astronauts ended up “loping” or “bunny hopping” all over the surface of the Moon.  In interviews, the astronauts often said it just felt more natural and comfortable to move about that way.  Scientifically speaking, it’s a matter of metabolic efficiency.  Walking is a metabolically efficient way to get around on Earth, but without Earth-like gravity to help bring your foot back down to the ground, the metabolic efficiency of walking is diminished.  The lower the gravity gets, the less efficient walking becomes, and if the gravity gets low enough, then skipping, hopping, and jumping start to feel, by comparison, a whole lot easier.

Mercury is about the same size as the Moon, but due to Mercury’s ginormous iron core, Mercury is a whole lot denser than the Moon.  Higher density means higher gravity, and the surface gravity on Mercury is roughly twice the surface gravity on the Moon (or roughly the same as the surface gravity on Mars, even though Mars is a much larger planet).  But Mercury-like (or Mars-like) gravity is still only one-third of the gravity we’re accustomed to here on Earth.

So if you ever want to go for a stroll on the surface of Mercury, first: remember to wear a spacesuit that can handle the extreme temperatures.  And second, don’t feel embarrassed if you end up jumping, hopping, or skipping all over the place.  It’s all for the sake of metabolic efficiency.

WANT TO LEARN MORE?

Here’s a short video from the Apollo era, showing astronaut Gene Cernan bunny hopping down a slope on the Moon while talking about how it is “the best way” to travel.

And here’s a short compilation of videos, also from the Apollo era, showing various astronauts tripping and falling all over themselves in lunar gravity.

And lastly, here’s the paper I mentioned, titled “Human Locomotion in Hypogravity: From Basic Research to Clinical Applications.”  It’s not an easy read, but if you really want to understand what “human locomotion” would feel like on other worlds, this paper is the absolute best resource I’ve ever found.

Sciency Words: Barycenter

~ J.S. Pailly ~ 13 Comments

Hello, friends!  Welcome to Sciency Words, a special series here on Planet Pailly where we talk about those super weird (but super cool) words scientists like to use.  Today’s Sciency Word is:

BARYCENTER

Tell me if you’ve heard this one: every action has an equal and opposite reaction.  This is true even for moons orbiting planets, or planets orbiting stars.  Whenever a star exerts gravitational force on a planet, that planet exerts an equal and opposite gravitational force on the star.  As a result of this ongoing gravitational tug-of-war, we end up with a planet and a star spinning round and round their common center of mass, a point which scientists call a barycenter.

Definition of barycenter: In astronomy, a barycenter is the center of mass of two or more objects in space that are gravitationally bound together.  

Etymology of barycenter: The word barycenter traces back to a Greek word meaning “weighty” or “heavy.”  The word barometer has a related etymology (barometers measure atmospheric pressure—the “weight” of the atmosphere, in other words).

Sometimes a barycenter will be located deep inside the more massive of two celestial bodies, in which case the more massive body will appear to wobble in place.  This is the case for the Earth and the Moon.  The Earth-Moon barycenter is approximately 1700 km beneath Earth’s surface.  Other times, the barycenter will be somewhere in the empty space between objects.  For an example, look at Pluto and its largest moon, Charon.  The Pluto-Charon barycenter is more than 900 km above the surface of Pluto.

The concept of a barycenter dates back to Isaac Newton (though I can’t find any sources saying he coined the word, nor could I find any evidence that he ever used the word himself).  Newton’s Principia Mathematica, originally published in 1687, briefly discusses the Sun-Jupiter barycenter, saying, “[…] the common centre of gravity of Jupiter and the sun will fall upon a point a little without the surface of the sun.”  Newton also discusses the Sun-Saturn barycenter, which he describes as “[…] a point a little within the surface of the sun.”

And then there’s the barycenter of the Solar System as a whole: the “common centre of gravity of all the planets,” as Newton calls it.  Due to the combined gravitational forces of all the planets (most especially that of the giant planets: Jupiter, Saturn, Uranus, and Neptune), the Sun is constantly being pulled in multiple directions at once.

As a result, the Sun does not sit still in the middle of our Solar System.  It is “agitated by perpetual motion,” to quote Newton one last time.  Sometimes, as the Sun moves about, it happens to pass through the Solar System’s barycenter. Other times, it loops and spirals around the barycenter, as if performing an elaborate dance.

WANT TO LEARN MORE?

Here are a few articles that go into a little more detail about barycenters:

And here’s a link to the translation of Newton’s Principia Mathematica that I quoted in this post.  The relevant section is titled “Proposition XII.  Theorem XII.”