Sciency Words: The Anomalous Precession of the Perihelion of Mercury

If you’re anything like me, you’ve probably looked at planetary orbits and asked yourself: why does Mercury’s perihelion precess so anomalously? That simple, straightforward question is the subject of this week’s edition of Sciency Words.

Sciency Words is a special series here on Planet Pailly where we take a look at a new and interesting scientific term so we can all expand our scientific vocabularies together. Today’s term is:


I know, it’s a bit of a mouthful, but trust me… this anomalous precession thing is pretty cool.

Gravity According to Newton

Back in the 17th Century, Isaac Newton found a mathematical way to describe gravity, and his mathematical description worked for everything from falling apples to the orbits of all the planets. Well, all the planets except Mercury.

Mercury’s perihelion (the point where Mercury is as close to the Sun as it gets) moves.  That in and of itself isn’t so strange, but the perihelion moves a tiny bit faster than it should according to Newton.
Mercury’s perihelion (the point where Mercury is as close to the Sun as it gets) moves. That in and of itself isn’t so strange, but the perihelion moves a tiny bit faster than it should according to Newton.

The mystery of Mercury’s orbit (or the “anomalous precession of the perihelion of Mercury,” to use the technical lingo) baffled scientists for centuries. That is until Albert Einstein came along.

Gravity According to Einstein

Einstein’s theory of general relativity postulates that space and time are not separate entities but two aspects of what physicists now call space-time. General relativity predicts that the force of gravity causes space-time to bend or warp.

Needless to say, the Sun has a lot of gravity. Turns out that the warping of space-time around the Sun precisely explains Mercury’s weird orbit. In fact, every planet experiences some degree of this anomalous perihelion thing. It’s just that, because Mercury is so much closer to the Sun, the warping effect is significantly more noticeable.

Fe12 Time Warp

This is perhaps the planet Mercury’s greatest contribution to science. The anomalous precession of Mercury’s perihelion provided one of the earliest proofs that general relativity—and all the wibbly-wobbly, timey-wimey stuff that comes with it—is not just science fiction.

Fe12 Albert and Isaac


The 200-Year-Old Mystery of Mercury’s Orbit—Solved! from io9.

The Mysterious Orbit of Mercury from The Great Courses.

Accounting for General Relativity at Mercury from The Planetary Society.

Probing Mercury

In 1974, the first space probe form Earth skimmed past Mercury.

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Mariner 10 flies past Mercury for the first time.

As we discussed in a previous post, getting to Mercury is a challenge, so it wasn’t until the early 21st Century that we tried again. This second mission was more ambitious than the first.

The MESSENGER spacecraft achieves Mercurial orbit.
The MESSENGER spacecraft achieves Mercurial orbit.

Unfortunately, MESSENGER can’t maintain its orbit forever. By the end of 2015, the probe is expected to run out of fuel. After that, it will eventually crash somewhere on Mercury’s surface.

But don’t worry! The European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA) have teamed up to send two new probes to Mercury.

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The BepiColombo spacecraft will separate into two probes once it reaches Mercury.

The ESA-JAXA mission is named BepiColombo in honor of the Italian mathematician and scientist Giuseppi “Bepi” Colombo. It was Professor Colombo’s calculations that enabled Mariner 10 to safely approach Mercury back in the 1970s.

The BepiColombo spacecraft is scheduled to launch in July of 2016, and the two probes should reach Mercury in early 2024.



Is Mercury Shrinking? (Sciency Words: Lobate Scarps)

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Every Friday, we take a look at a new and interesting scientific term to help us all expand our scientific vocabularies together. Today’s word is:


Since the Solar System formed 4.5 billion years ago, Mercury has decreased in size by about 11 kilometers in radius. How do we know this? By studying wrinkly features on the planet’s surface called lobate scarps.

Fe09 Shrinking Mercury

Mercury’s interior is cooling off, and as it cools, the planet shrinks. Unlike Earth, Mercury doesn’t have active plate tectonics, so the planet’s surface is forced to contract like a deflating balloon. Lobate scarps are long, winding ridges standing over the still deflating landscape.

Recent data from the MESSENGER space probe has allowed scientists to measure the height of these scarps. From those measurements, they’ve extrapolated the size Mercury used to be compared to the size it is now.

And it just so happens that these measurements match the theoretical predictions for how much contraction a planet like Mercury should experience due to heat loss. It’s nice when theory and observation agree. It means we’re doing something right.


Mercury Shrinking More Than Thought from

Is Mercury Still Shrinking? Astronomers Confirm The Littlest Planet is Getting Littler from International Science Times.


Colonizing Mercury: How to Live There

If you hunger for a quaint, frontier life far from the noise and bustle of modern society, then moving to Mercury might be for you. Maybe. At first glance, Mercury appears to be the most godforsaken planet in the Solar System. But—assuming you can get there—living there might not be so bad.

One thing you and your fellow colonists won’t have to worry about is energy. You’ll have all the solar power you could ever need. In fact, you might get a little too much solar power. The MESSENGER space probe, currently orbiting Mercury, actually angles its solar panels away from the Sun to prevent overheating.

What about air and water? Turns out certain craters near Mercury’s north pole contain plenty of water in the form of ice. Colonists living in and around these craters would have easy access to that water, and they’d be able to extract oxygen from the water as well.

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Water ice lies in Mercury’s polar craters, safely concealed from the Sun’s harsh glare.


Despite Mercury’s proximity to the Sun, keeping cool isn’t impossible. Thanks to a simple sunshade and a series of “diode heat pumps,” MESSENGER’s computers operate at room temperature. In fact, if you’re living inside one of those polar craters, perhaps in some sort of subterranean bunker, your bigger concern should be how to keep warm.

But is a Mercurial colony economically feasible? Well, it’s a safe bet that helium-3 will be the optimal fuel source for any futuristic, Solar-System-spanning civilization. Being so close to the Sun, Mercury has loads of helium-3, so establishing a colony there might someday be highly lucrative.

Believe it or not, some people actually want to live on Mercury. They even argue that colonizing Mercury will be easier and cheaper than colonizing Mars. I doubt that, but after all the research I’ve done for this post and the post before it, I can at least agree that colonizing Mercury isn’t as crazy as it might seem.

P.S.: One of those craters near Mercury’s north pole is named after J.R.R. Tolkien. I can only assume that in the future, any subterranean dwellings in that region will be colloquially known as Hobbit holes.


Colonizing Mercury, Could It Be Done from Colony Worlds.

First Photos of Water Ice on Mercury Captured by NASA Spacecraft from

“The Hobbit” Author Gets a Crater on Mercury from Universe Today.

Colonizing Mercury: How to Get There

In several recent posts (here and here), I’ve said that nobody wants to live on Mercury. It’s too hot. It’s also too cold, and there’s no air, et cetera, et cetera. But apparently some people do want to colonize Mercury. If you’re one of them, your first obstacle is getting there.

Only two spacecraft have ever visited the First Rock from the Sun, and the process of getting there was not exactly straightforward. (Get it! No? You will in a minute.) The problem is the Sun.

The Sun is massive, and so is its gravity. By comparison, Mercury’s gravity is minuscule, so trying to achieve Mercurial orbit requires guts, pinpoint accuracy, and a little unconventional thinking.

The MESSENGER spacecraft did it by taking an indirect course through the inner Solar System. It looped past Earth once, Venus twice, and Mercury three times before settling into a safe planetary orbit.

This long, spiraling voyage took six years. A trip to Pluto would take just as long even though Pluto is five times farther away.

Barring some ginormous advances in anti-gravity technology, any effort to colonize Mercury would most likely have to follow MESSENGER’s convoluted path to get there.

Of course, living on Mercury presents a whole other set of challenges. More on that in Wednesday’s post.


How Long Does It Take to Get to Mercury? from Universe Today.

NASA Probe Becomes First Spacecraft to Orbit Mercury from

Mercury Orbit Insertion Press Kit from NASA.

Why Doesn’t Mercury Have a Moon? (Sciency Words: Hill Sphere)

In 2012, NASA announced the discovery of a moon orbiting the planet Mercury. Sadly, this turned out to be an April Fool’s Day prank. In reality, Mercury does not and probably cannot have a moon. Why? Because the Sun is a bully.

Fe06 Flick

For a more complete answer, let’s get to this week’s edition of Sciency Words. Every Friday, we take a look at some new and interesting scientific term so we can all expand our scientific vocabularies together. Today’s word is:


Named after American astronomer George William Hill, a planet’s Hill sphere is the region of space where that planet’s gravity has more influence over orbiting bodies than the gravity of the Sun.

In general, if a moon’s orbit lies within a planet’s Hill sphere, the moon will remain in orbit of the planet. Otherwise, the moon will probably escape the planet’s gravity and begin to orbit the Sun.

To determine the size of a planet’s Hill sphere, we must consider two factors against each other: the planet’s gravity vs. the proximity of the Sun. Even a large, Jupiter-sized planet will have a small Hill sphere if it’s too close to the Sun. Conversely, tiny planets can have surprisingly large Hill spheres if they’re far enough away.

Maybe at some point in the distant past, Mercury did have a moon. But Mercury is too small and way too close to the Sun to have a substantial Hill sphere. Sooner or later, this hypothetical moon would have been yanked away from Mercury and launched into a highly unstable orbit around the Sun.

P.S.: Pluto may not be considered a planet anymore, but it still has a Hill sphere, and because Pluto is so far away from the Sun, its Hill sphere is much larger than the Hill spheres of Mercury, Venus, Earth, or Mars. Large enough to hold onto at least five moons.

Fe06 Pluto's Moons


The Moon That Went Up a Hill but Came Down a Planet from Bad Astronomy

File: Hill Sphere of the Planets from Wikipedia

Artsy Science: Craters of Mercury

Artsy Science

Today’s post is part of a collection of posts on the artistic side of science. Through both art and science, we humans try to make sense of the world around us, and the two fields have a lot more in common than you might expect.

* * *

The International Astronomy Union has the sole power to name things in outer space, and they have a lot of rules and regulations for how the naming process works. This usually results in names like HD 856019b. But for Mercury, specifically the craters of Mercury, the I.A.U.’s process is actually kind of fun.

With only a few exceptions, the craters of Mercury are all named after famous authors, artists, or musicians. This means Mercury has a Shakespeare Crater, a Hemmingway Crater, a Van Gogh Crater, and a Beethoven Crater. For you Beetles fans out there, there’s a John Lennon Crater. Edgar Allen Poe has a crater. Mozart and Monet have craters. My personal favorite is, of course, Tolkien Crater.

It’s worth noting that Mercury has more craters than any other planet in the Solar System, and we keep discovering more of them. That means we need more artists and authors and musicians, and we need them fast before we run out of names. So like Neil Gaiman says, go out and make good art. Mercury is counting on you.

Fe05 Support the Arts

P.S.: Special shout out to Spacerguy, who mentioned this fun fact in a comment yesterday.  Please check out his Star Trek themed blog by clicking here.

Into Thin Air: Mercury’s “Atmosphere”

Poor Mercury. Since its formation, Mercury has substantially shrunk. It gets hit by more meteors than any other planet in the Solar System, and its “atmosphere” is so thin it doesn’t even deserve the name. Scientists call it an exosphere instead.

What little air Mercury has comes from several sources. Volatile chemicals seep out of the planet’s crust through a process called outgassing. In addition, those meteor impacts kick up plenty of dust particles. Mercury even gets some spare hydrogen and helium from the Sun.

But the Sun giveth and the Sun taketh away. The constant pressure of solar radiation blows most of Mercury’s gaseous materials away.

Mercury even has a comet-like tail of particles streaming away from it.
Mercury even has a comet-like tail of particles streaming away from it.

Mercury’s gravity and weak magnetic field hold some of the planet’s exosphere in place, but not enough. Just another reason why nobody wants to live on Mercury.


Extreme Effects: Seven Things You Didn’t Know About Mercury from NASA.

Atmosphere of Mercury from Universe Today.

The Planet Mercury Has a Comet Like Tail from Watts Up With That?

Sciency Words: Volatile

Sciency Words PHYS copy

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Every Friday, we take a look at a new and interesting scientific term to help us all expand our scientific vocabularies together. Today’s word is:


In colloquial English, the word volatile is an adjective used to describe things like the stock market or the Middle East. But as a technical, scientific term, volatile is a noun.

Volatiles are chemicals that tend to turn into gas at relatively low temperatures or when exposed to a vacuum (like the vacuum of space). Examples include:

  • Hydrogen
  • Oxygen
  • Ammonia
  • Methane
  • Water

Many volatiles are so eager to become gases that they’ll sublimate, meaning they’ll transform from solid to gas without bothering to be liquids first.

As we continue our exploration of the Solar System, we’ll be talking about this class of chemicals a lot. They’re especially important if you want to understand why Mercury is the way that it is. More about that on Monday.


Volatiles from Wikipedia

Outgassing from Wikipedia

Mercury: It’s Hot and It’s Cold

Throughout 2015, we’ll be exploring the Solar System. In January, we visited the Sun. Today, we begin our exploration of the planet Mercury.

Let’s start with a few basic facts that pretty much everyone knows. Mercury is the smallest planet in the Solar System (unless you insist on calling Pluto a planet). Mercury is the planet closest to the Sun. Mercury is one of the coldest places in the Solar System.

Wait… coldest?

Fe01 Mercury Singing 1

While it’s true that the dayside of Mercury is scorching hot (800 degrees Fahrenheit), the temperature on the night side can drop as low as -300 degrees Fahrenheit. The reason for this enormous temperature discrepancy is Mercury’s atmosphere, or rather Mercury’s lack of an atmosphere.

Atmospheres enable planets to trap and redistribute heat, maintaining a relatively constant global temperature. So planets without atmospheres (like Mercury) cook on one side and freeze on the other.

This is just one of many reasons why nobody wants to live on Mercury.

Fe01 Mercury Singing 2
Mercury is also the densest planet in the Solar System.



How Hot is Mercury? from

What is the Hottest Planet in the Solar System? from Universe Today.