Mercury A to Z: Zero Degrees Longitude

Hello, friends!  Oh my goodness, we made it!  We made it to the end of the A to Z Challenge!  For this year’s challenge, my theme is the the planet Mercury, and in today’s post Z is for:


Zero degrees longitude.  The prime meridian.  It’s an imaginary line that helps define the latitude-longitude coordinate system for mapping the surface of a planet.  On Earth, the prime meridian runs through the very English Royal Observatory in Greenwich, England.  On Mars, the prime meridian runs through Airy-0, a crater named after Sir George Airy, the very English scientist who decided where Earth’s prime meridian should be.  So where is the prime meridian on Mercury?

Actually, we talked about this in a previous post.  Mercury’s 0 and 180 degree longitude lines are supposed to run through the planet’s “hot poles,” the two points along Mercury’s equator where the temperature gets highest.  But the hot poles aren’t visible surface features, like Airy-0 or the Greenwich Royal Observatory.  So in the 1970’s, when NASA’s Mariner 10 space probe arrived at Mercury, scientists were hoping they could find an obvious surface feature to serve as an official prime meridian marker.

Mariner 10 visited Mercury three times.  It flew by Mercury, looped around the Sun, then flew by Mercury again, and then again one more time, before the space probe ran out of fuel.  During each of those three visits, only half of Mercury was visible to Mariner 10’s cameras, and unfortunately it was always the same half of the planet.  As a result, Mariner 10 never saw Mercury’s prime meridian, nor could it see any surface features on or near that imaginary line.

So Mercury’s prime meridian ended up being defined in a rather awkward way.  Scientists picked a tiny crater 20 degrees west of where the prime meridian was supposed to be.  They named the crater Hun Kal, which means twenty in an ancient Mayan language (this is one of the rare craters on Mercury not named after an artist, writer, or musician).  Scientists then officially defined Mercury’s prime meridian as a line of longitude exactly 20 degrees east of the center of Hun Kal Crater.

Thanks to NASA’s MESSENGER Mission, we now have photos of the entire surface of Mercury.  Presumably this means scientists could redefine Mercury’s prime meridian, if they wanted to, but nobody seems interested in doing that.  Using Hun Kal Crater to define the prime meridian may not be ideal, but it seems to work well enough.  And if it works, why fix it?


Here’s an article from, featuring an image of Hun Kal Crater as seen by MESSENGER.

Mercury A to Z: NASA Missions to Mercury

Hello, friends!  We are halfway through this year’s A to Z Challenge.  I have to admit when I picked the planet Mercury as my theme for this year’s challenge, I was a little worried I wouldn’t be able to find enough material for a full alphabet worth of posts.  But Mercury has not disappointed me.  There are more than enough Mercury facts to cover!  In today’s post, N is for:


Which planet is closest to the Sun?  More often than not, the answer is probably Mercury.  That may seem counterintuitive, since the orbital path of Venus (the 2nd planet) lies between the orbital paths of Mercury (the 1st planet) and Earth (the 3rd planet).  But consider it this way: every time Venus and Earth happen to be on opposite sides of the Sun, Mercury is somewhere in between.  So on average, Mercury ends up being the closest planet to Earth more often than Venus, Mars, or any other planet.

And yet, despite the fact that Mercury is so close to Earth so much of the time, Mercury is still one of the absolute hardest places for Earth-launched spacecraft to reach.  The problem is the Sun.  The Sun is very big, and the gravitational pull of the Sun is very strong.  For our purposes, imagine that the Sun is “down,” and you’ll start to see what the problem is.  Flying to Mercury is an awful lot like falling toward the Sun.

Now I do want to acknowledge that I’m glossing over a whole lot of technical details here.  The purpose of this blog post is not to teach you the science and mathematics behind orbital mechanics.  All I want is to give you a small taste of what makes flying to Mercury so very challenging, so that you can better appreciate the amazing accomplishments of NASA’s Mariner 10 and MESSENGER Missions.


NASA’s original plan for Mariner 10 was to aim carefully and fly by Mercury one time.  A certain Italian astronomer had a better idea, involving a never-before-attempted gravity assist maneuver near Venus.  This tricky maneuver allowed Mariner 10 to perform three flybys of Mercury for the price of one.

Gravity assist maneuvers, where a spacecraft uses a planet’s gravity to make a “for free” course adjustment, are standard practice in spaceflight today, but Mariner 10 was the first to ever attempt such a thing.  Mariner 10 was also the first spacecraft to visit two planets, collecting some data about Venus before continuing on its way to Mercury (Mariner 10 was also lucky enough to collect data from a nearby comet—another first in space exploration).

Mariner 10 flew by Mercury in March of 1974, September of 1974, and March of 1975.  During those three encounters, Mariner 10 discovered Mercury’s magnetic field and Van Allen radiation belt.  Mariner 10 also discovered Caloris Basin, Kuiper Crater, and many other important surface features.  Unfortunately, only half of the planet was in daylight during Mariner 10’s three flybys, and it was always the same half of the planet, so the other half of Mercury remained unseen and mostly unknown for decades thereafter.

Shortly after Mariner 10’s third flyby of Mercury, the spacecraft ran out of fuel for attitude control.  Without attitude control, the spacecraft couldn’t keep its communications system pointed toward Earth.  So before contact was lost, mission control ordered the spacecraft to shut down.  The now defunct spacecraft is still, presumably, orbiting the Sun somewhere near the orbit of Mercury.


MESSENGER is an acronym for MErcury Surface, Space Environment, Geochemistry, and Ranging.  The name is also a reference to Mercury’s role in Roman mythology as the messenger of the gods.  The MESSENGER Mission was funded through NASA’s Discovery Program, a highly competitive program for space missions that can be done on a tight and highly-restrictive budget.

MESSENGER launched on August 3, 2004.  Unlike Mariner 10’s series of flybys, the plan for MESSENGER was to enter orbit of Mercury.  This required a much longer and more intricate flight trajectory, with one gravity assist maneuver at Earth, two at Venus, and a series of three maneuvers at Mercury to help match Mercury’s orbital velocity.  MESSENGER achieved Mercury orbit on March 18, 2011, after seven-plus years of travel.

Over the next four years, MESSENGER photographed the entire surface of Mercury (including the half of the planet Mariner 10 couldn’t see), continued to study Mercury’s magnetic field, and revealed Mercury’s internal structure through a process called gravity mapping, which involved measuring subtle variations in a planet’s gravitational field.  Oh, and who could forget this?  MESSENGER also discovered water on Mercury.  Believe it or not, there is water (frozen as ice) inside craters around the north and south poles of Mercury.

In early 2015, MESSENGER ran out of fuel, and the spacecraft’s orbit around Mercury began to deteriorate.  On April 30, 2015, MESSENGER finally crashed into the planet’s surface, giving the most heavily cratered planet in the Solar System one additional crater.


The work of NASA’s Mariner 10 and MESSENGER Missions will be continued by BepiColombo, a collaborative mission by ESA (the European Space Agency) and JAXA (the Japanese Aerospace eXplotation Agency).  I wrote about BepiColombo in a previous post.

Now I want to correct something I’ve been saying about BepiColombo in previous posts.  I’ve said that BepiColombo will arrive at Mercury in 2025; that’s not quite right.  BepiColombo will enter Mercury orbit in 2025, but much like MESSENGER, BepiColombo needs to perform several gravity assist maneuvers near Mercury first.  Two of those gravity assists have already happened, and during those maneuvers, BepiColombo already started snapping photos and gathering science data.

So every time this month that I said only two spacecraft have ever visited Mercury, that was incorrect.  BepiColombo has already become Mercury’s third visitor.


NASA has posted some nice articles about Mariner 10, MESSENGER, and BepiColombo on one of their educational websites.  Click these links to check them out:

Sciency Words: The Unknown Absorber

Hello, friends!  Welcome back to Sciency Words, a special species here on Planet Pailly where we talk about those weird and wonderful words scientists like to use.  Today on Sciency Words, we’re talking about:


We’ve talked about this one before.  Several times now.  But given the recent news about Venus, I feel like this is a topic worth revisiting right now.

In 1974, NASA’s Mariner 10 spacecraft discovered that an unknown chemical in Venus’s atmosphere was absorbing copious amounts of ultraviolet light.  No one could figure out what this chemical could be.  And whenever science can’t figure something out, people’s imaginations tend to run wild.

What if this unknown ultraviolet absorber were a complicated chlorophyll-like molecule?  That would imply that some sort of organism, perhaps something like Earth’s cyanobacteria, was soaking up U.V. light and using it for some sort of alien version of photosynthesis!

Now you may be wondering how anything could live on a planet as absurdly hot as Venus.  Venus’s surface temperature is approximately 460°C (870°F).  But the unknown absorber wasn’t found on Venus’s surface; it was drifting around in the upper layers of Venus’s clouds, where the temperature is about 30°C (80°F)—almost Earth-like!  And as we learned in a previous Sciency Words post, microorganisms can (and do) use clouds as a habitat.

Don’t get too excited, though.  The unknown absorber was a mystery for a time, but in 2016 it was identified as a fairly simple sulfur compound.  At this point, there is no reason to think the formerly unknown absorber has anything to do with photosynthesis or any other biological process.  It’s just another weird chemical among the many, many weird chemicals found on Venus.

So when you hear about the discovery of phosphine in Venus’s atmosphere, and when you hear speculation about where that phosphine might be coming from, remember the story of the unknown absorber.