# Mercury A to Z: Solar vs. Sidereal Days

Hello, friends!  Welcome to another posting of the A to Z Challenge.  My theme for this year’s challenge is the planet Mercury, a planet that often gets overlooked by space geeks like me.  In today’s post, S is for:

## SOLAR vs. SIDEREAL DAYS

We’ve been talking about Mercury all month long, and we’ve been talking a lot about Mercury’s rotation rate.  In some posts, I’ve told you that Mercury has a rotation rate equal to approximately 59 Earth days.  In other posts, I said a day on Mercury is about 176 Earth days long.  That seems like a contradiction, but both of those numbers are correct.  It all depends on whether we’re talking about a solar day or a sidereal day.

A solar day can be defined as the time it takes a planet to rotate once relative to the Sun, while a sidereal day is the time it takes a planet to rotate once on its own axis.  A solar day is what we Earthlings usually mean by the word “day.”  It’s the 12 a.m. on Monday to 12 a.m. on Tuesday kind of day.  A sidereal day (pronounced si-der-e-al) is more like a planet’s true rotation period, relative to the rest of the universe.  Why are these things different?  Because planets orbit the Sun.  Because as they orbit, they change position relative to the Sun.

In Earth’s case, it takes about 23 hours and 56 minutes to rotate once on its own axis, but because Earth moves through space during that time (changing positions relative to the Sun), it takes an extra 4 minutes to rotate once in reference to the Sun. And according to my math, 23 hours and 56 minutes, plus an extra 4 minutes, makes Earth’s solar day 24 hours long.

The situation is similar for Mars.  A Martian sidereal day is about 24 hours and 37 minutes, while a Martian solar day is more like 24 hours and 40 minutes.  For both Earth and Mars, the difference is small.  Most of the time, it’s not worth mentioning.  But on Mercury, a sidereal day is 59 Earth days long, while a solar day ends up being 176 Earth days in length.  You see, Mercury rotates very, very slowly.  Over the course of one sidereal day, Mercury travels two-thirds of the way around the Sun.  That puts Mercury in a very different position, relative to the Sun, at the end of a sidereal day.  As a result, Mercury’s solar day ends up being longer—a whole lot longer—than Mercury’s sidereal day.

I have to admit I had a hard time understanding the distinction between solar and sidereal days the first time I heard about it.  I hope that I have done a decent job explaining it to you.  But if anyone was wondering why I quoted different numbers in different posts for Mercury’s day/rotation period, this is the reason I did that.  For some topics, the sidereal day matters; for others, it’s the solar day that’s important.  And for the purposes of the A to Z Challenge, I wanted to save this discussion for S-day, so I just glossed over it until now.

I’m going to recommend an article from Universe Today called “How Long is a Day on Mercury?”

I also want to recommend another article from Universe Today called “How Long is a Day on Venus?” because if you think Mercury’s day is confusing, wait until you hear how messed up a day on Venus is.

# Our Place in Space: Yestersol

Hello, friends!  Welcome to Our Place in Space: A to Z!  For this year’s A to Z Challenge, I’ll be taking you on a partly imaginative and highly optimistic tour of humanity’s future in outer space.  If you don’t know what the A to Z Challenge is, click here to learn more.  In today’s post, Y is for…

YESTERSOL

Do you ever feel like there just isn’t enough time in your day?  Like you just cannot get everything you need to do in a day done in a day?  Do you wish your day could be just a little bit longer?  If so, moving to Mars might be a good solution for you!  A day on Mars is nearly 40 minutes longer than a day on Earth!  Scientists call this slightly longer Martian day a “sol,” and several cute and clever new words have been introduced related to Martian timekeeping: words like yestersol, tosol, and solorrow.

As of yet, there are no humans on Mars (citation needed), but there are humans here on Earth who have to live and work and plan their whole schedules according to Mars time.  You see, the Mars rovers can only operate during Martian daylight hours.  Therefore, everyone back at mission control for those rovers needs to be awake, alert, at their desks and ready to go when it’s daytime on Mars (regardless of what time it is here on Earth).

Sometimes the discrepancy between a Martian sol and an Earthly day isn’t so bad.  Sometimes, when it’s daytime at Jezero Crater (current location of the Perseverance rover), it’s also daytime in southern California (where Perseverance mission control is headquartered).  But day after day, sol after sol, that forty minute difference adds up.  At some point, high noon at Jezero crater will be the middle of the night in southern California.

It’s important that the same crew of people always works with the same rover.  Therefore, NASA has had special clocks and watches made to help people keep track of what time it is on Mars.  NASA scientists and engineers associated with various Mars missions set their work schedules, meal schedules, and sleep schedules according to Mars time.  As a result, there is a small community of “Martians” here on Earth, living their lives about forty minutes out of sync from the rest of us.  And quite naturally, certain colloquial terms have developed within this little community of Mars researchers.

Yestersol refers to the sol before the current sol.  Tosol is the current sol.  And solorrow is the next sol, after the current sol.  Making a clear distinction between “yesterday” and “yestersol” is especially important for people who live on Earth and still have to deal with many Earthly concerns, but who also, in a very real way, need to think and act as if they’re living on Mars.

I like to think of the whole “yestersol, tosol, solorrow” phenomenon as a little preview of the future.  It’s one thing to think about big picture futuristic stuff, like space elevators and planetary protection laws; but it’s little bits of culture and daily life (sorry, sol-ly life) that help make the future feel like a real place.

NASA spacecraft engineer Nagin Cox gave a really neat TED Talk about what it’s like living on Mars time.  Click here to watch it.

# Our Place in Space: Utopia Planitia

Hello, friends!  Welcome to Our Place in Space: A to Z!  For this year’s A to Z Challenge, I’ll be taking you on a partly imaginative and highly optimistic tour of humanity’s future in outer space.  If you don’t know what the A to Z Challenge is, click here to learn more.  In today’s post, U is for…

UTOPIA PLANITIA

Earlier this month, we talked about Jezero Crater on Mars.  There’s strong and compelling evidence to suggest that Jezero Crater was once filled with liquid water.  In other words, Jezero Crater used to be a lake.  If Jezero Crater used to be a Martian lake, then the nearby region of Utopia Planitia was probably once a Martian ocean.

As you can see in the highly technical diagram above, much of Mars’s northern hemisphere was once covered in water.  Probably.  Okay, as a responsible science blogger who wouldn’t want to make you think there’s scientific consensus about a topic when there is not scientific consensus about a topic, I should make it clear to you that the topic of ancient Martian oceans is somewhat controversial in the scientific community.  Scientists are still arguing over what may or may not be ancient Martian shorelines, among other things.

But let’s assume that ancient Mars did have oceans of liquid water on its surface (that seems like a safe assumption to me, but I’m not a scientist—I’m just a guy with a blog).  If so, those oceans would have covered much of Mars’s northern hemisphere.  Today, Mars is sort of a lopsided planet, with generally low elevation terrain in the north and generally higher elevation terrain in the south.  So if ancient Mars did have large amounts of liquid water on its surface, common sense tells us that that water would have accumulated in the low elevation regions (i.e., the northern hemisphere).

Utopia Planitia is one of those low elevation regions in the northern hemisphere.  The terrain is also relatively flat, making Utopia Planitia a fairly easy place to land a spacecraft.  Several robotic missions to Mars have already landed there, the most recent being China’s Zhurong Rover.  And as if all that weren’t enticing enough, ground penetrating RADAR has detected frozen water underground in the southwestern portion of Utopia Planitia.

In the distant future, Utopia Planitia may end up being the site of a major human colony on Mars.  It’s a safe place to land, there’s a supply of water nearby, and it’s a scientifically interesting region.

On the other hand, if plans to terraform Mars ever come to fruition, Utopia Planitia may end up being part of a Martian ocean once again.  As I said before, it’s a low elevation region.  As we transform Mars into a more Earth-like world, water will start to accumulate in places like Utopia Planitia first, at which point we’d probably have to change the name from Utopia Planitia (plains of Utopia) to Utopia Mare (sea of Utopia).

P.S.: As a Star Trek fanatic, I’d be remiss if I didn’t mention this.  Many of the ships from Star Trek were built at the Utopia Planitia shipyards on Mars, according to Star Trek lore.

# Our Place in Space: Phobos

Hello, friends!  Welcome to Our Place in Space: A to Z!  For this year’s A to Z Challenge, I’ll be taking you on a partly imaginative and highly optimistic tour of humanity’s future in outer space.  If you don’t know what the A to Z Challenge is, click here to learn more.  In today’s post, P is for…

PHOBOS

Buzz Aldrin.  He walked on the Moon.  He also has ideas about how to get humans to Mars.  We talked about one of those ideas earlier this month, and now we’re going to talk about another.  What if, rather than going straight down to the surface of Mars, we first set up a little base for ourselves on Phobos, one of Mars’s two moons.

Whenever you want to land on a planet (or a moon), you’ll have to fight against gravity to do so.  That is assuming, of course, that you want to land safely.  Crashing into a planetary body is fairly easy.  Landing safely—that’s the hard part!  You need to control your descent.  If you’re controlling your descent using rocket engines, you’re going to use up a whole lot of fuel in the process.

But as you can see in this highly technical diagram, Phobos is very small.

Okay, maybe not that small.  But still, Phobos is much smaller than Mars, and Phobos’s surface gravity is significantly less than the surface gravity on Mars.  That means a rocket controlled descent onto the surface of Phobos will use up less fuel than a rocket controlled descent all the way down to the surface of Mars.

In his book Mission to Mars: My Vision for Space Exploration, Aldrin argues that we should set up a way station on Phobos before attempting to land humans on Mars.  From this Phobos way station, astronauts could get an up close and personal view of Mars.  They could get the lay of the land without actually landing.  Using remote controlled robots, they could explore the Martian surface and prepare the way for future missions.  And on the off chance that we discover alien life on Mars (current life, I mean, not fossils), then our astronauts on Phobos could study that life from afar without risking any sort of biological contamination.

Personally, I’m not 100% sold on this idea.  I kind of feel like if we’re going to go to Mars, let’s just go to Mars.  But Buzz Aldrin is Buzz Aldrin, and I’m just some guy with a blog.  The thing about the fuel costs for landing on Phobos vs. landing on Mars makes sense to me.  And if it does turn out that there’s life on Mars, contaminating the Martian ecosystem with our Earth germs (or having Mars germs contaminate us) does become a serious concern.

But otherwise, do we really need a way station on Phobos?  Is that a necessary prerequisite to landing humans on Mars?  I don’t know.  Maybe it would be helpful.  When the time comes, maybe we really will go to Phobos first and land on Mars later.  It’s possible.

Once again, I’m going to recommend Mission to Mars: My Vision for Space Exploration by Buzz Aldrin.  Lots and lots of ideas in that book about how we might one day travel to Mars and what we might do once we get there.

# Our Place in Space: Lava Tube Habitats

Hello, friends!  Welcome to Our Place in Space: A to Z!  For this year’s A to Z Challenge, I’ll be taking you on a partly imaginative and highly optimistic tour of humanity’s future in outer space.  If you don’t know what the A to Z Challenge is, click here to learn more.  In today’s post, L is for…

LAVA TUBE HABITATS

The surface of Mars is not a safe place for humans.  Martian dust storms can be really scary.  The temperature fluctuates wildly from a bit too cold to waaaaay too cold, and there’s basically no protection against all the deadly radiation raining down on the planet from space.  Fortunately, humans in the distant future won’t need to live on the surface of Mars.  Mars is offering us free housing in the form of underground lava tubes.

Mars was once a volcanically active world.  In fact, the largest volcano in the entire Solar System is on Mars.  Lower gravity means volcanoes can grow much larger on Mars than they ever could on Earth.  But Mars hasn’t been volcanically active for a long, long time.  The volcanos stopped erupting and the lava stopped flowing billions of years ago.  Today, all those oversized Martian volcanoes are extinct, and all the lava tubes around them are now empty.

So what exactly is a lava tube?  Well, have you ever seen rivers of lava (either in real life or in videos) flowing down the side of an active volcano?  You know how the surface of these lava rivers starts to cool off, forming a blackened crust?  Eventually, this crusty surface lava will become thick enough and solid enough to form a roof over the lava river, while the rest of the lava continues to flow freely underneath.  This is how lava tubes form.

On Earth, lava tubes can get pretty large.  They can be wide enough and tall enough for multiple people to walk through them comfortably.  On Mars, lava tubes could (theoretically) be even larger—almost half a kilometer wide, perhaps!  Once again, this is because of the reduced Martian gravity, which allows all sorts of natural structures to grow larger on Mars than they ever could on Earth.

In the future, sections of these lava tubes could be sealed off and pressurized with air.  Dust storms could rage on the Martian surface while human colonists remain safely underground.  All that natural rock would insulate us against the extreme temperature variations on the surface, and the rock would also serve as a natural barrier against all that radiation raining down from space.  With relatively little effort, we could convert the smaller lava tubes into comfortable and cozy human habitats.  Or, using those half kilometer-wide tubes, we could build much larger and more robust human communities.

At the moment, though, finding lava tubes that would be suitable for human habitation is tricky.  Lava tubes are underground.  Therefore, fully intact lava tubes are not visible in photos taken by our orbiting space probes.  The only Martian lava tubes we currently know about are the ones where the roof has either partially or fully collapsed.  This leaves us with a bit of a Catch-22 scenario: any lava tube we can currently find is structurally compromised and, therefore, might not be suitable for human habitation.

But that seems to me like a limitation of our current Mars exploration program.  As NASA, the E.S.A., and other human space agencies send more and more orbiters, landers, and rovers to Mars, I’m sure new techniques (seismography, gravity mapping, etc.) can be used to find all the lava tubes hidden beneath the Martian surface.

Here’s a short paper advocating for more research about lava tubes on Mars and also on the Moon.

And here’s a ten minute video from Fraser Cain describing what we currently know about Martian and Lunar lava tubes in more detail.

# Our Place in Space: Jezero Crater

Hello, friends!  Welcome to Our Place in Space: A to Z!  For this year’s A to Z Challenge, I’ll be taking you on a partly imaginative and highly optimistic tour of humanity’s future in outer space.  If you don’t know what the A to Z Challenge is, click here to learn more.  In today’s post, J is for…

JEZERO CRATER

Someday, I’d like to help dig up dinosaur fossils.  That’s apparently a thing pretty much anybody can volunteer to do.  Someday, I’d also like to live on Mars.  In the distant future, it may be possible to do both of those things.  Places like Jezero Crater on Mars may be full of ancient Martian fossils!

If you look at satellite images of Jezero Crater, it’s pretty obvious it used to be full of water.  You can see what appears to be a dried-up river bed snaking its way across the Martian landscape.  Where that river meets the crater, there’s a breach in the crater wall and a large river delta where the river would have spilled into the crater basin.

Right now, NASA’s Perseverance Rover is driving around that river delta, scoping the place out, examining the sediments and clays found in the region.

Okay, I may have taken some creative liberties with the cartoon above.  If life ever did evolve on Mars, it would have been short-lived.  All of Mars’s lakes, rivers, and oceans would have dried up fairly early in the planet’s history.  It is highly unlikely that anything as complex as fish or seaweed could have developed, and there certainly wouldn’t have been anything as awesome as a Martian dinosaur.

But in places like Jezero Crater, simple microorganisms could have been plentiful.  These microbes may even have joined together, creating larger structures like the bacterial mats we sometimes find here on Earth.  That’s kind of icky, I know, but it could have happened, and those bacterial mats may still be there, preserved as fossils beneath all that red dust.

I don’t expect questions about life on Mars (past or present) to be answered any time soon.  Even if one of our Mars rovers did stumble upon something that looked like a fossilized bacterial mat, there would be scientific debates for years—decades, even—over what that fossil-looking-thing really is and what it’s presence on Mars really means.  We’ve been through this before, when scientists found “bacteria shaped objects” inside a Martian meteorite.  Something can look like a fossilized bacterium, and yet not be a fossilized bacterium.

But someday in the distant future, we will know, one way or the other, if life ever existed on the Red Planet.  And perhaps in that distant future, humans living on Mars will volunteer to help dig up fossils in Jezero Crater, or other places very much like it.

Here’s an interactive map from NASA showing the Perseverance Rover’s current location.  You’ll have to zoom out a little to see all of Jezero Crater.  If you do, you’ll see that the dried-up river (marked Neretva Vallis) and river delta I mentioned are pretty obvious.

And here is a NASA press release from a few years back, announcing Jezero Crater as the Perseverance Rover’s landing site and explaining why the crater was selected.

Also, here’s an article from Space.com about that Martian meteorite I mentioned, the one with those “bacteria shaped objects” inside.

# Our Place in Space: The Aldrin Cycler

Hello, friends!  Welcome to Our Place in Space: A to Z!  For this year’s A to Z Challenge, I’ll be taking you on a partly imaginative and highly optimistic tour of humanity’s future in outer space.  If you don’t know what the A to Z Challenge is, click here to learn more.  In today’s post, A is for…

THE ALDRIN CYCLER

Even in the future, space travel will be expensive.  True, new technologies should make it less expensive than it is today, but there’s one problem that will never go away, no matter how advanced our technology gets: gravity.

Anywhere you want to go in space, you’re going to have to fight against gravity to get there: Earth’s gravity, the Sun’s gravity, the gravity of other planets and moons—at some point on your journey, you’re going to have to fight against any or all of these gravitational forces.  And fighting gravity uses up fuel.  Lots and lots and lots of fuel.

And yet, despite the unforgiving and unrelenting force of gravity, human civilization will eventually spread out across the Solar System.  I’m not going to tell you it will happen in the next twenty years.  I won’t tell you it will happen in the next century, even.  But someday, it will happen.  I’m sure of it!  And so today, I want to talk a little about what the future transportation infrastructure of the Solar System might be like.

American astronaut Buzz Aldrin is, of course, most famous for being the second person to set foot on the Moon.  Aldrin is also a highly accomplished scientist and engineer.  In 1985, he did some math and discovered a very special orbital trajectory that would make traveling from Earth to Mars (and also from Mars back to Earth) far more fuel efficient.

The term “Aldrin cycler” refers to that very special orbital trajectory Aldrin discovered.  The term can also be used to describe a spacecraft traveling along that special orbital trajectory.  The initial investment to build an Aldrin cycler (the spacecraft, I mean) would be really high.  We’d probably want to build a rather large spacecraft for this, and once it’s built, maneuvering the thing into the proper trajectory would require a stupendous amount of fuel.

However, once we’ve done all that, the cycler will cycle back and forth between Earth and Mars, over and over again, pretty much forever.  Traveling to Mars would be a little like catching a train.

Passengers would board the cycler as it flew past Earth; about five months later, they’d disembark and head down to the surface of Mars.  The cycler would then take a long journey (about twenty months) looping around the Sun before flying past Earth once more; then the “cycle” would begin again.

The trip from Earth up to the cycler would still require some amount of fuel.  So would the trip from the cycler down to the surface of Mars.   The cycler itself would also require a little bit of fuel for maneuvering thrusters; otherwise, over time, the ship could start to drift ever so slightly off course.

Obviously this is not a cost-free form of space travel, but I’m sure you can see how this could help keep the cost of space travel down.  And so I imagine in the distant future, the Aldrin cycler (or something very much like it) will be a key part of the Solar System’s infrastructure, just as trains are an important part of our modern day infrastructure here on Earth.

Click here to see a short animation of the Aldrin cycler orbital trajectory, showing several cycles worth of Earth-to-Mars and Mars-to-Earth journeys.

I’d also recommend Buzz Aldrin’s book Mission to Mars: My Vision for Space Exploration, where Aldrin describes the Aldrin cycler (and other cool Mars related things) in more detail. Click here to see the book’s listing on Amazon.

# October Is Europa Month Here on Planet Pailly!

Hello, friends!  Let’s talk about aliens!

If we want to find alien life, where should we look?  Well, if money were no object, I’d say we should look anywhere and everywhere we can.  Phosphorous on Venus?  Could be aliens.  Let’s check it out.  Melty zones beneath the surface of Pluto?  Let’s check that out too.  Ariel?  Dione?  Ceres?  Let’s check them all for signs of alien life!

But money is an object.  We simply don’t have the resources to explore all of these places.  Space exploration is expensive.  Space exploration will always be expensive so long as we’re stuck using rocket-based propulsion.  The Tsiolkovsky rocket equation makes it so.

Whenever you’re working within a restrictive budget, you need to think strategically.  With that in mind, astrobiologists (scientists who specialize in the search for alien organisms) have focused their efforts on four worlds within our Solar System.  Their names are Mars, Europa (moon of Jupiter), Enceladus (moon of Saturn), and Titan (another moon of Saturn).

This month, I’m going to take you on a deep dive (no pun intended) into Europa.  In my opinion, of the four worlds I just listed, Europa is the #1 most likely place for alien life to be found.  I don’t mean to denigrate Mars, Enceladus, or Titan.  There are good reasons to think we might find life in those places, too.  But there are also good reasons to think we might not.

• Mars: Life may have existed on Mars once, long ago.  But then the Martian oceans dried up.  We’re unlikely to find anything there now except, perhaps, fossils.
• Enceladus: Enceladus’s age is disputed.  She may be only a few hundred million years old, in which case she may be too young to have developed life.
• Titan: If you want to believe in life on Titan, you have to get a little imaginative about how Titanian biochemistry would work.

Europa doesn’t have those issues.  Unlike Mars, Europa has an ocean of liquid water right now, in modern times.  Unlike Enceladus, Europa’s age is not disputed; she’s definitely old enough for life.  And unlike Titan, Europa doesn’t require us to get imaginative about biochemistry.  The same carbon-based/water-based biochemistry we use here on Earth would work just as well for the Europans.

There are still good reasons to search for aliens on Mars, Enceladus, and Titan.  Finding fossils on Mars would be super exciting!  Enceladus’s age is, as I said, in dispute, with some estimates suggesting she’s very young, but others telling us she’s plenty old.  And while life on Titan would be very different than life on Earth, scientists don’t have to imagine too hard to find plausible ways for Titanian biochemistry to work.

But if I were a gambler, I’d put my money on Europa.  And if I were in charge of NASA’s budget, I’d invest heavily in Europa research and Europa missions.  Europa just seems like the safest bet to me, if we want to find alien life. And in the coming month, I plan to go into more detail about why I feel that way.

If you’re interested in learning more about the Tsiolkovsky Rocket Equation, you may enjoy this article from NASA called “The Tyranny of the Rocket Equation” (because NASA is the American space agency, and anything Americans don’t like is tyranny).

As for astrobiology, I highly recommend All These Worlds Are Yours: The Scientific Search for Alien Life, by Jon Willis.  Willis frames the search for alien life just as I did in this post: alien life could be anywhere, but you only have a limited budget to use to find it.  So how would you spend that money?

# Going to Mars is My Dream, But Not My Passion

Hello, friends!

So this post isn’t really about Mars.  I mean, if NASA ever announces that they desperately need to send a writer/illustrator to Mars, I’d volunteer.  I’d love to go to Mars!  That would be awesome!

But I don’t expect that to happen.  Even if we do send humans to Mars, and even if that does happen in my lifetime, those humans will be scientists and engineers.  They’ll be people who are good at math.  I’m not a math person, nor do I wish to become a math person.

So while I dream about standing on the surface of the Red Planet, my passions lie elsewhere.  And I think it’s important to know the difference between your dreams and your passions.  Dreams matter.  Your dreams say a lot about who you are as a person and what you believe (and do not believe) about the world.  Cherish your dreams, but pursue your passions.

I have a passion for writing and also a (slightly lesser) passion for art.  If I could spend every day of my life writing and drawing, that would be glorious.  If I had to spend every day doing math, I’d be miserable.  And that’s why I write blog posts about Mars rather than sitting in a laboratory somewhere trying to figure out how to actually get to Mars.

Of course, no matter what your dreams and passions happen to be, there will still be closed-minded people trying to stand in judgement over you.  Ignore those people.  Cut them out of your life, if you can (maybe consider moving to another planet, if the opportunity comes up).

So what are your dreams, and what are your passions, and what are you doing to pursue them?

# Sciency Words: Heartbeat Tone

Hello, friends!  Welcome back to Sciency Words, a special series here on Planet Pailly where we talk about those weird and wonderful terms scientists use.  Today’s Sciency Word is:

## HEARTBEAT TONE

Last week, I watched NASA’s live coverage of the Perseverance rover landing on Mars.  Naturally, I had a notepad ready, and I picked up quite a few new scientific terms.  My absolute favorite—the one that brought the biggest smile to my face—was “heartbeat tone.”  I love the idea that Perseverance (a.k.a. Percy, the Mars Rover) has a heartbeat.

As this article from Planetary News describes it, Percy’s heartbeat tone is “similar to a telephone dial tone.”  It’s an ongoing signal just telling us that everything’s okay.  Nothing’s gone wrong, and everything’s still working the way it’s supposed to.

Of course, other NASA spacecraft use heartbeat tones as well.  According to two separate articles from Popular Mechanics, the Curiosity rover on Mars and the Juno space probe orbiting Jupiter also send heartbeat tones back to Earth.  And that article about Juno offers us a little bit of detail about what Juno’s heartbeat actually sounds like: a series of ten-second-long beeps, sort of like very long dashes in Morse code.

Based on my research, it seems like the earliest NASA spacecraft to use heartbeat tones (or rather, the earliest spacecraft to have this heartbeat terminology applied to it) was the New Horizons mission to Pluto, which launched in 2005.  As this article from Spaceflight 101 explains it, New Horizons’ onboard computers monitor for “heartbeat pulses” that are supposed to occur once per second.  If these pulses stop for three minutes or more, backup systems kick in, take over control of the spacecraft, and send an emergency message back to Earth.

So, I could be wrong about this, but I think this “heartbeat pulse” or “heartbeat tone” terminology started with New Horizons.  To be clear: I’m sure spacecraft were sending “all systems normal” signals back to Earth long before the New Horizons mission.  I just think the idea of using “heartbeat” as a conceptual metaphor started with New Horizons.  But again, I could be wrong about that, and if anyone has an example of the term being used prior to New Horizons, I would love to hear about it in the comments below!

P.S.: I recently wrote a post about whether or not planets have genders.  With that in mind, I was amused to note in NASA’s live coverage that everyone kept referring to Perseverance using she/her pronouns.  However, the rover has stated a preference for they/them on Twitter.  So going forward, I will respect the rover’s preferred pronouns.