Making Marks

This week’s post is brought to you entirely by the fact that I saw a picture that was sent to Earth from the Perseverance rover on Mars and found myself unexpectedly enamored of it. It’s a very simple picture. The rover used its left NavCam to take a picture of some tracks it had left behind it (presumably from the wheels on the rover’s left side).

The rover frequently takes shots of its own tracks. It’s one of the ways they can monitor the state of the wheels, for one thing (something that became a topic of concern all the way back in 2013 when images revealed that Curiosity’s wheels were getting slowly shredded by Martian rocks). It’s helpful for scientific purposes, since seeing how deeply the rover’s wheels have dug in tells us things about the dirt. It’s also helpful for hazard avoidance—ever since the Spirit rover got trapped in a sandpit in 2010 every rover driver has made avoiding deep sand a priority. 

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As for the picture itself, there’s not much in it other than the tracks. The rover is not visible in the image. There are only a few rocks visible. The sand itself isn’t appreciably different looking from sand you’d find on Earth. Taken on artistic merits, there’s not much to appreciate about this image. 

But it wasn’t artistic merits that caught my attention, and it certainly isn’t artistry that has kept me thinking about this image repeatedly since I first came across it. No, that has to do with what this picture means. Those are tracks that were left behind by a little piece of Earth that we humans built and sent out into the void.

There’s something beautifully simple and yet profound about that picture of rover tracks. So many of our spacecraft operate in deep space and don’t leave much behind. But sometimes it’s nice to think of the quiet ways human ingenuity is making its mark on our solar system. So let’s take a moment and appreciate the vistas our spacecraft have reached, and what they’ve left behind.

Mercury and Venus

We haven’t done a whole lot of Mercury exploration, with only one flyby and one orbiter under our belt. Having never put a lander or rover on Mercury, you can’t say that we’ve left tracks. But we definitely left a mark, courtesy of a very special messenger. By which I mean MESSENGER, the MErcury Surface, Space ENvironment, GEochemistry, and Ranging mission (at least they didn’t try to count any letters randomly in the middle of the words for that one).

MESSENGER spent just over four years orbiting Mercury from 2011 to 2015. And when it began to run out of fuel it became one with the planet it had studied so faithfully, purposefully slamming into Mercury at over 8,000 mph (12,800 kph). It’s highly doubtful any bit of the spacecraft itself survived that, but there’s definitely a MESSENGER memorial crater estimated to be at least 50 feet (15 m) across. Perhaps it will be spotted by the BepiColombo spacecraft when it enters Mercurian orbit later this year.

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Venus also doesn’t have any tracks, but it has other marks left by human technology. The Soviet Union made something of a project out of Venus in the 1960s and 70s. It sent nearly 30 spacecraft to the pad to be flybys, atmospheric probes, and landers. Several never made it out of Earth orbit, others never made it to Venus. The first mark humans ever left on any planet other than Earth was when Venera 3 slammed into the surface of Venus in March 1966. 

Mind you, it was supposed to land softly. Still, it was the first and you can’t take that away from it.

Veneras 4, 5, and 6 were atmospheric probes never designed to live to see the surface. Still, they probably did make it. Venera 7 was the first soft landing on another planet, although it didn’t land well and didn’t last long. Also scattered on the surface will be the litter of Veneras 8-14 and Vegas 1 and 2, all Soviet missions.

What about NASA missions, you ask? We know Pioneer Venus 2, meant to be an atmospheric probe, made it to the surface, so that will be there. As for what would be left of any of these craft after spending several decades in the hell that is Venus’s surface, who knows. Nothing has landed on the surface of Venus since Vega 2’s lander in 1985.

Mars

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Whoo boy have we left tracks on Mars! Six rovers have successfully left their treads behind in the Martian soil, from the bitsy Sojourner in 1997 to the mighty Perseverance starting in 2021. Also in the club are Curiosity and Spirit, as well as Spirit’s twin Opportunity and the sole successful non-NASA rover, Zhurong. Of these, Perseverance and Curiosity are obviously still making tracks.

There have also been six landers (which don’t leave tracks but do leave marks) that were able to successfully carry out their full missions on Mars, five from NASA and Tianwen-1 which arrived with Zhurong. And I would never, ever leave out Perseverance’s early partner in crime, the Martian helicopter Ingenuity, which planted its little bug-like feet in over 70 different spots.

Of course, Mars is also riddled with the still bodies of landers and rovers that successfully made it to the ground and then couldn’t, for one reason or another, continue to function. And it’s got a few memorial craters of its own left by spacecraft that didn’t stick the landing. For instance, I like to say Mars has two Schiaparelli craters, the actual Schiaparelli Crater and the crater left behind when the spacecraft Schiaparelli slammed into the ground at near terminal velocity. In case you were unaware, landing on Mars is hard. 

Giant Worlds

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Here’s the thing about those outer planets: they’re gassy. You can’t land on them. So no rover tracks. They do have plenty of moons though, and one of them does have something from Earth sitting on it. The Huygens probe rode the Cassini spacecraft to Saturn orbit and then entered the atmosphere of the big moon Titan in January 2005. It didn’t last long, and it never was meant to. But it did touch down on the surface and there’s nothing viciously destructive about Titan’s environment, so it would still be there.

`That’s it for landings though. We’ve mostly been trying to actively avoid landing things on the moons of Jupiter and Saturn, considering how many of them have underground oceans. We like spacecraft around those worlds to burn up in the planets’ atmospheres instead (RIP Galileo and Cassini). Which is kind of like leaving a mark, if you think about it. 

As for Uranus and Neptune, they got a flyby once in the 1980s and that’s it. I would love to one day talk about some mark, however ephemeral, we left on these planets or their moons.

Tiny Worlds

Rovers and asteroids or comets really wouldn’t mix well, what with the rocks’ universally tiny gravities, so we’ve never put a rover on anything smaller than our Moon. We have put a lander on a comet though, although as I’ve blogged before, the landing didn’t go so well (you tried Philae. You tried).

Two small world orbiter missions ended with soft touchdowns of the spacecraft on the worlds they had been orbiting. The Philae lander’s orbital partner Rosetta joined Philae on the surface of 67P/Churyumov-Gerasimenko with a gentle landing in 2016. The Near Earth Asteroid Rendezvous spacecraft (NEAR, redesignated NEAR-Shoemaker mid-flight) ended its mission in 2001 with our first soft touchdown on an asteroid, a gentle landing on Eros. Following these touchdowns, Rosetta and NEAR went to sleep for the last time. It’s possible they’ve been dislodged in the years since, but it’s also entirely possible they’re still resting where they landed.

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Another thing we’ve done with some of these tiny worlds is sampling. This is a gentle way of leaving a mark, usually a brief touch of the surface to scoop some material. Since they weren’t made very long ago it’s very likely the sampling marks left by the spacecraft Hayabusa and Hayabusa2 are still there on the surfaces of the asteroids Itokawa and Ryugu.

In the same vein, the asteroid Bennu probably still has a big old hole in it caused by OSIRIS-REx’s thrusters firing desperately to get the spacecraft back off the surface after the asteroid tried to swallow the spacecraft when it took its sample. Serves it right, the greedy rock. 

Then of course there are the tiny worlds we’ve left our marks on because we punched them. This includes the comet Tempel 1, which was the target of the Deep Impact mission. This mission lived up to its name by sending an impactor flying into the comet. And then there’s the poor asteroid Dimorphos, which was the target of the Double Asteroid Redirect Test (DART), whose entire mission was to slam itself into the asteroid. You know, for science. Leaving behind a giant crater isn’t quite as evocative as tire tracks, but it is unquestionably a way of leaving a mark behind.

Standing the Test of Time

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I left the Moon for last, and that’s for a reason. The thing is littered with our spacecraft. Live ones, dead ones, craters left by ones that never got a chance, rovers, landers, all of it. I don’t want to list them all, it would take too much time and I just don’t feel like it. Suffice to say there are absolutely tracks on the Moon, along with many other kinds of marks.

But that’s not actually why I left it for last. It’s because if there’s anything more profound than the sight of the tire tracks we create in our explorations, it’s the sight of a footprint. There’s a reason why one of the first images ever taken on the Moon by an astronaut was a shot of a single footprint in the otherwise unmarred lunar surface. And the thing about the Moon is that there isn’t much that will erase those marks. Barring some statistically unlikely scenarios, those are there for the long haul. They’re a kind of testament to the sorts of remarkable things our species is occasionally capable of. 

One of the things I would most love to see in my lifetime is a similar mark left behind in the red Martian dust. Even if it won’t last, even if the Martian wind will erase it as it may have already erased Perseverance’s tire tracks, the important thing is that it will have been there. We will have been there. That’s the monument I want to see.