If you were around in the mid-1970s you may remember the Pet Rock fad. For those of you who weren’t — the Pet Rock was the brain child of advertising copywriter Gary Dahl, who had responded to his friends’ complaints about destructive pets that his pet rock was the ideal pet and never gave him any trouble. The joke turned into a marketing idea, and Dahl developed a small box resembling a critter carrier you might get at a shelter, along with an instruction manual about care, grooming and feeding (note: none of that was required) and teaching it to do tricks, such as “play dead” (at which, as you know, rocks excel). The joke took off and made Dahl a millionaire, but like any true fad it was a short-lived one.
Even though the original Pet Rock can still occasionally be found for sale online (at multiple times its original cost) it’s pretty much out of the public’s consciousness these days. However, the concept of a pet rock has survived — every kid has at some point brought home a favorite rock, and some of us have never outgrown our fondness for pet rocks, except that we tend to not buy them anymore, but find them somewhere and take them home because something made us take notice — its shape, color, texture or special features such as veins. If you don’t have a pet rock, find one on your next nature walk. It’s a lot of fun!
It stands to reason that a place like Mars, which is a geologist’s wonderland even on a bad day, would be a great place to find a pet rock. And while it will be years before one of us will plod through the red dust and pick up a pebble here and there, there are robotic geologists on Mars as you read this.
Granted, these robots photograph, zap, analyze, drill and examine rocks all the live-long day with no particular fondness for one particular pebble or boulder. It’s quite a different thing for the human beings who direct, program and operate those robots: the science and engineering teams behind the Curiosity and Perseverance rovers spend their days deciding which rocks to target for maximum science return while being unable to actually ever see those rocks in person. Perseverance has been stashing small canisters with samples along its path, in the hopes that in a future mission a robot will pick them up, put them on a rocket and send them back to Earth.
And every once in a while a rock will hitch a ride on a rover. Not on purpose – like their terrestrial cousins Mars rocks prefer to “play dead” when a robot comes along (and probably even when no one is looking). Due to the varied terrain it’s not uncommon for a small rock or pebble to get trapped inside a Mars rover wheel. Generally that’s not a problem, and while the rock may rattle around and around and around inside the wheel it will usually fall back out after a few days or weeks.
Perseverance picked up a rock in one of its wheels on February 4, and it’s been its little travel companion since then. Four months is a record for a rock to be carried in a rover wheel, and since it keeps making appearances in one of the forward hazard avoidance cameras we know it’s still there. By now the rock is far from its original site – several miles, in fact – and it’s a complete stranger in the current terrain, having little in common with the other rocks in the area. If a future geologist should find the rock where it is these days it would be quite a mystery on how it could have possibly ended up there.
Perseverance itself has no awareness of its little pal, but back home everyone refers to the hitchhiker as a pet rock. And just like the original fad Pet Rock, this one is a patient companion that does not need to be fed, groomed or entertained. It gets a workout for sure whenever the rover is in motion, and at Perseverance’s slow traveling speed it would take a very long time for it to be tumbled into dust. And as long as it doesn’t get stuck in a gear or other vital part there is no harm done and the two unlikely friends could stay together for a long time — maybe forever.
However, it’s probably just a question of time before a sudden move will toss the rock out of the wheel back onto the surface of Mars. But until then, Perseverance is not alone. Its progress may be a little noisier with a rock in its wheel but other than that it’s business as usual for this Martian explorer. It will be interesting to see how much longer the rock will accompany the rover; luckily there are ways to check on it, and every so often it may make another appearance, as cameras are trained on the wheels to assess their condition.
Meanwhile, you can follow both Mars rovers’ adventures at https://mars.nasa.gov/. There’s always interesting news about our robotic explorers InSight, Curiosity, Ingenuity and Perseverance and the most famous Pet Rock of them all.
Beate Czogalla is the Professor of Theater Design in the Department of Theatre and Dance at Georgia College & State University. She has had a lifelong interest in space exploration and has been a Solar System Ambassador for the Jet Propulsion Laboratory/ NASA for many years. She can be reached at firstname.lastname@example.org