NASA's Curiosity rover has a pet rock on Mars

Mars is probably a pretty lonely place.

The red planet is littered with rusty red rocks and, as far as we know, devoid of any kind of life. But even so, rovers still roam the surface of the world on humanity's behalf, making their solitary treks along previously unexplored paths.

And sometimes, they make friends.

NASA's Curiosity rover — which has been exploring the red planet's Gale Crater since 2012 — appears to have a little pet rock caught in one of its wheels.

The rock got stuck in the right front wheel at some point in the last couple weeks, but scientists aren't exactly sure when it started hitching a ride because the rover doesn't snap photos of its wheels every day.

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This isn't the first time a rock has gotten caught in Curiosity's shoe, according to Emily Lakdawalla, a planetary scientist writing a book about the rover.

Lakdawalla said that NASA and other interested folks back on Earth keep an eye on Curiosity's wheels and the rocks that might pop up in them as it roams the Martian surface.

Scientists also spotted another rock, which appeared in that same right front wheel starting in January and disappearing by April, according to Ashwin Vasavada, a Curiosity project scientist.

The current pet rock was seen in photos taken on Sol (Mars day) 1729 and 1730, which translates to June 17 and June 18 on Earth.

"We don’t exactly know how the rocks get in the wheels, but it’s likely that they can hop in as the wheels (and heavy rover) break rocks and sink into the soil around loose rocks," Vasavada said via email.

As for the rock's effect on Curiosity's wheels, it seems to be pretty benign.

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The rocks are small and relatively soft, Vasavada said, so they shouldn't pose too much of an issue to the long-running rover's movements. However, it's always possible that something unexpected could happen.

"There are some bad scenarios, such as when a rock gets jammed and makes a larger hole in a wheel, or if a rock interferes with some cables that run to the wheel motors," Vasavada said. "But we don’t think those are credible problems with these small rocks."

Scientists have Curiosity take these photos of its wheels to keep an eye on any holes that have developed in the rover's machinery.

The holes in Curiosity's wheels are a sign of the wear and tear the rover has taken on during its years of work on the red planet, but for the moment, the worn wheels are still functioning well.

And hey, at least Curiosity has a friend.