When a planet's entire population is just five denizens, your travels can be pretty lonely. Not so for Perseverance, the plucky Mars rover, which not only has the first flying vehicle on another world as its travel buddy but also has a hitchhiking companion in the form of a pet rock. 

According to NASA, this is now a record-breaking hitchhiking pet rock. 

The small interloper was first spotted lodged inside the rover's wheel back in early February. Most likely the rover kicked it up while trundling around Jezero Crater, where it’s been exploring since February last year. 

One of Perseverance's missions is to select rocks to sample and send home, but as Eleni Ravanis, a student collaborator from the University of Hawaiʻi at Mānoa, adorably pointed out in a NASA mission blog post: "How do you choose a rock on Mars? Sometimes you don’t – it chooses you." 

So, how is it record-breaking? Well, it's not the first time a Mars rover has acquired a small rocky friend. Spirit managed to scoop up a "potato-sized" pebble in its rear wheel well back in 2004, although it made its way out again the very next day. Curiosity has picked up a few hitchhikers in the last decade, though none have managed to stay with the veteran rover for more than a few weeks.

Perseverance's companion was first imaged on February 6, meaning the duo have been traveling together for four months, a hitchhiking record on Mars. 

Can't Percy just kick it out? Its four-month trip suggests apparently not – though it doesn't seem to be hindering the rover. It is, after all, the "biggest, heaviest, cleanest, and most sophisticated six-wheeled robotic geologist ever launched into space."

Its wheels – designed to withstand the wear and tear seen on Curiosity's – are 52 centimeters (20.4 inches) wide, with titanium spokes, built to withstand regolith from another planet. T'is but a rock. One of the most well-traveled rocks on the planet, however.

First thought to have been picked up while Percy was exploring the “Máaz” formation in the crater floor, it has now traveled 8.5 kilometers (5.3 miles) with the rover to the western Jezero delta, a long way from home. If it falls out during Perseverance's future ascent of the crater rim, it will be amongst a rocky brethren very different from itself. 

As one of the rover's scientists back on Earth quipped: "We might confuse a future Mars geologist who finds it out of place!”