Roaming Mars is a lonely existence for NASA's Perseverance, but the exploratory rover now has a traveling companion: a hitchhiking "pet rock" that got stuck in one of its wheels. Luckily, the Martian stone won't impact the rover's science mission and is only a minor inconvenience — like having a pebble stuck in your shoe.
Perseverance's front-left wheel accidentally picked up the pet rock on Feb. 4, or Sol 341 — the 341st Martian day of the Martian year, according to a statement by NASA. The rock has periodically photobombed images taken by the rover's front-left Hazard Avoidance Camera (Hazcam). Recent images show that the rock is still tumbling along with Perseverance 126 days (123 sols) after it first hitched a ride. (A sol, or Martian day, is just 37 minutes longer than an Earth day.)
The rock has been hitchhiking with Perseverance for just over a quarter of the rover's mission on the Red Planet. When the rock first made a home for itself in Perseverance's wheel, the rover was exploring the Máaz formation — a section of the Jezero crater that researchers suspect is made from ancient lava flows. Since then the rover has traveled 5.3 miles (8.5 kilometers) though the Octavia E. Butler landing site, where Perseverance first touched down on Mars in February 2021, and past the remains of the Kodiak delta, which once linked an ancient river and lake. The rover will shortly be gearing up for an ascent of one of the Jezero crater's steep slopes, which may dislodge its stoney stowaway.
When the pet rock does eventually fall out of the rover's wheel, it will likely be surrounded by rocks that are very different from itself because it is likely of volcanic origin. "We might confuse a future Mars geologist who finds it out of place," one mission scientist joked in a recent meeting, according to the statement.
Perseverance, or Percy, has picked up several other small rocks in its front-right wheel during its mission, but these have all fallen out within a few days or weeks. This makes the newest pebbly passenger a Martian hitchhiking record-breaker, according to the statement.
But Percy is not the only Mars rover to pick up a pet rock. In December 2004, operators of NASA's Spirit rover — which roamed Mars between January 2004 and March 2010 — had to perform a sharp turn maneuver to shake out a "potato-sized" rock from its right-rear wheel because scientists feared it would cause significant damage, according to NASA.
Previously, picking up unwanted rocks in other parts of the rover has been a much more serious problem for Perseverance. On Dec. 29, a bunch of small pebbles fell into part of the rover's machinery, causing Percy to shut itself down for almost a week. Mission scientists eventually worked out a way to remove the pebbles after forcing the rover to detach its drilling arm to properly photograph the affected area.
Originally published on Live Science.
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Harry is a U.K.-based senior staff writer at Live Science. He studied marine biology at the University of Exeter before training to become a journalist. He covers a wide range of topics including space exploration, planetary science, space weather, climate change, animal behavior, evolution and paleontology. His feature on the upcoming solar maximum was shortlisted in the "top scoop" category at the National Council for the Training of Journalists (NCTJ) Awards for Excellence in 2023.