This story is part of Welcome to Mars, our series exploring the red planet.

Mars does not take kindly to our robotic explorers. It chokes them with dust, roughs up their wheels and messes with their probes, and now it’s throwing a new challenge at NASA’s Perseverance rover. There’s some pebble-size debris getting in the way of the machine’s rock sample collection system.

Everything went smoothly at first. On Dec. 29, Perseverance drilled into a rock nicknamed Issole and extracted a sample of it. “However, during the transfer of the bit that contains the sample into the rover’s bit carousel (which stores bits and passes tubes to the tube processing hardware inside the rover), our sensors indicated an anomaly,” wrote Louise Jandura, chief engineer for sampling and caching, in a blog post on Friday.

The rover halted its sampling activities when it met unusual resistance during the process of stowing the sample. As the rover team expressed in a tweet on Friday, the upshot is the debris is preventing the rover’s robotic arm from properly handing off the sample-filled tube for sealing and storage. 

Perseverance is the first rover to attempt to collect samples of Mars in sealed tubes. It’s a key part of a mission that’s also seeking out signs of ancient microbial life on the red planet. NASA is planning to send an ambitious future mission to pick up the samples and bring them back to Earth for study. 

The rover team commanded the machine to backtrack by pulling out the drill bit and tube. It snapped some images along the way to help diagnose the problem. “These most recent downlinked images confirm that inside the bit carousel there are a few pieces of pebble-sized debris,” Jandura wrote. NASA expects the pebbles fell out of the sample tube.

While the debris presents a challenge, there’s no reason to fret yet. Jandura said the designers of the bit carousel have considered this possibility, but that it will take time to work through a solution that allows the pebbles to “exit in a controlled and orderly fashion.”

NASA has proven resourceful when it comes to working through the red planet’s temper tantrums. The Perseverance team plans to take this one slow to ensure the sampling system is clear and ready to go for future work.