Over the past few years, Perseverance has been exploring an ancient river delta in Mars’s Jezero Crater, with the aim of finding signs of past life. The rover’s belly is now stuffed with 17 tubes of Martian rock, dirt and air that scientists say represent an astounding geological collection.

However, bringing Perseverance’s samples back could cost as much as US$11 billion, an independent panel concluded in a scathing engineering analysis last year. That’s more than NASA can afford. By the end of this month, it and ESA are supposed to find a cheaper way to achieve Mars sample return — or risk leaving the carefully collected rocks where they are.