When Sarah Stewart Johnson was eleven, her father took her to the medical laboratory where he worked and showed her a 141-year-old toenail. It came from one of America’s less well-known presidents, Zachary Taylor, who’d been exhumed so that his untimely death could be investigated. Johnson saw the sophisticated machines that would determine by chemical analysis if the unfortunate president had, as rumoured, been poisoned. It turned out he hadn’t, but for Johnson the experience provided a foretaste of her own future career: ‘Although I could sense the enormous power of these instruments, I had no inkling then of their potential as tools for space science.’ Professor Johnson is part of the science team monitoring NASA’s Curiosity Mars rover, a car-sized buggy that has been trundling across the planet since 2012. After reading her fascinating, elegantly written book on the red planet, I’m glad she didn’t become a chiropodist instead.