Mars

Perseverance's 2021 landing and its instruments have delivered exactly what was engineered, precise navigation, a surprise helicopter programme, oxygen from thin air, but the sample-return mission that would confirm any life finding is stalled.

In 1976 two Mars landers ran parallel experiments meant to catch microbial life in the act, and one of them produced a signal its own designer still calls biological, decades after most scientists moved on to a chemical explanation.