Within the lunar dirt is a type of helium so rare on Earth that a palm-sized container is estimated to be worth millions In the silent vacuum of space, five autonomous robots churn through the lunar surface, digging up a loose layer of rock and dust and leaving rows of uniform tracks in their wake. Stopping only to recharge at a central solar power station, the car-sized machines process the lunar dirt internally to extract a type of helium so rare on Earth that a palm-sized container is