An old story … Once there was a preacher who brought his congregation to the river’s edge. “If you have faith,” he proclaimed, “I can walk on this water! Do you believe?” They all cried out, Yes! Two more times he challenged them, two more times they shouted out their faith. “Well then,” he finally said, “if you already believe, I don’t have to do it.”

An old story … Once there was a preacher who brought his congregation to the river’s edge. “If you have faith,” he proclaimed, “I can walk on this water! Do you believe?” They all cried out, Yes! Two more times he challenged them, two more times they shouted out their faith. “Well then,” he finally said, “if you already believe, I don’t have to do it.”

Nasa’s Dart Mission, launched last week, seems to have a similar issue. The “Double Asteroid Redirection Test” spacecraft will fly to a 700-metre-wide asteroid called Didymos and then crash itself into that asteroid’s even smaller moonlet. By observing that moonlet carefully from Earth, with telescopes and radar, we’ll be able to measure just how much the impact changes its orbit.

But at first glance that seems to be a wholly unnecessary experiment. We trust Newton’s laws of physics even more than the preacher’s followers believe in his water-walking skills. Plough a spacecraft of known mass and velocity into a moonlet of known mass and velocity? It’s a problem a high-school student could solve: apply conservation of momentum.

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