Comet 3I/ATLAS has already puzzled astronomers with its odd trajectory, twin tails and sudden bursts of activity. Now a rhythmic 16.16-hour “heartbeat” has become the latest mystery. Most researchers see a rotating, venting comet. Harvard astrophysicist Avi Loeb argues the pulse points instead to jets driving the brightness “like a bloodstream”, keeping open, cautiously, the possibility of something more engineered.