An icy chunk the size of a small town hurtles through space. As it approaches the Sun, the ice starts to vaporize, and jets of gas and dust burst from the comet. These jets act like tiny rocket engines: they push the comet and make it spin. That's exactly what astronomers observed with comet 3I/ATLAS after it rounded the Sun.
To figure out how fast the comet is spinning, scientists used two independent methods. The first involves tracking the gas jet erupting from the comet's nucleus. Images from the Hubble Space Telescope show the jet shifting position like the hand of a clock. The second method measures the comet's brightness: as it rotates, its brightness waxes and wanes like a spinning flashlight. Both methods gave similar results: the comet completes one rotation in about 7 hours.
Such a precise match between two different methods is a stroke of luck. It confirms that the comet truly rotates with a period of about 7 hours, and the small discrepancies are due to measurement uncertainties. Understanding comet rotation helps predict how they will behave in the future and how sunlight influences their trajectories.
The rotation period of comet 3I/ATLAS, about 7 hours, is nearly the same as a human workday. The comet manages three full rotations in one Earth day.