A rotating green comet unexpectedly slowed its spin.
A rotating green comet that orbits the Sun every 5.4 years dramatically slowed in its spin. Images taken in May 2017 by NASA’s Neil Gehrels Swift Observatory revealed that comet 41P/Tuttle-Giacobini-Kresák, or 41P for short, was spinning three times slower than it was in March 2017, when it was observed by the Discovery Channel Telescope at Lowell Observatory in Arizona. The abrupt slowdown is the most dramatic change in a comet's rotation ever seen. Comets can slow their rotation when ice on their surfaces melt directly to gas, creating jets of gas and dust that act like thrusters. The jets can create a torque, or twisting force, that can either enhance or counter the comet’s rotation. In 41P's case, its small size at less than a mile across meant that more than half its surface area was covered by these jets – leading to a dramatic slowdown from once every 20 hours to once every ~46-60 hours in two months. Watch the video to learn more.