Comet vs. Sun

To the naked eye, a comet’s most distinctive feature is its tail. On April 20, 2007, NASA’s STEREO A spacecraft witnessed something never before recorded: a comet losing its tail. Comet Encke is a short-period comet that circles the sun every 3.3 years. As the icy rock hurtles through the inner solar system, it produces two tails: one made of dust and another made of gases. The gas tail, known as an ion tail, points directly away from the sun, aligning along magnetic field lines within the solar wind. But these field lines can be put into complete disarray by a coronal mass ejection (CME), powerful eruptions of plasma and magnetism from the sun. When a CME blew past comet Encke, the wave of solar material disturbed the magnetic field surrounding the comet and snapped its tail off. The effect, however, was only temporary—within minutes a new tail formed. Watch the video to see the event unfold.