On November 18, 2013, Comet Encke will pass within 0.025 AU of Mercury, followed a day later by Comet ISON at 0.24 AU (1 AU is the distance between the sun and Earth, 150 million km).

NASA's MESSENGER spacecraft, now orbiting Mercury, will turn away from the sun's innermost planet for a time and toward the passing comets. Astronomer Ron Vervack at the Johns Hopkins University Applied Physics Lab and a member of the science team for MESSENGER, called it:
... a unique coincidence and a golden opportunity to study two comets passing close to the sun.