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NASA Mars Exploration Program
Mars Exploration Program


Mars and Comet ISON (C/2012 S1)

Comet ISON (C/2012 S1) streaks across the darkness of space, with shining stars and distant galaxies in the background

Comet ISON (C/2012 S1) took about three million years to reach the inner solar system on its first journey from the Oort cloud, which swarms with small icy bodies in the outer regions of the Solar System.

Because it had never made a journey close to our Sun, Comet ISON's icy materials had never been altered since they first formed along with our solar system. Comet ISON thus contained a record of materials from that ancient time, about 4.5 billion years ago. Vitali Nevski and Artyom Novichonok at the International Scientific Optical Network (ISON) near Kislovodsk, Russia discovered Comet ISON in September, 2012, when it was about 585 million miles away.

Comet ISON in four image frames looks like a fuzzy dot against a star filled sky from Mars
Image Credit: NASA/JPL-Caltech/U of Arizona

Comet ISON (C/2012 S1) flew by Mars at a distance of about 6.5 million miles (0.07 AU), about 6 times closer than it came to Earth. Mars rovers and orbiters tried to get a close-up view, but NASA's Mars Reconnaissance Orbiter was the only spacecraft at Mars able to see the comet.

Despite its close distance from Mars, Comet ISON was very faint and relatively inactive at that stage in its journey toward the Sun. Mars Reconnaissance Orbiter used its large, high-resolution telescopic camera called HiRISE to image Comet ISON, However, engineers did not design this camera for astronomy. To take great, highly detailed images of Mars as the orbiter passes over a particular place of interest on the Red Planet, the maximum exposure time is short. To take pictures of comets, it's much more effective to have longer exposure times, especially when they are faint.

Sun-grazing comet diagram shows a comet getting brighter with the longer tail as it nears the Sun.
Due to camera limitations and low activity in the comet's coma at the time, the HiRISE images were not as visually interesting as later ones taken by other spacecraft. However, the HiRISE image helped scientists estimate the size of comet ISON's nucleus, as well as understand the patterns of comets on their first voyage toward the Sun. After Comet ISON (C/2012 S1) left Mars, it grew brighter as it approached the Sun and ice and gases in its nucleus "melted" and "evaporated," forming a larger coma and tail.

Because it came so close to the Sun, Comet ISON was known as a sun-grazing comet. Learn more about sun-grazing comets.

On November 28, 2013, when Comet ISON passed only 724,000 miles (~1.16 million kilometers) from the surface of the Sun, the Sun's gravitational forces and intense radiation tore apart the comet.

NASA's Comet ISON Toolkit: More information on the observation campaign on Earth and throughout the Solar System to image Comet ISON. Sixteen spacecraft attempted to take pictures of it, and Earth dwellers got out telescope cameras too!