Comet C/2013 A1, called "Siding Spring" after the observatory where it was discovered, is making its first-ever pass into the inner solar system.
On Oct. 19, 2014, the comet passed Mars at a distance of 87,000 miles (139,500 kilometers). The comet was traveling 35,700 miles per hour (57,400 km/h) relative to the planet.
The nucleus is the small, solid core of the comet. The nucleus of Comet Siding Spring is 2,300 feet (700 meters) in diameter, compared with the height of One World Trade Center tower, which is 1,792 feet (546 m) tall.
Ice and dust emitted by the comet forms an atmosphere called a coma, which can be up to a hundred thousand miles or more across. Comets often have multiple tails of gas and dust, up to millions of miles long. Tails always point away from the sun.
Although the comet will did hit Mars, the comet’s considerable atmosphere (the coma) may intersect with the thin Martian atmosphere. This could react with the weak, localized magnetic field of Mars to create auroras in the sky.