The comet will make a close approach to Mars on 19 October, passing about 138,000 km from the planet. That’s should be close enough that gas and dust in the outermost reaches of the comet’s atmosphere, or coma, will interact with the atmosphere of Mars.
The closest recorded Earth approach by a comet was by the now-defunct comet Lexell. In 1770, it came within 2.3 million km. That’s roughly 6X the distance from the Earth to the Moon.
Image Credit: NASA