By Jessica Orwig from Business Insider

Mountain-sized comet Siding Spring has spent its entire life in the outer reaches of our solar system, but on Sunday Oct. 19 it will enter the inner solar system for the first time, heading toward the red planet, Mars — incredibly close to the planet’s surface.

It’s going to be so close to the planet, NASA had to take precautions by moving its Martian satellites behind the planet, protecting them from the comet’s path, which will be the closest comet flyby Mars has ever seen in recorded history.

The comet won’t hit the planet itself, but it may spray it with meteors. It’s a once-in-a-million years event, according to astronomers.

The comet is about the size of an Appalachian Mountain. As it whizzes by, NASA’S Mars rovers Curiosity and Opportunity will direct their cameras skywards for the spectacular show. Below is animation of why it might look like from a one of NASA’s satellites in orbit around Mars.

The comet will reach its closest approach at 2:28 pm EDT (11:28 am PDT) when it will be about 87,000 miles from Mars. If the comet were passing by Earth instead of Mars, it would be closer to the surface than our moon

Observers in Australia, South America, and other parts of the Southern Hemisphere will have the chance to see the comet in their night sky with a pare of binoculars or a telescope. The live online observatory, Slooh, will be streaming the event for those of us in the Northern Hemisphere (livestream below), who can’t see it with our own eyes.

This an event that only happens once every million years, and scientists are waiting with anticipation.

“This particular comet has never before entered the inner solar system, so it will provide a fresh source of clues to our solar system’s earliest days,” astronaut and Associate Administrator for NASA’s Science Mission Directorate in Washington, John Grunsfeld, told MailOnline.

The images above show — before and after filtering — comet C/2013 A1, also known as Siding Spring, as captured by Wide Field Camera 3 on NASA’s Hubble Space Telescope. Image released March 27, 2014.

Scientists think that Comet Siding Spring formed between one to two million years after the birth of our solar system, which was about 4.6 billion years ago. The comet spends most of its time at the very fringes of our solar system where billions of comets reside in what is called the Oort Cloud, imaged below. Only ever one million years or so has Comet Siding Spring ventured closer in to Uranus, Jupiter, and now its closest approach ever — to Mars.

The Oort Cloud is made up of billions of comets.

Comet Siding Spring will be the first comet from the Oort Cloud to come near enough for us to take close-up observations of it with spacecraft — especially those around Mars. 

“We can’t get to an Oort Cloud comet with our current rockets,” said senior astrophysicist at the Johns Hopkins University Applied Physics Laboratory during a NASA news conference last week. In a way, it’s a “free flyby” that will give scientists the chance to learn more about the very early stages of our solar system and how the planets formed.

Slooh will be hosting two shows of the event: The first show “Close Call — Comet Siding Spring zips by Mars” will begin at 2:15 EDT (11:15 PDT). The second show “Comet Siding Spring — The Outcome” will start at 8:30 pm EDT (5:30 pm PDT). You can watch both shows live on Slooh’s website or right here. 

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