A billion year old comet is hurtling through space and is set to make a pass by Mars. This is a very rare encounter with the Red Planet, an occurrence that only happens around once in a million years. While the comet will not make contact with the planet’s surface, it will pass very close, somewhere within eighty eight thousand miles.

The comet itself, which has been named Siding Spring, is quite large. The interior of the comet is around a mile in diameter, and the entire thing is around the size of a small mountain. However, unlike most comets, this particular comet is soft. Scientists say that Siding Spring has a consistency much closer to powder than solid rock. The reason for this comes from the fact that the comet is composed of different volatile ices such as methane and carbon monoxide. These ices are so volatile that if they were treated with any real heat the comet would simply burn away.

The reason that Siding Spring, despite its fragile make up, has managed to exist as a construct of ice for so long is due to its origin. According to reports from NDTV  the comet originated from a deep part of space referred to as Oort Cloud. This region is home to many comets that were created billions of years ago and still exist today. A large part of this is due to how far Oort Cloud is from the sun, which allows the comets to stay in a deep freeze. It is this reason that Sliding Spring is still around to make its pass by Mars.

Despite the fact that it will not collide with Mars, scientists still are eager to keep an eye on the comet. The rovers on the Red Planet have already moved to make sure they do not get damaged by falling debris, and will keep their cameras toward the sky. The comet is set to pass on October 19th, and scientists hope that the rovers will be able to capture the very rare phenomenon.