Far off at the most distant edge of the solar system is the Oort cloud, a shroud of ice and dust largely unchanged since the very beginning of the solar system. Looking at the Oort cloud is like looking back in time—a superpower hampered by the fact that the Oort cloud is very, very, very, very far away. This weekend, however, a piece of the Oort cloud is coming to us.
Comet Siding Spring was discovered just last year and, at first, astronomers thought it might have been on a collision course with Mars. Eventually, says NASA in the video above, astronomers realised that Siding Spring and Mars would not meet, but they would pass very, very near each other.
Mars is currently a hotspot of space research, with a small fleet of satellites and probes in orbit around the red planet. Siding Spring's close approach—at a minimum distance of 84,000 miles, just a third of the distance between the Earth and the Moon—combined with the bevy of telescopes and sensors, says Amy Shira Teitel for Motherboard, means that this weekend will offer astronomers their best look yet at an Oort cloud object.
Millions of years ago, says University of Maryland astronomer Michael Kelley, something disturbed the orbit of Siding Spring, sending it hurtling toward the Sun. The odds of it passing by so near to Mars are astronomically small, he says: “we may never see another Oort cloud comet breeze past Mars so closely again.”
Though Siding Spring will give us our best-yet look an an Oort cloud object, this isn't the only time one of these ancient relics graced the inner solar system. Just last year, all eyes were on Comet ISON, a different Oort cloud object that passed close by to the Sun.