C/2013 A1 (Siding Spring) is the name given to an Oort cloud comet that was discovered in 2013 and had a close encounter with Mars the following year.
The comet is believed to have fallen from the distant Oort cloud millions of years ago. Its eventual arrival to the inner solar system meant that it passed close to various orbiting spacecraft close to Mars, giving us a unique glimpse of what happens when a comet has a close encounter with a planet.
The Discovery
Robert H. McNaught is credited with the discovery of the C/2013 A1 Siding Spring. This event occurred on January 3, 2013, at the Siding Spring Observatory in New South Wales, Australia. The discovery was made using the Uppsala Southern Schmidt Telescope, which McNaught has also used to locate more than 400 asteroids close to Earth, as well as the Great Daylight Comet.
When spotted, the comet was in the Lepus constellation at a distance of 7.2 AU from the Sun. It was estimated to have spent millions of years traveling from the Oort cloud.
A spherical mirror with a diameter of 0.5 meters captured the first images of the object, It was named by following the tradition of using the name of the observatory where the initial discovery was made. The C/ part of the name tells us that it’s a non-periodic comet, while the 2013 A1 section refers to the fact it was discovered in the first half of January 2013.
The Close Encounter with Mars
On October 19, 2014, f C/2013 A1 had a brush with Mars, passing the planet at a distance of 0.000939162 ± 2.7×10−8 au.
This meant that the comet’s nucleus missed Mars by a distance that was only around a third of the distance between the Earth and its moon. In other words, it was about 10% of the closest distance that any identified comet has come hitting to the Earth.
Initially, it was feared that the comet might collide with Mars. However, that idea was proven to be incorrect following the 200-day observation arc that allowed its orbit to be calculated more precisely. Other estimates suggested that its passing could cause a meteor shower to rain on Mars or damage the spacecraft. These fears proved to be unfounded.
It approached Mars at 35 miles per second. It gave the five spacecraft orbiting Mars (2001 Mars Odyssey, Mars Reconnaissance Orbiter, MAVEN, Mars Express, and the Mars Orbiter Mission) a chance to capture a large quantity of data about this interaction.
These crafts sheltered behind Mars as C/2013 A1 Siding Spring approached the planet head-on and at great speed. The size of the trail meant that there was ample opportunity to collect data as it passed through.
The Comet Dust and Nucleus
The close encounter with Mars the NASA Mars orbiters to check the comet on its flyby, with all of them reporting a healthy status afterwards. The orbiters were able to detect a huge quantity of comet dust, measured at thousands of kilos per hour. They are made up of elements, including sodium, nickel, potassium, zinc, and magnesium.
It turned out to be smaller than orginially thought, with the comet nucleus estimated at somewhere between 0.2 and 0.4 miles in diameter. The nucleus is reported as rotating every eight hours.
Current calculations suggest that it will next be seen in our inner solar system around 740,000 years from now.