One of the most anticipated astronomical events of 2014 is the close passage of Comet C/2013 A1 Siding Spring to the planet Mars on October 19, 2014. The comet’s tiny nucleus, or core, will miss Mars by about 82,000 miles (132,000 kilometers). For comparison, Mars’s small outer moon Deimos orbits at a distance of about 15,000 miles (24,000 km). As comets travel through space, though, they leave behind a trail of dust particles, and this trail of debris might be wide enough to reach Mars and encounter its thin atmosphere … or might miss it, too. If the comet dust does reach Mars, it has the potential to damage our spacecraft in orbit there. That’s why, with the idea that luck will favor the prepared, NASA is now taking steps to protect its Mars orbiters. NASA also plans to gather valuable scientific data on the comet in the days before and after closest approach. Follow the links below to learn more:
Comet Siding Spring / Mars encounter on October 19, 2014 Hurtling through space at about 35 miles (56 kilometers) per second, this comet will come very close to Mars. It will be less than one-tenth the distance of any known previous Earthly comet flyby. The comet’s icy core, or nucleus, will not strike Mars, but the high speed of the comet relative to Mars means that even the smallest grain of dust from the comet could cause “significant damage” to our spacecraft.
There are two live NASA spacecraft orbiting Mars now, with a third on its way and scheduled to go into orbit around Mars about a month before the comet flyby. The Mars Reconnaissance Orbiter (MRO) and Mars Odyssey spacecraft are already orbiting Mars. MAVEN – the Mars Atmosphere and Volatile EvolutioN mission – will arrive at Mars on September 21, 2014. NASA plans to have all three spacecraft positioned on the opposite side of Mars from the comet, when the comet is most likely to pass by. Rich Zurek, chief scientist for the Mars Exploration Program at NASA’s Jet Propulsion Laboratory in Pasadena, California, said in a NASA release on July 25:
Three expert teams have modeled this comet for NASA and provided forecasts for its flyby of Mars. The hazard is not an impact of the comet nucleus, but the trail of debris coming from it. Using constraints provided by Earth-based observations, the modeling results indicate that the hazard is not as great as first anticipated. Mars will be right at the edge of the debris cloud, so it might encounter some of the particles — or it might not.
NASA says the period of greatest risk will come not when the comet is closest to Mars, but about 90 minutes later. Space engineers expect the risk period to last about 20 minutes, during which time Mars will come closest to the center of the widening dust trail left behind by Comet Siding Spring. NASA said:
NASA’s Mars Reconnaissance Orbiter (MRO) made one orbit-adjustment maneuver on July 2 as part of the process of repositioning the spacecraft for the Oct. 19 event. An additional maneuver is planned for Aug. 27. The team operating NASA’s Mars Odyssey orbiter is planning a similar maneuver on Aug. 5 to put that spacecraft on track to be in the right place at the right time, as well.
NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft is on its way to the Red Planet and will enter orbit on Sept. 21. The MAVEN team is planning to conduct a precautionary maneuver on Oct. 9, prior to the start of the mission’s main science phase in early November.
When did astronomers discover Comet C/2013 A1 Siding Spring? Astronomer Rob McNaught discovered the on January 3, 2013 using the Uppsala Schmidt Telescope at Siding Spring Observatory in Australia. At the time of discovery it was 7.2 AU (7.2 times Earth’s distance) from the sun. Shortly after its discovery, astronomers in Arizona confirmed the comet in images taken a month earlier as part of the Catalina Sky Survey. Those early observations, plus others from around the world, quickly showed that the comet has a highly inclined, retrograde orbit orbit around the sun. We learned that the comet won’t come very close to the sun at its perihelion (closest point to the sun), no closer than 140 million miles (in contrast to Earth’s average distance of about 92 million miles). So the comet will not come near Earth. But its perihelion is just about at the orbit of Mars. This means the comet will be at its most active around the time it encounters Mars.
By the way, Comet C/2013 A1 Siding Spring probably took millions of years to come into the inner solar system from the Oort cloud. Once it passes closest to the sun and heads back out to deep space, we will not see it again.
How will the comet look from Mars? If only we could transport ourselves there! Sky & Telescope said in an article in March 2013:
… Comet Siding Spring is going to put on one helluva show as seen from Mars. As calculated by small-body aficionado Bill Gray, the comet will approach Mars from the south and sweep into its northern-hemisphere skies over just a few hours.
Hopefully, some of the NASA spacecraft will be able to capture its picture. Although Mars’ atmosphere is thinner than Earth’s, it is thick enough to protect the Opportunity and Curiosity rovers that are transmitting from Mars surface, NASA has said, “even if dust particles from the comet hit the atmosphere and form into meteors.” What a sight that would be! NASA also said:
Rover cameras may be used to observe the comet before the flyby, and to monitor the atmosphere for meteors while the comet’s dust trail is closest to the planet.
In the meantime, NASA also plans to study the comet with its orbiters in the days before and after the comet’s flyby. They’ll use instruments on the Mars orbiters to study the comet nucleus, the coma surrounding the nucleus, and the tail of Siding Spring, as well as the possible effects on the Martian atmosphere. MAVEN, in particular, will study gases coming off the comet’s nucleus into its coma as it is warmed by the sun. MAVEN also will look for effects the comet flyby may have on the planet’s upper atmosphere and observe the comet as it travels through the solar wind.
Will I be able to see this comet in Earth’s night sky? Despite the spectacular show from Mars, we on Earth will not be able to see this comet with the eye alone. However, southern hemisphere observers with telescopes have been seeing it for some time now, and some have captured the comet’s picture. Click here for info on the position of Comet C/2013 A1 Siding Spring.
On the other hand, on October 19, 2014 – the night of the comet’s Mars encounter – you will be able to see Mars. I’ll be up in the early evening, located more or less in the direction toward the center of our Milky Way galaxy. That’s a star-rich region of the heavens, but Mars’ reddish color should help you spot it. EarthSky’s planet guide will tell you more.
Bottom line: The tiny nucleus of Comet C/2013 A1 Siding Spring will not strike Mars when it comes closest on October 19, 2014. But material shed by the comet could damage our spacecraft in orbit around the Red Planet. NASA is taking steps now to prevent damage to its orbiting spacecraft.