In October, Mars will encounter comet C/2013 A1 Siding Spring, which will come closer to the Red Planet than any recorded comet has passed to Earth. This spectacular event isn't just an astronomical curiosity, it’s also a potential hazard to NASA’s armada of orbiting explorers, so the space agency is taking steps to protect them from damage by the cosmic visitor.
Comet C/2013 A1 Siding Spring was discovered in 2013 by Robert H. McNaught at Siding Spring Observatory. It’s unusual because its orbit is so elongated that this is the first time it’s visited the inner solar system in several million years. That would be enough to peak astronomers’ interest, but early calculations indicated that it would pass so close to Mars that it could result in a once in a million years impact on the Red Planet.
As Siding Spring approached Mars, new observations and calculations proved that the comet wouldn't strike, but its nucleus would pass within 82,000 mi (132,000 km) of the planet at a relative speed of 35 mi/sec (56 km/sec, 126,000 mph, 200,000 km/h) on October 19. This is only a tenth of the distance of the closest observed comet encounter with Earth.
While the nucleus of the comet won’t hit, Mars will pass through its tail. This tail is made up of gas so tenuous that it’s almost a vacuum, but it also includes solid particles of various sizes down to 0.5 mm that could destroy one of the orbiters like a bullet going through a smartphone
NASA is now preparing its Mars Odyssey and Mars Reconnaissance Orbiter (MRO) orbiters for the encounter, as well as the approaching Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, scheduled to arrive a month before Siding Spring, and its two surface rovers Opportunity and Curiosity. It’s also configuring the craft to take advantage of the event to take a closer look at the passing comet.
The agency solicited three computer models of the comet’s passing from the University of Maryland in College Park, the Planetary Science Institute in Tucson, Arizona, and the Jet Propulsion Laboratory (JPL) in Pasadena, California. These were used for calculating how best to protect the orbiters.
NASA is putting all three of the orbiters through course maneuvers that will place them on the opposite side of the planet during the most dangerous 20 minutes of the encounter, which will occur 90 minutes after Siding Spring makes its closest approach. It will be during this period that debris from the comet will reach Mars.
According to the space agency, the MRO made a course correction on July 2, and the Mars Odyssey will do the same on August 27. Meanwhile, MAVEN will arrive in Mars orbit on September 21, after which it will make a protective maneuver on October 9. Though the Martian atmosphere is extremely thin, NASA says that it’s thick enough to keep the two rovers out of danger by burning up any comet particles.
In a cosmic example of making lemonade out of unexpected lemons, NASA will use the instruments on the various spacecraft to observe Siding Spring’s passing and its effects on the Martian atmosphere as it encounters the tail. In addition, the rovers’ cameras will record the comet’s passing and any meteors burning up in the Martian atmosphere. NASA hopes that the very long orbit of the comet will provide new insights into the early history of the Solar System.