One plan becomes two plans

Editor’s note: Those who have been following our blog will know that the MEX Flight Control Team at ESOC have been actively preparing for the flyby of comet C/2013 A1/Siding Spring on 19 October. Initial estimates gave the possibility that Mars Express might have to contend with a large particle flux – and that several (2? 3?) very high-speed (~56 km/sec!) particles might bash into the spacecraft. Happily, additional observations by ground and space telescopes (including the ESA/NASA Hubble Space Telescope) have allowed initial estimates to be refined and the risk is now understood to be much lower – and perhaps even as low as zero. In today’s blog post, the team explain how this (happy!) real-life, real-time development is affecting their preparations for fly-by.

Comet C/2013 A1 Siding Spring seen on 6 September 2014 from Argentina. Image credit: César Nicolás Fornari https://www.facebook.com/cesar.fornari

Comet C/2013 A1 Siding Spring seen on 6 September 2014 from Argentina. Image credit: César Nicolás Fornari https://www.facebook.com/cesar.fornari

Late last year, estimates given in scientific papers estimated that over the duration of the encounter, the number of large cometary particles per square metre would be around 1. As MEX’s area in the most protected attitude is about 3m2, we could then expect about 3 potentially significant impacts. Not good!

By the middle of this summer, published estimates (based on new images and additional modelling) were indicating a flux of around 10-6 particles per m2, which, for Mars Express, very roughly equates to a 1-in-300,000 chance of being hit. It’s starting to look like our comet C/2013 A1/Siding Spring will manifest itself as a more friendly passer-by than initially thought and that it won’t be hurling clouds of large particles at unthinkable speeds towards Mars and its man-made satellites.

Closest approach: If Mars were Earth. Credit: NASA/JPL

Closest approach: If Mars were Earth. Credit: NASA/JPL

So why the big change?

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