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Using both ground-based and Rosetta observations scientists have now measured when the comet reached its peak brightness as well as how much material it lost during this orbit’s closest approach to the Sun.
Based on Rosetta’s pre-perihelion measurements that indicate the dust:gas ratio was approximately 4 , that means roughly 80% of the material being lost is dust, with the rest dominated by water, CO, and CO2 ices. (Note: at the time of that blog post an estimate of 3 was made for perihelion, but the actual data has yet to be analysed.) In any case, using 3 and 4 respectively, the total mass loss rate at its peak is likely in the range of about 100,000–115,000 tonnes per day.
Of course, that’s not a huge amount compared to the comet’s overall mass of around 10 billion tonnes. But nevertheless, a very simple calculation reveals that if, for example, the comet lost that much mass continuously for 100 days, it would correspond to roughly 0.4-0.5 metres of its surface being removed in that time.
In other words, the surface lost about 1.5 feet during close approach.
Peak brightness occurred near the end of August, and has been declining since.