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Secrets of the comet’s tail


A simulation traces the complex physics that makes a comet visible, writes Michael Lucy.


Comet Catalina.
NASA/JPL-Caltech

Comets are dull, most of the time. In the outer reaches of the solar system they are frozen, inert lumps of rock and ice. It’s only when they swing in closer that they start to warm up in the sunlight.

When they thaw a little, some of the ice vaporises and the vapor and associated dust is lit up by the charged particles of the solar wind to form the bright, streaming tail – a cloud of plasma, or ionised gas – that you probably think of when you think of a comet.

While this general process has been understood for some time, a paper to be published in Physical Review Letters this week sheds new light on the complex details.

The four interacting fluids, along with the directions of the solar wind (Vsw) and the interplanetary magnetic field (Bimf).
Deca et al.

Jan Deca of the University of Boulder, Colorado, and colleagues built a detailed computer model to simulate the comet as a dynamic interaction of four fluids: electrons from the comet, ions from the comet, electrons from the solar wind, and protons from the solar wind. (These are not what we conventionally think of as fluids, but they behave in similar ways.)

The model was checked against data from the Rosetta mission to comet 67P/Churyumov-Gerasimenko, and found to accurately replicate features of the comet’s electron distribution. In news that will come as no surprise, the researchers conclude that “a detailed kinetic treatment of the electron dynamics is critical to fully capture the complex physics”.

The simulation showing gas and dust from the comet nucleus interacting with the solar wind.
Deca et al.

Michael Lucy is features editor of Cosmos.
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