Comet's song

Okay. Forget the grand title. What we're talking about is a scientific phenomenon that also has audio implications and offers us the opportunity to discuss one of the most fascinating space missions of recent years.

Rosetta is a space mission developed by the European Space Agency and launched in 2004. The mission's ultimate goal is to study comet 67P/Churyumov-Gerasimenko, a periodic comet that returns every 6.45 Earth years.

The mission consists of two elements: the Rosetta probe itself and the Philae lander. The latter is intended to detach from the probe and land on the comet's nucleus, of which the probe has already sent back splendid photographs. The separation occurred today, November 12, and the lander is descending toward the nucleus (fingers crossed).

This is the audio part of the story. Obviously, the probe carries many instruments to study the comet up close. Five of these constitute the so-called Rosetta's Plasma Consortium (RPC) and are used to study the plasma environment surrounding the comet (plasma is a state of matter that appears as an electrically conductive gas that surrounds magnetic fields and electric currents).

Already in August, the RPC discovered that the comet's magnetic field oscillates with a frequency of around 40-50 millihertz. Now, any more or less periodic oscillation can be transformed into sound. In this case, the problem is that the frequency is too low to be audible. The lowest frequencies we can perceive are around 20 Hz, while here we're dealing with 0.05 Hz.

The problem can be solved by transposing (i.e., speeding up) the whole thing by about 10,000 times. So 0.05 Hz becomes 500 Hz, which is perfectly audible, and the resulting sound is this:

Here's an image of the comet's nucleus (the tiny 747 you see in the center isn't alien; it's there for comparison and gives you an idea of the object's size; click the image to enlarge)

A final note. Before you rave mystically about the comet's song, realize that any periodic phenomenon can be converted into sound. For example, the Earth's rotation is periodic. The average solar day lasts approximately 24 hours, or 86,400 seconds. Consequently, since frequency is the inverse of the period, it has a frequency of 0.000011574 Hz, too low to hear, but transposing it 24 octaves gives approximately 194.18 Hz, which corresponds to a perfectly audible note: a slightly flat G (G2 = 196 Hz with A3 = 440 Hz).

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