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Rosetta team find the source of their comet's song

Paul Sutherland, Feature writer
Aug 22, 2015, 2:06 UTC

Sen—We are used to seeing remarkable images of Rosetta’s comet, 67P/Churyumov-Gerasimenko. But on top of the raft of other information garnered since the space probe’s arrival was the surprise discovery that the comet can sing!

The cosmic chorus is at frequencies usually beyond the hearing range of the human ear, but was converted to an audio track that we can listen to by ESA’s versatile team of experts. When they did so, they were still not sure why the comet was singing at all.

But now the cause of the celestial sounds has been found by the Rosetta Plasma Consortium (RPC) that recorded them following the probe's arrival last year. They confirm that the melody emitted by the comet is actually from oscillations in the magnetic field that surrounds its nucleus. For although 67P has no magnetic field of its own, a stream of electrically charged gas, or plasma, from the Sun, called the solar wind, interacts with the atmosphere of gas and dust around the comet and magnetises it.

Rosetta first noticed the singing—large-amplitude fluctuations in the magnetic field—when the spacecraft arrived at the comet in August, 2014. They detected around 3,000 examples of wave activity with frequencies of about 40 millihertz. The broadcasts came as a surprise because much weaker wave activity had been measured by spacecraft that flew past other comets, including Giacobini-Zinner and Halley.

The team concluded that there had to be a different mechanism producing 67P’s “music”. The data was collected when the comet was still 400-540 million km (250-335 million miles) from the Sun, and its activity was still low, compared to the fizzing jets seen now around perihelion. 

From the start, the scientists suspected the “singing” was linked to the comet’s activity, even if low, and the release of neutral particles into space. They realised that ultraviolet radiation from the Sun was ionizing these atoms and molecules. As they crossed the magnetic field, they formed an unstable electric current which produced the sounds. A paper explaining the phenomenon has been published in the journal Annales Geophysicae.

RPC principal investigator Karl-Heinz Glassmeier, Head of Space Physics and Space Sensorics at the Technische Universität Braunschweig, Germany, told ESA’s Rosetta blog: “The physical process is somewhat difficult to understand without a deeper understanding of plasma physics, but we can use a simple analogy to have a better idea of what’s going on. 

“Consider your garden hose. If you start the water flow, there is a chance that the hose starts to oscillate, generating waves. This is about what happens in the plasma. Of course, the flow we have in the cometary situation is not like water, but is a flow of charged particles. But somehow the analogy is suitable.”

For the moment, 67P’s song appears to be over. Or rather, since February, the meteoroid melody has been drowned out by the increasing levels of other activity. Professor Glassmeier explained: “Around this time, the activity is changing, other features show up, the plasma interaction region becomes much more violent. Singing comet waves are still present, but buried under a variety of other features we are currently trying to understand.”

Rosetta’s chief scientist Matt Taylor, a space plasma expert, told Sen today: “The magnetometer result is a fascinating revelation of how the solar wind interacts with the cometary coma, the marvel of which was further exemplified by the recent outburst which completely blew the solar wind away from the comet!

“The early interaction signal indicated the birth of the comet-solar wind interaction , just as it started to kick off and gives us a wonderful insight into the complexity of this interaction, where current sheets are formed and rapidly evolve. The sonification of the signal is a wonderful way to represent the data for those strange people who, unlike me, don’t get excited by periodic squiggly lines. :)”

Since Dr Taylor is also famously a rock fan—he picked up the Spirit award at the Metal Hammer Golden Gods event at London’s IndigO2 arena in June—we asked him if he was disappointed that 67P had been singing aetherial sounds rather than something more like a burst of death metal. He replied: “A massive number of people have  downloaded the sound and it formed the theme tune for The Sky at Night’s comet-landing special. One guy on YouTube had it mixed in with the Beatles’ Across The Universe, and someone else did something more dub step. For me, it would be awesome if it was tuned to an E flat to be a bit heavier. :)”