Comet gives birth to baby bow shock

Artist’s concept of Rosetta spacecraft spying a bow shock – similar to the wave that forms at the bow of a ship – for the comet it orbited beginning in 2014. It saw the shock both before and after the comet’s closest point to the sun in 2015. Image via ESA

Data from the Rosetta mission to comet 67P/Churyumov-Gerasimenko – which took place from 2014 to 2016 – are still being studied. A new study shows that, contrary to what scientists first believed, Rosetta did detect signs of an infant bow shock – a result of the interaction between the solar wind and the comet’s outer atmosphere – which is expected for comets, and had been seen before, but had never been seen before at such an early stage of formation, anywhere in the solar system.

Rosetta arrived at comet 67P/Churyumov-Gerasimenko on August 6, 2014, after a 10-year journey through space. The spacecraft eventually orbited the comet as closely as 6 to 19 miles (10 to 30 kilometers). As it turns out, the craft flew directly through the comet’s bow shock several times both before and after 67P reached its closest point to the sun – its perihelion – a year into the orbital phase of the mission, on August 13, 2015.

Rosetta first encountered the bow shock on March 7, 2015, when the comet was inbound toward the sun, but still over twice as far from the sun as Earth’s orbit. Rosetta data from that time indicate signs of a bow shock beginning to form. The same indicators were present after the comet and the craft had rounded the sun, and were on their way back outward, on February 24, 2016.

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Material jetting from comet 67P/Churyumov-Gerasimenko on July 7, 2015, about a month before the comet swept closest to the sun. Image via ESA/ Rosetta/ NAVCAM.

Rosetta was the first spacecraft to so closely examine a comet both before and after its perihelion. A statement from ESA explained more about how cometary bow shocks form and about what Rosetta saw:

Comets offer scientists an extraordinary way to study the plasma in the solar system. Plasma is a hot, gaseous state of matter comprising charged particles, and is found in the solar system in the form of the solar wind: a constant stream of particles flooding out from our star into space.

As the supersonic solar wind flows past objects in its path, such as planets or smaller bodies, it first hits a boundary known as a bow shock. As the name suggests, this phenomenon is somewhat like the wave that forms around the bow of a ship as it cuts through choppy water.

Bow shocks have been found around comets, too – Halley’s comet being a good example. Plasma phenomena vary as the medium interacts with the surrounding environment, changing the size, shape, and nature of structures such as bow shocks over time.

Rosetta [had] looked for signs of such a feature over its two-year mission, and ventured over 1,500 kilometers [about 1,000 miles] away from 67P’s center on the hunt for large-scale boundaries around the comet – but apparently found nothing.

But that was before the new analysis of the data. Herbert Gunell of the Royal Belgian Institute for Space Aeronomy, Belgium, and Umeå University, Sweden, one of the two scientists who led the new study, said:

We looked for a classical bow shock in the kind of area we’d expect to find one, far away from the comet’s nucleus, but didn’t find any, so we originally reached the conclusion that Rosetta had failed to spot any kind of shock.

However, it seems that the spacecraft actually did find a bow shock, but that it was in its infancy. In a new analysis of the data, we eventually spotted it around 50 times closer to the comet’s nucleus than anticipated in the case of 67P. It also moved in ways we didn’t expect, which is why we initially missed it.

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Artist’s concept of a bow shock taking shape at comet 67P/Churyumov-Gerasimenko, with the Rosetta spacecraft looking on. Image via ESA.

The infant bow shock for comet 67P/Churyumov-Gerasimenko was observed to be asymmetric. It was also wider than the fully developed bow shocks observed at other comets. Charlotte Goetz of the Institute for Geophysics and Extraterrestrial Physics in Braunschweig, Germany – a co-leader on the new study – said:

Such an early phase of the development of a bow shock around a comet had never been captured before Rosetta.

The infant shock we spotted in the 2015 data will have later evolved to become a fully developed bow shock as the comet approached the sun and became more active – we didn’t see this in the Rosetta data, though, as the spacecraft was too close to 67P at that time to detect the ‘adult’ shock. When Rosetta spotted it again, in 2016, the comet was on its way back out from the sun, so the shock we saw was in the same state but ‘unforming’ rather than forming.

The statement from ESA continued:

Herbert, Charlotte, and colleagues explored data from the Rosetta Plasma Consortium, a suite of instruments comprising five different sensors to study the plasma surrounding 67P. They combined the data with a plasma model to simulate the comet’s interactions with the solar wind and determine the properties of the bow shock.

The scientists found that, when the forming bow shock washed over Rosetta, the comet’s magnetic field became stronger and more turbulent, with bursts of highly energetic charged particles being produced and heated in the region of the shock itself. Beforehand, particles had been slower-moving, and the solar wind had been generally weaker – indicating that Rosetta had been ‘upstream’ of a bow shock.

Matt Taylor, ESA Rosetta Project Scientist, said:

These observations are the first of a bow shock before it fully forms, and are unique in being gathered on-location at the comet and shock itself.

This finding also highlights the strength of combining multi-instrument measurements and simulations. It may not be possible to solve a puzzle using one dataset, but when you bring together multiple clues, as in this study, the picture can become clearer and offer real insight into the complex dynamics of our solar system – and the objects in it, like 67P.

Bottom line: Rosetta spacecraft data reveals an infant bow shock – similar to what forms at the bow of a ship – at the comet this craft explored for 2 years. It’s the 1st seen forming anywhere in our solar system.


December 19, 2018

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