In 2014 , the European Space Agency’s (ESA) Rosetta spacecraft made history when it rendezvoused with Comet 67P/Churyumov-Gerasimenko. This mission would be the first of its kind, where a spacecraft intercepted a comet, followed it as it orbited the Sun, and deployed a lander to its surface. For the next two years, the orbiter would study this comet in the hopes of revealing things about the history of the Solar System.
In this time, Rosetta’s science team also directed the orbiter to look for signs of the comet’s bow shock – the boundary that forms around objects as a result of interaction with solar wind. Contrary to what they thought, a recent study has revealed that Rosetta managed to detect signs of a bow shock around the comet in its early stages. This constitutes the first time in history that the formation of a bow shock has been witnessed in our Solar System.
As noted, bow shocks are the result of charged particles (plasma) emanating from the Sun (aka. solar wind) intercepting objects in its path. This process leads to the formation of a curved, stationary shock wave in front of the object. They are so named because when visualized, they resemble a bow and their behavior is similar to waves that form around the bow of a ship as it cuts through turbulent water.
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In addition to planets and larger bodies, bow shocks have been detected around comets. Over time, the interaction between the Sun’s plasma and an object can have an effect on the object itself, its bow shock, and the surrounding environment. Since comets are an excellent way to study plasma in the Solar System, the Rosetta team was hoping to detect a bow shock around Comet 67P and study it up close.
To accomplish this, Rosetta flew over 1500 km (932 mi) away from 67P’s center between 2014 and 2016 in search of large-scale boundaries around the comet. Unbeknownst to the mission team at the time, Rosetta actually flew directly through the bow shock several times, before and after the comet reached its closest point to the Sun along its orbit.
“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.”
The first detection took place on March 7th, 2015, when the comet was over 2 astronomical units (AUs) from the Sun – i.e. twice the distance between the Earth and the Sun. As the comet approached the Sun, Rosetta data showed signs of a bow shock beginning to form. The same indicators were detected on February 24th, 2016, when the comet was moving away from the Sun.
A clear indication that this was a bow shock in the early stages of formation was its shape. Compared to fully developed bow shocks observed around other comets, the boundary detected around Comet 67/P was asymmetric and wider than usual. As Charlotte Goetz, a researcher from the Institute for Geophysics and Extraterrestrial Physics who co-led the study, explained:
“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.”
To determine the properties of the bow shock, the research team explored data from the Rosetta Plasma Consortium – a suite of five different instruments designed to study the plasma environment surrounding Comet 67P. Combining this data with a plasma model, they were able to simulate the comet’s interactions with the solar wind.
What they found was that as the bow shock formed around Rosetta, its magnetic field became stronger and more turbulent. This was characterized by highly energetic charged particles being periodically produced and heated in the region of the bow shock itself. Prior to this, these particles had been moving more slowly and the solar wind was generally weaker.
This, they concluded, was the result of Rosetta being “upstream” of a bow shock when the first readings were obtained, then “downstream” when the second readings were obtained – which accorded with the comet approaching and receding from the Sun. As Matt Taylor, a ESA Rosetta Project Scientist, indicated:
“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.”
In addition to being an historic discovery, the detection of this bow shock in formation provided a unique opportunity to gather in-situ measurements of the Solar System’s plasma environment. Even though Rosetta ended its mission by impacting on the comet’s surface two years ago, scientists to continue to benefit from the data it gathered during the time it orbited Comet 67/P.