Rosetta has detected a powerful jet of activity emitted from the comet 67P/Churyumov–Gerasimenko (67P). The force of the outburst, which is believed to be travelling at 10 m per sec (32 ft per sec), was strong enough to temporarily repel the solar wind – a constant stream of charged particles emanating from the Sun, that work to convey our star's magnetic field across the solar system.
Both comet and spacecraft made their closest approach with the Sun, otherwise known as perihelion at 02:03 AM GMT. The Rosetta science team expected extreme comet activity in the weeks following perihelion, and note that outgassing activity would be unpredictable even before, but the sheer power of the recent activity display appears to have caught the team by surprise.
"This is the brightest jet we’ve seen so far," states OSIRIS team member Carsten Güttler, of the Max Planck Institute for Solar System Research, Göttingen, Germany. "Usually, the jets are quite faint compared to the nucleus and we need to stretch the contrast of the images to make them visible – but this one is brighter than the nucleus."
Rosetta, which is in the process of transitioning to a safer orbit of 300 km (186 miles), was able to detect the outburst from a distance of 186 km (116 miles) with a number of its onboard instruments. While being showered in the ejected dust particles, ROSINA, the element of the probe's scientific suit designed to map the comet's atmosphere and ionosphere, recorded significant structural and compositional deviations in 67P's coma.
The orbiter's OSIRIS camera captured a sequence of images of 67P during the episode, first capturing the jet at 13:24GMT, with the outgassing subsequently appearing to subside a mere 18 minutes later. However, whilst the jet itself may have been short-lived, its effects lingered for some time after, with Rosetta's GIADA instrument recording 30 dust hits on the spacecraft per day, a significant increase from the standard 1-3 per day usually recorded.
As the event unfolded, the gas and dust being expelled from the comet pushed back the solar wind. Due to the fact that the comet itself is not magnetized, Rosetta was able to make detailed studies of how this solar wind interacted with 67P using its onboard Plasma Consortium Magnetometer, as any magnetic field readings detected could only be attributed to the solar wind.
"The solar wind magnetic field starts to pile up, like a traffic jam, and eventually stops moving towards the comet nucleus, creating a magnetic field-free region on the Sun-facing side of the comet called a 'diamagnetic cavity'," explains Charlotte Götz, science team member for Rosetta's magnetometer instrument at the Institute for Geophysics and extraterrestrial Physics, Braunschweig, Germany.
The effect only lasted for a few minutes, but managed to push the solar wind back as far as 186 km (116 miles) from 67P's nucleus, with the event providing a valuable insight into the power of the comet's activity as it nears perihelion.
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