A dramatic short-lived outburst from 67P/Churyumov-Gerasimenko was captured by the OSIRIS narrow-angle camera aboard ESA’s Rosetta orbiter on July 29, 2015.
This image was taken from a distance of 186 km from the center of Comet 67P/Churyumov-Gerasimenko; the jet is estimated to have a minimum speed of 10 m/s and originates from a location on the comet’s neck, in the rugged Anuket region. Image credit: ESA / Rosetta / MPS for OSIRIS Team / MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA.
Now 67P/Churyumov-Gerasimenko and Rosetta are two days from perihelion, the point on the comet’s 6.5-year orbit that takes it closest to the Sun.
On 13 August 2015, they will be 186 million km from the Sun, about a third of the distance at rendezvous last August.
“The period around perihelion is scientifically very important, as the heat from the Sun and the resulting outflow of gas and dust build to a maximum, providing us with important insights into this key time in the overall life cycle of the comet,” explained Dr Nicolas Altobelli from the European Space Astronomy Center, acting Rosetta project scientist.
“For example, surface changes may reveal fresh material that has yet to be altered by solar radiation or cosmic rays, giving us a window into the comet’s subsurface layers – this will be the first time ever in cometary exploration that surface changes can be monitored in relation with increased activity.”
On 29 July, Rosetta observed a dramatic outburst, registered by several of its instruments from their vantage point 186 km from the comet.
Rosetta’s OSIRIS camera imaged the outburst erupting from the nucleus, other instruments witnessed a change in the structure and composition of the gaseous coma environment surrounding the spacecraft, and detected increased levels of dust impacts.
Perhaps most surprisingly, the spacecraft found that the outburst had pushed away the solar wind magnetic field from around the nucleus.
“This is the brightest jet we’ve seen so far. 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,” said Dr Carsten Güttler from the Max Planck Institute for Solar System Research in Göttingen, Germany, OSIRIS team member.
Soon afterwards, the ROSINA instrument detected clear indications of changes in the structure of the coma and recorded changes in the composition of outpouring gases.
For example, compared to measurements made two days earlier, the amount of carbon dioxide increased by a factor of two, methane by four, and hydrogen sulfide by seven, while the amount of water stayed almost constant.
“This first quick look at our measurements after the outburst is fascinating. We also see hints of heavy organic material after the outburst that might be related to the ejected dust,” said Dr Kathrin Altwegg from the University of Bern, Switzerland, ROSINA principal investigator.
The orbit of Comet 67P/Churyumov-Gerasimenko and its approximate location around perihelion, the closest the comet gets to the Sun. The positions of the planets are correct for August 13, 2015. Image credit: ESA.
“But while it is tempting to think that we are detecting material that may have been freed from beneath the comet’s surface, it is too early to say for certain that this is the case.”
Meanwhile, about 14 hours after the outburst, GIADA was detecting dust hits at rates of 30 per day, compared with just 1–3 per day earlier in July.
A peak of 70 hits was recorded in one 4-hour period on August 1, indicating that the outburst continued to have a significant effect on the dust environment for the following few days.
“It was not only the abundance of the particles, but also their speeds measured by GIADA that told us something ‘different’ was happening: the average particle speed increased from 8 m/s to about 20 m/s, with peaks at 30 m/s – it was quite a dust party,” said Dr Alessandra Rotundi from the University of Naples, Italy.
Perhaps the most striking result is that the outburst was so intense that it actually managed to push the solar wind away from the nucleus for a few minutes – a unique observation made by the Rosetta Plasma Consortium’s magnetometer.
