ESA’s Rosetta orbiter yesterday witnessed 67P/Churyumov-Gerasimenko making its closest approach to the Sun. The comet and the spacecraft were 186 million km from the Sun, about a third of the distance at rendezvous last August.
This Rosetta image was taken just one hour before 67P/Churyumov-Gerasimenko reached perihelion, the closest point to the Sun along its 6.5-year orbit. The image was taken around 327 km from the comet. Image credit: ESA / Rosetta / NAVCAM / CC BY-SA IGO 3.0.
In the year that has passed since Rosetta arrived, the comet has traveled some 750 million km along its orbit towards the Sun, the increasing solar radiation heating up the nucleus and causing its frozen ices to escape as gas and stream out into space at an ever greater rate. These gases, and the dust particles that they drag along, build up the comet’s atmosphere and tail.
The activity reaches its peak intensity around perihelion and in the weeks that follow – and is clearly visible in the spectacular images returned by the spacecraft in the last months.
One image taken by Rosetta was acquired around an hour before the moment of perihelion, from a distance of 327 km.
“Activity will remain high like this for many weeks, and we’re certainly looking forward to seeing how many more jets and outburst events we catch in the act, as we have already witnessed in the last few weeks,” said Dr Nicolas Altobelli from the European Space Astronomy Center, a scientist for the Rosetta project.
This series of images of 67P/Churyumov–Gerasimenko was collected between 2014 and 2015 using the Gemini Observatory. All images are centered on the comet and were obtained by stacking several short exposures; stars are also visible as streaks. In the first image of the upper row, taken on 20 September 2014, the comet appears nearly point-like. At the time, it was around 500 million km from the Sun. In the second image, taken on 14 November 2014 – two days after the landing of Philae – the comet exhibits a faint tail to the upper left; it was about 450 million km from the Sun at the time. The comet was observed again in the summer, as shown in the third image, taken on 30 June 2015, when the comet was around 200 million km from the Sun; the tail is visible towards the right. As the comet moved towards northern latitudes, it became visible from the northern hemisphere in July 2015. The fourth image, taken on 4 August 2015, shows a bright tail. The comet was around 186 million km from the Sun and about to reach perihelion on 13 August. The two images in the lower row show a zoomed version of the 4 August image. On the left, the original image is shown (visualized in a color scale) with the bright comet tail extending towards the right. On the right, the image was processed by subtracting a radially symmetric component from the coma to enhance possible anisotropies in its shape. While the bulk of the tail points to the right, there is a notable high-density region pointing in a different direction, to the lower left. The comet nucleus is too faint to be seen, lying within the innermost pixel of this image. The upper frames measure 100 000 km across and the lower frames 50 000 km across (at the distance of the comet). Image credit: Gemini Observatory / AURA.
Rosetta’s measurements suggest the comet is spewing up to 300 kg of water vapor per second. Then, it recorded an outflow rate of just 300 g per second.
Along with gas, the nucleus is also estimated to be shedding up to 1,000 kg of dust per second.
“In recent days, we have been forced to move even further away from the comet. We’re currently at a distance of between 325 km and 340 km this week, in a region where Rosetta’s startrackers can operate without being confused by excessive dust levels – without them working properly, Rosetta can’t position itself in space,” said Dr Sylvain Lodiot of ESA.
The comet’s average temperature has also been on the increase.
Not long after arriving, surface temperatures of minus 70 degrees Celsius were recorded. By April 2015, this had risen to only a few degrees below zero Celsius, and now highs of a few tens of degrees above zero are forecast for the next month.
This is a projected map of 67P/ChuryumovGerasimenko using data from Rosetta’s OSIRIS camera. It is based on a cylindrical projection, a procedure that is routinely used to chart the surface of planets, moons and other spherical celestial bodies. The intricate appearance is caused by the unusual, double-lobed shape of this comet: the small lobe is shown at the centre of the map, while portions of the large lobe are shown to the left and right. The blue points refer to possible origins, on the surface of the comet, of some of the dust jets recently observed by OSIRIS. The yellow line indicates the sub-solar line, which traces out the points on the surface where the Sun is directly overhead. Image credit: ESA / Rosetta / MPS / OSIRIS Team / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA.
Meanwhile, astronomers back on Earth have been following the comet’s evolution from afar.
Rosetta is far too close to the comet to see its growing tail, but images collected over the past few months with ground-based telescopes show that it already extends more than 120,000 km.
A lop-sided coma, with a notable high-density region away from the main tail, was revealed in various images.
“Combining these big-picture views from ground-based telescopes with Rosetta’s close-up study of individual jets and outbursts will help us to understand the processes at work on the comet’s surface as it approaches the Sun,” Dr Altobelli said.
