For weeks, comet Churyumov-Gerasimenko has been active, hurling dust and gas into space – but it will not reach the closest point to the Sun in its orbit, the perihelion, until 13 August 2015 at exactly 4:03 CEST. It will take another six-and-a-half years to get this close to the Sun once again. "Even though the comet is as the shortest distance from the Sun, they are still separated by roughly 185 million kilometres; the comet finds itself in between the orbits of Earth and Mars," says Ekkehard Kührt, a researcher at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR). Earth, for example, is only about 150 million kilometres from the Sun. In spite of this, it is hotter on the comet than on Earth due to the lack of an atmosphere: "The maximum temperature at 67P, which would currently be measured in the southern latitudes due to seasonal constraints, is around 80 degrees Celsius." In comparison, the heat record on Earth is a mere 56.7 degrees in Death Valley (United States). And even if 67P is vigorously losing mass, the Sun's gravitational pull is too weak at this distance to tear the comet apart as has often happened in the past with other comets that have come too close to the Sun or Jupiter.
Less mass, more gas
Roughly 100 kilograms of the comet’s mass are disappearing into space per second. In its approach to the Sun, the comet’s frozen ice has been heating up and turned into gas, which has dragged along some dust with it as it spills out into space. On 29 July 2015, the instruments on board the ESA Rosetta spacecraft succeeded in imaging and analysing the most powerful outburst yet, from a distance of 186 kilometres. Images acquired with the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS), taken at 18-minute intervals, show a 'Jet', a gas outburst, in which comet material is being hurled into space at high speeds.
The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) recorded a change in the composition of the coma, the dust and gas enveloping the comet: compared to measurements taken two days earlier, shortly after the gas outburst, the scientists discovered twice the amount of carbon dioxide, four times the amount of methane and a sevenfold increase in the amount of hydrogen sulphide, whereas the vapour production remained almost constant. Fourteen hours after the outburst, about 30 dust grains a day started to drizzle on the Grain Impact Analyser and Dust Accumulator (GIADA)– 10 times the amount of particles in comparison with measurements taken in early July 2015. This quantity continued to increase, and on 1 August 2015 no less than 70 dust grains hit the detector in four hours. This powerful outburst was so strong that it even pushed away the solar wind's magnetic field from around the nucleus for a few minutes.
At a safe distance from Churyumov-Gerasimenko
"The activity of the comet will likely increase slightly in the days after perihelion," says Kührt. At least, this is what has been observed in previous perihelion passages of 67P and of many other comets. "We are now excited to see how it will evolve in the coming days and weeks. The activity will depend mainly on where the active areas are with respect to the comet's seasonal cycle." When Rosetta arrived at 67P one year ago, the comet was more than 500 million kilometres away from the Sun and hardly active. "With the mission, we are for the first time accompanying a comet and monitoring its development for such a long period of time."
The active comet is making it difficult for the Rosetta spacecraft to fly any closer to the surface as the dust particles that are constantly being hurled into space confuse the star trackers and cause problems with the navigation. For this reason, at perihelion, Rosetta will be flying at a safe distance of approximately 300 kilometres from Churyumov-Gerasimenko. "For some instruments on the orbiter – for example, the magnetometer – that is the better option, but for others, such as OSIRIS, it would be nice to fly tighter." The comet's outgassing and its associated risks for Rosetta in this case warrant a safe distance.
Waiting for contact possibilities with Philae
The Philae lander is also on its way to perihelion on the comet's surface, although Rosetta's current trajectory is not favourable for establishing communications with it. "The orbiter, which is a kind of relay station to Philae for us, is flying over the southern hemisphere, which is particularly active," explains DLR engineer Koen Geurts, Technical Project Manager for the Philae lander. "From 11 August, Rosetta will once again be flying over latitudes where communication with Philae would be possible." However, the great distance between the orbiter and the comet could complicate communication with the lander. The last contact between Philae and the team at the DLR Lander Control Center in Cologne took place on 9 July 2015.
To make sure Philae could still carry out its job on the surface, even without communication with the ground team, the DLR engineers tested some commands on their ground model in Cologne. These commands were then sent "blind", in other words: without a response, to Philae. In the event that the lander receives these commands and executes them, it will initiate a sequence in which various instruments will be operated and the data stored until contact is resumed.
At least it will not be too warm for the lander during perihelion: at its landing site Abydos, Philae is in the shade and cool. "The lander and instruments can endure temperatures of at least 50 degrees Celsius," says Geurts. "And the temperature at Abydos will definitely not get higher than this."
Rosetta is an ESA mission with contributions from its Member States and NASA. Rosetta's Philae lander is contributed by a consortium led by DLR, MPS, CNES and ASI.