Credit: DLR
"This shape is most surprising," says comet researcher Ekkehard K"uhrt from the German Aerospace Center (Deutsches Zentrum f"ur Luft- und Raumfahrt; DLR). For 30 years, the project leader has been studying these celestial bodies for scientific experiments on the probe and lander.
"But it is not unlikely. Comets were formed by the collision of small building blocks far away from the Sun during the emergence of the Solar System." The effect of the comet's unusual shape on the landing cannot be estimated yet.
Fused together 4.5 billion years ago
Rosetta and the Philae lander are currently less than 10,000 kilometres away from their destination. So far, Churyumov-Gerasimenko has been an enigma for scientists: only recordings from a distance, such as those from the Hubble Space Telescope or the first images of OSIRIS as well as the navigation camera provide clues of what the orbiter and lander will encounter. Credit: DLR
Previous observations estimated the comet to be about three by five kilometres in size. The fact that two clearly distinguishable parts make up Churyumov-Gerasimenko is a surprise. "The two blocks likely formed 4.5 billion years ago, collided at low speed, stuck to each other and have since been moving together," says K"uhrt.
"Scientifically, it is now of course very interesting to find out whether the two components differ in their composition." If the two parts are from different regions, their structure might also differ.
Waiting for details
In the coming months, as the spacecraft get closer to their target, the scientists will learn more about the comet. "For the landing, it is especially important to have a detailed view of the comet and understand how the two parts are connected," says Koen Geurts, an engineer at the Lander Control Centre at DLR in Cologne.
This information will be incorporated into the planning of the trajectory of Rosetta spacecraft - and its course and height in turn has an impact on the landing of Philae, as it is the first time that a lander touches down on a comet and performs in situ measurements. "So far, it looks as though there are large flat regions on the comet."
The location where the two parts are connected will likely not be considered as the landing site. In addition to being a suitable, reasonably flat terrain, the landing site should also have a day-night cycle so that the Philae lander can cool down out of the sunlight and so that scientific research can be carried out under different conditions.
Regular communication with the Rosetta spacecraft is necessary for the lander team to send the recorded data to Earth and empty the data storage. "These aspects are currently still hard to assess."
Rosetta is a European Space Agency mission with contributions from its Member States and NASA. Rosetta's Philae lander has been contributed by a consortium led by DLR, MPS, the French space agency, CNES (Centre National d'Etudes Spatiales), and the Italian space agency, ASI (Agenzia Spaziale Italiana).
The European Space Agency (ESA) Rosetta spacecraft and its lander Philae were around two million kilometers from their target comet in May, 67P/Churyumov-Gerasimenko when images acquired by the OSIRIS (Optical, Spectroscopic and Infrared Remote Imaging System) camera system could already show the comet awakening on its way towards the Sun, enveloped in a cloud of small dust particles.
Credit; DLR
Using these observations, the OSIRIS science team, which includes planetary researchers from the German Aerospace Center (Deutsches Zentrum f"ur Luft- und Raumfahrt; DLR), have been able to determine the comet's rotation period with additional precision - 12.4 hours. In August, Rosetta will arrive at the comet, and will deploy the Philae lander onto the comet's surface in November - the first ever landing on a comet.
The sequence of images acquired by the OSIRIS camera system between 24 March and 4 May, during the flight towards Churyumov-Gerasimenko, reveal the expansion of the comet's dusty veil, or coma. Although 67P is still more than 600 million kilometres from the Sun, its surface has already started to warm up, and, as a result, volatiles have begun to evaporate from its surface, carrying tiny dust particles with them. The images acquired in early May show that the comet's gas and dust cloud has reached approximately 1300 kilometres into space.
The comet nucleus has a diameter of about four kilometres. "It's beginning to look like a real comet," says Holger Sierks from the Max Planck Institute for Solar System Research (Max-Planck-Institut f"ur Sonnensystemforschung; MPS), the OSIRIS Principal Investigator.
This image, acquired by the OSIRIS camera system on 30 April 2014, shows Comet 67P/Churyumov-Gerasimenko. The target of the Rosetta comet mission has a dust cloud formed around it, which stretches up to 1300 kilometres into space.
Credit: DLR
Unveiling the unknown
The OSIRIS team also revealed a new fact. Until now, comet researchers believed the rotation period of Churyumov-Gerasimenko to be 12.7 hours. Through observation and analysis of the changes in brightness of the comet, the research team was able to determine that the comet only takes 12.4 hours to rotate around its axis.
"Knowing the exact rotation period of the comet is of vital importance, both for the optimal planning of the mission and scientific data gathering and for the interpretation of the acquired data," said Stefano Mottola, who studies comets at the DLR Institute of Planetary Research and is a member of the OSIRIS team.
Any information obtained during the approach to the comet also helps to plan the manoeuvres required for the Rosetta spacecraft to enter orbit around the comet and the landing by Philae. One thing is certain - with the awakening of the comet, Rosetta and Philae will be able to study its coma and tail. The first manoeuvre to prepare the probe for its rendezvous with Churyumov-Gerasimenko in August has already been carried out.
In May, the engineers and scientists were also able to breathe again - all the instruments on the spacecraft and lander have survived two and a half years of hibernation during their journey through space - they have all passed their 'Health Check'. Eleven instruments are being carried on board the Rosetta orbiter and ten scientific experiments on the lander. For the first time ever, measurements will be conducted directly on the surface of a comet.