The European Space Agency’s (ESA) Rosetta spacecraft was launched in March 2004 and is currently en route to the comet Churyumov-Gerasimenko. Rosetta is due to rendezvous with the comet in two years time, and, if everything goes according to plan, will enter into orbit around it. A lander – named Philae, after the island in the Nile where an obelisk was found which helped decode the Rosetta stone – is intended to alight on the surface of the comet's nucleus in order to obtain in situ
measurements on its composition and physical properties.
Speaking at a seminar as part of the Euroscience Open Forum (ESOF) 2012, Project Scientist on the Rosetta mission, Rita Schulz explained why this is such a groundbreaking operation and what it is hoped will be achieved when Rosetta wakes from its three year hibernation in June 2014 to meet Churyumov-Gerasimenko.
"This is the first mission ever to orbit a comet nucleus and to deploy a lander onto its surface," Schulz stated. "It will enable us to study the evolution of the comet as it approaches the sun and the lander carries a full complement of instruments for in situ
chemical analysis. This will help us understand how comets in the early solar system contributed to its formation and how they influenced the Earth. It has been suspected for a long time that comets brought a lot of material – water and also organic matter – to Earth, and so they may have been crucial to the origins of life and formation of oceans on our planet."
Churyumov-Gerasimenko belongs to the so-called Jupiter family of comets as its abhelion, the point in its orbit when it is furthest from the Sun, is in Jupiter’s orbit. Relatively little is known about the activity and evolution of comets as those which are not in an elliptic orbit are practically impossible to follow; monitoring over an extended period is necessary for accurate observations of the changes undergone during different stages of a comet’s circuit around the Sun. Schulz explained that researchers are interested in the composition and activity of comets because their unique features allow us a glimpse into our past.
"They are small bodies which are believed not to have changed much since the formation of the solar system because they stayed in the cold for a long time and are too small for gravity to change their interior structure. This is why we believe there is primordial information contained in comets that can give us an insight into the formation of the planets."
An example of how tracking comets is more useful than recording fly-by events such as the passage of Halley’s comet are the results recorded on the occasion of Hale-Bopp’s most recent perihelion, which is the point in its orbit at which a celestial body passes closest to the Sun.
"Hale-Bopp had its perihelion in 1995 and we were able to track and measure it on its way," said Schulz. "We were able to get the production rates of certain gas species and also of dust, as a function of heliocentric distance. The findings showed the levels of OH, which is representative of water in a comet, and carbon monoxide changing as the comet came closer to the Sun and as it moved away again."
Since its launch in 2004, Rosetta has required a number of gravity assists; three at the Earth, one at Mars and one each at the asteroids Steins and Lutetia. No rocket on Earth has enough power to send Rosetta into orbit with a comet, hence the need to utilise angular momentum from these bodies. In 2014, the comet rendezvous manoeuvre will take place and Rosetta will go into orbit around Churyumov-Gerasimenko.
"The lander will be delivered in November 2014, and the comet will have its perihelion passage in August 2015," Schulz outlined. "The nominal end of the mission is December 2015, but we could go on for another half a year until the comet goes back into the outer solar system and it is too cold to function."
The 11 payload instruments on board the main craft and an additional ten on the lander will provide data on the physical and chemical make-up of the comet and on its electrical and magnetic properties. Even if the landing is not an unmitigated success (ie Philae does not land perfectly upright on a flat surface) a number of the instruments will still be able to carry out their functions.
The day when Rosetta awakes from the hibernation it went into on 8 June 2011 is eagerly awaited by researchers from ESA and elsewhere in the science community. The hibernation was induced because the craft is so far away from the Sun now that its solar panels cannot sustain the necessary power supply, but Rosetta will wake again automatically at 10 am on 20 January 2014.
"Rosetta will provide a lot of information, particularly on the nucleus of this comet," Schulz concluded. "From this we can learn about the physical and chemical processes of comet activity, a topic which we do not as yet know much about."