Lumpy, Bumpy, Fluffy and Layered
Observational and theoretical studies of Comet 67P/Churyumov-Gerasimenko, the target of ESA’s Rosetta mission, are building a detailed portrait of the comet’s nucleus as it travels around the Sun.
Observations of the comet using the 8.2 m-ESO Very Large Telescope (VLT) show an irregularly-shaped object that is about 4.6 kilometres in diameter with a rotational period of 12 hours 49 minutes. Cecilia Tubiana presented results at the second European Planetary Science Congress (EPSC) in Potsdam on Tuesday 21st August, and said, "These observations were taken when the comet was approaching the furthest point from the Sun in its orbit. Rosetta will rendezvous with the comet in 2014 at a distance of about 600 million kilometres from the Sun. While a quite detailed portrait of the comet at small heliocentric distance has been drawn, a profound description of Rosetta’s target comet at large heliocentric distance is missing."
A team of scientists, led by the Max Planck Institute for Solar System Research, observed the comet’s nucleus in June 2004, May and August 2006 and July 2007, when the comet was at least 680 million kilometres from the Sun. Surprisingly, although the comet was not active, they found that a faint dust trail is visible in the images of the comet, extending more than 500,000 km along the comet’s orbital path. Tubiana said, "We believe that this dust trail is composed of large grains that the comet shed over the many times it has travelled along this path."
Later at the EPSC, Jeremie Lasue, of the Service d’aeronomie in France, presented results of numerical studies that describe how a comet’s nucleus changes as it travels along its orbital path. Lasue explained, "Comets constantly evolve by ejecting material as their distance from the Sun changes and their temperature increases or falls. To land on a comet’s nucleus, you need to have a good idea of its structure, density and tensile strength. Comet 67P/Churyumov-Gerasimenko most probably has an irregular comet nucleus with crater-like depressions on its surface. Our team has developed a three-dimensional model of the internal processes in the nucleus, allowing us to predict the thermal evolution and surface activity as the comet moves along its orbit"
Recent mission results suggest that a comet’s structure is highly stratified. Lasue said, "Stardust showed that the dust ejected from the outer layers is composed of fluffy particles that can be relatively large. These particles are rich in silicates and organics, which are the building blocks of life. Our simulations, for the first time, take into account the relationship between the impact history of the comet and the forces holding the comet’s constituents together. This technique has enabled us to reproduce and interpret the amazing layered structure and surface features that Deep Impact observed at comet 9P/Tempel 1. This is a new means to quantify the tensile strength of comet nuclei, which gives us vital information in preparing for Rosetta’s rendezvous with 67P/Churyumov-Gerasimenko."
The teams of scientists from France and Italy in which Lasue works, are developing these numerical tools to support two of Rosetta’s instruments: VIRTIS, which will determine the composition of the ices in the comet’s nucleus as well as emitted gases and dust, and CONSERT, which will investigate the deep interior of the nucleus with radio waves.
ROSETTA, the cornerstone mission of ESA’s Horizon 2000+ program, aims for the thorough in-situ exploration of a cometary nucleus, one of the most primitive and primordial bodies from the formation period of the solar system. ROSETTA will be the first spacecraft to orbit a comet’s nucleus and to land on it. What makes this mission extraordinary is that the spacecraft will follow the comet in its journey towards the sun, examining how a frozen comet is transformed by solar heating and space weathering.
In 2003, due to a delay in the launch of the ROSETTA spacecraft, the target for the mission was changed from comet 46P/Wirtanen to the Jupiter family comet (JFC) 67P/Churyumov-Gerasimenko. Very little was known at that time about this comet and the physical properties of its nucleus. Finally, the spacecraft was launched in March 2004, with 20 experiments on-board including a lander probe with a nucleus surface science package. It will rendezvous with the comet in 2014 at about 4.1 AU (1AU = 149 597 870.691 kilometers) from the Sun. 67P/ Churyumov-Gerasimenko orbits around the Sun with a period of about 6.6 years; its perihelion is between the orbit of Mars and the Earth and the aphelion beyond the orbit of Jupiter.