Space scientists will be on the edge of their seats this Saturday as they wait for the first images to be received back from comet-chasing spacecraft Rosetta's rendezvous with her second asteroid, (21) Lutetia on 10 July.

As relics originating from billions of years ago, comets and asteroids offer a tremendous opportunity to effectively look back in time and elucidate more on the origins of the Solar System as a whole. To that end, Rosetta embarked on a 10 year rock tour back in March 2004 with three planned stops along the way. Her first, asteroid (2867) Steins was achieved in September 2008, and earned Rosetta the accolade of being the first spacecraft to study an E-type asteroid up close. Her cameras were able to detect the first occurrence of the “YORP” effect on a main body asteroid such as this – a phenomenon whereby adsorbed light energy from the Sun is re-emitted from a body as infrared radiation, which reduces the momentum and thus alters the rotation rate of the asteroid. In the case of Steins, the resulting high spin rate likely caused material to converge around the midriff of the asteroid, giving the asteroid its distinctive conical shape.

Now it is time for Rosetta to get up close and personal with the much larger, much more intriguing Lutetia on 10 July. To take it all in, Rosetta will fly by at a separation of 3,200 kilometres, allowing approximately two hours of high quality imaging to be achieved, which will be immediately piped back to Earth. The time of closest approach is currently estimated at 16:45 GMT, and the spacecraft's accompanying lander, Philae, will be switched on earlier that day in order to collect data throughout the flyby.

The ultimate questions of the Lutetia rendezvous are ascertaining the size, shape and composition of the asteroid. Initially estimated at 95 kilometres in diameter and with a mild elliptical shape, ground images now appear to suggest a larger diameter of 134 kilometres and a much more pronounced elongation. There is further confusion on the classification of the asteroid: most measurements thus far have pointed towards Lutetia being a C-type asteroid, i.e. carbon-rich, but there are some findings suggesting it may be a metallic M-type. Crucially, metallic asteroids are generally thought to be smaller fragments originating from the shattered core of larger asteroids, thus there would not be any surface metal expected on such a large, carbon-based rock. If Lutetia does indeed contain large amounts of metal, the findings could spark a move to rethink many of the classification criterion used currently. Rosetta will be able to shed much more light on these questions, and will act as a powerful tool in understanding how to interpret ground based data in the future.

After Lutetia, the chief target of the Rosetta mission is still four years away. In May 2014 she will rendezvous with the comet 67P/Churyumov-Gerasimenko at a period of closest proximity to the Sun. Upon reaching her destination, the Rosetta orbiter will gather data on the composition and characteristics of the comet as well as studying the gas and dust ejections from the surface during the solar approach. Even more impressive, Philae will attempt the first ever soft landing on the nuclear surface of a comet, allowing a direct, hands on acquisition of data.

Since launching in 2004, Rosetta has travelled five billion kilometres. You can keep up to date with Rosetta’s travels on the following blog: http://webservices.esa.int/blog/blog/5/.

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