67P - how much is a comet worth

I've been trying to learn a bit more about comets (call it summer-research) taking the chance of the visit of the ESA Rosetta mission to comet 67P (aka Churyumov–Gerasimenko).

Comets are small bodies of rock and ice thought to form in the outer regions of the solar system at the same time planets were formed, ca. 4.6 billion years ago. An important known unknown about comets is their relative contribution to the accumulation of water in the early Earth. So learning about them is learning about our planet too.

67P is a 4 km-long ice body orbiting around the sun every 6 years, following an elliptical orbit ranging between those of the Earth and Jupiter.
Barcelona and 67P, to scale
The shape of 67P suggests that it might be the result of the accretion of smaller comets. In fact, one thing that surprises many of us who are unfamiliar with comets is their low density. Most of the comet you see in these pictures has been left empty during its formation. 67P is about 10 2.5 times lighter than water: 102 400 kg/m3 (figures updated after Rosetta's approach), implying that it is a very porous body. Is this related to an accretion process?
Image taken on 2014-08-12 from a distance of 103 km. Credit: ESA/Rosetta/NAVCAM

Another curious fact: 67P used to have a perihelion distance of 2.7 AU (1 AU = distance from the Sun to the Earth), but in February 1959 an approach to Jupiter reduced this to only 1.3 AU, where it remains today. Comets are often shifted by the gravity field of planets, but recent events like this remind us that we are not in a static Solar System. The same process can lead to the split of comets in pieces: a beautiful example is given by the comets 42P/Neujmin and 53P/Van Biesbroeck, which appear to be fragments of a parent comet. This is based on computer integration, a reconstruction of their past position, showing that both comets were close to Jupiter in January 1850 and had nearly identical orbits before that. The debris produced by such comet disintegrations is often responsible for meteor showers like the Perseids seen worldwide in middle August.
Approach to a distance of 104 km.
67P rotates once every 12.7 hours.

Rosetta's won't be the first mission actually touching down on a comet (check this list of space missions that have approached comets, and see the unsubtle 'landing' of Deep Impact in the animation below). But it is the first mission ever to smoothly land on a comet (Philae lander) and to analyze its surface. And it is the first mission to orbit a comet, something remarkable since the escape velocity of 67P is only 0.5 m/s. It will also be the first mission to land a probe on the surface and, in the words of ESA, Rosetta will be "the first spacecraft to fly alongside a comet as it heads towards the inner Solar System, watching how a frozen comet is transformed by the warmth of the Sun". A lander will sample the composition and structure of the comet nucleus, drilling more than 20 cm into the subsurface for analysis at the onboard laboratory. 

Rosetta has costed the europeans around 1 billion euros (10^9 €) through a consortium of the German Aerospace Research Institute (DLR) with ESA, CNES, and european and american research institutes. The results will provide information on how comets form and also on the early stages of the Solar System. It should contribute to the discussion on where did the terrestrial water form and when did it arrive here. Previous studies have shown that the isotope ratios of hydrogen in other comets is different from that of oceanic water, but it remains unclear that these comets were representative enough of the comet orbits most likely to contribute to our waters. New answers will arrive soon, together with new questions. 

Deep Impact colliding with comet Temple 1 in 2005

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