On November 11, ESA's Rosetta spacecraft will put the Philae lander on comet 67P/Churyumov–Gerasimenko. The chosen primary landing site is seen above.
Comet 67/C-G has such weak gravity that Philae would simply bounce off when it hits the surface, were it not for the harpoons the lander will deploy to attach itself to the comet, while thrusters hold it firmly on the surface and its active legs orient the lander. Philae will drill into the comet and deliver soil samples for imaging and chemical analysis inside the lander. Instruments will cook the soil samples in ovens and examine evolved gases for complex organic molecules and isotope abundances. Other instruments will image the surface and obtain spectra, examine radio emissions from the orbiter after they have passed through the comet, study mechanical properties of the surface and dust, perform acoustic soundings, and study magnetic fields and the interaction of the comet with the solar wind.
COSAC (Cometary Sampling and Composition experiment) is the organic chemistry experiment, consisting of two ovens, a gas chromatograph and a mass spectrometer. While no one expects to find life on comets, comets are believed to have been the source of volatiles for Earth's oceans and for the chemical precursors of life. Material from 67/C-G drill samples is expected to be primordial -- unchanged from the time the Solar System formed. It may be rich in many of the kinds of organic molecules that have been detected in interstellar molecular clouds.
The COSAC instrument package, developed by the Max Plank Institute for Solar System Research.
More below the coma.
Immediately after touchdown, the First Science Sequence (FSS) of the lander will begin. It will last 2-3 days, and it is expected that it will receive the most press coverage, though the lander will continue operating for at least a week and possibly for many months. Each instrument will have at least one period of taking measurements during the FFS. Since one of the instruments (CONCERT) involves measuring radio waves from the orbiter after passing through the cometary nucleus, the orbiter must also fly to a suitable distance and orientation during this period. It will be a time of much tense activity.
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What else will be done during the first FSS block? CONCERT will start its first sensing of the cometary interior and the upper surface layers. This measurements needs to be phased properly, since the instrument units onboard the orbiter and onboard the lander need to have the same time reference and, of course, it is required that the nucleus is 'placed' in between the two instrument units. In practice, it means that ROSETTA has to fly an orbit that puts the nucleus in between orbiter and lander. And this has to happen when the orbiter tries to escape from the lander delivery orbit towards a safer distance from the nucleus; so, it is very demanding for the orbit and spacecraft engineering.
For the lander it means that it will lose the orbiter out of view after the time synchronization of the CONCERT units is done in order to get the cometary nucleus in place for the instrument sensing. The aim of the CONCERT measurements during FSS is to develop a first glance of the interior, i.e. is it homogeneous or more heterogeneous, nothing more. Greater details will only be possible through more measurement orbits of the instrument during the LTS phase.
The two gas analyzers onboard PHILAE, COSAC and PTOLEMY, will 'sniff' the local atmosphere around the lander for the C, N and O bearing compounds (PTOLEMY) and for evaporating organic material (COSAC). Naturally, they will also 'see' the evaporating water gas from the nucleus. ROMAP will follow the time evolution of the melange of solar wind and cometary plasma around the landing site through magnetic field and charged particle measurements and the MUPUS sensors in the heads of the Philae anchoring harpoons (shot into the ground for anchoring the spacecraft at touch-down on the surface) will measure the temperature evolution underneath the landing site surface for almost a full comet rotation. How deep the MUPUS sensors will be located is unpredictable – anything between a few cm to 2 ½ m - and will depend on the strength of the cometary surface which is currently unknown.
-- Hermann Böhnhardt, lead scientist for the Philae lander.