Someone asked me recently why the lander was called Philae. Despite knowing all the ins and outs of the landing operations and onboard instruments, I had to admit that the answer to this question escaped me! Thank goodness for Google – in case you didn’t know, apparently Philae is an Egyptian island in the river Nile. An obelisk was discovered at the Temple of Isis on the island in 1815 by explorer and egyptologist William Bankes, and the hieroglyphs and Greek inscriptions on the Philae obelisk helped 19th Century scholars unlock the secrets of the Rosetta Stone and the language of ancient Egypt. However, I digress … getting back to the 21st Century Philae lander residing on the alien surface of a comet half a billion kilometres away.
While Philae made its descent to the comet’s, the orbiters narrow-angle camera tracked the ‘fridge-sized’ landing craft as it headed for its first touch-down. Below shows an amazing time-tagged mosaic of images of this event.
The Philae power system comprises a primary (non-chargeable) battery system intended to give 60 hours of surface operations, a secondary (chargeable) battery system for extended surface operations and a primary power source in the form of body-mounted solar arrays to convert sunlight into electricity to charge the secondary batteries. However, due to the cold shaded place where Philae came to rest, there is insufficient sunlight to charge the batteries. The realisation of this situation after landing meant that the surface science teams had to work rapidly to devise a comprehensive plan to achieve as much as possible in the brief period before the primary battery expired – allowing some residual power, of course, to ensure that the results could be uplinked to the orbiting Rosetta spacecraft. Fortunately a great deal was achieved, and as the power levels dropped, Philae executed an automated process to put itself into hibernation. So the lander is not actually ‘dead’, and contrary to ‘popular opinion’ the lander mission may not be at an end. As the comet approaches closer to the Sun (closest approach between the orbits of Earth and Mars in August 2015), Philae’s temperature will rise, and solar intensity will increase by a factor of about 15. So there is a chance that we may hear from the surface of the comet again. Personally, I’m not sure how likely this is, but I probably wouldn’t bet on it – but it is possible.
In the meantime, the science teams have oodles of results to mull over in the coming weeks and years. Although some of these have been discussed in the media, many of them will require a great deal of work and further interpretation to provide a definitive view of the comet’s surface environment.
The results most highlighted by the media so far have been the claim that ‘carbon-based organic molecules’ have been detected independently by two onboard experiment packages. The COSAC instrument (COmetary SAmpling and Composition experiment) is designed to ‘sniff’ the comet’s thin atmosphere. If you’re into this stuff, COSAC is based upon a Gas Chromatograph/Mass Spectrometer (GCMS) system. The instrument’s principal investigators (PIs) could not definitively indentify exactly what molecules had been detected at the moment, and that they were ‘trying to interpret the results’. Another GCMS-based experiment called ‘Ptolemy’ later confirmed the COSAC finding, the PI commenting that “there is a rich signal there … there is clearly a lot of peaks (in the chromatograph’s trace). Sometimes a complicated compound can display a lot of peaks”. Ptolemy was programmed to ‘sniff’ the environment immediately after first touchdown, so presumably the observed surface impact cloud gave it plenty of raw material to work on.
Results concerning the nature of the surface were returned by an instrument called MUPUS (MUlti-PUrpose Sensors for surface and sub-surface science). This experiment deployed a hammer, which detected a dust layer about 10 to 20 cm deep overlying what is believed to be a hard surface of water ice. At the extremely low temperatures currently being endured by Philae, water ice is very hard – having characteristics similar to sandstone. Another key objective of MUPUS was to drill a sample of ‘soil’ and deliver this to COSAC’s oven for analysis – but unfortunately no such sample was delivered.
Reading all this back, I have to say the tone seems a little negative – but let me say that this is not intended. I think it’s amazing what has been achieved under very pressured and trying circumstances. And also it really is too early to understand the huge amount of information that Philae’s 60 hours of surface life has given us. It will take a while for it all to be assimilated, but I think there will some interesting and unexpected outcomes over the months to come. Congratulations to all on both the engineering and the science teams involved in the Philae landing mission!