At 00:36 UTC today (15 November) Philae put itself into idle mode, and all communication was lost - potentially for a long time.
The Esa team reckoned yesterday that they have managed to complete & receive 80% of all the anticipated scientific work (yet more was received after that point). They will now wait for sufficient light to fall on Philae's solar panels to wake it back up. The batteries can work at a minimum of 0°C (freezing point for water), and it is very much colder than that at the moment. Therefore, waking back up is a 2-stage process:-
Esa does not know exactly where Philae ended up on the 4km-wide comet 67P, but it appeared to be in the shadow of a cliff & was receiving sunlight for only 1.5 hours / day. They were able to work out that it was resting on only 2 of the 3 feet, and sent it commands yesterday (Friday) to raise itself by 4cm and then rotate the main housing by 35% which--hopefully--will cause the largest solar panel to be better exposed to sunlight.
Before going silent Esa decided to go for broke & deployed all science instruments on board, including those that required mechanical movement (and thus would suck the most power). That included the sd2 drill, which delivers bits of the comet to the PTOLEMY and COSAC instruments inside the lander (those work out what the comet is made of). One specific feature here is to find if the comet contains an excess of left-handed amino acids. That is a touch long to explain, but most organic compounds have just one of what should be 2 distinct forms. The name for this is `chiral / chirality', and it leads to 2 different compounds, each of which has the identical chemical formula. However, one will rotate light clockwise, and the other anti-clockwise. That may seem complicated, but we are all aware of it through sugars, because they are all chiral. The chemical name for ordinary sugar is `sucrose', whilst the human body metabolises sucrose to `dextrose' (derived from the Latin `dexter'). Both sucrose & dextrose are dextrorotatory (right-handed, or clockwise) whilst fructose (fruit sugar) is levorotatory (left-handed, or widdershins). You can perhaps understand the importance of this to our bodies if I point out that if we eat the left-handed version of sucrose or dextrose then, even though it has the identical chemical composition, our bodies just process it straight through without digesting it. To finish this bit off, even water rotates light (which is how Polaroid sunglasses work, and why they help you to see through the reflections on water or windows).
--------- Alex Kemp