Free Astronomy Magazine May-June 2015

SMALL BODIES ever, producing the desired result. It is though fair to say that the chances of suc- cess were rather small, given that with the comet travelling at about 300 million km from the Sun it is highly unlikely that the lander’s temperature could be warmer than the minus 45 degrees C which are needed to awaken it; as well as needing at least 5 watts of power (while for a stable radio link the watts needed are instead at least 19). That of March had, in short, been a rath- er unrealistic attempt. A second attempt to communicate with Philae was started on 11 April and it is still underway, with Rosetta that periodically listens out to any signals coming from an area of 30 by 50 metres within which it is believed that Philae ended up. The most suitable period for re- establishing a connection will be from May to June, also because in the months imme- diately after the comet will reach its maxi- mum surface activity and Philae could be rendered permanently unusable by blasts of gas and dust. A taste of what could hap- pen to it was had by the same Rosetta in late March, when racing at an altitude of just 14 km it was hit by a dense outflow of material from underground and suffered an unexpected deceleration due to the fric- tion on its solar panels. The worst conse- n F alse colour image, taken on 21 August 2014 from 70 km away, showing a bevelled region called Hapi, connecting the two large lobes that make up the comet. The dif- ferences in reflec- tivity have been emphasized with shades of blue, which could indi- cate water ice reserves. [ESA/Ro- setta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/ SSO/INTA/UPM/DAS P/IDA] Left, details of that same region taken 2 months later from 10 km away. [ESA/Ro- setta/NavCam – CC BY-SA IGO 3.0] quences were however suffered by its stars tracking navigation system, which by tak- ing as stars numerous glittering particles of cometary material caused the probe to lose its orientation. Consequently, the telecommunica- tions antennas started to point in the wrong direction, putting at risk the continuation of the mission. After several hours of intermittent or absent connections, Rosetta en- tered into safe mode, turning off all the instruments. Luckily, when the probe was about 75 km from the comet, the star trackers restart- ed to work correctly and allowed to restore the proper alignment with the Earth. At that point the engi- neers cautiously took Rosetta to an altitude of 400 km, confident of being able to gradually move it back to less than 100 km from the surface without running further risks, which would prevent the hoped reawakening of Philae that everyone wishes to see.