Philae calling ...
By Cinzia Fantinati and Koen Geurts
On 9 July, the team at the DLR Lander Control Center made contact with Philae for the last time. Towards the end of October, Rosetta will come closer and attempts to communicate will resume. ##markend##
On 12 November 2014, the Philae lander became the first spacecraft to land on a comet – touching down at the Agilkia landing site on the head of 67P/Churyumov-Gerasimenko at 15:34 GMT, and confirmation arriving back to Earth 28 minutes later. The harpoons did not deploy as intended, and the ice screws alone proved insufficient to secure the lander on the surface upon landing. The lander bounced, embarking on an additional two-hour flight before finally coming to rest at a site now known as Abydos – albeit on its side in the shadow of a cliff.
Philae’s 10 instruments sprung into action, sniffing the surroundings, drilling and hammering the comet – one covered with coarse material and a surprisingly hard surface, which complicated life for MUPUS (MUlti-PUrpose Sensors for Surface and Sub-Surface Science). SESAME (Surface Electrical, Seismic and Acoustic Monitoring Experiment) found the strength of the ice to be extremely high. Organic molecules were detected by COSAC (Cometary Sampling and Composition).
Close-up images of the comet’s surface were taken by ROLIS (ROsetta Lander Imaging System). Philae and Rosetta analysed the comet’s nucleus using CONSERT (COmet Nucleus Sounding Experiment by Radio wave Transmission). After 60 hours of work, its primary battery became exhausted and Philae entered hibernation – completing its science sequence and delivering the first data from the surface of a comet. Was this the end?
Touching base with the lander
To communicate with Philae, the Electrical Support System (ESS) unit on board Rosetta, responsible for the interface between Philae and Rosetta, must be switched on and configured to 'Research Mode'. Then, the orbiter sends signals to the lander and, upon receipt, the lander switches on its transmitter to reply.
This 'handshake' will result in a two-way communication link. Then, Philae immediately starts sending the acquired science and housekeeping data. Rosetta can also send commands to Philae – for example, to upload and execute scientific measurements.
Seven months later, on 13 June 2015, the lander contacted Rosetta. A two-way link was established for 78 seconds. Philae sent back 343 encouraging housekeeping packets with information from the thermal, power and on-board computer subsystems – alive and well, the lander was warm and its solar panels were receiving sunlight.
Since that day, the teams at the DLR Lander Control Center (LCC), the Science Operations and Navigation Center (SONC, CNES), Rosetta Mission Operations Center (RMOC, ESOC) and Rosetta Science Ground Segment (RSGS – ESAC), together with the mission scientists, have been working non-stop to establish regular and predictable contacts and resume scientific measurements with Philae.
Changing Rosetta's trajectory
The first signal triggered numerous teleconferences, discussions and immediate decision-making. It was clear that the orbiter’s trajectory had to be compatible with Philae communication – this was a priority. The favourable latitude range was defined between 0 and +55 degrees North and processed for implementation by the RMOC Flight Dynamics and Flight Control teams.
The second 'conversation' with Philae, on 14 June, lasted 04:04 minutes – albeit with frequent link interruptions. The third contact occurred on 19 June: 18:53 minutes, again with frequent interruptions. The fourth contact, on 20 June lasted 31:01 minutes, with many interruptions. The fifth contact occurred on 21 June and lasted 11:25 minutes, with a 10:37 minute interruption.
The sixth and last contact took place on 24 June and lasted 17:11 minutes with continuous link interruptions. All in all, the sparse stored and real-time data showed that temperatures had increased from -55 degrees Celsius in May to -5 degrees Celsius on 13 June, that the battery was charging and the comet days were getting longer.
Clearly, the team had hoped for more, and their focus was to improve this situation. At the time, Rosetta was at a distance of about 200 kilometres – at the very edge of the operational range. Communication was not established every comet day, so a dependency on certain latitude ranges was suspected. The scientists and engineers are still unsure of why the contacts suffered so many interruptions.
Comet 67P has a 12.4-hour rotation period, so Philae's location is not always visible to the Rosetta spacecraft. Contact opportunities should be possible approximately two times per Earth day during an overflight by the orbiter, and orbiter and lander antennas must be aligned. The performance of Philae’s antennas is partially affected by objects in the nearby environment and the orbiter must align its antenna as closely as possible towards the comet.
Hardware issues and CONSERT switch-on
The sparse contacts provided the team with enough data to puzzle the engineers further. The telemetry received during the contact on 19 June showed that only one of the two receivers was switched on, although the configuration was set to use two. Further inspection shed light on the situation: one of the receivers (RX1) suffered from a short circuit and was switched off by the hardware (HW) overcurrent protection. This was worrying, but is precisely why redundant units are provided.
After two weeks of silence, there were concerns that the second receiver could also be compromised. To reject this hypothesis, it was decided to use the CONSERT instrument on Rosetta and Philae with independent antennas. By sending commands to Philae 'in the blind', instructing it to switch on CONSERT, the unit on Rosetta should be able to tell if the unit on Philae is switched on.
This would prove that the receiver is still working. The first attempt failed as no CONSERT signal from Philae was detected. But the second attempt was a big surprise, as contact was re-established on 9 July for 22 minutes, of which approximately 12 were completely uninterrupted – the best so far! Paradoxically, no signal from Philae’s CONSERT unit was obtained.
What the future holds
Since that day, the comet-chasing duo have continued to travel with the comet. Rosetta is now 400 kilometres away from 67P, due to the increased cometary activity around perihelion on 13 August. Although communication with Philae at this distance is unlikely, Rosetta is trying nonetheless.
On 22 September, Rosetta will begin a comet tail exploration, increasing the distance up to 1500 kilometres. Towards the end of October, Rosetta will come closer and attempts to communicate will resume. Thermal and power predictions suggest Philae should be operational until late 2015, so attempts to connect will continue.
If you, too, are wondering: Will we hear from Philae again? Guess you will just have to wait and see.
Cinzia Fantinati is Operations Manager for the Rosetta Lander. Koen Geurts is Technical Project Manager for Philae. They work at the DLR Lander Control Center (LCC).