Contact with the European Space Agency’s (ESA) stranded Philae lander remains unstable despite continuous efforts to restore steady communication with the probe. The German Aerospace Center’s (Deutsches Zentrum für Luft- und Raumfahrt; DLR) reports that it received a signal from Philae on Thursday, July 9, lasting only 12 minutes. It was the first time ground controllers heard from the lander since June 24.
The brief contact revives the hopes for establishing a permanent radio connection with the probe currently lying on the comet 67P/Churyumov-Gerasimenko and it’s promising for the future use of the lander’s science instruments. However, the unsteady communication is troublesome.
"This sign of life from Philae proves to us that at least one the lander's communication units remains operational and receives out commands," said engineer Koen Geurts, a member of the lander control team at DLR Cologne, Germany.
Missing a Beat
Philae suddenly woke up on June 13, after nearly seven months in hibernation and “spoke" with its team on the ground, via the ESA’s Rosetta spacecraft that is orbiting the comet. Confirmed contacts between the lander and the orbiter were made on June 14, 19, 20, 21, 23, and 24. However, the value of these contacts varies greatly. For example, on June 23, there was a 20-second contact, but no stable link was established and consequently no telemetry data was received (alas). Therefore, the July 9 contact lasting 12 minutes gave is new hopes, as the team began to wonder lately if the lander had continued to survived on the comet’s frigid surface.
But unfortunately, since that contact, things have been quiet, and scientists remain puzzled as to what caused the two-week break in communication and subsequent silence. The data sent on June 24 did not suggest that the lander had experienced technical difficulties.
"We do not yet have an explanation for why the lander has communicated now, but not over the past few days,” Geurts admitted.
There are two opportunities for contact between the two spacecraft each Earth day, but their duration depends on the orientation of the transmitting antenna on Philae and the location of Rosetta along its trajectory around the comet. Similarly, as the comet rotates, Philae is not always in sunlight and thus not always generating enough power via its solar panels to receive and transmit signals.
Now, when the comet is closer to the sun, Philae is getting enough sunlight to produce enough electricity to power up its radio and some of its science instruments. The probe's internal temperature of zero degrees Celsius gives the team hope that the lander can charge its batteries. This would make scientific work again possible.
"We never gave up on Philae and remained optimistic," said Geurts.
The latest data came from the COmet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) instrument. CONSERT probes the comet's interior by studying radio waves that are reflected and scattered by the nucleus.
"We can already see that the CONSERT instrument was successfully activated by the command we sent on July 9," Geurts added. "This is extremely good news for us.”
It is hard to predict when a stable link might be made between Philae and Rosetta. One of the key issues being worked on is to adjust Rosetta’s trajectory to see whether a more reliable communications link can be established with Philae. ESA revealed that changes to orbiter’s trajectory are made depending on the latest information with regards lander communications and the performance of the orbiter’s star trackers in the days between each decision point.