Philae hasn’t communicated for nearly two weeks; ESA scientists worried

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Comet 67P/C-G on 7 July 2015. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

The European Space Agency’s Philae comet lander hasn’t communicated since July 9 and scientists at the space agency are concerned that the lander may possibly have moved on the space rock.

In a bid to get a better lock on the lander, ESA’s Rosetta mission scientists have been flying the Rosetta spacecraft along the terminator plane of Comet 67P/Churyumov-Gerasimenko, at distances from 180 km down to 153 km and at latitudes between 0 and 54 degrees.

However, owing to increased comet activity, mission scientists were forced to move the spacecraft back to a safer distance of 170–190 km.

The ESA revealed that they haven’t been able to contact Philae since Thursday 9 July. Currently mission scientists are busy investigating the data acquired at that time to try to better understand Philae’s situation.

“The profile of how strongly the Sun is falling on which panels has changed from June to July, and this does not seem to be explained by the course of the seasons on the comet alone,” explains Philae’s project manager, Stephan Ulamec at DLR.

Rosetta mission scientists are looking at two possible situations. One of the possible explanation behind the radio silence is that the position of Philae may have shifted slightly, perhaps by changing its orientation with respect to the surface in its current location.

The lander is likely situated on uneven terrain, and even a slight change in its position – perhaps triggered by gas emission from the comet – could mean that its antenna position has also now changed with respect to its surroundings. This could have a knock-on effect as to the best position Rosetta needs to be in to establish a connection with the lander.

The second issue that ESA is looking at involves the two transmission units of the lander appears not to be working properly, in addition to the fact that one of the two receiving units is damaged.

Philae is programmed to switch periodically back and forth between these two transmission units, and after tests on the ground reference model, the team has sent a command to the lander to make it work with just one transmitter. As Philae is able to receive and accept commands of this kind in the “blind”, it should execute it as soon as it is supplied with solar energy during the comet’s day.

The current status of Philae remains uncertain and is a topic of on-going discussion and analysis. But in the meantime, further commands are being prepared and tested to allow Philae to re-commence operations. The lander team wants to try to activate a command block that is still stored in Philae’s computer and which was already successfully performed after the lander’s unplanned flight across to the surface to its final location.

This “safe block” set of activities includes temperature measurements by the thermal probe MUPUS, measurements by ROMAP and SESAME, and analysis by PTOLEMY and COSAC in sniffing mode, and do not involve moving any mechanism on lander. No detailed commands are needed: if operated in the currently stored configuration, the “safe block” only needs to be activated.

If this commanding works, Philae could re-start its scientific measurements and, if a link is established with Rosetta, it would be able to send its data back to Earth, via the orbiter.

The team has already tried to ‘call’ ROMAP in a similar way to the commands sent to the CONSERT instrument earlier in the month, but so far no confirmation signal has been returned. The situation continues to be analysed with the available data.

“Philae is obviously still functional, because it sends us data, even if it does so at irregular intervals and at surprising times,” adds Stephan Ulamec. “Several times we were afraid that the lander would remain off – but it has repeatedly taught us otherwise”.

From the end of this week, Rosetta will begin alternating lander communication attempts with its own scientific observations of the comet, including exploration of the southern latitudes, which have only started to become illuminated in recent months.