After all the excitement of the astonishing info from Pluto it seems a real downer to have to report that, even as 67P approaches Perihelion (closest approach to the Sun) in August, the orbiter Rosetta has had zero communication with the Philae lander, with the last contact being on 9 July 2015.
Here’s a picture of all the instruments on Philae:-
26 June 2015 : 7 further connections, but all very intermittent (13, 14, 19, 20, 21, 23, and 24 June)
It requires 50 minutes to dump all the stored info from Philae’s 2 mass memories (assuming a stable link). The best was 19 June, but that was just 2 sessions, each of 2 minutes. Others have been worse, including longer but unstable throughout and some with zero telemetry.
Communications are line-of-sight (3GHz RF subsystem working in S band (Sarthou, 1998)). 67p has a 12.4 hour rotation period, which means 2 opportunities each day to communicate. The current predicted contact windows vary between a few tens of minutes and up to three hours. The process is that Rosetta sends a calling signal to which Philae responds, establishing a link. Philae then uses that link to transmit it’s stored telemetry to Rosetta. Rosetta then relays the telemetry back to Earth (only Rosetta has a radio link powerful enough to transmit the (current) 300 million kilometres to Earth).
At this time Philae was unlikely to store energy & thus needed to be in direct sunlight to have enough power to transmit. Another factor was that the orbiter needed to be ~200km from the comet due to the volumes of dust being emitted. Dust affects the ‘star tracker’ unit which keeps it orientated; in the worst case it goes into “Safe Mode”; this is an autonomous mode that could take days or weeks to recover from.
Radio power varies as the square of the distance (twice as far away means ¼ as strong) so they were doing their best to get Rosetta as close as they could, but they worried that Safe Mode may get triggered again. Safe Mode was last triggered back in March. That was an event that fractured everybody’s nerves, and nobody wants it to happen again. Nevertheless, they were planning on getting as close as 165 km on 30 June.
9 July 2015 : 12 minute stable connection (the first since 24 June)
The DLR Lander Control Center (LCC) sent an initial test command to turn on the power to CONSERT on 5 July 2015, but the lander did not respond. CONSERT is a transponder on 90MHz, and is likely to respond independently of the rest of Philae, which suggests a reason why they sent the command.
Koen Geurts, a member of the lander control team at DLR Cologne:-
We can already see that the CONSERT instrument was successfully activated by the command we sent ... We do not yet have an explanation for why the lander has communicated now, but not over the past few days.
The DLR team have been sifting every last bit of info uploaded on 9 July to try to work out why no further communication:
The pattern of sunlight on the panels has altered. Project manager, Stephan Ulamec at DLR:-
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.
They are trying to work out why (hit by a dust jet? etc.).
One of the two transmission units of the lander appears not to be working properly; in addition to that, one of the two receiving units is damaged.
Do you know the famous line by Lady Augusta Bracknell in Oscar Wilde’s 1952 play “The Importance of Being Earnest”? (Jack Worthing informs Lady Bracknell that he is an orphan; he lost both parents whilst a young boy):-
To lose one parent, Mr. Worthing, may be regarded as a misfortune; to lose both looks like carelessness.
Philae is programmed to switch periodically back and forth between the two transmission units. They have a “Reference” Lander back at control and, after some tests on it, sent a command to Philae to make it work with just one transmitter. Philae is able to receive and accept commands of this kind in the “blind”, so it should execute it as soon as it is supplied with solar energy during the comet’s day.
DLR tried to ‘call’ ROMAP in a similar way to the commands sent to the CONSERT instrument, but has yet to receive a confirmation signal.
There is a command block that is still stored in Philae’s computer (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 the lander. No detailed commands are needed: if operated in the currently stored configuration, the “safe block” only needs to be activated, and that is very much what they would like to do. However, they would also need to get the telemetry back, and they need to be able to empty the currently-stored telemetry first.
Rosetta’s altitude (from 180 km down to 153 km) + attitude (latitudes between 0 and 54 degrees) have been altered to try & find the best place to link up with Philae. Unfortunately, on 10–11 July the ‘star tracker’ unit began to wobble, so they uplifted Rosetta back to 170–190 km.
The orbiter keeps on pressing Philae’s doorbell (and getting zero response). In the meanwhile, she is beginning to scan 67P’s southern hemisphere, which has only started to become illuminated in recent months.