Subject: Rosetta: Nothing heard from Philae Lander since 9 July
From: Alex Kemp
Date: Tuesday, 21 July 2015 23:38:05 +0100
To: Oliver Kemp, Micaela Kemp, Liisa Kemp, Davin Kemp

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:-

Original at (4936 × 3762 pixels)

This is the recent story:

  1. 7 months of hibernation
  2. 13 June 2015 : Philae phones home with status info
  3. 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.
  4. 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.
  5. 20 July 2015 : No further communication since the last.

    The DLR team have been sifting every last bit of info uploaded on 9 July to try to work out why no further communication:

Hopefully, much more information to come...

Alex Kemp