[meteorite-list] Philae Wake-Up Triggers Intense Planning

[Ron Baalke]

http://www.esa.int/Our_Activities/Operations/Philae_wake-up_triggers_intense_planning

Philae Wake-Up Triggers Intense Planning
European Space Agency
15 June 2015

The receipt of signals from Rosetta's Philae lander on 13 June after 211 
days of hibernation marked the start of intense activity. In coordination 
with its mission partners, ESA teams are working to juggle Rosetta's flight 
plan to help with renewed lander science investigations. 

Philae has woken up after seven months in hibernation on Comet 67P/Churyumov-Gerasimenko. 
Hidden by shadows, Philae shut down on 15 November 2014 at 00:36 GMT after 
completing its main science operations sequence on the comet when the 
primary battery expired as expected after about 60 hours.

Since March 2015, when Philae's environmental conditions started to improve 
with higher surface temperatures and better illumination, the orbiter's 
receiver had been turned on periodically to listen for signals from the 
lander when the orbital geometry was thought to be optimum.

On the evening of 13 June, a weak but solid radio link between Rosetta 
and the lander was finally established for 85 seconds. More than 300 "packets"
- 663 kbits - of lander housekeeping telemetry were received. This information 
had been stored on board at an as-yet-to-be determined time in the past, 
as much as several days to a few weeks, so does not necessarily reflect 
the lander's current status.

Rosetta then relayed the signal to ESA's European Space Operations Centre, 
ESOC, in Darmstadt, Germany, at 20:28 GMT.

Lander subsystems working nominally                                   
                                        
"We are still examining the housekeeping information at the Lander Control 
Centre in the DLR German Aerospace Center's establishment in Cologne, 
but we can already tell that all lander subsystems are working nominally, 
with no apparent degradation after more than half a year hiding out on 
the comet's frozen surface,' says DLR's Stephan Ulamec, Philae Lander 
Project Manager.

A second, smaller burst of lander data was received on Sunday, 14 June, 
at about 21:26 GMT, lasting just a few seconds. These data were confirmed 
to give the current status, showing the lander's internal temperature 
had already risen to –5ºC.

Philae's memory has stored over 8000 packets of additional status data, 
but it is unclear from when in recent days they were recorded.

Here comes the Sun

Engineers at the Lander Control Centre have determined that Philae is 
already being exposed to sufficient sunlight to heat it to an acceptable 
operating temperature and to generate electricity.

"Power levels increase during the local 'comet day' - the part of the 
about-12 hour comet rotation when Philae is in sunlight - from 13 W at 
comet sunrise to above 24 W,' notes ESA's Patrick Martin, Rosetta Mission 
Manager. 'It needs at least 19 W to switch on the transmitter."

The telemetry downloaded covered the lander's status for a full night-day 
cycle of the comet, which is helping ground teams to understand how the 
Sun is shining on the lander. The solar panels appear to be receiving 
power for over 135 minutes in each illumination period.

"While the information we have is very preliminary, it appears that the 
lander is in as good a condition as we could have hoped," says Dr Ulamec.

The task at hand

The main task now for all the mission partners - ESA for Rosetta operations 
and DLR and France's CNES space agency for lander operations and science, 
respectively - is to determine how to optimise Rosetta's orbit so as to 
facilitate contact and enable new science investigations.

It is believed that there is sufficient power now being generated to allow 
some science measurements during the time Philae is illuminated, with 
initial activities focusing on low-power measurements. This first phase 
would also likely include measurements that did not previously generate 
science in November.

However, the mission teams first must establish a more robust link between 
Rosetta and Philae before uploading the first batch of science operations 
commands.

The quality of the communication link is also possibly related to the 
trajectory Rosetta is flying and the orientation it adopts.

Optimising an orbit 305 million km away

Currently, Rosetta experiences two possible communication slots per 24 
hours - once per 12-hour comet rotation.
 
Until 23:35 GMT on Tuesday, 16 June, Rosetta will be flying an orbit set 
by already-uploaded commands on the terminator - the plane between comet 
day and night- moving out from about 200 km to 235 km altitude.

This orbit is not optimised for lander communication, so longer periods 
of contact may not be possible until the trajectory has been changed.

"With work done by the flight dynamics and operations team at ESOC and 
based on intense planning being conducted with the mission partners today, 
a new orbit will be devised that ensures optimum lander communications 
beginning with the next command upload later tonight,' says Paolo Ferri, 
ESA's Head of Mission Operations.

This new orbit will include an already-planned reduction of distance from 
the nucleus, down to 180 km versus 200 km, and 'nadir pointing' - continuously 
pointing Rosetta's communications unit at the comet. In the coming days, 
the orbiter may also be moved closer to the comet, without compromising 
the safety of the spacecraft, to help communications.

The new orbit will be flown by Rosetta starting after 23:25 GMT on 16 
June until 19 June, aiming to enable more and longer contacts with Philae, 
especially towards the end of this period.

Ready to react quickly

Establishing a regular and predictable pattern of contacts is a prerequisite 
for performing a complete assessment of the lander's status and for planning 
science operations.

"If we manage to achieve and maintain a predictable contact pattern," 
continues Paolo Ferri, "the lander teams can devise a strategy for a new 
sequence of scientific operations."

"Regardless, we will stay very flexible and be ready to react quickly. 
It's clear this incredible mission continues to stimulate and challenge 
us, developing in ways we could never have predicted."

As a bonus, any operation of Philae's instruments up to or through perihelion 
on 13 August - the comet's closest point to the Sun along its orbit -
will allow in-situ study of a comet during its peak activity.

Had Philae landed at the planned site, at Agilkia in November 2014, its 
mission would likely have ended in March because of the higher temperatures 
of that location as solar illumination increased.

Philae was contributed by a consortium led by DLR, the Max Planck Institute 
for Solar System Research (MPS), CNES and Italy's ASI space agency. It 
made the first-ever soft landing on a comet on 12 November 2014.