November 7, 2014

Post Wake-Up Phases

Post Hibernation Commissioning: PHC

The Post Hibernation Commissioning starts after the probe has returned in its nominal mode of functioning (stable pointing frame based on the star tracker).

Until end of February, all sub-systems of the orbiter platform will be tested to ensure a complete checkup.

In March, the commissioning of each orbiter instrument will start. The lander Philae will be switched on for the first time on the 28 of March, after 39 months of hibernation (the last working period was on the 8 of December 2010).

In April, 3 periods of test will be dedicated to the commissioning of Philae: central software change, refresh of instrument flight software, tests of right functioning of the various experiment working modes, last interference tests between instruments, ... are planned.

PreDelivery Calibration and Science: PDCS

The PreDelivery Calibration and Science is a period of time between end of April and end of October when Philae instruments will be used in conditions close to those after landing. Though Philae remains attached to the orbiter experiments can be performed when the probe navigates inside the comet tail and comes closer and closer:

  • Magnetism and plasma measurements, interaction with the solar wind,
  • Temperature measurements,
  • Gas sniffing and components analyses.

Landing Site Selection Process: LSSP

The Landing Site Selection Process (LSSP) starts as soon as the first resolved images of the comet nucleus have been acquired at the beginning of July.

These first images provided by the Osiris instrument allow to reconstruct a more precise shape of the du comet nucleus, as well as a good first approximation of its rotational parameters: direction of the rotation axis, period of rotation.

Rosetta coming closer and closer to the comet, new other data are provided by Virtis, Miro, Alice, Rosina instruments. They allow to determine the comet activity, the outgassing characteristics and the dust flows.

In the second half of August, the comet mass and gravitational potential disturbing the Rosetta trajectory, the comet nucleus mass can be computed.

Exploitation of all these data allows to create various models of the comet. They can be taken into account at different periods of time in the software that are set up to compute the descent trajectory. These softwares determine the possible landing zone: if the nucleus is complex in term of shape, outgassing, the possible landing areas can be very limited.

Flatness of the landing zones, the probability to land on boulders are important parameters too. At last, scientists select the landing areas on other different criteria as soil composition, or outgassing flow of the zones.

This selection process is planned in 3 steps : a first choice of 5 landing sites performed with little data at TL - 90 days (Philae responsibility), that is 90 days before landing, then 2 sites selected at TL - 60 days (Philae responsibility). At TL - 30 days, the final landing site, the landing time and the descent trajectory are chosen (ESA responsibility under proposals of the Philae consortium).

Landing Delivery Preparation: LDP

The lander delivery preparation starts when all following constraints are known:

  • Trajectory of the Close Observation Phase (COP),
  • Trajectory of preparation to the delivery comprising orbiter maneuvers to approach the comet surface,
  • Philae descent and landing trajectory (SDL, Separation, Descent, Landing),
  • Post-delivery trajectory comprising orbiter maneuvers to come back to higher orbits.

This Flight dynamics data allow to preparer the scenarios of science operations during the descent and during the First Science Sequence (FSS).

Beside this, numerous lander sub-systems are tested before separation. The primary battery is switched on, the rechargeable secondary battery is filled at its maximal charge.

Separation, Descent, Landing: SDL

The Separation, Descent, Landing (SDL) is the most complex and the most risky phase.

Philae release from the orbiter is performed by the Mechanical Support System (MSS) that allows to adjust the separation velocity. In case of failure, a spring is activated, but the provided velocity ix not adjustable.

A few minutes after separation, the landing gear is deployed, simultaneously with the Consert antennae and the Romap beam.

According to the descent duration and the uncertainty on the time of landing, the flight software is put in a waiting mode, waiting for impact of the feet on the soil. Images of the overflyed zones are performed by the Rolis instrument.

As soon as the landing gear detects the impact on the surface, the cold gas thruster (Active Descent System, ADS) is activated to prevent the rebound or the tip over of Philae in case the soil would be very chaotic. The 2 harpoons are thrown one after the other, their threw being pulled to definitely fix the lander to the comet.

In the following minutes, the panoramic cameras are activated. They will allow to determine the location of the landing site and the Philae attitude after landing.

All these operations do not prevent to operate other instruments, according to the descent duration and the number of parallel activities involved.

The SDL phase ends with the complete emptying of the Philae mass memory.

First Science Sequence: FSS

The First Science Sequence (FSS) consists in the activation of all instruments in hours following the landing. Primary battery is the main source of energy; its use will last according to the descent duration, periods of memory downloading, durations of science activities. It is roughly estimated to 2.5 days.

Science operations scheduling is the result of an optimization performed by a software that took into account a big number of constraints: temperatures, efficiency of converters, efficiency of primary batteries, instrument memory capacities, central memory capacity, data transfer velocities, data downloading capacities, orbiter-Philae visibilities...

Nowadays, 3 blocks of operations are planned:

  • Block 1 : Consert, Romap,
  • Block 2 : SD2, Cosac, ptolemy,
  • Block 3 : Mupus, Sesame, Apxs.

The exact sequence will be determined around 20 days before landing. It must be robust to uncertainty on the location of the landing site and lander attitude at landing.

Long Term Science: LTS

The scientific sequence called Long Term Science (LTS) is a period during which Philae will work on the secondary battery. The recharging is planned every 2 days for activating instruments according to its level of charge.

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