During the second DEMETER International Symposium that took place at CNES headquarters on October 2011, from 10th to 12th, 80 geophysicists from all over the world met to present new results obtained from the vast quantity of data acquired during the six and a half years mission of this satellite.

Read the complete Press Release on CNES website (in French).

To limit the number of debris growth around the Earth, most of the space agencies have adopted near 2000 rules implementing good practices for disposal of satellites. Coming into force at the end of 2010, the French Law on Space Operations, together with the associated Technical Regulation prepared by CNES, gave a mandatory aspect to these rules (tank passivation, electric passivation, re-entry within less than 25 years for the low orbits).

Tank passivation:
Demeter was the first satellite of the Myriade family designed with a low cost approach. The propulsion system was using hydrazine stored in a bladder tank and the gauging system was presenting significant uncertainties. When preparing the retirement operation, the main concern was when will occur the final depletion and what will be the behavior of hydrazine. This last point was considered as a technological experiment with an extensive analysis of the process to define a strategy avoiding any risk of tank explosion. Indeed, as the rationale for propellant depletion is to prevent break-up or further generation of orbital debris, the implementation should not result to an explosion. Finally a safe strategy was made available to the operational team and the tank passivation was performed beginning of 2011.

DEMETER tank passivation operations were consisting in a series of semi major axis manoeuvres to empty gradually the tank while keeping control of the satellite.
During each manoeuvre, the variation of the semi major axis commanded was ±1.6 km corresponding to 2 minutes burn, which enables to avoid any risk of auto-explosion of the hydrazine. One should note that, in this case, there was no requirement for large reduction of DEMETER orbit semi major axis because on one hand the estimated duration of the entry in the atmosphere was still lower than 25 years on the operational orbit, and on the other hand the lower reachable orbits were in potential conflict with other satellites.

In total, 15 manoeuvres have been realized, DEMETER orbit have been lowered by 1.4 km.
The loss of efficiency of the propulsion system has been reached during the 13th manoeuvre, then two additional manoeuvres have been realized to observe the system behaviour during low hydrazine pressure burn.
The satellite was declared non manoeuvring at the end of tank passivation operations, which lasted 5 weeks.

Electric passivation:
By design, a satellite is extremely robust to any event that could lead to its loss. Particularly, the sub-systems are able to restart automatically and it is impossible to definitively cut off the power supply from the ground. The bypass strategy was to orient the solar panel in a direction opposed to the Sun until the batteries were completely discharged. But, without attitude control, the solar panel could eventually be oriented towards the Sun and recharge the battery, which could result in an automatic restart of the computer and a switch on of the satellite equipments (notably the on board emitters). To avoid this situation, a flight software, only composed of inoperative code, was uploaded with the last command.
The electric passivation operations took place on March 17, 2011, date of the upload of the last commands, which was performed with a lot of emotion, in presence of the operational team as well as the project development team.

DEMETER team during the last satellite operations

The end of a chapiter... Demeter, as first satellite of the Myriade family, demonstrated magnificently the capabilities to offer innovative missions at low cost and to comply with debris mitigation rules.

The satellite turned off on March 17 at 17:37:45 in direct visibility of Kiruna, after more than 5 years of good and faithful services for an initial planned lifetime of two years.

The final science commands established by the science ground segment at LPC2E (Chemistry and Physics of the Environment and Space Laboratory) was recieved on board Demeter at 9:03 on 7/12/2010. This command brought to a conclusion Demeter's Science Mission on the 9/12/2010. This was an emotional moment for all of the CNES team.

The Demeter mission, the first MYRIADE microsat developped and launched by the CNES, leaves us a rich heritage of six years of data which have allowed scientists to significantly improve our knowledge of the ionosphere and to think up new experiments sur as the Taranis mission. This data will continue to be used in studying the link between ionospheric perturbations and seismic and volcanic activity.

The final satellite operations will begin shortly, including a procedure to burn-up remaining fuel on board and passivation of the satellite.

DEMETER was above HAITI 3 days before the earthquake

Observation performed by DEMETER 3 days before the Haiti earthquake (magnitude 7) which occurs on January 12, 2010 at 21.53.09 UT (epicentre located at 18.451°N, 72.445°W). The top panel shows the variation of the electron density recorded at the satellite altitude during night time. The red triangle in the bottom panel indicates the time when the satellite is just above the future epicentre. A decrease of the density is locally observed around this area.

DEMETER observations 7 days prior to the M8 Samoa Earthquake

DEMETER observations 7 days prior to the M8 Samoa Earthquake which occurred on September 29, 2009 at 17.48.11 UT (location 15.51°S, 187.97°E). From the top to the bottom, the panels show the electron density, the electron temperature, the O+ ion density, and the earthquake occurrences along the satellite orbit. The red triangles indicates the closest approach to the Samoa earthquake and the many aftershocks. It can be observed an increase in the densities and a decrease in the temperature which are well localized above the future epicenter.

DEMETER Exploitation Review at CNES Toulouse

SCIENTIFIC RESULTS

Influence of VLF (Very Low Frequency) transmittters on the ionosphere

NWC in Australia is a very powerful transmitter of the US army (1000 kW) which sends waves at 19.8 kHz. The figure shows the perturbations induced in the ionosphere by this transmitter around 14.52.30 UT. These perturbations are registered when DEMETER is close. The top panel represents a spectrogram in the HF (High Frequency) range where we can observe a characteristic frequency of the plasma which is excited around 1.8 MHz. The following panel shows a VLF (Very Low Frequency) spectrogram of an electric component with a dramatic increase of the signal over all the frequency range (the transmitter frequency can be observed at 19.8 kHz). Then the electronic density and temperature measured by the Langmuir probe are plotted. The curves indicate a large variation of these ionospheric parameters as in the bottom panel which shows the ion temperature measured by the plasma analyzer. During two years more than fifty typical events as this one have been registered and the orbit projections at the time of the perturbations have been plotted on the following map.

The star indicates the transmitter position in the North-West of Australia and the square indicates the geomagnetic position of this transmitter at the DEMETER altitude (700 km). One can see that these perturbations are centred at 700 km around the magnetic field line whose foot corresponds to the ground position of the transmitter. This ionospheric heating covers an area of about 500000 km².