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).
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.
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 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.