A LONG AND EVENTFUL TRAVEL

Cruise and releasing

The four NetLander probes should be brought to Mars periphery by a carrier satellite which will provide ressources and operation management necessary to the housekeeping of the probes during the cruise phase which will last about 10 months.

When the satellite will arrive near Mars, it will inject each NetLander probe on a trajectory aimed at Mars to reach the targeted landing site. This manoeuvre is particularly critical because the probes have no attitude control or thrusters. If the trajectory is not correct, either the probe bounces on the martian atmosphere and continues its cruise in the interplanetary medium, or the probe enters the atmosphere with a too high incidence and burns. In NetLander case, this operation is more complicated because it must be realised for each probe, which means four times consecutively. Due to the time necessary to know accurately the trajectory and attitude of the satellite between each ejection as well as the delay to prepare the Mars orbit injection phase (MOI) for each probe, the first NetLander will be released 28 days before the MOI. This constraint enforces very high accuracy for the injection.

The NetLanders in cruise configuration

Once installed on Mars surface, the NetLander probes should use a satellite in orbit around Mars to relay data to the Earth. Indeed, the dimension, mass and available power constraints do not enable to have a system to communicate directly with the Earth, thus data must transit via a relay satellite.

Installation

The descent and landing phase is describe in EDLS.

After detecting the motionless of the probe in its airbag, the surface module's on board computer will command the ejection of the airbag and the opening of the upper part of the station. The opening system is designed to return the station in the case where it falls on the wrong side after the airbag ejection. Then, the "petal" carrying the solar generators will be opened to recharge the batteries and the UHF antenna will be deployed to communicate with the relay satellite and to give a first health status of the lander to the mission center.

The next step consists in deploying successively the different scientific instruments, beginning by the camera to know the station environment and to supply complementary information about its status. Once all the instruments are installed, scientific measurements will begin following pre-established scenarii and will regularly be sent to the surface module via the relay satellite. However the on board computer has an autonomy to be able to adjust the work load according to the ressources available. The scientific data volume is of about 100 Mbit/week with a minimum volume by day of 3 Mbit.

The targeted lifetime is one martian year (about 2 terrestrial years) to realise the observations during a complete seasonal cycle. However according to the technical solution imposed by mass constraints, the uncertainties about the quantity of dust covering the solar generators and the environmental conditions which can be very different in one site compared to another site, this objective appears very ambitious.

Earth/NetLander communications transit via a relay satellite.

The network formation

The selection of the landing sites is the result of a compromise between the scientific objectives and the technical constraints.

The scientific requirements are mainly in the form of the network, the latitutes and longitudes available as well as the distances between the stations. The best configuration is three stations 1000 km appart in the north hemisphere and one station at the antipode of the triangle formed by the first three stations.

The constraints on the available landing sites derive from the manoeuvre capacity of the satellite during the NetLander ejection phases which limit the reachable longitudes. Other constraints come from the EDLS system (Entry, Descent and Landing System), as for example a 2 km maximum altitude of the landing site to have an atmospheric column sufficient to break the probe with a parachute with a reasonable size or to have an area relatively flat with not much rocks not to damage the "airbag" system during the impact. At last, the latitude is limited by the needed insolation to provide the energy to the station and keep an acceptable mean temperature.

In colour, the areas accessible for the installation of the NetLander network taking into account the technical constraints.