Lunar Lander (spacecraft)

Launching from Centre Spatial Guyanais, Kourou in late 2018 on a Soyuz launcher, the Lander is injected into a Highly Elliptical Orbit (HEO) by a Fregat-MT upper stage, through a series of intermediate orbits. Following Fregat separation, the lander uses its own propulsion to enter the final Lunar Transfer Orbit and, after an overall transfer time of several weeks, injects itself into a polar orbit around the Moon. After a series of Apolune and Perilune lowering manoeuvres, the Lander reaches its final Low Lunar Orbit (LLO) at 100 km altitude.[6]

Once in low lunar orbit the Lunar Lander prepares for the final phase of the mission waiting for the correct constellation of orbit, Earth and Sun geometry and performing the checkout and calibration operations of the systems critical for landing. The time spent by the Lander in LLO before the start of the landing operations is expected to last from a number of weeks up to a maximum of 3 months.[6]

The descent and landing phase of the mission starts when the Lander performs a de-orbit burn close to the lunar north pole. This burn decreases the orbit's perilune to about 15 km, some 500 km ahead of the lunar south pole, half an orbit later. During the coasting period, automatic visual recognition of landmarks on the lunar surface is used to determine the lander's precise location and to ensure correct positioning at the beginning of the final descent. Heading towards the south pole, the lander enters the final powered descent phase. Using its cluster of thrusters, the lander decelerates and descends. During this phase, a varying thrust needs to be applied as the lander approaches its landing site. Finer thrust levels are achieved using the ATV engines in pulse modulation, as shown in this video of the hot-firing tests on YouTube. At an altitude of a few kilometres, the Hazard Detection and Avoidance system (HDA) is able to see the primary landing site and evaluate it. If the primary site is deemed unsafe, due to the presence of surface hazards (like steep slopes, craters, boulders, shadows, etc.), the HDA has the opportunity to command re-targetings to a secondary landing site. When a safe landing site is found, the lander performs a soft touch down using its legs.

Once landed on the surface, the lander carries out critical operations such as deployment of its antenna and camera mast, and relays the complete package of data relating to the descend and landing sequence back to Earth. The lander relies on direct line-of-sight communication with Earth as no relay satellite is planned for the mission. This configuration implies periods where no communication with Earth is possible because of Earth moving outside the lander's field of view. Similarly to the Sun, Earth will be below the horizon following a monthly cycle due to the Moon's tilted axis of rotation with respect to its orbital plane.

Nominal surface operations are then initiated which include the deployment of specific payloads onto the lunar surface via a robotic arm, the activation of other static monitoring payloads on board the lander, and ultimately the acquisition of surface samples using the robotic arm for analysis by instruments on the lander.

Lunar Lander dimensions [mm] with astronaut as size comparison

The main body of the Lander is cylindrical, with four landing legs extending from the sides. The circumference of the main body is covered with solar cells. The bottom side is dominated by the nozzles of the main thrusters while the top offers space for sensors and payload.

The lander will employ a robotic arm to retrieve soil samples for on-board analysis.

Polar landing sites offering long periods of continuous illumination have been found to be limited in extent, by analyses of the data from the Lunar Reconnaissance Orbiter and Kaguya.[7] Therefore, a landing precision of few hundreds of metres is required to the Lunar Lander. Compared to previous robotic lander missions (for example Surveyor[8]), the technologies required to the Lunar Lander mission have the potential to increase the achievable landing precision by one or two orders of magnitude, reaching the performances only achieved by crewed vehicles (Apollo 12 LM landed only ~150 m from the Surveyor 3 probe.[9])
Potential landing sites in the polar regions are also likely to be partly covered by shadow, and may present areas of steep slopes or large rocks. In order to avoid landing on unsafe terrain, an autonomous Hazard Detection and Avoidance (HDA) system is employed. The system is composed of a LIDAR and a camera, which generate 2D and 3D images of the surface, and by the on-board computer, which uses these images to characterise the landscape underneath the lander during the final descend. If the area is deemed unsafe, the system orders a retargeting to a safe landing area, compatibly with the propellant left.

Planetary exploration missions have often turned to Radio-Isotope devices, whether RHUs or RTGs, to support thermal control and power generation in what are often extreme temperature and energy poor environments. However, for Europe, where these technologies are currently unavailable, employing such devices have important technical and programmatic impacts. While activities investigating the development of RHUs and ultimately RTGs are proceeding within Europe, it is not expected that European devices would be available in the 2018 timeframe of the Lunar Lander mission.[10]
Instead, the Lunar Lander is powered by solar arrays which are wrapped around the body tube. Once landed, the vehicles axis of symmetry will be almost perpendicular to the direction of the Sun ensuring continuously good illumination of the solar cells as the lander rotates with respect to the Sun (due to the rotation of the Moon).

Batteries are used to bridge short periods without solar power. Solar power is unavailable in LLO when the lander goes into lunar eclipse and on the ground, when mountain peaks at the horizon cover the Sun. Landing operations will also be conducted relying solely on battery power.

The spacecraft employs three types of engines:

1. Six 220N ATV thrusters[11] operated in pulsed mode[12] to deliver a variable impulse along the descent, as the engine itself has a fixed thrust level, unlike the Descent Propulsion System used on Apollo for example.
2. Five 500N European Apogee Motors.[13]
3. Sixteen small attitude control thrusters

All of the 500 and 220 N engines will be needed to deliver sufficient thrust to decelerate the lander from low lunar orbital velocity for controlled final descent.

The vehicle uses traditional means of navigation during its transfer trajectory to the Moon. This includes employment of an IMU (combined unit of accelerometer and gyroscope), star trackers and sun sensors. Furthermore, range and Doppler measurements from Earth will help to determine the spacecraft's position and velocity, respectively.

In LLO and during descend, other means of navigation need to be considered. Early study phases identified the need to use high altitude vision-based absolute navigation, along with relative visual navigation.[2] These advanced techniques allow an improvement of the navigation performances, as compared to traditional techniques, such as inertial navigation and Earth-ground-based orbit determination. Furthermore, in order to guarantee soft landing and to reach the start of the approach phase within a tight corridor, an on-board long-range altitude estimate is required, which will be available through a combination of visual navigation and altimeter measurements.