Astrobotic expeditions ride to the lunar surface using a 10-foot wide spacecraft built in cooperation with Carnegie Mellon University. The Falcon 9 launch vehicle throws this lander and attached rover to a trans-lunar injection – in other words, sailing toward the Moon with enough force to reach lunar orbit in four days.
The lander structure is built around a square deck plate of aluminum, with its underside hollowed out into a pattern of triangular ribs in order to reduce its mass. A cone descends from the deck to attach to the Falcon 9, and a second cone rises up from the deck to hold the rover. This primary structure was built in the first half of 2011, and successfully tested on a “shake table” that reproduced the intense vibration and acceleration that it will encounter during Falcon 9 launch. NASA paid Astrobotic $500,000 for data generated during the testing. This was the first task order in a $10 million contract awarded to Astrobotic for information it generates during its initial lunar landing.
Next Astrobotic will add flight-quality legs and two ramps for delivering the rover down to the surface. The dual ramps give mission controllers a fallback option if one ramp ends up in a crater or blocked by large rock. Solar panels will provide electrical power during flight and on the surface. The heaviest portion of the lander will be its propulsion system: four spherical tanks for fuel and oxidizer, a main engine mounted in the center between the upper and lower cones, and eight attitude control thrusters mounted on the four corners of the deck.
Unlike some lunar missions, the lander does not need a separate braking stage to slow it down into lunar orbit. The lander uses its own propulsion to reduce its speed into a circular orbit at 100 km altitude, where it updates its navigation data to be ready for a precision landing. Astrobotic engineers will test an innovative navigation technology in which a stored photographic map of the lunar surface is matched to the images being gathered by the lander’s cameras. All key orbital parameters (such as speed, orientation and altitude) can be derived from the matching process.
After six to twelve hours to refine position estimates, the engine fires again to put the lander into an elliptical orbit of 100 km by 15 km. Near the low point of this orbit, the propulsion activates again to slow still further, eventually transitioning to a slow vertical descent from 100 meters down to 2 meters. At this height, the engine cuts off and the lander drops the final two meters to the surface. The mild impact is absorbed by crushable material in the footpads and lander struts.
Landing is expected about 36 hours after local sunrise, when a low Sun angle will show the surface with high contrast for better photo matching. (Daytime on the Moon lasts for two weeks, followed by two weeks of night.)
After the rover rolls off to begin exploration, several payloads attached to the lander begin operation. The lander supplies payloads with electrical power and communication back to Earth. The lander also has a short-range radio link to the rover, providing an alternate communications path in the event of any problems with the rover or lander’s Earth-range transmitter or antenna. When night falls, the lander systems go into hibernation during the extreme cold of minus 180 degrees C – about as cold as liquid nitrogen. When dawn warms the lander 14 days later, systems will be switched back on. However, key components may crack or otherwise fail during the deep freeze, likely from the contraction and expansion caused by the huge temperature swing of the day-night cycle.
The spacecraft launches with legs and solar panels fully deployed; only the two ramps need to be stowed to fit within the Falcon 9 fairing limits.
Top photo shows the cone that attaches the spacecraft to the Falcon 9 and the eight bulkheads that provide stiffness to restrain the deck plate (bottom photo) from excessive bending as launch vibrations shake the heavy fuel tanks (to be placed in the four circular openings).
The gold box in the center represents one of the locations where lander payloads can be attached. The four gold spheres represent the fuel and oxidizer tanks that will be added when the propulsion system is installed.
The primary lander structure was shipped to a shake-table facility for testing that confirmed it has the stiffness required to resist the accelerations experienced during a Falcon 9 launch.
