European Space Agency plans for a robotic Moon landing have been boosted by successful testing of a rocket motor which ESA engineers plan to use to control the lander's descent on the 2018 expedition.
The thruster is the same unit as used on ESA's Automated Transfer Vehicle, selected to save development cost and for its known reliability - as demonstrated on International Space Station resupply missions. But specific testing at EADS Astrium's facility in Lampoldshausen, Germany to simulate a lunar descent and touchdown in a vacuum has convinced ESA the ATV thruster will do the job.
According to mission study manager Bérengère Houdou, the landing will take about 1.5h from a 100km lunar orbit, but the final 10min will be extremely challenging, with dynamics similar to those experienced at launch. In the lunar vacuum, all the braking is carried out by rocket power, which means a huge fuel burn.
Thus, says Houdou, thrust must be adjustable to account for a tremendous change in the lander's mass as it approaches the Moon, as a rocket powerful enough to slow the craft's early descent would be powerful enough to cause it to "bounce back" off the surface.
However, while much of the purpose of the mission is to prove European technologies suitable for use on later, manned international missions - for cargo supply, for example - Europe does not have a single engine capable of such modulation.
So, says Houdou, the lander will carry a cluster of motors which can be switched on or off individually to vary the total thrust. One of the key milestones achieved in the latest tests was to show that the ATV engine is accurately controllable in high-frequency pulses.
The next stage of testing, she says, will attempt to verify the interaction of several engines and their performance when connected by shared fuel lines. A design review should, in the coming months, outline the mission requirements and cost. Pending budget approval at this November's meeting of ESA member-state government ministers, Houdou hopes to have the mission ready for final preparation in 2015.
Unlike for a manned mission, ESA's flightplan is leisurely. A Soyuz-Fregat launch from Kourou, French Guiana will carry the lander to Earth orbit about halfway to the Moon, followed by a one- or two-month cruise to lunar orbit.
The landing site will be near the south pole, where "several months" of constant sunshine would allow a lengthy mission by sparing the spacecraft the devastating effects of temperatures that can plunge as low as -170˚C.