The craters on the Moon’s poles are in permanent shadow. But they’re also intriguing locations, due to deposits of water ice and other materials. The ESA is developing the idea for a rover that can explore these areas with power provided by lasers.
Everybody thinks they know what lasers are: they’re beams of light. Well, basically, yeah.
Laser stands for Light Amplification by Stimulation of Electromagnetic Radiation. A laser is basically focused light. Whereas most light is considered incoherent, lasers are coherent. They can be spatially coherent, meaning the beam will stay focused over long distances without widening. And they can be temporally coherent, meaning that they can emit light in a very narrow spectrum, basically one color.
The first laser was invented in 1960 in the USA. Now they’re used for all kinds of things, including in astronomy. Observatories use laser pointers to create laser guide stars, which are then used to calibrate adaptive optics.
Lasers have been put forward as a type of spaceship propulsion, too. Theoretically, a powerful enough laser could be used to power spaceships through the Solar System, in the form of a laser-pushed lightsail.
But this is the first time we’ve heard of using a laser to propel a rover on the Moon.
The idea comes from the European Space Agency’s (ESA) Discovery and Preparation program. The goal of the program is to “lay the groundwork for the Agency’s future activities in the short to medium term.” And in the short and medium term, the Moon is definitely on the menu.
The Moon isn’t like the Earth. As the Earth rotates on its axis, it experiences pronounced seasons. The polar regions feel them the most. Sometimes the poles have 24-hour darkness, sometimes 24-hour light. The Earth’s axial tilt is about 23.5 degrees, while the Moon’s is 1.5 degrees to the ecliptic.
So on the Moon, at the highest latitudes, the Sun is low on the horizon, thanks to its 1.5 degree tilt. Crater floors can be in permanent shadow. That’s actually a good thing for us, because there’s water there. But it’s bad, because without light they’re hard to explore. No solar-powered rovers there.
NASA has toyed with the idea of a rover that could explore those dark craters. Called VIPER, it would visit shadow areas on battery power, but would need to find sunlight regularly to recharge itself. That’s a pretty important limiting factor.
If you follow rover design at all, then you know that there basically two options for powering them: solar power, or radioisotope thermoelectric generators (RTG), which capture the heat from a decaying radioactive element to create electricity. Plutonium 238 is most often used, and that’s how MSL Curiosity operates.
But according to the ESA, an RTG might not be the best choice for this mission. They produce a lot of heat, which could complicate the study of ice.
“The standard suggestion for such a situation is to fit the rover with nuclear-based radioisotope thermoelectric generators,” commented ESA robotics engineer Michel Van Winnendael. “But this presents problems of complexity, cost and thermal management – the rover could warm up so much that prospecting and analysing ice samples actually becomes impractical.”
It’s the role of the ESA’s Discovery and Preparation program to think outside the box, and to solve upcoming problems. To do that, they looked at things like laser-powered drones, such as DARPA’s (Defense Advanced Research Project Agency) Silent Falcon. It’s a solar-electric drone that uses a laser to recharge its batteries, and in theory, fly forever.
“As an alternative, this study looked at harnessing a laser-based power system, inspired by terrestrial laser experiments to keep drones powered and flying for hours on end,” said Van Winnendael.
The laser powering the rover wouldn’t be on Earth, nor in lunar orbit. It would be on the surface of the Moon, between 4 to 15 km (2.5 to 9.3 miles) away. The laser power source would itself be solar-powered.
The ESA awarded a contract to develop this idea. They gave it to Italy’s Leonardo company and Romania’s National Institute of Research and Development for Optoelectronics. It involves both a lander and a rover. The lander would be somewhere near the South Pole’s de Gerlache and Shackleton craters, in a location with near-permanent sunlight. From there, it would beam a 500 watt solar-powered infrared laser to a 250 kg (550 lb) rover. As the rover entered the shadowed areas, the laser would keep itself trained on the rover.
The rover itself would receive the laser on modified solar panels, and convert it into elecricity. Photodiodes on the sides of the solar panels would control the laser’s aim.
The 10-month study, called PHILIP (Powering rovers by High Intensity Laser Induction on Planets), included finding the right routes to make this all work. They looked for routes down into craters that had a gentle 10 degrees of slope, and that would keep the rover in contact with the laser.
The laser could also do double duty as a communications device, according to the ESA. It could be used as a two-way communications link. The second of the rover’s two panels could host a retroreflector, which would send pulses of light as signals back to the lander.
The ESA has already done some testing. They used Tenerife, one of the Canary Islands, to work on the idea of operating rovers in near-permanent darkness. While laser propulsion hasn’t been tested there yet, it may be in the future.
“With the PHILIP project completed, we are one step closer to powering rovers with lasers to explore the dark parts of the Moon,” said the ESA’s Van Winnendael. “We’re at the stage where prototyping and testing could begin, undertaken by follow-up ESA technology programmes.”