Illustration of dark craters near the moon’s south pole
Science Photo Library / Alamy
Scientists want to build a laser inside one of the moon’s coldest craters that could help lunar landers and rovers navigate accurately.
Ultrastable lasers are vital for timing and navigation systems that require extreme precision. They work by bouncing a beam between two mirrors inside a cavity. The beam reflects between the mirrors at a highly precise rate, in part because the chamber stays almost exactly the same size rather than expanding or contracting. To keep this beam length stable, the mirrors are usually kept inside a vacuum at extremely low temperatures, isolated from external vibrations.
On the moon, there are hundreds of craters around the poles that never receive any direct sunlight because the moon doesn’t tilt much as it spins around. This makes these permanently shadowed regions exceptionally cold, with some craters predicted to be around -253°C (20 kelvin) in lunar winter.
Jun Ye at JILA in Boulder, Colorado, and his colleagues have proposed that these frigid conditions – along with the moon’s natural lack of vibrations and virtually non-existent atmosphere – could make these craters the perfect location for an ultrastable laser, with a potential stability far greater than any laser on Earth.
“The whole environment is stable, that’s the key,” says Ye. “Even as you go through summers and winters on the moon, the temperature still varies between just 20 to 50 kelvin. That’s an incredibly stable environment.”
Ye and his team’s laser device would be similar to devices they have already built in JILA’s labs, called optical cavities, which consist of a chamber made from silicon with two mirrors.
The best optical cavity lasers on Earth can only remain coherent, which means the laser’s light waves remain in sync, for a few seconds. However, the researchers think a moon-based laser could stay coherent for at least a minute.
This would allow it to act as a reference laser for many different lunar activities, such as maintaining a time zone on the moon or coordinating satellites that fly in formation and use lasers to measure their distance from one another. It could even be used as a reference laser for activities on Earth, because it takes just over a second for a beam to reach Earth from the moon, says Ye.
While it will be difficult to implement, the underlying idea makes sense and could help with future moon landings, says Simeon Barber at the Open University, UK. “We have seen various recent lunar polar landers have suboptimal landing events because of illumination conditions, which hinder the use of vision-based landing systems,” says Barber. “Using a stable laser to support positioning, navigation and timing could increase the reliability of successful high-latitude landings.”
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