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 precisely.
Ultra-stable lasers are essential for timing and navigation systems that require extreme precision. They work by bouncing a beam between two mirrors inside a cavity. The beam is reflected between the mirrors at a very precise rate, in part because the chamber remains 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 get direct sunlight because the moon doesn’t tilt much as it spins. This makes these permanently shadowed regions exceptionally cold, with some craters predicted to be around -253 °C (20 kelvin) during the 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 nonexistent atmosphere—could make these craters the perfect place 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 if you go through summers and winters on the moon, the temperature still varies between only 20 to 50 kelvin. It’s an incredibly stable environment.”
Ye and his team’s laser device will be similar to devices they have already built in JILA’s labs, called optical cavities, which consist of a chamber made of silicon with two mirrors.
The best optical cavity lasers on Earth can only remain coherent, meaning that the laser’s light waves stay in sync for a few seconds. However, the researchers believe that a lunar-based laser could stay coherent for at least a minute.
This will 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 each other. It can even be used as a reference laser for activities on Earth, because it takes a little over a second for a beam to reach Earth from the Moon, Ye says.
Although it will be difficult to implement, the underlying idea makes sense and could help with future moon landings, says Simeon Barber of the Open University, UK. “We have seen several recent lunar polar landers have suboptimal landing events due to lighting conditions, which preclude 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 landings at high latitudes.”
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