When NASA’s Lunar Reconnaissance Orbiter (LRO) arrived in orbit around the Moon in 2009, scientists immediately started firing lasers at it.
More specifically, they were firing lasers at a small reflector array roughly the size of a paperback book, trying to bounce light back to Earth. And after almost 10 years, they have finally succeeded.
It’s the first time photons have been successfully reflected back to Earth from a lunar orbiter. And it not only gives us a new way to perform measurements of and around the Moon – it can help us understand the conditions on the lunar surface that could be degrading instruments placed there over 50 years ago.
The Apollo program saw astronauts visiting the Moon from 1969 to 1972. But they weren’t just in it for the short term. They left behind (among other things) equipment for continued monitoring, such as seismometers and three laser reflectors. The Soviet space program also put reflectors up there on robotic rovers – two of them, for a grand total of five between the two space agencies.
Why laser reflectors? Well, if you send a really powerful laser beam at the Moon and time how long it takes to bounce back, you can make a really accurate measurement of the distance between the two points, based on the speed of light. Thus, we can determine how far away the Moon is, with millimetre precision.
Over time, those measurements can paint a picture of how the Moon moves around. That’s how we know the Moon has a fluid core, based on how it rotates; and, if there’s solid material in that fluid core, that in turn could tell us how the Moon once powered its magnetic field.
Such precise measurements are also how we know the Moon is slowly moving away from Earth at a rate of about 3.8 centimetres (1.5 inches) per year. Distance measurements can tell us so much, if we’re patient.
“Now that we’ve been collecting data for 50 years, we can see trends that we wouldn’t have been able to see otherwise,” said planetary scientist Erwan Mazarico of NASA’s Goddard Space Flight Center. “Laser-ranging science is a long game.”
But there’s a problem. Over time, the amount of light returned from those lunar reflectors has dimmed, to as little as 10 percent of what it should be. And it’s not clear why.
However, if there’s one thing the Moon has in spectacular abundance, it’s dust. Although there’s no atmosphere, and therefore no wind to stir that dust up, impacts from tiny micrometeorites could be dislodging just enough to slowly coat the reflectors.
So, this is where the LRO reflector comes in. If we can receive signals bounced off its reflector, scientists can compare the results from the surface reflectors.
With the help of modelling, this could help determine the cause of the surface reflectors’ declining efficiency – and, perhaps, reveal just how much micrometeorite bombardment the Moon is subject to, and how much dust this bombardment kicks up.
It is a lot easier said than done, though. It’s difficult enough to bounce a laser off lunar surface reflectors, in large part due to Earth’s atmospheric effects and electromagnetic attenuation. The LRO’s reflector is even more challenging. It’s a small, fast-moving target just 15 by 18 by 5 centimetres (6 by 7 by 2 inches), and it is, on average, 384,400 kilometres (238,900 miles) from Earth.
The team’s initial attempts to reach the reflector using green visible light were unsuccessful. But then they teamed up with scientists at the Université Côte d’Azur in France, who had developed an infrared laser – light much more efficient at penetrating gas and cloud.
On 4 September 2018, the Laser Ranging Station in Grasse, France recorded infrared laser light bouncing back from the LRO for the very first time.
Then, in two sessions on 23 and 24 August 2019, the result was repeated – except this time, the team also slewed the spacecraft to orient the reflector towards Earth, demonstrating how to create opportunities for two-way laser ranging, rather than just waiting for the LRO to swing around the right way.
The light returned was minimal – just a few photons. It’s not enough, yet, to be able to figure out what’s blocking the reflectors on the lunar surface. But over time, even a few photons can build enough of a picture to tell us more.
It’s not just the LRO measurements that are of interest here. The team’s work demonstrates the improvements that can be made using infrared laser instead of optical, penetrating farther and potentially enabling the use of much smaller reflectors.
“This experiment provides a new method of verifying theories of dust accumulation over decades on the lunar surface,” the researchers wrote in their paper.
“It also showed that the use of similar arrays onboard future lunar landers and orbiters can support LLR lunar science goals, particularly with landing sites near the lunar limbs and poles, which would have better sensitivity to lunar orientation.”
The research has been published in Earth, Planets and Space.