In the third season of “The Big Bang Theory” sitcom, Sheldon Cooper and his nerdy friends are seen standing on the roof of a house in Pasadena one night. Their plan is to shoot a laser beam to the moon. This raises a question for Penny’s new boyfriend Zach, who is more handsome than clever: Will the laser beam make the moon explode?
Their experiment centers on a true story about a quartz glass reflector placed on the surface of the moon by the crew of Apollo 11 in July 1969.
Zach’s clueless concerns for the moon infuriate Sheldon—he really could have asked some better questions. For example, how is it possible for a human-made laser beam to travel roughly 384,000 km through space and hit an object the size of a small suitcase? This feat is equivalent to a bullseye on a penny coin at about 120 kilometers. Or: How could this moon mirror still be working after more than 50 years? How has it survived numerous solar eruptions and the extremely aggressive cosmic UV radiation that destroys nearly all the materials we know of here on Earth within a short time?
The answers to these questions lie within the small device itself. The reflector, which consists of an inclined metal frame sitting on a base plate, appears relatively unremarkable at first glance. But inside it holds something unbelievable: 100 glass lenses that have been trained on the Earth for more than 50 years. These lenses are not made of regular glass, but rather high-tech quartz glass. Thanks to these quartz glass lenses, the reflector has been bouncing laser beams reliably back to Earth for almost five decades now. It is the only Apollo space experiment still running.
This amazing longevity is only possible because quartz glass can withstand the extreme temperatures on the moon (swinging from -150°C at night to 135°C in the day) and the aggressive UV radiation. Fifty years after the reflector went to work, its lenses are still unclouded. Quartz glass does not deteriorate with age. It is so pure—99.9999%—that impurities have to be measured in parts per billion (ppb). Because of this extreme degree of purity, the few laser photons that reach the reflector after a nearly unimaginable distance of 384,000 kilometers are sent back to Earth with minimal scattering. The concentrated laser pulse sent by scientists at ground control may be extremely powerful and strong, but it still fans out over two kilometers on the way to the moon. In order to even be able to conduct measurements, the reflection must not be scattered due to impure materials. Impure quartz glass would send the few photons that actually hit the reflector into the depths of outer space rather than reflecting them back to Earth. NASA engineers and Bendix Corp. (now Honeywell Aerospace), which was responsible for the Apollo 11 experiments package, already recognized back in the 1960s that Heraeus quartz glass is extraordinarily homogenous. That’s how Heraeus became part of the Apollo 11 mission and helped write moon landing history.
The friends on The Big Bang Theory celebrate when the laser beam they shot at the moon actually comes back and their monitor registers a few returning photons. Zach wants to know what’s so great about that. The friends respond that the laser’s return is proof that there are man-made objects on the moon, which also means that the moon landing didn’t take place on a Hollywood soundstage. But how did they keep the laser from blowing up the moon? One of the friends, who is famously a Star Trek fan, smiles. Don’t worry, “we set the laser on stun.”