For billions of years, the Earth and the Moon have danced together.
Their performance began, many scientists believe, when a celestial body the size of Mars slammed into Earth. The debris jettisoned from the impact would swirl and coalesce into a molten sphere of magma, eventually crystalizing into the gray, luminescent specter we see in the night sky.
We only see one side of the Moon from our earthly vantage. It is tidally locked, meaning it completes a rotation on its axis in the same time it takes to orbit Earth. This behavior is a natural phenomenon caused by the interactions of gravity in planetary systems with relatively large moons, like Pluto and Charon, Pluto’s largest moon.
But we are also tidally locked to our Moon in another sense. The Moon’s gravity causes our oceans’ tides to rise and fall. The tides create currents and play a large role in our ocean systems. Some scientists believe that life came from tidal waters, with the Moon playing a critical role in the evolution of humanity.
The Moon has defined many of Earth’s physical characteristics, and has loomed large (both literally and figuratively) in culture. Civilizations have relied on the Moon to make calendars. They’ve used the Moon to predict the seasons — to know when to harvest. They’ve worshipped her as a god.
The Moon is, arguably, one of the best-studied objects in space. We can thank brave Apollo astronauts for much of our knowledge. They returned home from the Moon with samples of lunar soil, or regolith, for scientists to study. The insights their instruments recorded laid the foundation for much of modern planetary science. Even the stories they brought back had enormous value, inspiring the next generation of space pioneers.
But what did they leave behind? Flags and footprints — of course. They left their rovers. They left physical and biological waste.
They left mirrors.
I’m Danny Baird. This is the Invisible Network.
A standard mirror, like the one you might have in your bathroom, doesn’t reflect light back at its source, unless it is exactly perpendicular to the mirror’s surface. If you go to your bathroom, turn off the lights and point a flashlight at your mirror, you will only be illuminated if you stand directly in front of the mirror and hold the flashlight steady, parallel to the ground and perpendicular to your chest.
Even a small deviation in angle will illuminate something else. A slight swivel to the left redirects the light away from you, at an angle from the mirror. A swivel to the right does the same.
If you pointed a flashlight at the mirrors the Apollo astronauts left on the Moon, the light would return to you, no matter where you stood. They weren’t standard mirrors, but corner cubes, a type of retroreflector. These prisms are essentially mirrors designed to reflect light back towards its source, no matter the angle at which the light hits the device.
The Apollo 11, 14 and 15 missions left arrays of corner cubes atop the gray regolith of the Moon. They require no power. They need no maintenance. They simply rest there.
Yet, they’ve empowered terrestrial scientists to pursue bold inquiry into the nature of the cosmic dance between Earth and the Moon, long after NASA ended the Apollo program. Because of these mirrors, scientists can measure the distance between Earth and the Moon with unbelievable accuracy. In fact, because of these measurements, NASA has shown that the distance between Earth and the Moon increases by about 1.5 inches each year.
How can such discoveries be made using devices with no electronics or means of communications? For decades, NASA has been shooting lasers at them, calculating the time it takes for those lasers to return to Earth.
This process is called lunar laser ranging.
The term “laser” began its life in the 1960s as an acronym of “light amplification by stimulated emission of radiation,” but, thankfully, the word has become so ubiquitous that the acronym is seldom used.
A laser emits light “coherently,” or all in the same direction. This allows the light to be focused on a tight spot and to stay narrow over vast distances. Compare a flashlight with a laser pointer — the difference is pretty clear.
Lasers have numerous commercial applications, like in welding or in laser cutting. They’re used in entertainment as the laser light shows you might see at a concert or music festival. And they’re used in science, like in laser ranging that has enhanced NASA’s understanding of the Moon.
NASA has long used lasers to judge distances, but not just between Earth and the Moon. Satellites can use Earth-facing lasers to map the topography of the land, the height and depth of clouds and the volume of ice at the poles. Missions across practically every science discipline use techniques like Light Detection and Ranging, or LIDAR, to study everything from our oceans to our forests.
In space communications, laser technology promises to deliver more data than ever before. By using infrared lasers, missions can encode more data into each transmission than they could with radio or microwaves. This technology, dubbed optical communications, uses instruments that take up less size, weight and power than traditional communications systems. A smaller size means more room for science instruments. Less weight means a less expensive launch. Less power means less drain on the spacecraft’s batteries.
In December of 1992, in the dead of night, faint beams of laser light flew through the sky above California, soaring to altitudes 15 times the distance between Earth and the Moon. These lasers were the first-ever demonstration of optical communications with a deep-space vehicle, Galileo.
Galileo was bound for Jupiter to study its moons. On the journey, it would take photos of Venusian clouds, become the first spacecraft to visit an asteroid and provide the first observations of a comet colliding with a planet.
As it performed a fly-by of Earth after a gravity assist from Venus, Galileo offered a unique opportunity to communications engineers. As the spacecraft receded from Earth en route to Jupiter, they could fire lasers at it from the Table Mountain Facility in California and the Starfire Optical Range in New Mexico. Galileo’s Solid-State camera could then act as an optical communications receiver. This demonstration, known as the Galileo Optical Experiment, or GOPEX, would prove that NASA could use lasers to talk with deep space.
In the following decades, NASA continued to develop optical communications technologies. We’ve demonstrated direct-to-ground communications from the International Space Station and from a spacecraft in lunar orbit. Those missions led to this moment, where NASA is on the cusp of realizing optical communications as a robust capability that our networks can provide missions.
The Laser Communications Relay Demonstration, LCRD, an upcoming optical communications mission, will relay data between ground stations in Hawaii and California. It’s still primarily a technology demonstration, but once the Integrated LCRD Low-Earth Orbit User Modem and Amplifier Terminal, or ILLUMA-T, completes testing on the International Space Station, NASA will have the first operational end-to-end, bidirectional optical communications relay system.
Many of the upcoming Artemis missions will have optical terminals capable of streaming live, high-definition video to Earth from the Moon. Moving past the boundaries of lunar orbit, the Deep Space Optical Communications project will test laser technologies against the unique challenges presented by deep space science and exploration. The optical terminal is on a larger mission that will visit 16 Psyche, a large asteroid which scientists believe is the remnant of a planet stopped in mid-formation by a celestial collision.
Galileo Galilei, Renaissance astronomer and namesake of NASA’s Galileo mission to Jupiter, changed the way we look at the stars. Recording systemic observations through his telescope, Galileo gleaned insights into the way our solar system and the celestial bodies within it danced. He championed heliocentrism, the model of the solar system where the Earth orbits the Sun, at a time when it was dangerous to do so. Peering deep into the night sky, he saw the rings of Saturn, the phases of Venus and the four largest moons of Jupiter.
When Galileo began his study of the sky, the Moon was the only moon we knew. Now, through persistent study and inquiry, we know over a hundred moons in our solar system alone. With missions like the Transiting Exoplanet Survey Satellite, or TESS, we’re studying planets around other stars. One day soon, we might know their moons as well.
When Apollo astronauts landed on the Moon, laser technologies were still in their infancy. The first lasers had only been built in the preceding decade. Yet, laser technologies matured rapidly. From the fiber optic technologies that bring high-speed internet to your home, to the precision lasers used to map Earth from space, lasers are as pervasive as they are vital to modern life.
While massive breakthroughs loom large in science and tech reporting, incremental technology development is the true driver of human achievement. Centuries of scientists, engineers, entrepreneurs and innovators have lent their talent to a vast timeline of advancement leading to today’s technological landscape.
Optical communications technologies are this in a microcosm. NASA, as a long-standing government agency, has been uniquely situated to take this innovation from infancy to fruition. From Galileo and GOPEX to the optical terminals on board the Artemis missions, NASA has changed — and will continue to change — the technology that supports spaceflight, allowing humanity to join in the cosmic dance between Earth, the Moon and all that lies beyond.
This season of “The Invisible Network” debuted in November of 2019. The podcast is produced by the Space Communications and Navigation program, or SCaN, out of Goddard Space Flight Center in Greenbelt, Maryland. Episodes were written and recorded by me, Danny Baird, with editorial support from Matthew Peters. Our public affairs officers are Peter Jacobs of Goddard’s Office of Communications, Clare Skelly of the Space Technology Mission Directorate and Kathryn Hambleton of the Human Exploration and Operations Mission Directorate.
Special thanks to Barbara Adde, SCaN Policy and Strategic Communications director, Rob Garner, Goddard Web Team lead, Amber Jacobson, communications lead for SCaN at Goddard, and all those who have lent their time, talent and expertise to making “The Invisible Network” a reality. Be sure to rate, review and subscribe to the show wherever you get your podcasts. For transcripts of the episodes, visit NASA.gov/invisible. To learn more about the vital role that space communications plays in NASA’s mission, visit NASA.gov/SCaN.