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What’s a surefire way to know your rocket has reached the Moon? Well, if it’s 1916, and you’re one of the founding fathers of modern rocketry, you might propose a bunch of magnesium flash powder which would go off after your rocket crashed into it. No, seriously. After some experiments, Robert Goddard concluded it’d take just 6.2 kilograms of the stuff to be quote “strikingly” visible. Unfortunately, a flash of light isn’t much communication beyond “Hey! I got here.” But Goddard was on to something. A bunch of light is how we communicate through the vacuum of space. But scientists have spent over a century working out what kind of light works best. And one day, astronauts might even use lasers to phone home.
At the turn of the 20th century, decades before most people thought it was even possible to launch stuff into space, scientists wondered how we could communicate with other worlds. They knew sound couldn’t travel through a vacuum, and outer space was very nearly a vacuum, so building a cosmic megaphone to shout at the universe was completely out of the question. The answer was electromagnetic radiation, a.k.a. light. After all, we’ve got a big ball of plasma in the sky reminding us that light can traverse the vacuum of space. But what you and I typically think of as light is just one tiny band in the middle of the electromagnetic spectrum. Over here rocking the longest wavelengths and lowest frequencies, you’ve got radio and microwaves. Hovering on either side of visible light is infrared and ultraviolet. Then your x-rays and gamma rays are up here with the shortest wavelengths and highest frequencies. These are all light. Most are just invisible to us. And by the 1930s, everyone from scientists to early morning shock DJs were using certain frequencies in the radio band of that spectrum to send messages around the world. They didn’t need line of sight because the radio waves were bouncing off of a layer in the atmosphere called the Heaviside layer. Today we know it’s a section of the ionosphere. But that means they couldn’t be used for interplanetary communications. They wouldn’t go through Earth’s atmosphere.
Lucky for scientists, they found a subset of radio waves that could make it into space: the ones with the highest frequencies. And while we haven’t exchanged any radio messages with aliens, we do use those frequencies for all kinds of space communication, from keeping in contact with astronauts on the ISS, to deep space spacecraft. Ever wonder how we get those amazing high-resolution images from the James Webb Space Telescope, or send instructions to spacecraft like New Horizons, on the other side of the Solar System? Yep. It’s all radio.
But there’s a problem. Radio waves are kinda crap in terms of how fast you can transmit data. The longer the wavelength of light, the less information you can transfer per second. And remember, the radio band has the longest wavelengths of the entire electromagnetic spectrum. So when New Horizons flew by Pluto in 2015, it took over 15 months to get all the data it collected back to Earth, at the painfully slow rate of 1 kilobit per second. We’re communicating across billions of kilometers of near empty space, but that feat might still not impress some people used to fiber optic cables and cat-6 ethernet cables. And that’s not the only disadvantage our radio signals have. They spread out as they go. The farther out a spacecraft is, the weaker its signal, and the harder it is to pick up its call home, or vice versa.
Radio has served us well for so long, but if humanity wants to continue expanding our space-based activities, our system is going to need a bit of an upgrade. While lasers may evoke visions of spaceship battles and death stars, scientists are using them to transmit data, not destruction. Optical lasers use near-infrared wavelengths, which are shorter than radio waves, allowing for more data to be transmitted per second. Tests of two-way laser communication were conducted in the 1990s, and NASA has even used laser light to transmit an image of the Mona Lisa to the Lunar Reconnaissance Orbiter. As we continue to explore the Solar System, our need for faster and better communication through deep space will grow. In the meantime, optical space lasers are used in Earth’s orbit for satellite constellations like Starlink.
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