Image: NASA Goddard scientists transmitted an image of the Mona Lisa from Earth to the Lunar Reconnaissance Orbiter at the moon by piggybacking on laser pulses that routinely track the spacecraft. Credit: NASA Goddard Space Flight Center

Talking Back from Alpha Centauri


Back when I was working on my Centauri Dreams book, JPL’s James Lesh told me that the right way to do communications from Alpha Centauri was to use a laser. The problem is simple enough: Radio signals fall off in intensity with the square of their distance, so that a spacecraft twice as far from Earth as another sends back a signal with four times less the strength. Translate that into deep space terms and you’ve got a problem. Voyager puts out a 23-watt signal that has now spread to over one thousand times the diameter of the Earth. And we’re talking about a signal 20 billion times less powerful than the power to run a digital wristwatch.

Now imagine being in Alpha Centauri space and radiating back a radio signal that is 81,000,000 times weaker than what Voyager 2 sent back from Neptune. But lasers can help in a major way. Dispersion of the signal is negligible compared to radio, and optical signals can carry more information. Lesh is not a propulsion man so he leaves the problem of getting to Alpha Centauri to others. But his point was that if you could get a laser installation about the size of the Hubble Space Telescope into Centauri space, you could send back a useful datastream to Earth.

The probe would do that using a 20-watt laser system that would lock onto the Sun as its reference point and beam its signals to a 10-meter telescope in Earth orbit (placed there to avoid absorption effects in the atmosphere). It’s still a tough catch, because you’d have to use optical filters to remove the bright light of the Alpha Centauri system while retaining the laser signal.

Read more: Talking Back from Alpha Centauri — Centauri Dreams.

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