By Jonny Lupsha, Wondrium Staff Writer
NASA’s spacecraft Voyager 2 has sent back data from beyond the solar system, according to a statement released last week. The unmanned Voyager 2 is only the second spacecraft to travel beyond the heliosphere. How did it send data 11 billion miles?
NASA said that Voyager 2 left the atmospheric reach of Sol on November 5, 2018. Around that time, it sent information about the area beyond our sun back to Earth. That data—including figures about the density of plasma in the vast, empty regions between stars—took almost a year to the day to reach us. Even more incredibly, it was sent using equipment that’s only as strong as a cell phone tower here on Earth. The science of signals and bandwidth is marvelous.
“The Voyager radio transmitter power is about 20 watts,” Dr. Benjamin Schumacher, Professor of Physics at Kenyon College, said. “The power that matters, of course, is the power that reaches Earth. If Voyager broadcast those 20 watts equally in all directions, the radio signal on Earth would be incredibly weak.”
If cell phone towers can only transmit a few miles, how does Voyager 2 get its info back to us from several billion times that distance? The answer is how specifically Voyager directs its broadcasting, which it does using a specialized antenna.
“The large parabolic dish antenna focuses the energy to a beam less than one degree wide,” Dr. Schumacher said. “This means that the spacecraft must keep this antenna aimed very precisely at Earth. The narrow beam means that the radio power is greater in that direction.”
In order to “catch” this data as it comes in, NASA makes use of several Earth-based parabolic antennae in California, Spain, and Australia called the Deep Space Network. According to Dr. Schumacher, each of these antennae is 70 meters in diameter and has a total area of 4,000 meters.
However, each antenna must filter out noise from other radio waves, whether they’re from terrestrial sources like other radio signals or from the natural waves resonating from elsewhere in deep space. To do this, NASA has ensured that Voyager broadcasts at a specific radio frequency legally reserved for communications from spacecraft.
With as clear a channel as possible for Voyager 2 to send data back to Earth, the next factor to consider is the speed at which data can be transmitted. As Dr. Schumacher puts it, as Voyager 2 gets farther away, the signal-to-noise ratio is diminished, which reduces the information rate.
“When the spacecraft visited Jupiter, less than a billion kilometers from Earth, data was returned at 115,000 bits per second,” he said. “At Saturn, the rate was less than half that. When Voyager 2 flew past Uranus and then Neptune, the rate was halved again.”
Its predecessor, Voyager 1, is now so far beyond Pluto that it’s reaching the edge of being able to send information back at all.
“Its data can be transmitted only at 160 bits per second—almost 800 times more slowly than when it was by Jupiter,” Dr. Schumacher said. “Voyager is not designed to transmit at any slower bit rate. Thus, at some point in the next decade, Voyager 1 will simply be too far away for us to hear its messages.”
“Though for a few years longer, radio telescopes will be able to find its carrier wave, shining faintly in the radio sky like a star.”
Dr. Benjamin Schumacher contributed to this article. Dr. Schumacher is Professor of Physics at Kenyon College, where he has taught for 20 years. He received his Ph.D. in Theoretical Physics from The University of Texas at Austin in 1990.