On November 18, something will happen that has never happened before in human history.
Voyager 1 will become the first human-made object to reach one light-day from Earth.
One light-day. About 16.1 billion miles.
Voyager 1 left Earth in 1977, flew past Jupiter and Saturn, and has spent nearly half a century racing away from us. Today it travels through interstellar space at roughly 38,000 miles per hour. After 49 years, it will finally have traveled the same distance light covers in just 24 hours.
When we think about light, we usually think about flipping on a light switch or waiting for the Sun to come up. But visible light is only a tiny part of the electromagnetic spectrum. Radio waves are light, too. So are microwaves, infrared, ultraviolet, X-rays and gamma rays. In the vacuum of space, all of them travel at the same cosmic speed limit: about 186,000 miles per second.
That includes the radio signals we use to talk to Voyager.
Once Voyager crosses the one-light-day mark, a command sent from Earth will take about 24 hours just to reach it. If Voyager could answer immediately, its response would need another 24 hours to get home.
Send a message Monday. Hear back Wednesday.
But Voyager isn’t the only thing we’ve been sending out into space.
For more than a century, Earth has transmitted radio and, later, television signals. The electromagnetic waves that escape Earth continue outward at the speed of light. Just like your car radio gets fuzzy as you drive away from the station, these signals become weaker with distance. Across light-years of space, “weaker” is putting it mildly. Voyager uses a 12-foot-wide antenna to hear our faint signals from Earth, while NASA’s Deep Space Network uses enormous antennas, some as wide as 230 feet, to hear Voyager whisper back.
So, assuming our alien neighbors have a ridiculously advanced antenna, an inexplicable fascination with American media, and somehow managed to subscribe to the right network, we can use a few familiar broadcasts to put these distances into perspective.
On December 8, 1941, Americans gathered around their radios as President Franklin Roosevelt addressed Congress and the nation following the attack on Pearl Harbor. The wavefront carrying that broadcast has now traveled nearly 85 light-years from Earth.
Ten years later, on October 15, 1951, I Love Lucy premiered. Somewhere about 75 light-years from Earth, Lucy and Ricky’s antics are only now arriving.
Fast-forward another 60 years to Game of Thrones, which premiered in 2011, and we can actually go outside and put a star to this concept.
Sirius, the brightest star in our night sky, is about 8.6 light-years away. Find Orion and follow the three stars of his belt down and slightly to the left, where you’ll run straight into the unmistakably bright Sirius.
Now imagine someone living near Sirius with our absurdly good antenna and, apparently, an HBO subscription. The first episodes of Game of Thrones reached their neighborhood about when you were enjoying Thanksgiving dinner in 2019. They already know all about the Starks and Lannisters, but the 2019 series finale won’t reach them until around 2028.
Our extraterrestrial friends are still waiting to find out who will ultimately sit the Iron Throne.
No spoilers.
These distances are why astronomers eventually give up on miles. The Moon is about 239,000 miles away on average, but light crosses that distance in only 1.3 seconds. The Sun is about 93 million miles away, or roughly eight light-minutes. On November 18, Voyager will be one light-day away.
Then we leave our Solar System.
Our closest neighboring star system, Alpha Centauri, is more than four light-years away. Sirius is 8.6. Even the light from our nearest stellar neighbors has been traveling for years before it finally reaches your backyard.
Distance and time become inseparable.
Look at the Sun and you’re seeing about eight minutes into the past. Hear from Voyager and you’re receiving yesterday’s message. Look at Sirius and you’re catching photons that left the star more than eight years ago.
So this month, find Orion rising over the Arizona sky and follow his belt down to Sirius. While our radio and television signals carrying more than a century of human history race outward through the Galaxy, light carrying the history of the stars is racing back toward us.
Somewhere in between, Voyager keeps plugging along.
On November 18, after 49 years of travel, the most distant machine humanity has ever built will finally be one light-day down the road.
Space is big.
Even light needs time to cross it.
Clear skies and happy viewing.
Adam England is the owner of a local financial services firm and moonlights as an amateur astronomer, writer, and interplanetary conquest consultant. Follow him on Instagram @TheBackyardAstronomerAZ and at http://www.ManzanitaInsuranceAndAccounting.com
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