Voyager 1 is about to become the first spacecraft one light-day from Earth

Next week, Voyager 1 will set another distance record. NASA says the probe, already the most distant human-made object, will become the first to reach one light-day from Earth: the distance light travels in 24 hours.

The numbers

• When: 10:16:07 UTC on Wednesday, November 18 (5:16:07 a.m. EST, 2:16:07 a.m. PST).
• How far: about 16.09 billion miles, or 25.9 billion kilometers.
• How fast: Voyager 1 is moving at roughly 38,000 mph (61,000 km/h).

A light-day also means a full day of delay. Any command NASA sends will take a day to arrive, and the reply will take another day to come back, so every exchange with the spacecraft takes two days.

Wave at Voyager

NASA is marking the moment with an event called "Shine Bright for Voyager's Light-Day." After sunset on November 17, people are invited to flash the mission's three-letter designation, VGR, in Morse code, using a flashlight or NASA's website. The agency describes the idea as a wave of light rolling around the planet as the sun goes down.

Voyager won't see any of it. Its cameras were switched off decades ago, after it took the famous "Pale Blue Dot" image of Earth. NASA has run events like this before: in 2013, people were asked to look up and smile while Cassini photographed Earth from Saturn.

Still going, just

Voyager 1 launched in 1977 and will mark its 50th anniversary next year. It had a communications hiccup last month, a reminder that the aging probe is running on shrinking power and decades-old hardware.

Its twin, Voyager 2, took a different route past Uranus and Neptune and isn't expected to reach one light-day for another nine years or so. With its power margins already razor thin, it's uncertain whether it will still be able to talk to Earth by then.