Astronaut Tim Peake will officially start the London Marathon - 250km away in space.
Major Peake has recorded a special good-luck message from the International Space Station - where he will run the full 26-mile race - which will be shown to his fellow runners down on Earth on a big screen.
Race organisers said the message - which ends with a 10-second countdown - will trigger the start for 40 elite men and more than 37,000 mass race runners who are following the course from Shooters Hill in south-east London to Westminster.
Floating to the finish line would be cheating, so he will be strapped firmly to a treadmill while watching the streets of London pass under his feet in real time on an iPad.
He finished the London Marathon in a time of three hours, 18 minutes and 50 seconds in 1999. He does not expect to match that performance when he takes part in the Digital Virgin Money event this weekend.
He is aiming to cross the virtual finish line after around four to four-and-a-half hours of running just as thousands of club runners and charity fund-raisers make their way towards The Mall in the heart of central London.
Speaking in a live link to a press conference on Earth earlier this week, Peake said: "I've been doing longer runs at 7.5mph and I'm confident I can keep that going.
"The fitness training is going well. I'm feeling good. We have a great team that support us up here and they've been keeping me in good shape."
One of his biggest challenges will be having the determination to keep going without the company of spectators or other runners. To help him, an iPad will screen a moving image of the run which can be adjusted to his pace. In addition, he hopes to have access to a TV showing scenes from the marathon beamed from Earth.
"It will be a huge boost to me to know that I'm running alongside everybody down there," he said.