“Timothy Peake, official portrait” by NASA/Robert Markowitz – https://www.flickr.com/photos/nasa2explore/18306699143/. Licensed under Public Domain via Commons – https://commons.wikimedia.org/wiki/File:Timothy_Peake,_official_portrait.jpg#/media/File:Timothy_Peake,_official_portrait.jpg

British astronaut Tim Peake is to take part in this year’s London Marathon — despite the fact he’ll be in orbit at the time.

Peake won’t be an official competitor, but will be starting a 26.2 mile run at the same time as the more conventional runners on April 24. The big difference is that he’ll be on a treadmill on the International Space Station, watching an iPad screen which displays a video of the course, “moving” at the same speed as the treadmill.

While he’s in good physical condition, Peake forecasts he’ll complete the run in somewhere between three hours thirty minutes and four hours, slower than his personal best of three hours eighteen minutes on Earth.

That slower time isn’t related to being in space and in microgravity, but rather to a couple of other factors. One is that Peake won’t be trying to run at his full capacity because that could risk a lengthy recover time to return to peak fitness, something that’s needed for his return to Earth eight weeks later.

The other limitation is that he’ll need to wear a harness with shoulder straps and a waistbelt to keep him on the treadmill. Peake notes that this becomes uncomfortable after about 40 minutes of use. The tension also puts a limit on his maximum speed.

Sports Illustrated has calculated that if you actually count the speed at which Peake (and the ISS) is travelling through space, he’ll be completing a marathon every five and a half seconds.

Peake is running the marathon to promote The Prince’s Trust, an organization which provides financial and other support for young people to train for jobs, take part in education, or start a business.