NASA billed it as a year in space, but Mr. Kelly’s trip will actually be a few weeks short of that. (An unexpected delay could extend his stay slightly.) Still, it dwarfs the previous NASA record, held by Michael López-Alegría, who spent 215 consecutive days in space in 2006 and 2007.

But it is far short of the all-time record of almost 438 days. That was achieved by Valeri Polyakov on the old Russian Mir space station back in 1994 and 1995.

This is Mr. Kelly’s fourth mission, so his cumulative time in orbit will be 540 days, assuming an on schedule return.

The International Space Station zips around Earth at more than 17,000 miles per hour, or once every 90 minutes. That means over the course of Mr. Kelly’s stay, the space station will have made 5,440 orbits, and the sun will have gone up and down 10,944 times from the perspective of the astronauts aboard. Of course, Mr. Kelly did not see all of them. He is not continuously looking out the window, and he sleeps, too.

That is the distance that Mr. Kelly will have traveled during this mission, or roughly the distance for a one-way trip to Mars. Of course, on the space station, Mr. Kelly was never more than about 250 miles from Earth.

That is the amount of recycled urine and sweat that Mr. Kelly will have drunk, according to NASA. Water is heavy and expensive to transport from Earth, so for efficiency, water is continually recycled. That technology will be essential for interplanetary missions in which astronauts will not have anywhere to stop to pick up supplies.

That is about how far Mr. Kelly ran on a treadmill. (In space, bungee cords and power racks take the place of gravity.) Exercise is one of the strategies NASA uses to limit weakening of bones and muscles in zero gravity. In all, he will have exercised more than 700 hours.

One of the treadmills on the space station is called Colbert, a shortening of Combined Operational Load Bearing External Resistance Treadmill, and yes, it is named after Stephen Colbert, the host of “The Late Show With Stephen Colbert.” Back in 2009, NASA polled the public on what to name a new module for the space station. Mr. Colbert mobilized the viewers of “The Colbert Report,” his Comedy Central show at the time, to write in “Colbert,” which won, but NASA ended up naming the module Tranquility.

As a sort of consolation prize, the treadmill was named Colbert.

Mr. Kelly worked on more than 400 experiments aboard the International Space Station. One of the main purposes of the space outpost is as a platform to perform biology, physics and health studies and see what is different when everything is floating.

With the nearly one-year mission, NASA health experts hope to gain more insights on the effects of long-term weightlessness. They will be able to compare data gathered during this trip with that from Mr. Kelly’s earlier six-month stay on the space station – and also with data from his twin brother, Mark Kelly, a retired NASA astronaut.

Whatever Earthly things Mr. Kelly may have been missing during the mission, the Internet was not one of them. He has averaged more than two Instagram posts per day during the mission. And he has connected with his 900,000 followers on Twitter more than 2,000 times, although some posts were certainly sent before the launch.

Last week, Mr. Kelly put out a video on Twitter that raised the question: How does a gorilla suit get to space (especially given that it costs about $10,000 a pound to lift something from ground to orbit)?

Brandi Dean, a NASA spokeswoman, explained: “The short answer on the gorilla suit is that friends and family are given a certain amount of cargo space to use as they like to send things up to the crew on the station. It is checked to ensure it meets safety requirements, but otherwise is considered private and confidential. The gorilla suit was sent up to Scott as part of one of those deliveries.”

“Needed a little humor to lighten up a #YearInSpace,” he wrote on Twitter when he posted the video. “Go big, or go home. I think I’ll do both.”