Scott Kelly is coming home tonight after almost a year in space. At around 11:25 p.m. Eastern Time, Kelly and Russian cosmonauts Mikhail Kornienko and Sergey Volkov will touch down southeast of Dzhezkazgan, Kazakhstan and set foot on solid ground. You can watch parts of the trip on NASA’s website as they happen, and follow along with the minute-by-minute schedule here or here, but this is how the whole return trip to Earth works.
With the United States’ Space Shuttle Program shut down for good, Kelly, Kornienko, and Volkov will not so much fly as plummet back to Earth in the Russian Soyuz capsule. The Soyuz, mounted on top of a rocket, is one of a few spacecraft that can both take supplies and passengers to the ISS and, critically, bring them safely back to Earth. Soyuz capsules come and go from the ISS every few months, but this will be the first time Kelly gets a trip planetside in 340 days. In December, he watched three of his colleagues detach from the ISS and head home, waving farewell from a window.
Leaving the ISS is a bit more complicated than driving out of your garage. For one, your garage isn’t orbiting the earth at 28,000 kilometers per hour. First, the ISS crew packs up the Soyuz vehicle and runs simulations and tests to make sure all of the craft’s systems are operating. When it’s ready, the crew says their goodbyes (which happened today around 4:15 p.m. Eastern) and gets in the craft. They spend the next three hours doing careful checks, especially around the seals of the Soyuz hatch (leaving the door unlocked can be a problem in space), put on their spacesuits, and get into the descent module of the Soyuz.
The Russian portions of the ISS (which is a hodgepodge Franken-station of the world’s best technology) have several docking points for Soyuz craft. Depending on where the Soyuz craft is docked, the station may have to change its orientation, so the Soyuz can undock from the station safely. When everything is ready, the station commander — as of yesterday, American astronaut Tim Kopra — will give the go ahead to release the Soyuz’s hooks, the only mechanical pieces holding the craft to the station. After 3-4 minutes, the hooks are open, and special “pushers” slowly move the Soyuz away from the station, at around 12 centimeters per second. The Soyuz can’t use its thrusters this close to the station, as the dust and chemicals could damage the ISS.
After another few minutes, the Soyuz is far enough away to engage its thrusters for about 15 seconds, moving it away from the station at about 2 kilometers per hour, getting some much needed space from the ISS.
The De-Orbit Burn
At this point, Kelly and his comrades are still orbiting the Earth at 28,000 kilometers per hour. They spend the next few hours doing final preparations for the de-orbit burn — which has to be perfectly calculated so the craft falls back to Earth at precisely the right speed and angle. For the Soyuz, and the men inside, this is the point of no return (to the ISS, at least). When they’re ready, they’ll fire the Soyuz’s main engine for four minutes 49 seconds and slow the craft down, pulling it out of orbit and starting its long fall toward Earth.
The Earth’s atmosphere acts as a giant brake, slowing the craft down even more but creating huge amounts of friction and heat. Around 10:59 p.m., 30 minutes before the Soyuz is scheduled to land, the craft will blow apart, 140 kilometers above Earth. Don’t worry, it’s supposed to do that. Explosive bolts fire off and break the craft into three parts, only one of which, the Descent Module (with Kelly and the Russians inside) will make it back to Earth intact.
The other two parts will burn up in the atmosphere, victim to the brutal friction heat of plowing through air after the smooth vacuum of space. The Descent Module is covered in heat shields, but the astronauts inside will still go through some rapid climate changes and have to handle significant G-forces, up to nine times Earth’s gravity depending on the conditions of the descent. At this point, Kelly and crew will begin to hear the sounds of wind rushing past the craft, which is traveling at almost the speed of sound. When the craft is 8 kilometers off the ground, the Soyuz’s giant main parachute, 1,000 square feet of life-saving fabric, flies open and slows the craft down to about 22 miles per hour. When the capsule is five and a half kilometers up, the heat shield, front window glass, and any excess fuel and oxygen are jettisoned, and helicopters will start to buzz around the capsule as it drifts down to Earth.
Long before the Soyuz leaves the ISS, ground teams carefully select, prepare, and map out their ideal landing zone in Kazakhstan. About a week before, they start running training simulations to run ground crews through the different scenarios and circumstances they may face. Right before the Soyuz hits the ground, its retro-rockets fire, slowing it down to just five kilometers per hour.
“The soft landing is not really soft,” said European Space Agency astronaut Paolo Nespoli, a former mission commander, in an ESA video. “You’re not talking, to not put your tongue in the middle of your teeth … and you’re waiting for this soft landing which for me felt like the collision between a truck and a small car, and of course, you’re in the small car.”
The European Space Agency has a wonderful video explaining, and showing, the entire process. You can watch the full 20-minute clip below.
If the not-so-soft landing goes well, the astronauts will be back on Earth before midnight tonight. For Kelly, it’ll take quite some time to re-adjust after being chased by gorillas and watching the Super Bowl, but fortunately we’ve prepared a handy guide of all the stuff he needs to catch up on. Welcome home, Commander Kelly.