The first ever test flight of SpaceX’s new Crew Dragon capsule will come to an end tomorrow, when the spacecraft detaches from the International Space Station and attempts to splash down in the Atlantic Ocean using a suite of parachutes. This is the last major milestone of the capsule’s mission — and perhaps the biggest challenge the Crew Dragon faces yet.

The vehicle needs to prove its novel shape and parachute system can survive the plunge through Earth’s atmosphere in one piece, while keeping its inner cargo safe. There won’t be any living passengers inside Crew Dragon when it makes its descent tomorrow. But as a vital part of NASA’s Commercial Crew Program, SpaceX’s vehicle will be tasked with transporting NASA astronauts to the ISS in the years ahead. And it will of course have to bring them back to Earth safely afterward.

it’s been decades since people have returned from space this way

The Crew Dragon’s landing technique is similar to that of its predecessor, SpaceX’s Dragon cargo capsule, which has been traveling to and from the ISS since 2012. After re-entering Earth’s atmosphere from space, the cargo capsule deploys parachutes in order to safely touch down in the Pacific Ocean. But it’s been decades since people have returned from space this way. NASA astronauts used to land in the ocean via parachutes during the Gemini, Apollo, and Skylab missions of the 1960s and ‘70s. But once the Space Shuttle started flying in the 1980s, all astronauts have returned to solid ground when coming back from space.

For that reason, there will be a lot of scrutiny over this landing, especially when the capsule re-enters Earth’s atmosphere. Crew Dragon may be similar to the cargo Dragon, but there are slight differences in the shapes of the vehicles that could cause a problem. The cargo Dragon has a smoother cone shape while the Crew Dragon is more asymmetrical. And that could cause some instability when the Crew Dragon streaks through the atmosphere.

“hypersonic re-entry is probably my biggest concern.”

The asymmetrical shape has to do with the fact that the Crew Dragon has an important feature the cargo version doesn’t have: an emergency abort system. Embedded in the outer walls of the Crew Dragon are eight thrusters called SuperDracos that are needed in case there’s a major catastrophe during a future flight. If for some reason a rocket carrying the Crew Dragon starts to deteriorate during launch, the SuperDracos will ignite and quickly propel the capsule away to safety.

This asymmetric shape may cause the Crew Dragon to roll during hypersonic re-entry — when the vehicle is going faster than the speed of sound through Earth’s atmosphere. It’s something that’s worrying SpaceX CEO Elon Musk. “I say hypersonic re-entry is probably my biggest concern,” he said during a press conference after the capsule launched on Saturday. However he noted that he thinks this rolling is unlikely, based on simulations the company has run 1,000 times.

Additionally, both NASA and SpaceX will be closely watching the performance of the Crew Dragon’s parachutes during tomorrow’s landing. Four main parachutes should deploy in order to slow the vehicle’s descent into the water. The company has done 17 parachute drop tests so far, according to SpaceX’s vice president Hans Koenigsmann, but NASA is still in the middle of qualifying this system for human spaceflight. “We’re looking at the physical parameters of how the chutes operate and if we covered all the corners of the envelope and testing,” Bill Gerstenmaier, NASA’s associate administrator for human exploration, said during a press conference a week before the launch.

In fact, NASA specifically picked the launch dates it did for this mission so that the Crew Dragon would return when the Sun is out. That way the agency can fully see how the parachutes look when they deploy and what the splashdown will look like. Once the vehicle has made it to the ocean, a SpaceX recovery ship will meet up with the spacecraft, lift it out of the water, and transport it back to land.

While no people will be on board for this landing, SpaceX’s “smart” dummy named Ripley will be riding inside, wearing one of the company’s custom flight suits. Equipped with multiple sensors for gathering data, Ripley rode to space in the vehicle on Saturday and has remained at the ISS since the Crew Dragon docked on Sunday. Soon the mannequin will be getting a feel for what the descent to Earth is like, which will help NASA know how many extra Gs future astronauts will feel. Ripley will also be riding with more than 300 pounds of return cargo that has been packed inside the capsule.

Today at 12:39PM ET, the crew on board the ISS closed the hatch on the Crew Dragon in preparation for tomorrow’s departure. The capsule’s exit begins super early on Friday, with undocking scheduled to take place at 2:31AM ET. It will slowly creep away from the ISS and about five hours later, the Crew Dragon will ignite is thrusters to take itself out of orbit. That’s when the harrowing plunge will begin, with Crew Dragon slated for splashdown at around 8:45AM ET.

For those waking up early on Friday, you can catch NASA and SpaceX’s live coverage of undocking starting at 2AM ET and then coverage of the landing starting at 7:30AM ET.