SpaceX made 6 major changes to its spaceship and rocket to be ready to launch its first full crew of NASA astronauts

• SpaceX is set to launch its first full astronaut crew for NASA on Sunday in its newly certified launch system.
• Since its test launch of two NASA astronauts this summer, SpaceX has made major adjustments to its Falcon 9 rocket, Crew Dragon spaceship, and spaceflight processes.
• The Crew Dragon has a reinforced heat shield, the Falcon 9 has been inspected for clogs, and the spaceship now knows how to move to new ports on the space station.

Update: SpaceX successfully rocketed the four Crew-1 astronauts into orbit.

SpaceX is ready to send its first full astronaut crew into space for NASA, on a launch system newly certified for human spaceflight. The Falcon 9 rocket is upright on the launchpad, a Crew Dragon spaceship secured firmly to its nose. The engines have been test fired. Four astronauts are anxiously waiting for the countdown — launch is scheduled for Sunday at 7:27 p.m. ET.

Since SpaceX's first human launch, a demonstration that rocketed NASA astronauts Bob Behnken and Doug Hurley to the International Space Station (ISS), the company has ironed out several wrinkles with its system.

In that mission, called Demo-2, the Crew Dragon spaceship's fiery fall through Earth's atmosphere wore away its protective shield more than expected. Its parachutes also deployed a little late, and then boats full of spectators swarmed the toasted capsule once it landed. Additionally, a few weeks after the astronauts returned, another Falcon 9 launch revealed that a nail-polish-like substance was clogging up vents in the engines.

NASA and SpaceX have worked quickly to correct these malfunctions, adapt their processes, and upgrade the hardware. As a result, the upcoming mission, called Crew-1, is going to look different from Behnken and Hurley's.

"We're flying a lot of upgrades on this flight compared to Demo-2," Steve Stich, the manager of NASA's Commercial Crew Program, said in a briefing on Tuesday. "So we continue to evolve and be safer as we move toward these flights."

Here's how SpaceX changed its system.

Checking tiny holes for nail-polish-like buildup

SpaceX was set to launch a routine satellite mission on October 2, but seconds before liftoff, a flight computer sensed an issue and automatically shut everything down.

The problem turned out to be in the Falcon 9's Merlin engines: A tiny hole on a relief valve had gotten clogged with a lacquer used to cover certain parts of the rocket while a treatment to prevent corrosion gets applied to other areas. This could have resulted in an explosion that would have rattled and damaged the engines — like a car backfiring.

So SpaceX examined all its engines for similar remnants of the nail-polish-like substance.

"SpaceX and the NASA team looked across the launch vehicle for any other areas that might have a small passage that could get blocked by this similar masking process. And we cleared all that hardware," Stich said.

Separately, the engines also have new bladed, jet-engine-like turbine wheels that pump in the liquid fuel. The upgrade makes the wheels less prone to vibrations called "resonances," which can damage hardware.

"These wheels are a little bit more robust to the kind of throttle profile that the Merlin engines fly," Stich said. "So we're happy to have that as a safety upgrade."

New, long-lasting solar panels

The crew of the upcoming mission, called Crew-1, consists of NASA astronauts Mike Hopkins, Victor Glover, and Shannon Walker, and Japan Aerospace Exploration Agency astronaut Soichi Noguchi. They're slated to dock to the space station around 11 p.m. ET on Monday after a 27-hour journey. After that, they'll spend six months on the orbiting laboratory.

Their Crew Dragon capsule will stay at the ISS for all that time, too. Keeping a spaceship in orbit may sound easy, but as it sits there, the vehicle gets doused in solar radiation and weathers extreme temperatures. In fact, the capsule Behnken and Hurley flew, which they named Endeavour, probably wouldn't have survived a full-length six-month mission in space. Its solar panels likely would have degraded significantly after 110 days in orbit.

So Resilience, as the new capsule is known, has a set of solar panels that can last 210 days (nearly seven months).

New software allows the Crew Dragon to switch ports on the ISS

Docking to a port on the space station requires the Crew Dragon to execute a series of complex maneuvers. But it might not be able to stay docked in the same place for the full six months. Unlike the capsule used in the Demo-2 flight, this one is programmed with the ability to move itself to another port in order to make room for other incoming spaceships.

"It's getting a little crowded in space. And that's a really good thing," NASA astronaut Suni Williams said in a Friday briefing.

Williams is set to fly on the first operational mission of Boeing's CST-100 Starliner — that company's spaceship for astronauts, which was funded and designed through the same NASA program that gave rise to SpaceX's Crew Dragon. The Starliner is set to reattempt an uncrewed test flight to the space station in 2021, since the first test failed. That might require the Crew Dragon to move to a different port.

To relocate the spaceship, the crew will climb back in and run new software that should maneuver the Crew Dragon away from its original docking point, the Forward Port, and re-dock to the station's Zenith Port.

The flight is programmed to be autonomous, but the crew will monitor the process. They can manually maneuver the spacecraft if anything goes wrong.

A fortified heat shield for the plunge back to Earth

After Behnken and Hurley splashed into the Gulf of Mexico and a recovery crew pulled them out, SpaceX studied the toasted Endeavor spaceship up close. Examiners found something unusual: deep erosion on the Crew Dragon's heat shield.

This thermal protection system is a collection of heat-resistant tiles that line the spaceship's underbelly. It deflects and absorbs heat that can reach 3,500 degrees Fahrenheit as the space capsule plummets through the atmosphere on its return to Earth.

One of the heat shield's tiles suffered "a little bit more erosion than we wanted to see," Hans Koenigsmann, SpaceX's vice president of build and flight reliability, said in September.

So SpaceX reinforced the new Crew Dragon's heat shield with stronger materials. NASA then tested five samples of the new tile in a wind tunnel that simulates the environment of reentry.

"I'm confident that we fixed this particular problem very well," Koenigsmann said.

Parachutes should open higher up

As the Crew Dragon approaches Earth toward the end of its descent – traveling 350 miles per hour — it should deploy its first two parachutes at about 18,000 feet. But on Behnken and Hurley's reentry flight, that happened closer to the ground than NASA and SpaceX expected.

As it turned out, a filter that vents air into the sensor that measures atmospheric pressure had gotten clogged.

"That particular filter has been opened up a little bit so that it's less constrained," Koenigsmann said. "That has the effect that we measure the barometric pressure more accurately and we come down and supply the parachute right on time."

Additionally, Stich said, "we've made some improvements to the [spaceship] structure so that we can handle a little bit more wind at the landing zone, which gives us a few more opportunities to land."

Keeping boats away from the landing site

When the Endeavour capsule splashed down in the Gulf of Mexico, a beeline of onlooker boats swarmed the toasted spaceship. Some passed alarmingly close.

That's a hazard for the astronauts and a danger for the boaters, since after plowing through Earth's atmosphere, the capsule was shrouded in low levels of a poisonous gas called nitrogen tetroxide.

To keep would-be spectators away from the Resilience capsule when it eventually returns to Earth, SpaceX and NASA plan to set up a 10-mile no-boat perimeter around the splashdown site. They also expect to have more Coast Guard boats there to enforce it.

"We ask that all boaters respect all notifications and guidance from our partners with the Coast Guard and the Navy," Benji Reed, SpaceX's senior director of human-spaceflight programs, said in a briefing on Tuesday. "Any distraction or interference with those operations can cause significant delays and actually can put the crew themselves and our recovery teams at risk."