• SpaceX on May 30 launched the NASA astronauts Bob Behnken and Doug Hurley to orbit in the private company's first flight of humans.
  • Behnken and Hurley rode SpaceX's new Crew Dragon spaceship, developed with NASA's help, to the International Space Station.
  • The crew is now wrapping up its test mission and plans to return to Earth in early August.
  • But before Behnken and Hurley return, NASA and SpaceX will inspect Crew Dragon's heat shield, which protects the spaceship from temperatures of 3,500 degrees Fahrenheit during atmospheric reentry.
  • In the rare event that Crew Dragon's shield has been damaged by micrometeoroids or orbital debris, the two men can stay aboard the ISS until another spaceship arrives to take them home.
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After living and working in space for more than two months, the NASA astronauts Doug Hurley and Bob Behnken are due back to planet Earth on August 2.

But before the astronauts can return, their Crew Dragon spaceship — designed, built, and launched by SpaceX with most of the $3.14 billion NASA awarded to SpaceX through the agency's Commercial Crew Program — must pass a crucial inspection.

SpaceX launched Behnken and Hurley with a Falcon 9 rocket from NASA's Kennedy Space Center on May 30. Their experimental flight, called Demo-2, is the aerospace company's first with humans. Once in orbit, the astronauts named their ship "Endeavour" (after the first space shuttle each man flew on), and they docked the new vessel to the football-field-sized International Space Station a day later.

Aboard the ISS, Behnken and Hurley have performed spacewalks to upgrade the power-supply system, assisted with chores, and taken mesmerizing photos of Earth, space, and even Comet Neowise. But until they've safely landed, their mission can't be deemed a success.

"I will start sleeping again when they when they're back safely on the planet," Gwynne Shotwell, the president and chief operating officer of SpaceX, said during a televised briefing on May 1.

The astronauts plan to board Crew Dragon and undock from the ISS on August 1. Depending on weather conditions, the duo should splash down in the Atlantic Ocean on August 2, NASA said on Friday.

Before that happens, the agency plans to use a Canadian-built robotic arm attached to the ISS and onboard cameras to survey the spacecraft for damage.

Looking for rare yet dangerous space-debris hits

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Even grains of sand and flecks of paint can seriously damage a spacecraft. This is a radiator perforated by micrometeoroids or orbital debris, or MMOD.
NASA

NASA and SpaceX need to ensure that Crew Dragon's underbelly hasn't been damaged by objects in space.

In its estimated risk calculations for Demo-2, NASA determined there was a 1-in-276 chance the mission proves fatal. Especially worrisome is the threat of strikes by MMOD, or micrometeoroids (bits of asteroids and comets) and orbital debris (human-made space junk).

Even grains of sand and flecks of paint can inflict serious damage, since they can move much faster than a speeding bullet before slamming into a spacecraft. MMOD has struck NASA spacecraft hundreds of times, perforating radiators (as in the image above), cracking circuit boards, chipping windows, and more.

When Crew Dragon returns to Earth, it will do so with its rounded bottom facing in the direction it's flying. That underbelly is covered with a heat shield, or thermal protection system, made of tiles of a NASA-pioneered, SpaceX-improved material called PICA-X.

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An illustration of SpaceX's Crew Dragon spaceship returning to Earth with a blaze of plasma ahead of its heat shield.
SpaceX via YouTube

The PICA-X tiles are designed to deflect and absorb the energy of atmospheric reentry, when Crew Dragon is moving at about 25 times the speed of sound. During reentry, a superheated plasma of gas molecules builds ahead of the heat shield, and temperatures can reach 3,500 degrees Fahrenheit, NASA told Business Insider.

It's unlikely that any MMOD has damaged the heat shield over the past couple of months, and whatever harm it may have inflicted should be minimal. But NASA is working with SpaceX to be on the safe side and check for any chips, cracks, holes, or other damage.

After the undocking, SpaceX is responsible for ensuring that Crew Dragon is safe to return, which entails reviewing imagery data and running analyses, a NASA representative, Stephanie Schierholz, told Business Insider in an email.

"Dragon was designed for the current MMOD environment, so we anticipate the spacecraft would be able to execute the planned deorbit/entry, and it would be very unlikely that significant damage would occur," Schierholz said.

Schierholz said similar checks are done with Russia's Soyuz spaceships before they return to Earth.

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A test placement of the PICA-X tiles on the first Cargo Dragon's heat-shield carrier structure. The lightweight tiles are designed to withstand temperatures as high as 3,620 degrees Fahrenheit.
Roger Gilbertson/SpaceX via NASA

Getting a peek at Crew Dragon's plasma-proof belly might at first seem difficult, given that a cylindrical "trunk" is attached to the capsule. (The solar-panel-covered module, which provides power to the capsule in space and propels it with a rocket engine, is discarded just before reentry.)

But the trunk is hollow and permits a view.

"The trunk does not completely cover the thermal protection system, and the survey will be able to visually inspect those areas," Schierholz said. "Also, if micrometeoroid damage is observed on the trunk, then NASA can take additional inspection activities if deemed necessary."

If significant damage is found after inspection, NASA and SpaceX wouldn't risk sending the astronauts home on the vehicle. Instead, the crew would stay aboard the space station until a new spaceship could collect them.

"If there is concern about returning the crew on a damaged vehicle, the ISS systems, consumables, and logistics chain supports leaving the crew on the ISS indefinitely as part of a 'safe haven' capability," Schierholz said. "That will provide NASA and the ISS International Partners with the time needed to work with all ISS launch service providers to assess available options for returning the crew safely."

But hunting for damage to Crew Dragon's heat shield is not the only concern for the crew's safety.

'We've looked at this 6 ways to Sunday'

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SpaceX's Crew Dragon is retrieved by the company's recovery ship in the Atlantic Ocean, about 200 miles off the eastern coast of Florida, on March 8, 2019.
NASA/Cory Huston

Before the Demo-2 mission's launch, Elon Musk, SpaceX's founder, told Irene Klotz of Aviation Week that he was most concerned about the landing phase.

"The part that I would worry most about would be reentry," Musk said.

Musk cited the capsule's asymmetric design, owed to an onboard emergency escape system. That system, powered by pods with small SuperDraco rocket engines, is designed to whisk the capsule away from a failing Falcon 9 rocket at any point during launch.

If the asymmetry Musk described somehow causes Crew Dragon to rotate or wobble at hypersonic reentry speeds, it could prove dangerous.

"If you rotate too much, then you could potentially catch the plasma in the SuperDraco escape thruster pods," Musk said. "We've looked at this six ways to Sunday, so it's not that I think this will fail. It's just that I worry a bit that it is asymmetric on the backshell."

In all likelihood, the risks of either heat-shield damage or rotation won't impede the astronauts' voyage. And for their part, Behnken and Hurley accepted those risks long ago.

Behnken told Business Insider ahead of his launch that he and Hurley had worked with SpaceX on Crew Dragon for roughly five years, allowing them to gain more insight into the ways the mission could fail "than any crew has in recent history."

"I think we're really comfortable with it," Behnken said of the mission's estimated risk level.

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This story has been updated.