Before 2 astronauts can return to Earth via SpaceX's Crew Dragon capsule, they'll use a robotic arm to check their ship for space-junk damage

• SpaceX on May 30 launched NASA astronauts Bob Behnken and Doug Hurley to orbit — 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 (ISS).
• The crew is now wrapping up its test mission and plans to return to Earth in early August.
• Before Behnken and Hurley come, though, NASA and SpaceX will inspect Crew Dragon's heat shield, which protects the spaceship from 3,500-degree-Fahrenheit temperatures 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.

After living and working in space for more than two months, NASA astronauts Doug Hurley and Bob Behnken are due back to planet Earth on August 2.

Before they can safely return, the astronauts' Crew Dragon spaceship — designed, built, and launched by SpaceX — must pass a crucial inspection.

SpaceX launched Behnken and Hurley with a Falcon 9 rocket on May 30 from NASA's Kennedy Space Center, and their flight is the aerospace company's first with humans on board. Crew Dragon's development, the astronauts' current experimental mission, called Demo-2, and six future flights were funded with about $2.7 billion of the $3.14 billion in funding awarded to SpaceX through NASA's Commercial Crew Program.

The astronauts dubbed their ship "Endeavour" upon reaching orbit, naming it after the first space shuttle each man flew on, and docked the new vessel to the football field-sized International Space Station a day later.

While aboard the ISS, Behnken and Hurley performed spacewalks to upgrade the power supply system, assisted with chores, and took mesmerizing photos of Earth, space, and even Comet Neowise. Until they've safely landed, though, 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 COO 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 Friday.

Ahead of those maneuvers, the agency plans use a Canadian-built robotic arm attached to the ISS and on-board cameras to survey the spacecraft for damage.

Looking for rare yet dangerous space debris hits

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

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

Even grains of sand and flecks of paint can inflict significant serious damage, since they can move many times faster than a speeding bullet before slamming into a spacecraft. MMOD is known to have struck NASA spacecraft hundreds of times. It has perforated radiators (as in the above image), cracked circuit boards, chipped windows, and more.

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

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 undocking, SpaceX is responsible for ensuring the Crew Dragon is safe to return, which entails reviewing imagery data and running analyses, NASA spokesperson Stephanie Shierholz 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 noted similar checks are done with Russia's Soyuz spaceships before they return to Earth.

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 also propels it with a rocket engine, is discarded just before reentry.) However, 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 can be scrambled to 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."

Hunting for damage to Crew Dragon's heat shield is not the only concern for the crew's safety, though.

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

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

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

Musk said his worry is placed on the capsule's asymmetric design, which is owed to an on-board emergency escape system. That system is designed to whisk the capsule away from a failing Falcon 9 rocket at any point during launch, and it is powered by pods with Draco rocket engines.

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," he said.

This story has been updated.