The Crew Dragon spacecraft, produced by private company SpaceX, is scheduled to return from the International Space Station (ISS) and splash down in the Atlantic ocean on August 2.
Contingent on a favourable weather forecast and a successful final week at the ISS, NASA astronauts Robert Behnken and Douglas Hurley will begin the undocking procedure on August 1, and re-enter Earth's atmosphere the next day – a total of 64 days since lift off.
The historic launch took place on May 30 from NASA's Kennedy Space Center in Florida, marking the first time a commercial space company has carried humans into orbit around Earth.
But while the launch was a nail-biting experience to watch, reentry will be even more risky – presenting a tense moment for mission control. SpaceX founder Elon Musk said that the reentry is indeed his "biggest concern".
The joint SpaceX and NASA mission was successful in docking with the ISS, so that astronauts could complete scientific and maintenance work, including four spacewalks.
Importantly, the mission's primary purpose is to test and demonstrate the vehicle's capability to safely carry crew to and from Earth orbit, as the first step in the plan of commencing regular ISS missions and commercial space flights.
Reentry danger points
The extreme velocities and temperatures the vehicle must endure present a major challenge to engineers and makes reentry the most perilous part of a mission.
The danger starts with finding the right angle of the trajectory as the spacecraft enters the upper atmosphere. If it is too steep, the astronauts will experience potentially fatal g-forces, and the friction of the air drag could cause the spacecraft to explode. If it is too shallow, the capsule will instead catastrophically skip off the atmosphere and back into Earth orbit.
The spacecraft will enter the upper atmosphere at 27,000 km/hour. That is 7.5 km/second, or more than 20 times the speed of sound. In whichever units you prefer – this is fast.
At these velocities, a very strong shock wave forms around the front of the vehicle, compressing and superheating the air. Managing the immense thermal load is a huge reentry engineering challenge.
At the most extreme stage, the temperature of the air in the shock layer exceeds 7,000°C. By comparison, the temperature at the surface of the Sun is around 5,500°C.
This makes the vehicle's heat shield so hot that it starts to glow — a process called incandescence. SpaceX's new and advanced PICA-X material heat shield has managed to protect the capsule in test flights, later being recovered in a very charred state.
The air molecules around the vehicle also break down into positively charged atoms and free electrons – a so-called plasma. When some of the molecules recombine, excess energy is released as photons (light particles) – giving the air around the vehicle an amber glow.
This plasma layer may be beautiful, but it can cause radio blackouts. When an electron travels along a conductive wire, we have electricity.
Similarly, when free electrons move through the plasma around the vehicle, we have an electric field. If the electric field becomes too strong, it can reflect and attenuate the radiowaves trying to reach the spacecraft.
Blackout not only leads to a loss of connection to on-board crew and flight data, it can also make remote control and guidance impossible. The Apollo missions, the Mars Pathfinder and the recent, failed 2018 Soyuz rocket launch all incurred communications blackout on the order of minutes.
NASA mission control are anticipating a nervous six minutes of blackout during the peak heating phase of Crew Dragon's return – if anything goes wrong during this time, it's in the hands of the astronauts.
Another risky stage is the parachute-assisted landing. The Crew Dragon will deploy four parachutes upon the final stage of reentry, as the vehicle descends toward a gentle splashdown in the Atlantic Ocean off the coast of Florida.
This manoeuvre has been tested by SpaceX 27 times prior to next week's crewed landing, so it should work.
A successful landing will have huge implications – lowering the cost of space exploration through the use of reusable rockets and enabling private space exploration.
While SpaceX engineered the Crew Dragon vehicle under contract to NASA, the company is free to use the spacecraft for commercial flights without NASA involvement after operational certification.
SpaceX has a partnership with commercial aerospace company Axiom Space, which has the ultimate goal of building the world's first commercial space station.
The proposed commercial activities for the station are broad: from in-space research and manufacturing to space exploration support.
Then there is space tourism. Private citizens are already queuing for their ticket to space, and with a successful Crew Dragon splashdown, they won't be waiting long.
American space tourism company, Space Adventures (partnered with SpaceX), are planning to offer zero-gravity atmospheric flights, orbital flights with a spacewalk option and laps of the Moon by late 2021.
Whether the costs, environmental impact and dangers of spaceflight is justified for space tourism is debatable. As this articles shows, the required safety briefing for Space Adventure ticket holders will be much more comprehensive than your regular "please take a moment to read the safety card in the seat pocket in front of you".