How Do Astronauts Get Back To Earth?

The science behind entering the Earth's atomosphere.

When an object from space enters Earth's atmosphere, it encounters the gas molecules that make up the air. To move through the atmosphere, the object must displace the molecules. However, this is not a major concern until the object approaches Mach 1, the speed of sound. At this point, molecules cannot move out of the way quickly enough, creating a "wall" that the object must break through, producing a sonic boom.



What does the sonic boom sound like?

Spacecraft and reentry

In the case of a spacecraft entering from low Earth orbit, it falls at about Mach 25, or 25 times the speed of sound. At this speed, the chemical bonds of the air molecules are broken, creating an electrically charged plasma around the spacecraft. This plasma is extremely hot, and spacecraft require special thermal barriers or heat shields to protect both the craft and its occupants during re-entry.

It is important to note that the requirements for launch are essentially opposite to the requirements for re-entry. During launch, the aim is to minimize drag to boost thrust and lift, which often leads to a typical rocket shape. Conversely, during re-entry, drag needs to be maximized to decrease speed, which requires a broad and blunt design.

So how do different spacecraft manage the physical demands of re-entry?

Space shuttles and reentry

The aluminum skin of the space shuttle was insulated by special silicone tiles, while the areas that were exposed to the most heat, such as the nose and wing edges, were reinforced with a carbon-carbon composite material. Unfortunately, the Columbia disaster occurred when the RCC on the shuttle was damaged during liftoff, causing it to burn up during re-entry.

During re-entry, the space shuttle acted like a glider and adjusted its flight path to help manage the heat buildup. The process began with the shuttle flying backward to reduce speed from orbit. The orbital maneuvering engines (OMS) would then thrust it out of orbit and toward Earth. The shuttle would turn nose-first and enter the atmosphere with its belly down, using drag from its blunt bottom to slow it down. It would then pull the nose up to about 40 degrees, utilize S turns and other flight patterns to increase drag and reduce speed and heat, and finally land on runways with landing gear and a parachute deployed to further slow down to about 200 mph.



Once on the ground, rescue teams would assist the crew with exiting the shuttle, and flight surgeons would perform medical checks.

Ballistic capsules and reentry

Ballistic capsules are spacecraft that astronauts use to return to Earth, including programs such as Mercury, Gemini, Apollo, Shenzhou, and Soyuz. Unlike the space shuttle, these capsules are essentially rockets and feature ablative heat shields made of special ceramic material that slowly burns away as the spacecraft encounters the high-temperature plasma flow during re-entry. Re-entry in ballistic capsules is often described by astronauts as a series of car crashes rather than a smooth glider-like landing.



Returning to Earth aboard a Soyuz vehicle takes approximately 3.5 hours. After closing the hatch and sealing the vehicle away from the ISS, springs launch the vehicle away from the space station, and engines fire for nearly five minutes to decelerate it from the ISS's orbital speed of 17,500 mph and reduce its orbit to re-enter the atmosphere. The Soyuz separates into three parts, with only the descent module carrying the astronauts returning to Earth. The orbital module and service module burn up in the denser layers of the atmosphere. As the capsule slows down in the atmosphere, astronauts feel up to five times their weight in pressure and are pushed into their seats. Parachutes further decrease the speed, and rockets fire just before touchdown, with shock-absorbing seats softening the landing, usually at around 5 km/hr. Rescue teams then open the hatch and assist the astronauts. While parachute descent with a splashdown in the ocean is common, landing on land is also possible.

SpaceX Dragon reentry

Following its partnership with NASA, SpaceX's Dragon spacecraft has become the primary spacecraft for transporting astronauts and supplies to and from space. Like the Soyuz spacecraft, the SpaceX Dragon functions as a ballistic capsule, and the following outline is derived from Axiom-1 descent footage.

• Initially, the astronauts use the Dragon's Draco thrusters to initiate a downhill phase burn and lower the altitude.
• Next, the "trunk," or the cylindrical unpressurized section of the spacecraft, detaches to expose the heat shields, and the Dragon switches to battery power.
• A de-orbit burn of around 10 minutes is performed using forward bulkhead thrusters to return to Earth, after which the astronauts close and lock the nose cone in preparation for re-entry.
• The atmospheric re-entry process commences, resulting in a brief loss of communication for approximately 7 minutes. During descent through the atmosphere, Nitrox is used to cool the interior while the ablative heat shields are utilized on the outside of the craft.
• To stabilize and decelerate the craft, 2 Drogue parachutes deploy at roughly 18,000 feet, followed by the 4 main parachutes less than a minute later at approximately 1,000 feet.
• Over the course of three minutes, these parachutes reduce the speed of the spacecraft from 350 mph to around 15 mph for landing or splashdown.
• A rescue team retrieves the capsule, uses a hydraulic lift system to transport it onto a boat, and then safely repressurizes the cabin before opening the hatch.
• The medical doctor or flight surgeon is the first to check on the astronauts, followed by the rescue team, who reminds the crew of the egress procedures.
• The Rescue Crew assists the astronauts out of the capsule, typically with an opportunity for photos and live video, before immediately escorting them to medical bays to address any concerns.

Future reentry plans: LOFTID

NASA is currently exploring the use of an inflatable aeroshell with a flexible heat shield for atmospheric re-entry. This saucer-shaped decelerator can be easily collapsed for launch and then deployed to maximize surface area during re-entry.

The purpose of the inflatable decelerator is to aid in Mars missions, as the planet's thinner atmosphere is insufficient to decelerate a craft upon arrival. The fully inflated decelerator will increase drag and slow the craft, leading to a safe and soft landing.



On November 10th, 2022, the Inflatable Decelerator underwent a successful test. Further updates on testing and deployment for future human spaceflight missions are highly anticipated.