By Paul RinconScience editor, BBC News website
NASAOrion is Nasa's new spaceship for humans, designed to visit destinations such as the Moon and Mars. Here's our guide to America's replacement for the space shuttle.
When astronauts return to the Moon this decade - under a Nasa plan called Artemis - they'll travel there in Orion.
Orion is a vehicle for the future, with something distinctly retro about it. The conical shape of the capsule recalls the Apollo modules that visited the Moon half a century ago.
That's no surprise - the physics of returning to Earth from space haven't changed. A teardrop-shaped object is stable when travelling through our atmosphere at supersonic speeds.
But Orion is packed with technology that couldn't have been imagined when Neil and Buzz landed on the Sea of Tranquility in July 1969.
These advanced capabilities are essential for travelling in deep space, where astronauts face different challenges to those encountered closer to Earth, such as on the International Space Station (ISS).
Orion consists of four main elements:
- A crew module to carry astronauts and cargo
- A European Service Module (ESM) to supply propulsion and electrical power
- A Launch Abort System (LAS) to propel Orion away from danger if a launch emergency happens
- A Spacecraft Adapter to attach Orion to the Space Launch System (SLS) rocket for launch
Building Orion
Conceived under the George W Bush presidency as a successor to the space shuttle, Orion has cost more than $18bn since work began in 2006.
The structural backbone of the crew module is called the pressure vessel, made of an aluminium-lithium alloy. It's full of pockets that give it a honeycomb structure, maintaining strength while reducing the weight.
Nasa / Michael DeMockerThe olive-green pressure vessel forms the backbone of the Orion crew moduleThe pressure vessel is manufactured in different sections which are fused together using a process called friction stir welding. This turns metal into a plastic-like state before stirring it under pressure to create a seamless weld.
The olive-green vessel is then enclosed under panels, which are in turn covered with heat-resistant tiles.
A metallic coating over the tiles further protects the craft from extremes of heat and cold.
The base of the crew module is covered by the largest heat shield ever designed for human missions. It bears the brunt of the extreme heating encountered when Orion returns to Earth.
NASAThe underside of the crew module is covered by the largest heat shield ever designed for human missionsDeep space travel
Inside Orion's cabin are four adjustable seats, which can accommodate a range of different body sizes and shapes. While the astronauts are strapped into them, they can see out through four windows located above the head rests.
Once Orion reaches orbit after launch, crew members can dismantle and stow some or all of the seats to give themselves more room.
NASAThe crew module attached to the European Service ModuleA control console with three display screens and 67 physical switches allows the pilot and commander to monitor the spacecraft during flight. For comparison, each space shuttle had 10 displays along with more than 1,200 switches, dials and gauges.
"You don't have to carry around all the switches and all the wiring, so you save weight," Jeff Fox, Nasa's Orion cockpit lead, told a Nasa podcast. Software now assumes the functions previously performed by those physical switches.
Although the consoles allow crew members to assume manual control, Orion's four flight computers can look after navigation and other critical operations without the need for human intervention. This leaves astronauts free to attend to other tasks.
Robert Markowitz - NASA - Johnson Space CenterTesting the control console inside a mock-up of the crew moduleThe computers are based on a type built for commercial airliners, but are adapted and ruggedized for the rigours of space travel, where G-forces, vibration and radiation can damage delicate hardware.
"That's why we have four sets of flight computers, it's not because we expect them to burn out through electrical failure. It's to handle that deep space environment," Rob Chambers, from Lockheed Martin, which is building Orion for Nasa, told BBC News.
During high-risk parts of the mission, such as launch and ascent, astronauts wear a brightly coloured orange spacesuit called the Orion Crew Survival System (OCSS).
NASAAstronauts and engineers evaluate the design of the docking hatchIt plays the same role as the "pumpkin suits" used aboard the space shuttle, providing air, cooling and communications - as well as protecting the wearer against a depressurisation event, where air is lost from the cabin. But there are numerous enhancements: the OCSS is custom-fitted for each astronaut and provides greater comfort and mobility than the shuttle suit.
Radiation and rocks
Orion is designed to operate beyond the protective magnetic bubble that surrounds Earth - and protects astronauts on the space station. Out in deep space, astronauts are exposed to roughly three times the radiation dose per day that they receive on the ISS.
Space radiation comes from different sources, but the most insidious threat is from solar particle events (SPEs). This is where the Sun periodically releases clouds of charged particles into space.
These bombardments increase the long-term risk of developing cancer or cardiovascular disease.
NASAThe Sun periodically releases clouds of charged particles which can threaten infrastructure on and around EarthWhen Orion astronauts are notified of an SPE, they will climb down into storage lockers under the capsule floor. These are close to the heat shield, which is one of the most heavily protected areas of the spacecraft.
In order to complete the dense radiation shelter, crew members must build a "pillow fort" around themselves using stowage bags that contain supplies and equipment.
Jason Hutt, Orion crew systems integration lead at Nasa, says each 3ft-deep locker can accommodate two people. "They're going to take some food, they're going to take their laptops," Hutt told a Nasa podcast.
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"You're going to be very good friends with your locker buddy," Hutt said.
Radiation isn't the only threat during flight. If a small space rock or piece of orbital debris were to puncture the pressure vessel, it could cause the air to leak out. Orion has been designed to get the crew back to Earth safely with an airless cabin.
Crew members could survive by wearing the OCSS suits, but the onboard electronics also need to function. These generate lots of heat, which is usually lost with the help of fans or airflow. Those heat loss solutions can't function without air, however, so Orion's electronics have been specially designed so that the spacecraft frame absorbs the heat and siphons it away.
Getting to the Moon
Orion lifts off from Kennedy Space Center in Florida atop Nasa's Space Launch System (SLS) rocket - which is more powerful than the Apollo-era Saturn V.
From launch until just before re-entry, the Orion crew module is attached to the cylindrical European Service Module (ESM), which supplies air, water, electrical power and propulsion.
Measuring 4.5m wide and 3m long, the ESM is provided for Orion missions by the European Space Agency (Esa). It has four 7m-long solar arrays that extend outwards from the main body. These tilt and rotate to track the Sun, generating enough electricity to power two three-bedroom homes.
NASA/ESA/ATG MedialabThe ESM has three different kinds of engine, to propel the crew module to its destination and manoeuvre it in multiple directions.
For the first mission to land on the Moon in the 2020s, known as Artemis-3, Orion will dock with a landing craft - expected to be Elon Musk's Starship - in lunar orbit. Crew members will then move from Orion to the lander, which subsequently undocks to carry its passengers down to the lunar surface.
For later missions, the plan is to have both Orion and a lander dock with a small space station in lunar orbit called Gateway. This configuration could support more complex missions.
Returning to Earth
Crew members put the seats back in place to prepare for re-entry. Shock absorbers beneath the seats protect the astronauts from the force of splashing down in the ocean.
The Orion capsule separates from the service module and fires its thrusters to begin the process of returning to Earth.
As the crew module plunges through the atmosphere at supersonic speeds, the heat shield has to withstand temperatures of roughly 2,760C (5,000F).
NASAIt's covered with blocks of a substance called Avcoat, which is "ablative" - it slowly burns off during re-entry, carrying away heat from the capsule.
As the crew module falls to Earth, a total of 11 different parachutes - made from a hybrid of Kevlar and Nylon - are deployed to slow the vehicle down to the speed of 17mph required to safely splash down in the Pacific Ocean.
An amphibious transport ship will be waiting nearby to retrieve the astronauts. The ship tows the Orion capsule onto a flooded well deck and lifts it into a recovery cradle for the journey back to the mainland.
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