NASA is building a huge rocket called the Space Launch System (SLS) to send astronauts to the moon in the future - and then to Mars.
With its debut performance scheduled for November 2021, the SLS is the most powerful launch vehicle built since the 1960s.
NASA has plans to send a woman and a man to the moon by 2024, which will be the first human landing since Apollo 17 in 1972.
For the past 20 years, astronauts have routinely traveled from I to the International Space Station (ISS).
But the Moon is almost 1,000 times farther from the ISS; in order for astronauts to reach it, a monstrous rocket is needed.
SLS is the modern equivalent of Saturn V, a huge launcher built during the Apollo era. Like Saturn, this rocket is divided into segments, stacked on top of each other. But this rocket also contains space shuttle technology.
The first version of the SLS will be called Block 1. In the coming years, it will undergo a series of improvements so that it can launch increasingly heavy loads to originals farther from the low Earth orbit.
Block 1 SLS will rise 23 floors above the launch pad - which will be higher than the Statue of Liberty
"It's a really huge rocket. You'll be amazed at how tall it is, "said John Shannon, vice president and program manager at SLS at Boeing, the primary rocket contractor. He told BBC News in 2019: "When you see SLS assembled, you realize that nothing similar has been seen since Saturn V."
The rocket will launch astronauts in NASA's future generation of crew vehicles - Orion, bringing it to the speeds necessary to get out of low Earth orbit and move further towards the Moon.
How a rocket works
The SLS consists of a giant central core with two solid fuel boosters on either side. There are two large tanks in the central core: one for liquid hydrogen, fuel, and the other for liquid oxygen, the "oxidizer," which makes that fuel burn.
Together, they make up the fuel.
At the bottom of the central core are four RS-25 engines, the same ones that powered the space shuttle orbiter, withdrawn in 2011.
When liquid hydrogen and oxygen are injected into the engine chambers and ignited by a spark, the chemical reaction produces a huge amount of energy and steam.
The steam comes out of the engine nozzles at a speed of 16,000 kilometers per hour to produce thrust - a force that pushes the rocket through the air.
Solid fuel boosters give the rocket extra power to wriggle out of the jaws of gravity. These two booster twins are over 17 floors high and burn six tons of solid propellant every second. They provide 75 percent of the total thrust during the first two minutes of flight.
The most powerful rocket of all time?
If we use thrust as a measure, in 2021 the SLS will be the most powerful rocket ever launched into space. Block 1 SLS will generate 8.8 million pounds (39.1 megawatts) of thrust at launch, which is 15 percent more than Saturn V.
In the 1960s, the Soviet Union made a rocket called the H1, in an attempt to reach the moon. Its first stage could produce 10.2 million pounds (45.4 megawatts) of thrust. But all four test flights were unsuccessful.
The future version of the SLS - called Block 2 Cargo - should approach the degree of thrust possessed by the H1. But the aircraft called Starship, made by the company Ilona Maska SpaceX, should surpass both - producing as much as 15 million pounds (66.7 megawatts) of thrust. Starship is currently under construction, but there is no scheduled date for its first flight.
How the shuttle technology was used again
The central core of the SLS is based on the outer tank of the space shuttle covered with foam. This tank supplied propellant to three RS-25 engines at the rear of the shuttle orbiter. Solid fuel boosters play a very similar role in both aircraft.
But SLS is a very different beast. A large number of components and constructions taken from the shuttle have undergone significant design changes due to the different pressure levels to which they are exposed by the SLS.
As an example of the difference in these pressures, in the space shuttle, the RS-25 engines were tilted and placed away from the solid fuel booster. Moving them closer to the boosters exposes them to greater shaking. As a result, every system in the complex part of the SLS engine had to be rigorously tested to ensure that it could withstand vibrations.
