Typical spacecrafts orbiting the Earth aren’t extremely far away. They’re only a few hundred miles above our heads. So, when it’s time to return to solid ground, it’s a relatively short trip.
However, spacecrafts and similar objects in orbit are also moving at exceptionally high speeds. We’re talking 17,000 mph for a low-Earth orbit satellite or spacecraft, and even faster for higher orbits.
Combined, these two factors create a dilemma — and a whole lot of heat — when bringing spacecrafts back to Earth: How do we decrease an object’s speed from 17,000 mph (or, roughly 283 miles per minute) to 0 mph, within the relatively short distance of a couple hundred miles?
The Parachute Option
One solution is to use parachutes.
Reentering spacecrafts do indeed deploy multiple parachutes to help them slow down — but only for the final stretch of a high-speed journey.
That’s because parachutes work best closer to the ground where the density of air is greater than the thin air at extreme altitudes. Also, supersonic speeds pose significant challenges for parachutes, and physicists still struggle to predict and control these dynamics.
So, while they’re great for the last leg of a return trip, parachutes are not typical the best at removing the bulk of a spacecraft’s excess velocity.
What About Rockets?
In theory, we could use rockets to slow an aircraft. After all, rockets are great for propelling things up into space in the first place, so why not use them on the descent phase as well?
The problem here is twofold.
First, rockets are extremely heavy — giant shells weighing up to hundreds of thousands of pounds. In fact, to deal with this excess weight and size, most space shuttles jettison their massive rocket engines just minutes after a successful launch.
Second, in order to power rockets, you need lots of fuel. To use rockets on the return journey, you would have to haul them up along with extra tanks of fuel during an entire space flight.
Considering how hard it is to propel things into space, and how the current strategy is to eject fuel tanks after launch, adding extra weight and capacity is impractical and counterintuitive for a mission.
Eather's Atmosphere as a Brake
So, what about forces outside the spacecraft? This line of thinking takes us to the Earth’s atmosphere, which NASA has described as “a jacket for our planet.”
The first person to think of this force as a benefit for reentry was rocketry pioneer Robert Goddard.
Goddard observed that meteorites survive passage through the atmosphere, generating an enormous amount of heat in the process while maintaining cool interiors.
After traveling through the atmosphere, meteorites seem to slow down enough that they don’t obliterate the moment they make impact with the ground. Perhaps humans could do the same.
In the case of a spacecraft, hitting our atmosphere at more than 10,000 mph could slow the object down, at the price of excessive heat.
This heat, more than 4,500 degrees Fahrenheit on some missions, comes from the friction of the spacecraft encountering the more-compressed air of Earth’s atmosphere compared to the vacuum of space.
Customized Spacecraft and Flight Path
If you design your spacecraft just right, it can slow down enough, without cooking the astronauts inside of it, and safely deploy parachutes before landing on Earth. This success required an enormous amount of work.
For starters, experts had to design heat shields that could withstand the temperatures of reentry. In addition, flight paths are now designed for missions so that spacecraft loop around the Earth as they descend, ensuring that they spend long enough in the atmosphere to slow down sufficiently.
In a sort of twist of fate, we don’t have this tool (or challenge, depending on how you view it) with missions to the Moon or Mars, because they don’t have a substantial atmosphere.
That means that for landings on these other places, we do haul rockets and fuel with us, which is one of many reasons that missions to land on other worlds are much more complex.