Typical spaceships orbiting the Earth are not too far away. They are only a few hundred miles above our heads. So when it’s time to get back on solid ground, it’s a relatively short trip.
But spacecraft and similar objects in orbit also move at extremely high speeds. We’re talking 17,000 mph for a satellite or spacecraft in low Earth orbit, and even faster for higher orbits.
Combined, these two factors create a dilemma—and a lot of heat—when returning spacecraft back to Earth: How to reduce an object’s speed from 17,000 miles per minute (or roughly 283 miles per minute) to 0 miles per hour, within the relatively short distance of a few hundred miles?
The parachute option
One solution is the use of parachutes.
Re-entering spacecraft do deploy multiple parachutes to help slow them down—but only for the final stretch of high-speed travel.
This is because parachutes work best closer to the ground, where the air density is greater than the thin air at extreme altitudes. Also, supersonic speeds represent significant challenges for parachutesand physicists still struggle to predict and control these dynamics.
So, while they’re great for the final leg of the return trip, parachutes aren’t typically the best for removing most of a spacecraft’s excess velocity.
What about the rockets?
In theory, we could use missiles to slow down a plane. After all, rockets are great for propelling things into space in the first place, so why not use them in 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 cope with this excess weight and size, most space shuttles eject their massive rocket engines just minutes after a successful launch.
Second, to propel rockets, you need a lot of fuel. To use rockets on the return trip, you’ll need to download them along with additional fuel tanks for the entire space flight.
Given how hard it is to propel things in space and how the current strategy is to discard the fuel tanks after launch, adding extra weight and capacity is impractical and counterintuitive for a mission.
Read more: The biggest space missions to look for in 2023
The atmosphere of Eather as a brake
So what about the forces outside the spacecraft? This line of thought brings us to Earth’s atmosphere, which NASA describes as “jacket for our planet.”
The first person to think of this power as a re-entry advantage was rocketry pioneer Robert Goddard.
Goddard observed that meteorites survive passage through the atmosphere, generating a tremendous amount of heat in the process while maintaining a cool interior.
After traveling through the atmosphere, meteorites seem to slow down enough to not be obliterated the moment they hit the ground. Maybe people could do the same.
In the case of a spacecraft, hitting our atmosphere at more than 10,000 mph can slow the object down, at the cost of excessive heat.
That heat, more than 4,500 degrees Fahrenheit on some missions, comes from spacecraft friction encountering the more compressed air of Earth’s atmosphere compared to the vacuum of space.
Custom spacecraft and flight path
If you design your spaceship right, it can slow down enough without cooking the astronauts inside, and safely deploy parachutes before landing on Earth. This success required a lot of work.
For starters, experts had to design heat shields to withstand re-entry temperatures. In addition, flight paths are now designed for missions so that spacecraft orbit the Earth as they descend, ensuring that they spend long enough in the atmosphere to slow down sufficiently.
By some twist of fate, we don’t have that tool (or challenge, depending on how you look at it) with missions to the Moon or Mars because they don’t have a substantial atmosphere.
This means that to land on these other places, we haul rockets and fuel with us, which is one of the many reasons that landing missions on other worlds are so much more complicated.
Read more: Skycrane: How NASA’s Perseverance rover will land on the Red Planet