When Neil Armstrong was narrating his landing, he says that he took over manually and “flew it like a helicopter” on out to the West to find a good site. At that point you depend on your aeroshell to protect you from the heat.Įxample: On December 11, 2015, the Soyuz TMA 17M mission started from a low Earth orbit at an altitude of 416.7 km, and re-entered with a change in speed of 128 meters per second or 286 miles per hour.īut couldn’t you just hover above the atmosphere much in the same way that the lunar module pilots hovered above the Moon’s surface? You may remember that the Apollo 11 landing module nearly ran out of fuel as they were landing, because the computer was taking them to the steep slopes of a crater with rocks the size of automobiles. You will hit the atmosphere at thousands of miles per hour, and will re-enter in a fiery descent. If you slow down by a tiny amount below that speed, even by just a few hundred miles per hour, as you skim the atmosphere, you will fall too far towards Earth before you complete your orbit. Your spacecraft still has to travel at 16,500 mph relative to our atmosphere to stay in orbit.

So, you can reduce your re-entry speed by orbiting in the same direction that the Earth spins. The Earth itself, with its atmosphere, is spinning eastward below you, at around 1,000 mph. To skim the Earth’s atmosphere in orbit, your spacecraft has to travel at least as fast as 7.8 km / second, or about 17,500 mph. That’s how satellites such as the ISS stay in orbit, and that’s why it is often called free-fall. If you throw it fast enough, what happens is that it gets beyond the horizon before it can hit the Earth, and Earth’s gravity continues to pull it around into a curve until it gets back to its starting point. If you could throw a ball from above our atmosphere, gravity still pulls it down in the same way as on Earth.

To see why it is so difficult with ordinary rockets, here is a quick refresher on orbits.

Ksp race into space