How to land on planets

This is one of NASA's most difficult tasks. Getting off the Earth is easy. All you need is plenty of rocket power. Take off and landing of rockets is similar to flying an airplane. It's relatively easy to take off, but landing is really tricky and dangerous.

When you're flying a spacecraft in open space, all you have to worry about are the gravitational forces of celestial objects like the Sun, planets and moons. Basically, you are in orbit around something. Even the Sun is in orbit around the Milky Way Galaxy's center.

There are two things you can do when you approach a planet. One is to slow to the correct velocity to orbit around the planet. This velocity is a product of the planet's gravity, which is determined by its mass. There's a formula for that. A simplification of this is that the velocity needed is equal to the escape velocity of the planet divided by the square root of 2. The escape velocity of Earth is roughly 11.2 km/s so it comes out to around 7.9 kilometers per second.

On the other hand, you might want to land on the planet. This is where it gets tricky. If the planet or moon has no atmosphere, all you do is plot the trajectory you want and then slow down enough to fall into the planet's gravity and then use retro rockets to slow down before hitting the surface. This takes some careful calculations and precise timing. This is how the lunar lander worked on the Moon landings. Neil Armstrong managed to land using his flying skills when the radar-guiding computer malfunctioned.

If the planet or moon has an atmosphere, the landing task is more complicated. The way that most landings have been made from orbit around Earth is to slow up so that the capsule or lander begins to hit the atmosphere and causes friction to slow the vessel so that it can drop down at a normal speed, ultimately using a parachute to break the fall. The trouble with this idea is that at tens of thousands of miles per hour, the friction generated with the atmosphere rises to thousands of degrees. NASA equipped the shuttle with carbon tiles. Carbon can be heated to thousands of degrees without damage, and the good part is that they don't transfer this heat to other materials. It works as long as the tiles are intact and not damaged. An earlier method employed an ablative shield that was sacrificed during reentry.

There is another method, but it requires the burning of fuel. If the rocket can slow down before it hits the atmosphere dead on, rocket thrust can break the approach to the ground. I remember this being the method that pulp science fiction used. It turns out that this may become the landing method of choice. Recently, this retro-rocket method was demonstrated with a rocket shot up to orbit in a demonstration of how the rocket booster could be saved and reused.

So, when you see those crazy science fiction movies like Star Wars where they seem to have no trouble landing anywhere they want, just keep these realities of planet landing in mind.

Thanks for reading.