Landing on Mars - or not!
When we launch a spacecraft from Earth, we need to speed it up to more than 7 km/s to get it into orbit around Earth, or more than 11 km/s to escape Earth and head off towards some other planet. A spacecraft returning from Mars will already be moving pretty fast relative to Earth. If it simply headed straight down into the atmosphere, it would first get intolerably hot (and perhaps break up) and then crash-land just like a meteorite. This would be very hard on any instruments or people in it.
We could slow it down with rockets, just as we used rockets to speed
it up, but that would use a lot of fuel and isn’t really necessary.
With a bit of care, we can use the Earth’s atmosphere to slow
the spaceship down and bring it in for a safe landing.
How does aerobraking – braking using the atmosphere – work?
A simple analogy is what happens to someone who jumps, dives, or belly-flops into a pool of water. At high speed, the water doesn’t have time to move out of the way, and the person’s body will be stopped – or at least slowed down a lot – before it gets very far into the water. You can figure out how far it gets from the following argument. To make the person’s body stop, the mass of material (in this case the water) it connects with needs to be about the same as the mass of the person’s body. The mass of material (water) it connects with is the cross-section of the person’s body as it goes in times the distance the person travels through the water before stopping. This cross-section is a small number for a diver who goes into the water smoothly and a large number for someone doing a bellyflop. (That’s why you don’t want to dive into shallow water, and also why a belly-flop hurts.)
We can use this same idea for a spacecraft traveling into the Earth’s atmosphere. The more air the spaceship runs into, the slower it goes, but the friction with the air will also heat the spaceship (and the rapid deceleration might crush the astronauts). The safest path is therefore one that brings the spacecraft in along a shallow dive, giving it time to cool down while it is being slowed by friction with the air. If the spacecraft travels into the atmosphere too steeply, and it gets too hot; too shallow and it skips back out into space like a stone skipping on the surface of a lake. (The movie Apollo 13 includes a nice discussion of this issue; those who remember the mission remember being very nervous about whether the astronauts would be able to land safely.)
The atmosphere of Mars is thinner (lower pressure, less gas) than Earth’s, and also not as deep. This makes the problem of slowing down using the atmosphere much harder. One way to solve this problem on Mars is to make many trips through the atmosphere, slowing a little each time. If we start in a very elliptical High Mars Orbit (HMO) and slow down just enough to bring the closest approach – perimars – into the atmosphere of Mars, the atmospheric drag will shrink the orbit as in this picture:
When the rockets are fired at perimars (closest approach), the big change is in the apomars distance (farthest from Mars), but the perimars distance also shrinks. Before this process would put our spacecraft into a close enough orbit it would crash. We can fix that, however, if we use our rockets to speed us up a little bit near apomars, just enough to make perimars pass through the right part of the atmosphere. If we keep doing this, we can achieve the following gentle approach, slowing down a little each time we pass through the atmosphere of Mars.
We can use parachutes to slow the final descent, as was done for early manned space missions returning to Earth. On Mars, however, this will probably still lead to rather a hard jolt on landing since the atmosphere is much thinner. NASA has been using giant airbags to cushion Mars landers so the landing is less destructive (and someday less uncomfortable) for the contents.