In less than a month, NASA’s Mars Science Laboratory (MSL) will land on Mars. But to just nonchalantly say “it will land on Mars” overlooks just how hard it is to land on another planet, especially one with an atmosphere. In science fiction, it’s commonplace to see ships land on planets like it’s no big deal, so I thought it would be worth taking a look at what NASA has to do to land MSL safely on Mars next month.
MSL launched on the day after Thanksgiving in November of last year and it has been drifting through space on a collision course with Mars since then. The spacecraft will hit the top of the martian atmosphere at around 8000 miles per hour, and it has to walk a fine line as it slows down and descends to the surface. Slow down too fast and you burn up in the atmosphere. Slow down too slowly… well, then you’re just another crater.
And it’s not just a matter of killing all that excess speed safely. You also want some control over where you end up on the surface. MSL has the most precise landing system ever used for a Mars mission, allowing us to drop the rover into the floor of Gale Crater at the base of an 18,000 ft tall mountain of layered rocks. To do this, MSL actually can steer itself as it is hurtling through the upper atmosphere. Early in the descent, the capsule drops a couple of tungsten bricks, offsetting the center of mass. This shifted center of mass means that the capsule is tilted so that it actually generates lift as it decelerates. Computer controlled jets fire to adjust the trajectory, giving us pinpoint landing capabilities.
Once the capsule has slowed down to a mere 1000 mph, it is no longer in danger of burning up, so it gets rid of the heat shield and releases a supersonic parachute. This parachute claws at the thin atmosphere and slows the rover down to a few hundred miles per hour.
For previous missions, once the parachute brought the rover close enough to the surface, the rover would disconnect from the parachute and inflate a tetrahedron of giant airbags, allowing it to bounce and roll to a stop. (If you ever had to do an egg-drop project in physics class, it’s like that, but the egg costs hundreds of millions of dollars and if it breaks you will have destroyed a decade’s worth of work by more than a thousand people.)
MSL is too heavy to land on airbags, so the engineers decided to use rockets. The problem is, rockets kick up dust, which can damage the rover’s delicate moving parts and scientific instruments. The solution? Wear the rockets like a jetpack, and then lower to rover on a winch when it gets close enough to the ground.
When the wheels finally touch down, explosive bolts cut the bridle and the jetpack blasts away to crash safely in the distance.
All of this takes about 7 minutes. Mars will be about 14 light-minutes away, so the rover’s computer does it all on its own. All of us on the mission will just be watching helplessly. They call it the “seven minutes of terror”:
So, next time you are watching or reading (or writing) science fiction, spare a moment to consider: How are the spacecraft going to solve the problem that NASA’s engineers had to solve for MSL? What are the requirements for the landing? Does it have to be precise, or do they just need to get down safely? How fast is the ship going? How is it going to kill off all of that kinetic energy without killing off the crew? Is it safe for the propulsion system to kick up dust and contaminate the surface, or is a bit more creativity called for? And then, if the ship is like most in science fiction, how is it going to do all of that again and again as it hops from planet to planet? Does it need its heat shield replaced every time? What about parachutes? Fuel?
There’s a reason all Mars missions so far have been one-way trips. It’s hard enough to survive the seven minutes of terror once. Launching from the surface and surviving it again when returning to Earth is not possible. Yet.