But how would the computer know when our rover had landed? Would we need to add complex touchdown sensors in each of the rover’s wheels?
It was Miguel San Martin who stumbled on the answer. Mig, always curious, had been playing around with some of his computer simulations. He noticed that the throttle setting dropped by half when the rover made contact with the ground. He realized that the rover weighs about as much as the descent stage does at this point—so an instant after touchdown, the thrusters would have only half as much lifting to do. He had stumbled onto the solution.
Imagine a helicopter pilot lowering a heavy container to the ground. In flight, he needs to keep a lot of power to the main rotor in order to counteract the downward pull of the container. As he descends gradually toward the delivery spot, he’s monitoring carefully; he knows that as soon as the container is on the ground, this extra power will cause the helicopter to start rising, so as soon as it begins rising, he will quickly roll off the throttle. The start upward is all the confirmation he needs that his container had landed.
What Miguel realized was that the moment of needing less thrust was the moment that the rover had touched down. When Miguel’s trick had been pointed out to us, we could hardly believe it. The issue we had been struggling with for months had a quite simple solution that required no special touchdown detection sensors at all! The rover software had to do just two things for touchdown: maintain the slow and steady descent rate during the skycrane phase, continuing that slow rate until the ropes started to become slack, and then monitor the engine throttle setting to be sure that once the power had been cut, the situation remained stable, confirming that the rover was firmly on the ground. That’s the system that made history by safely landing our one-ton rover Curiosity on Mars.
Landing humans on Mars would be a much tougher problem. To kick off our panel’s first session, after everyone had introduced himself or herself and provided a brief background, I asked them all to share what I called their “care-abouts.” What did they think the major issues were?
Kent Joosten, a brilliant design architect of human Mars missions, explained NASA’s “short- and long-stay” mission designs. Long stays would require the astronauts to be on Mars for 545 days (530 Martian days, or “sols”) and away from Earth for two and a half years. Short stays would require “only” 40 days on the surface and a little more than one and a half years away from home. In either case, the stuff that needed to be landed for the astronauts would be massive.
From Kent, we heard the description of a whole stocked-up camp that would be placed robotically prior to the astronauts’ arrival. To save weight, water would be extracted from the Martian air, and the oxidizer for the fuel needed to take off for the journey back to Earth would be manufactured on Mars. Radioactive power plants and solar arrays would be brought in by spacecraft, ready to be set up by humans. Even the crew’s Mars ascent vehicle for the return home would be positioned in advance, fueled and ready to go.
Jack Schmitt next took the floor and explained what a Mars mission would look like from an astronaut’s point of view. Being in space was lonely enough, he said. Astronauts would need people to talk to, people in mission control to help work them through the tough spots. But mission control in Houston is between 11 and 22 light-minutes away, depending on where Mars and Earth are located in their orbits. You could talk for 10 or 20 minutes before the first word you uttered ever arrived at an Earthling’s ears.
An unexpected issue surfaced when Jack commented, “To do this landing safely, of course, we need the ability for the astronauts at any time to hit the abort button, wave off the landing approach, and go back into outer space.”
Bobby, Dick, and I glanced at one another, appalled, and we all looked at Jack with stunned expressions that translated as, “What? Are you kidding? How the heck could we do that?”
With Mars landers, aborting raises a much bigger problem than landing. All of our Mars landers and rovers, as they approached the planet, shed an assemblage of intricate hardware and components, leaving a trail of debris on the way to the surface. We assumed that a human mission would do the same. How could it do all of those transformations and undressings, while still being able to turn around to fly back up into outer space in an emergency?
When we explained that the Mars entry was more like landing on Earth than landing on the moon, the astronauts were quite surprised. We gave them an explanation that went something like this: “Imagine you’re going at Mach 15 surrounded by a bubble of hot gas as you plunge through the Martian upper atmosphere. You’re decelerating at up to 6 Gs. If you wanted to change your mind and head back to space, how would you do that? You’re going so fast that there’s no way to undress yourself from the heat shield and turn your craft into a rocket. Worse, you’re headed in the wrong direction going extremely fast. The amount of fuel you would have to have at that point to get back into space would be enormous.”
The robotic EDLers in the crowd were thrown for another loop when shuttle astronaut Commander Barry (Butch) Wilmore argued the need for the astronauts to be able to take over control during landing on Mars, just as Neil Armstrong had to do during the first lunar landing. I know that if I were an astronaut, I’d want to be able to control my spaceship. But those of us who design rovers to land on Mars also know that those landings are nothing like landing on the moon or landing the space shuttle back on Earth. The Martian atmosphere is shallow; its top is so close to the surface of the planet that events during landing unfold at lightning speed.
We’re talking about an astronaut who has been traveling through deep space in zero G for well over a year and is now undergoing extreme entry deceleration. Silicon chips running at 120 million operations a second can handle it, but I wouldn’t expect any human—not even a highly trained, exceptionally fit astronaut—to be able to do the same. The entire sequence of events for entry, descent, and landing would almost certainly need to be automated.
Over the course of three months, we gathered for two additional multi-day workshops. Gradually we started to home in on a few key observations and details about putting human-scale landers on Mars. What became clear to me was that in the 50 years of its existence, NASA had never really had the focus, resources, or real Mars landing experiences to study these problems clearly.