How Will We Get Off Mars?

We know how to get to Mars. We know how to land on Mars. Now comes the hard part: figuring out how to leave.

Monday, April 8, 2019,
By Mark Strauss
Professor Brian Cox believes that we need to establish a working base on Mars before sending astronauts to live there.
Photograph by Photograph Twentieth Century Fox

When NASA engineers look at Mars, they see a planet-sized Venus flytrap.

It lures us with the promise of scientific discovery—but the moment we land there, gravity and a harsh climate will conspire to keep us stuck on the surface.

And that’s not an option. If The Martian holds one lesson for real-life space exploration, it's that the public won't stand for spending billions of dollars only to leave astronauts stranded on another world. The most crucial part of any NASA plan for visiting the red planet, arguably, is getting off it.

The spacecraft that NASA would build to get the job done, the Mars Ascent Vehicle (MAV), represents a formidable engineering challenge. When fully loaded with fuel, it’s too heavy to launch from Earth and land safely on Mars.

Instead, the vehicle would need to be pre-assembled and sent to the red planet—years before the astronauts arrive—where it would make its own propellant by squeezing it out of the thin Martian atmosphere.

And after that? The MAV must be built tough enough to remain fully operational despite being pummeled by massive dust storms and punishing UV radiation. When the cramped vehicle does finally take off, it needs to sustain the astronauts for days, as they maneuver to rendezvous with the orbiting vessel that will finally take them home.

The Mars Ascent Vehicle will be a mission within a mission: a crewed spacecraft launched into orbit from the surface of an alien planet.

And there’s only one chance to get it right.

Hauling All Our Stuff

A mission to Mars will be humankind’s first deep space caravan. As many as five separate spacecraft might be needed to ferry the astronauts and their cargo to the red planet.

Some of that cargo can be broken down into smaller components and then reassembled by the astronauts upon their arrival. Not so the MAV. “You don't want to be on Mars trying to bolt engines on, in your space suit, essentially wearing mittens in a dusty environment,” says  Michelle Rucker, a system engineer at NASA’s Johnson Space Center.

In NASA-speak, that makes the MAV the “largest indivisible payload element” on the mission, weighing an estimated 18 tons. To date, the most massive object that we’ve sent to the Martian surface is the one-ton Curiosity rover.

Landing an object on Mars—especially one that that weighs several tons—is not as easy as landing it on Earth, where a capsule basically falls from the sky, relying on the atmosphere to reduce the speed of its descent.

On Mars, where the air is a hundredth the thickness of Earth’s, “there's just enough atmosphere to be a pain in the butt, but not enough to do anything useful for you,” says Rucker. Or, put another way, it will burn you up but it won’t do much to slow you down.

That’s why NASA is developing technology such as the Hypersonic Inflatable Aerodynamic Decelerator—a massive, cone-shaped inflatable heat shield that would also act as a braking system.

The shield would deploy upon entering the Martian atmosphere, slowing the lander from hypersonic to merely supersonic speeds. At that point, rocket engines would kick in for a controlled landing.

Here's the kind of math astronaut Mark Watney would do to make it work: The landing will burn up around five to seven tons of propellant. When it comes time to take off from the Martian surface, the MAV will need 33 tons of propellant to break free of the red planet’s gravity, push through its atmosphere, and safely ferry the astronauts and their scientific cargo into orbit, where they can rendezvous and dock with their Earth Return Vehicle.

And that's too much to send ahead. The propellant will need to be manufactured on Mars.

Living off the Land

If expeditions to the red planet are going to have any chance of succeeding, they’ll need to live off the land.

By making fuel on Mars, NASA can shave several tons off the initial payload mass. And, after the first mission is over, the equipment can be left on Mars to serve as nascent infrastructure for expanded facilities to process not only fuel, but also water and air for future explorers.

The engines of the MAV will be powered by methane and liquid oxygen. All the ingredients needed to make that fuel—carbon, hydrogen, and oxygen—can be found on the red planet, if you know where to look.

In theory, oxygen can be extracted from the Martian atmosphere, which is 95 percent carbon dioxide (CO2), and from liquid and frozen water (H20) buried beneath the surface. The leftover carbon and hydrogen would be combined to make liquid methane.

An inflatable shield would deploy when the Mars Ascent Vehicle and its lander enter the Martian atmosphere.
Photograph by Illustration courtesy NASA

Drilling for water, however, would add an unwelcome element of uncertainty to an already difficult mission. Excavating and processing is a lot more complex than simply taking atmosphere from Mars. “The other problem with underground water propellant production is that it drives you to land where you're pretty sure there's water,” says Rucker. If you need to dig and “you land somewhere where it turns out you're on top of bedrock, then all bets are off,” she says.

If hydrogen won’t be extracted from Martian water, then Plan B would be to send a payload of hydrogen to Mars as seed stock for making methane. But, for an initial mission, that idea is also off the table. Although hydrogen isn’t heavy, it requires large tanks for storage that would take up a lot of precious space.

“We've got a lander design; it kind of has a flatbed deck on top,” says Tara Polsgrove, an aerospace engineer at NASA’s Marshall Space Flight Center.  “Right now, the MAV is taking up most of the room on that deck. There's not a whole lot of room there for a hydrogen tank.”

NASA engineers could accommodate hydrogen tanks by making the MAV taller instead of wider. But, increasing the height of the spacecraft is a scenario they’d like to avoid. They’re concerned that if the vehicle is too tall, there’s a greater risk of it tipping over after landing.

And, Rucker says, a taller MAV could place a difficult physical burden on the astronauts. If one or more of them are incapacitated during the mission, then climbing a tall ladder is the last thing they’d want. Easy access needs to be a high priority.

As such, the current plan envisions sending an ascent vehicle fully loaded with liquid methane and equipped with a chemical plant that would manufacture liquid oxygen from the Martian atmosphere.

The process is expected to take one to two years. When the MAV’s tanks are full, the human crew will be sent to Mars, secure in the knowledge that they’ll have a gassed-up vehicle waiting to get them back into space.

But NASA engineers won’t be ready to hang up any “Mission Accomplished” banners. “One of the challenges is that we're using cryogenic propellants,” says Rucker. “Once you make your propellant on Mars, then you've got to keep it cold for a couple years before you actually use it, without it boiling off.”

“We've got propellants, and right now we don't have any valves that have zero leakage,” adds Polsgrove. “You've got to think about that, which is why we're prioritizing technology development in the area of low-leakage valves.”

More broadly, engineers are concerned that time is not on their side. The MAV will require one to two years to manufacture its fuel. Then, the human crew will spend 200 to 350 days traveling to Mars, followed by their exploration of the red planet, which could last up to 500 days.

Add it all up, and that means the MAV must remain operational and ready for takeoff for as many as four years after its initial landing on Mars. “It's been sitting in the Mars environment,” says Rucker. “It's sitting in dust. There's intense UV radiation. How does your patio furniture look after it's been sitting outside for that long? That's on Earth, where it gets considerably more protection than there.”

Suit Up!

Among the many questions that engineers need to consider when designing the MAV, one of the most important is, “What will the astronauts be wearing?”

“You've seen pictures on the space station,” says Rucker. “They're hanging out in shorts and t-shirts. When you're in stable flight with a big vehicle, you can get away with that. In the ascent vehicle, there's nowhere else to go. If you pop a hole in it somewhere, you better have a suit on.”

But, which suit? The ones that the astronauts will have been wearing while exploring the surface of Mars—the extravehicular activity suits—are heavy and bulky. If the astronauts wore those aboard the MAV, engineers would have to increase the cabin size.

And then there’s the problem of Martian dust that will be clinging to the suits. That’s not the type of stuff that astronauts should be bringing home without proper planetary protection protocols.

Rucker believes the best solution is to leave the bulky suits on Mars, where a future mission could salvage them for parts. Instead, the departing astronauts would don “intra-vehicular activity” (IVA) suits—those puffy, orange outfits that the shuttle crew wore aboard their spacecraft during launch and reentry.

The IVA suits weigh less and are slightly more flexible. And they can be kept dust-free by restricting their exposure to the “outdoor” Martian environment. The astronauts would leave their habitat and get into a rover by means of a docking port. While in the rover, they would change into their spiffy clean IVA suits and drive over to the MAV, which they would enter by means of a specially designed, pressurized tunnel.

The space suits that astronauts would wear on the Martian surface are too bulky for the trip into orbit. Instead, they’ll don “intra-vehicular activity” suits.
Photograph by Robert Markowitz, NASA/Johnson Space Center

The downside of bringing a tunnel to Mars is that it adds the weight of a piece of equipment that would be used only once. Rucker, though, thinks the tunnel could have other uses.

“I look at it as a cool thing to have,” she says. “Now, instead of a big, single habitat, you can maybe take smaller habitats and use the tunnel to join them together….It's never good to add a new element, but if it's an element that solves a lot of problems, then it might be an advantage.”

Homeward Bound

Finally, it’s time to go.

The interior of the MAV will be spartan to minimize weight. This is a one-way space taxi, not a habitat. In fact, the engineers might not even include seats—in which case, the astronauts would stand for the duration of the trip.

The rocket-powered ascent will last seven minutes. But the journey doesn’t end there. The astronauts will need to burn more fuel to maneuver into an orbit that will allow them to rendezvous and dock with the Earth Return Vehicle (ERV).

That means the astronauts could be aboard the ascent vehicle for up to 43 hours, assuming that the ERV is parked in a “one sol orbit”—an elliptical orbit ranging in altitude from 155 to 21,000 miles above the Martian surface. But, Rucker says, this remains an unresolved issue among Mars mission planners.

“The in-space propulsion guys, they want to keep that big, fat transit habitat up as high as they can,” she says. “They don't want to dip down into the Mars gravity well. They'd really love to stay at five or ten sol and make the ascent vehicle come up to it.”

The problem with that, Rucker says, is that a longer stay onboard the MAV will require additional facilities.

“You can stay in your spacesuit, and you can do without hot soup and a bathroom for forty-three hours, probably,” she says. “But you start dragging into three days, or five days, or seven days, you have to start adding all those things in and that's going to drive up the size of the ascent vehicle.”

Once docking is finally achieved—and the crew and cargo are transferred to the spacecraft that will take them to Earth—the MAV detaches and performs a final disposal maneuver, placing it into an orbit that won’t interfere with future Mars missions: an ignoble end for a small spacecraft that will have played a pivotal role in human history.

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