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.
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?”
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.
“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.
Adapted
from:
http://news.nationalgeographic.com/2015/10/151002-mars-mission-nasa-return-space/
