At first glance, Mars seems pretty nice. The sun warms its rusty surface to a balmy 70 degrees Fahrenheit during the day, and gentle breezes ruffle its dirt. Spacecraft touch down and either plant their legs, so they can scrape and till its umber soil, or roll away, so they can trundle over rocks, up hills and down craters. Eventually, humans may be able to stake their flags in that solid ground, and build habitats, and maybe grow food.
No other world in the solar system offers us this chance. Mercury is way too close to the sun. Nearby Venus has far too much atmosphere, whose pressure and noxious gases would crush and choke visitors from Earth. Jupiter spews bullets of radiation, which will endanger human and robot exploration on its rocky or watery moons. Our own moon is airless, and it’s dark for two weeks at a time. So Mars is pretty much it, at least for the foreseeable future.
Too bad it’s such a jerk.
At night, temperatures drop to -100 degrees Fahrenheit. Dust devils and shifting sands cover up solar panels and will test even the most tightly sealed spacesuits and habitats. During dust storm season, Martian winds can stir up haboobs that cover the entire globe in clouds of sun-blotting microscopic particles. Mars has no global magnetic field, so the sun and cosmic sources freely bombard it with radiation, which will corrupt computers and bodies alike. And that’s assuming we make it there in one piece, which history suggests will be difficult.
Humans have been slinging spacecraft Marsward for 57 years, and we’re still not even batting .500. Since 1960, humans have attempted to launch 52 flybys, landers, orbiters or rovers toward Mars, and we’ve learned a lot about what works and what doesn’t — and why. Just 23 of those spacecraft have succeeded. The majority never left Earth, skipped right past Mars or crashed. The trouble is legendary: Scientists joke about a Great Galactic Ghoul, a monster that rips Mars probes from the skies.
Americans have beaten the ghoul 18 of those 23 times. So far, the U.S. is the only country to land anything on Mars, and we’ve stuck the landing on eight of nine attempts. Other space agencies and nations have delivered flybys or orbiters. The European Space Agency attempted a landing with its Schiaparelli probe Oct. 19, but it crashed in a spectacular explosion.
Mars may be our best hope in the solar system for settlement outside Earth, but Mars does not make it easy. When it’s humans versus Mars, Mars usually wins.
Mars is a jerk all the way from the atmosphere to the ground, but the air is the first challenge. Landing on a planet with an atmosphere is both easier and harder than landing on an airless rock like the moon. An atmosphere provides a natural brake, because an incoming spacecraft (or meteor, or what have you) encounters resistance. A parachute dramatically increases this effect, slowing a spacecraft even further. But all that resistance also produces heat, so you need a sturdy, heat-resistant covering — and that’s heavy, which makes it harder to slow down.
Mars offers the worst of both worlds, said Ashwin Vasavada, project scientist for the Curiosity rover at NASA’s Jet Propulsion Laboratory.
“It’s like this annoyingly middle value,” he said. “The atmosphere is thick enough to cause you all the problems you have on Earth, but too thin to really stop you like on Earth.”
Scientists have employed a suite of spacecraft solutions for this in the past several decades, from parachutes to airbags to retro-rockets that provide upward lift. All American Mars landers have used the same basic architecture: A pie-shaped capsule called an aeroshell, in service of aerodynamics and heat protection, with a parachute that can deploy when the capsule is falling at supersonic speeds. We are using the same design that Viking used in the 1970s, said Rob Grover, who leads the entry, descent and landing team for NASA’s next Mars lander, the InSight mission, launching in 2018.
“It’s good because we’re leveraging all the technology we developed in the ’70s, so it saves money. And we know it works, too, so why change?” he said.
Here’s what will happen when InSight arrives, or so the plan goes: It will slam into the Martian atmosphere bottom-first, at 13,000 mph. Its heat shield will reach 3,800 degrees F; that heat will burn away some of the spacecraft’s speed. Its parachute will deploy when InSight is roaring through the atmosphere at 900 mph. This takes out a lot more energy. By the time InSight cuts its chute loose, it will be flying 135 mph, Grover said. Retro-rockets will fire to take the speed down further, and the lander will settle on aluminum shock-absorbing legs at roughly 5 mph.
This process is a bit different for bigger craft. Anything weighing a metric ton or more, as the Curiosity rover does, needs more than a parachute. The Spirit and Opportunity rovers landed in a cocoon of airbags, bouncing around like beach balls before climbing out and driving away. But that wouldn’t work for the car-size Curiosity. Enter the most audacious landing ever attempted or achieved, the “seven minutes of terror” that brought Curiosity to the surface four years ago. Curiosity had the traditional capsule and parachute, but it also threw itself off balance midway through the atmosphere so it could fly like a wing rather than dropping like a cannonball. About a half-mile from the surface, it cut its chute, dropped out of its shell and free fell for a few seconds (nerve-wracking for the ground crew). Then it fired up its hovercraft-style delivery system, which NASA dubbed a “sky crane.” The sky crane’s rockets slowed the craft’s descent before a tether unspooled, dropping the rover the final few feet.
“It had its own little rocket jetpack, and that basically flew the rover down. The way we solve that very last bit without airbags is to have that jetpack practically hover and then lower the rover down on its tether,” Vasavada explained.
Curiosity touched down on its wheels, and you can imagine the cloud of dust that mushroomed from under it as six shock absorbers creaked under the rover’s weight.
Discussing this with Vasavada again, I started laughing. Who came up with this? He laughed, too. “You can work in a bubble and convince yourself that it makes sense. But then when you describe it to somebody else, you realize how ridiculous it sounds,” he said.
Schiaparelli had a similar retro-rocket system, and the engines were supposed to fire for 30 seconds to slow the lander’s descent before it fell safely onto a crushable cushion. But the engines cut off after three seconds. Engineers at ESA determined that an instrument measuring Schiaparelli’s altitude didn’t work properly, so the lander thought it was already on the ground when it was still 2.3 miles high.
If this all sounds absurd, early plans for crewed missions seem even more outlandish. SpaceX engineers want to land standing straight up, as the company’s reusable rockets do. At NASA, Grover said, engineers are talking about skipping the parachute entirely and going straight from a heat-shielded capsule to a rocket-powered descent. “Big parachutes are even more unpredictable,” he said.
That’s cruel Mars’s fault, too. The Martian atmosphere is not thick enough to provide much of a cushion, but it’s thick enough to carry dust great distances. Things get really bad during Martian dust storm season, when storms the size of Earth continents rage for weeks at a time. Roughly every three Mars years, or about five and a half Earth years, dust storms go global and the entire planet can be clouded in a haze of stinging particulates. Schiaparelli arrived at the height of dust season, and InSight will, too, to Grover’s chagrin. Thanks to the orbital march of the planets, Mars and Earth are at their closest point every 26 months, and space agencies time their launches accordingly to save fuel and money. But Martian weather does not always cooperate with this schedule.
“When we’re designing InSight, we have to make sure the spacecraft can land on a nice day with no dust storm, or the worst global dust storm,” he said. “Even in a global dust storm with very high winds, the air is so thin that you wouldn’t feel the same kind of pressure on you that you would expect for the same wind on Earth. The challenge is actually where the air is during a dust storm.”
Dust storms warm the upper atmosphere and cool the lower atmosphere, and this changes the atmospheric density, which changes how quickly InSight will plummet. InSight’s parachute deployment altitude needs to be lower in a dust storm than on a clear day — a risky prospect.
Researchers don’t have the best handle on these processes. Humans have sent wind-measuring instruments called anemometers to Mars, but for various reasons they haven’t worked well or at all. An anemometer was one of the key instruments on Schiaparelli, said Francesca Esposito, a Italian Space Agency scientist who designed an atmospheric monitoring package.
“One of the big questions to answer is how the dust is lifted on Mars. There are some hypotheses, but this is something that should be constrained on the surface,” she said in an interview before the landing attempt. Those hopes were dashed on the Martian surface, along with Schiaparelli.
Even when we manage to navigate the quirks of landing on Mars, this jerk of a planet will still throw plenty of problems our way. One is temperature fluctuations. The atmosphere isn’t thick enough to stabilize temperatures the way Earth’s does, so Mars experiences 100-degree-plus temperature shifts from day to night. This is hard to fathom on Earth, where most people live in places that undergo 20- to 30-degree diurnal swings, at most.
“In L.A., I can’t leave my laptop outside in my yard overnight and expect it to work the next morning. It’s barely designed to survive that,” Vasavada said. “If things are not built in a way to deal with that on Mars, they’ll just peel apart.”
By the way, that is what will happen to your skin and eyes if you step onto Mars without a pressurized spacesuit. Mars’s atmospheric pressure is only 0.6 percent of Earth’s, so the water in your eyes, lungs, skin and blood would turn instantly into steam, killing you in less than a minute.
Even if you have a spacesuit, you’re not really safe: Radiation will eventually get you.
Earth’s magnetic field blocks most high-energy subatomic particles from reaching us. Those that do make it through will find another blockade in the form of the atmosphere. But Mars doesn’t have a magnetic field, and its atmosphere is, well, you know. More radiation thus penetrates to the surface, where robots — and humans — will be.
To deal with this, eventual human habitats will have to include radiation shielding for the journey and the Martian settlement. Some conceptual plans suggest enclosing the spacecraft in a cocoon of drinking water, which will act as an insulator for the trip to Mars, but this is less feasible once humans reach the ground. More likely, Mars colonists will live in domed huts, or even underground caves.
There’s more bad news: When colonists venture outside to work or take in the Martian vistas, the very ground will pose a threat. Without water, the only force of erosion on Mars in the past three and a half billion years has been wind. Mars is the most wind-dominated planet in the solar system, said Mackenzie Day, a doctoral student at the University of Texas at Austin who studies wind erosion. Wind takes its time when it comes to altering the landscape, but that doesn’t mean it isn’t a force to be reckoned with. The upshot is that there is a lot of soft, slippery sand, with rocks rudely carved into sharp, dangerous points.
“It’s sort of a pick your poison from a rover perspective. You can drive through the nice, soft sand dunes, where you’re going to get stuck, or you can drive across the hard, sharp bedrock, where it will chew up your wheels,” Day said.
This has been a problem for the six-wheeled Curiosity rover, and Vasavada said rover drivers are being more careful to avoid jagged-looking terrain.
InSight has no wheels, but rocks are still a threat, Grover said.
“That is part of the challenge of a legged lander — what if you ‘stub your toe’ and you flip the lander over? What if you land on a rock that causes you to turn over? You definitely have to choose a landing site that is pretty flat and is devoid of larger rocks,” Grover said.
The InSight team scrutinized landing sites before picking a relatively clear area near the equator, in a region known as Elysium Planitia.
It’s an interesting name for this volcanic region, Mars’s second-largest. Elysium is named after the Greek abode for the blessed dead, as opposed to Hades, the realm of the cursed. Elysium may not harbor the Great Galactic Ghoul but instead welcome settlers both human and metallic to a place, as Virgil described it, of perpetual spring, with its own sun and lit by its own stars. A much friendlier vision of Mars, indeed.