Video: Mars parachute overcomes billowing
THE cramped room smells of hot breath. About 50 people are clustered in front of a floor-to-ceiling window, straining for a better view of the desert outside. The silence is only broken when one of them muffles a cough with the crook of his arm, a gesture that exposes the dark stain at his armpit.
Down on the ground outside, gleaming pickup trucks mingle with dusty cars, all with their windows down so that the glass doesn’t shatter with the force of what’s coming next.
A high-stakes test is about to begin at China Lake Naval Air Weapons Station in the Mojave desert in California. It will gauge the performance of a vital new technology for landing on Mars, and the results will shape the future of the planet’s exploration.
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You might remember the “seven minutes of terror” in August last year as NASA’s Curiosity Rover hurtled through the Martian atmosphere. The most dramatic moments were those involving the skycrane, but by the time this hovering sci-fi contraption gently winched Curiosity to the ground, the diciest moment had already passed. “Everyone was looking at skycrane, but the highest-risk equipment on that descent was the parachute,” says Ian Clark of NASA’s Jet Propulsion Lab (JPL) in Pasadena, California. A parachute of its size had never been tested, and to make things worse, Curiosity was the heaviest mission ever landed on Mars.
If we’re going to explore other corners of Mars and stage missions like rock-sample returns or human landings, we need to rethink the way we land on the Martian surface. That’s why NASA has given Clark and his colleagues $189 million to stage the most comprehensive redesign of their Mars landing system since the cold war. Testing it on Earth requires helicopters, rocket boosters and a supersonic sled in the Mojave desert, and success depends on the mood in Washington DC and the weather in Hawaii. This punishing testing regime is essential – when failure means the destruction of a multi-billion dollar interplanetary mission, you cannot afford to cut corners. “You can’t believe the number of things that can go wrong,” says Clark.
Landing safely on Mars is much tougher than on Earth: the atmosphere is too thin for a parachute to work until about 10 kilometres from the surface, where a lander is still zooming in at a blistering 1600 kilometres per hour. The forces on the unfurling nylon canopy are immense. “It’s like an explosion,” says and previously led the engineering team of the parachute redesign project at JPL.
Curiosity’s landing was particularly stressful, because the parachute’s 21.5-metre diameter was more than 5 metres wider than the biggest ones ever used on Mars. Would it inflate? Would it rip into pieces? Would it send the lander into a spin? “You’re playing this extrapolation game,” says Clark, “and that just made everyone really nervous.”
If mission controllers want to add a single additional kilogram to their spacecraft, the parachute will likely fail. That is down to what Rivellini calls the Matrioshka effect, after the Russian nesting dolls. He likes the metaphor so much, in fact, that a set of the dolls stands on a shelf in his office at JPL. One kilogram of extra cargo – one more tiny instrument, say – means you will need to add a container to hold it or enlarge an existing box, adding 3 to 4 kilograms. You will also need a larger craft to carry it, and more fuel. And so, like the dolls being swallowed up by bigger containers, your 1 kilogram turns into many. You can find occasional exceptions to this rule – for example, by scheduling landings at times when the atmosphere is denser, which helps to slow the descent. But sample-return and human missions are out of the question, since so much extra fuel and cargo are required.
Weight is not the only problem. Even if advances in materials science allowed us to keep adding equipment without adding mass, we would still only be able to land in low-lying areas – about 43 per cent of the planet’s surface (see diagram). Higher altitudes are off limits because there isn’t enough atmosphere to slow down a lander in time. Yet it is exactly these high-altitude sites – comprised of older, more ancient terrain, and less altered by water – that could yield revelations about the early history of Mars. “There are places there we really want to go,” says Mark Adler, the overall manager of the JPL parachute project.
Brake from the norm
Faced with this limitation, Rivellini and Clark realised that the only option was to rethink the entire descent. That had not been done since the 1960s, and for good reason. Previous attempts to introduce new parachutes have a history of being punished. For example, the parachute for the UK’s Beagle 2 lander in 2003 needed a last-minute redesign to add 50 per cent more surface area, because calculations suggested touchdown would be too rough. The new parachute was cited by some as a possible reason for .
Unfortunately, proper parachute testing costs a king’s ransom. “The chute itself is just nylon – a couple hundred thousand bucks,” says Clark. “It’s the testing that gets you.” Wind tunnel tests would be cheaper, but no facility in the world is big enough. Mission planners would be reluctant to add the necessary cost to their budget. Rivellini and Clark were in limbo.
That all changed in 2010. Under US president Barack Obama’s administration, new money has suddenly been made available for basic technology research, thanks to the reopening of the Office of the Chief Technologist, which had been shuttered in the early 2000s. In 2011, the parachute team won the $189 million to forge ahead.
But how to redesign a 40-year-old system? Before coming up with skycrane, Rivellini had pored through scores of other ways to land Curiosity in one piece, including a , and a to successfully land NASA’s Mars Pathfinder. But none of those would help a giant parachute to land heavy equipment. It’s not as easy as simply making the existing parachute bigger, because beyond a certain surface area, it would rip apart under the explosive forces of unfurling.
The good news was that Clark was the pre-eminent authority on a landing technology that had been mothballed in the 1960s. He had found it in half-rotted technical reports and footage of parachute tests on reels of grainy 16-millimetre film: an inflatable flat decelerator that deploys before the main parachute. “It’s the same principle as holding your hand flat out the window of a moving car,” Clark says. A prototype of this decelerator was tested in the 1960s but never flown.
So Clark, Rivellini and their team used this cold war-era idea to build the Supersonic Inflatable Aerodynamic Decelerator (SIAD). It is essentially a 6-metre-diameter fabric doughnut that bridges the gap between a heatshield, which aids deceleration, and a parachute. Clark’s calculations suggested that the SIAD, positioned beneath a descending lander, would slow it enough to protect a new, bigger, 30.5-metre parachute from collapse (see diagram). By October last year, it was ready to be put through its paces.
At China Lake, the testing begins. A man in white protective headgear moulded into the shape of a cowboy hat is running the final checks on the decelerator, which has been mounted atop a steel vehicle. The whole rig is coupled to a rocket sled on rails called SNORT (), which looks like the wingless fuselage of a bomber from the second world war, complete with toothy shark smile on its nose. The sled, which zooms along the ground on its rails, is usually used to test aircraft ejector seats at supersonic speeds, missile impacts and armoured car penetration. The rocket motors strapped to its back are capable of accelerating the sled to Mach 8.5, making it one of the planet’s fastest land vehicles.
Due to the pressure wave the test will produce, the only safe place to watch is from behind reinforced glass about half a kilometre away. “The pressure from the thrust would rip a fair bit of the flesh from your bones,” says Adler. “I’d guess your remains would be dispersed over a good-sized area.”
While SNORT won’t reach the velocity of a Mars descent, the rushing air about to batter the decelerator will apply a far greater force. “It’s different aerodynamics because the atmosphere is so much thicker here,” Clark says. He is worried that the device will fail. That’s because he has had to make several changes to the 1960s design. Back then, researchers had few clues as to what Mars’s atmosphere would be like; they assumed it would be similar to Earth’s. This misunderstanding was revealed in how the original decelerator inflated upon deployment. The footage shows it blossoming and billowing like a jellyfish. That’s exactly what Clark does not want to see today. If it inflates unevenly in this way during the Mars descent, the lander could spin out of control. “When we inflate it, it needs to expand into its final shape in a single shot,” he says, “almost like it’s a piece of metal.”
It’s time to find out. Except for a light breeze stirring the patches of colourless desert scrub, the area around the rocket sled is silent. In the observation room, Clark holds his breath as the operations chief pushes the button. In the next instant, a solid 50-metre-long plume of flame bellows out behind SNORT’s six missiles. A second later, a thunderous blast of pressure batters the observation room, rattling the reinforced glass and triggering complaints from the alarms of nearby vehicles.
Meanwhile, the entire roaring fury has accelerated to 400 kilometres per hour, until a fair distance down the track the decelerator puffs out like a bullfrog’s throat to absorb the shock in a fraction of a second.
“The roaring fury reaches 400 kilometres per hour, until the decelerator puffs out like a bullfrog’s throat”
Everyone stampedes down the building’s rickety stairs. The engineers are itching to examine their decelerator, but that section of the track is off limits until the rocket motors’ toxic fumes have dispersed. Until then, the only option is to crowd into the dim bowels of the observation building. There, banks of monitors and workstations let them watch the test as it was captured by the cameras.
To get a sense of how the decelerator behaved, the engineers are using a technique called photogrammetry, which relies on a camera to track several hundred black dots that have been meticulously hand-stencilled at regular intervals all over the decelerator’s fabric. Clark, Adler and Rivellini hunch over the same screen, watching the decelerator pop open in slow motion. They pause, rewind, and pause and rewind again in search of any worrying deformations that would cause a lander to lose control.
Judging by the smile breaking across Clark’s face, the news is good. “The deflections were on the order of a centimetre,” he says into his phone. It means the decelerator popped open like solid steel. Across the room, Rivellini instructs someone to cancel the back-up tests.
It’s celebration time now, but the next stages of their testing regime will raise the stakes once again. Next will be a trial of the new 30.5-metre-diameter parachute, which is 9 metres wider than the one used to land Curiosity, and twice the area.
In October, the JPL team will return to China Lake, where a helicopter will first lift the new chute to an altitude of about 3000 feet. The parachute will be yanked back down to Earth by a rope made from a stronger cousin of Kevlar, attached to SNORT, whose rockets will exert a force of 560,000 Newtons on the chute. If the parachute, rope engagement and rocket ignition aren’t choreographed perfectly, the shock wave could disrupt the timing of the helicopter’s blades and send everything crashing to the ground.
Land of hope
All the parts will come together for a final test next year, under conditions that are the closest you can get to Mars on Earth. In June, the full package of decelerator and parachute will be hoisted by balloon into the stratosphere over the Pacific Missile Range in Kauai, Hawaii. In the Hawaiian summer, the wind blows almost exclusively westward – which is vital to prevent the rocket and chute drifting over a populated area.
If all goes well, at 120,000 feet, a Star 48 rocket motor – the third-stage engine that powered the Pathfinder and Phoenix landers – will take over, sending the whole kit screaming upwards a further 40,000 feet. At this altitude, the air density is similar to Mars’s atmosphere. The decelerator and parachute will deploy as the motor drags everything horizontally through this thin slice of stratosphere at Mach 4.
Will all this be worth the $189 million? After all, it seems an enormous price tag. Adler sees it as a long-term investment. The first parachute tests in the 1960s and 1970s provided the foundation for four decades of the US’s exploration of Mars, he says. “We think our chute will have a similar legacy.”
For Adler and the others at JPL, the faith NASA has shown in this project is emblematic of a renewed vigour at the space agency: it’s time to start thinking big again. It also means that sample-return missions and human landings are finally on the table.
Back at China Lake, it is time to pack up the decelerator, ready for its next round of punishment. The technicians have separated it from the still-smouldering remains of the rockets. Now, using cranes and ropes, they gently decouple the decelerator from the rig. Clark, Rivellini and Adler hover around their creation like nervous mother hens. Little wonder: the future of US Mars exploration relies on it.
This article appeared in print under the headline “Chute for Mars”
