Landing the Hard Way
Imagine you’re an Olympic high platform diver. You’re planning a dive never attempted before, something with more elements than believed to be possible and no room for error. You have less than two seconds to make all your moves, which include a unique and dangerous entry into the water. And while you’ve practiced each of the maneuvers individually, you’ve never had access to the high platform to actually practice the dive. The difficulty factor: 10.5 out of 10.
Now imagine you’re in charge of landing a $2.5 billion spacecraft on Mars that is carrying the hopes and possibly the future of the nation’s entire Mars program. You also have what amounts to seconds to execute all your maneuvers—many of them untried—and you begin your entry into the Martian atmosphere at 13,200 miles an hour. Within six to seven minutes, the spacecraft has to position its heat shield so it can block the full 3,500-degree blast of friction heat that will do much of the slowing; it will make three broad S curves to correct its trajectory toward the landing site; a solar parachute has to properly open and operate; dozens of rockets and pyrotechnic devices large and small have to fire correctly to further slow the capsule and blast free those components no longer needed; and an additional vehicle tucked into the original capsule must be detached so it can drop further and position itself and begin a rocket-powered hover. But you’re still not done, because that hovercraft now must release the Curiosity rover itself and, using three tethers, lower it down to the Martian surface. Since the rover-robot is headed for a site inside a crater, the landing ellipse has to be smaller than any selected before—by a lot. And unlike the Olympic diver, the rover is speeding toward not water but very hard rock.
“Yes, thinking about all the things that have to go right is terrifying—I can’t deny it,” admits Adam Steltzner, NASA’s chief engineer for the entry, descent, and landing, and one of a handful of intrepid souls who came up with the Curiosity landing formula.
“When we proposed this plan, we were almost laughed off the project. People said it couldn’t possibly work. Well, we now have a risk analysis that says it has a 99 percent likelihood of succeeding. That’s higher than any Mars lander that’s flown before. Reassuring, for sure, but with a big caveat: That low risk assumes all the parts will operate and deploy as they should. Given the circumstances and essential timing of all the maneuvers, we’re definitely asking a lot. That’s what keeps me up at night.”
The landings are so difficult in part because the thin atmosphere makes for a super-speedy drop, and slight changes in atmospheric conditions can quickly turn a picture-perfect landing into a disaster. There’s no opportunity to fine-tune the landing from Earth while it plays out because of the time lag of about 14 minutes between when something happens on Mars and when Mission Control learns about it. In other words, the landing will be successfully or unsuccessfully completed before NASA Mission Control knows for sure it has started. Overall difficulty factor for the descent and landing: You don’t want to know.
The need for such a complex landing is straightforward: Curiosity is much larger and much heavier than any of its predecessors. Some have likened this increase in size and weight to a kind of “monster SUV” effect and suspect that NASA has fallen into a bigger-is-better way of thinking. But actually, the extra weight is due to the fact that Curiosity will carry to Mars both an organic chemistry lab and a geochemistry lab, each far more complex and capable than anything delivered to the surface before. It will also have a much larger robotic arm with more tools to do geology work. And it needs a sturdy chassis to move around and climb Mount Sharp. By themselves, the ten science instruments together weigh only 165 pounds and are marvels of compaction. But the structure needed to deliver and protect them still makes Curiosity—at 10 feet long, 7 feet high, and almost 2,000 pounds—twice as large and more than five times as heavy as the two rovers currently on Mars, Opportunity and Spirit (5.2 feet long, 4.9 feet high, and 375 pounds).
Those earlier rovers were dropped onto Mars wrapped in large and highly engineered protective “air bags,” and Steltzner and his team very much wanted to use that landing architecture again. But Curiosity had outgrown that technology, and landing inside the crater required a different level of accuracy. So Curiosity had to arrive on Mars differently—leading to those six or seven “minutes of terror,” as the landing is often described.
On its way down through 80 miles of Martian atmosphere, Curiosity will make use of at least three technologies or procedures never before tried on Mars. The first is “guided entry,” a process by which the capsule can reposition itself based on changed atmospheric conditions. This is necessary as part of the first phase of the descent, when the entry capsule turns so its heat shield takes the full force of the friction. If the turn is slightly too sharp, the capsule drops too fast and most likely crashes. If it’s too flat, then the capsule rides the atmosphere boundary too far and skips well past its landing site. Since humans can’t make the tweaks in real time needed to make sure the heat shield is properly positioned, the computers aboard the capsule have to do the job. A more rudimentary version of this technology was used when the Apollo capsules returned to Earth from the moon back in the late 1960s and 1970s, but it hasn’t been used since and certainly never on faraway Mars.
The descent will also use retro-rockets in an entirely new way. After being slowed down significantly by the force of the atmosphere on the heat shield and the deploying of a supersonic parachute, a final “powered descent vehicle” stage begins that will bring the rover and its carrier to a height of about 60 feet above the Martian surface. Eight rockets (called Mars Lander Engines) are used to slow the powered descent to a near halt and allow the duo to hover. Advanced throttleable engines were used on the Viking missions in 1976, but not since then on Mars. And they were never used to provide the kind of hovering platform required for this landing.
After a several-second hover, a set of pyrotechnic bolts are fired to detach the rover from the descent shell. By then, Curiosity has changed from its stowed flight configuration and opened up to prepare for landing. But the descent vehicle is still some 60 feet from the ground, and that’s when the most novel (and controversial) aspect of the landing commences: the “sky crane” maneuver. The rover is connected to the hovercraft by three nylon tethers and a line for power and communications, and the tethers are released to allow Martian gravity to pull Curiosity slowly to the ground. If all goes well, there will be one ton of rover, dangling on three tethers, inching its way to the surface. When it finally hits, it will be falling at about 1.7 miles an hour
Excerpted from Mars Landing 2012 by March Kaufman. Copyright © 2012 by March Kaufman. Excerpted by permission of National Geographic, a division of Random House LLC. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.