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How Did They Do It?
David Bearden and Matthew Hart
The Mars rovers have outlasted their 90-day design life by more than three years, and they continue to provide clues to past water activity on Mars.
The United States and the former Soviet Union have attempted trips to Mars for 40 years. But two out of three missions to the red planet have failed because of the great difficulty involved. The most recent journey to the surface of Mars was in mid-2003, with the launch of twin rovers Spirit and Opportunity on June 10 and July 7. To get to the red planet, the rovers flew through approximately 483 million kilometers of deep space toward a precise landing spot.
The seven-month journey was far from easy. Hazards ranged from "single event upsets"—which occur when a stray particle passes through a chip in a spacecraft's computer, causing a glitch and possibly corrupting data—to massive solar flares that can damage or even destroy spacecraft electronics. Adjustments to the rovers' flight paths were made along the way, but a small trajectory error could have resulted in a big detour, or even missing the planet completely.
If getting to Mars was hard, landing proved even tougher. The rovers had to decelerate into the tenuous atmosphere to safely reach the surface. The challenge of entry, descent, and landing—sometimes described as six minutes of terror—is how to decelerate something that weighs half a ton traveling at 19,300 kilometers per hour enough to ensure it has a chance of survival.
Testing of a rover prior to its flight into outer space and mission to Mars. The application of selective redundancy to the single-string design allowed for optimal use of the available mass and volume to maximize reliability and mission success. |
During the first 4 minutes of descent, friction with the atmosphere considerably slowed the rovers. Even then, they were traveling at 1600 kilometers per hour with only 100 seconds remaining as they reached the altitude where commercial airliners typically fly. Parachutes slowed the spacecraft, and then, with only 6 seconds left, retrorockets fired to bring it down to zero velocity—at the height of a four-story building above the surface. The spacecraft fell the rest of the way cocooned in airbags to cushion the blow of hitting the ground at 48 kilometers per hour or more and bounced before coming to rest.
Spirit landed in Gusev crater on January 4, 2004. Once there, it had to extract itself from the airbags used for landing, deploy its gear, and finally, check out its systems and instruments. The landing captured the world's attention. In the following week, NASA's Web site recorded 1.7 billion hits and 34.6 terabytes of data transferred, eclipsing records set by previous NASA missions. Three weeks later, Opportunity landed in the Meridiani Planum on the opposite side of Mars.
Chief among the scientific accomplishments of the rovers is the discovery of conclusive evidence that water was common and flowed freely on the surface of Mars in its ancient past. Both rovers have outlived and outlasted expectations for reliability and performance, surviving two Martian winters, climbing into and out of craters, hills, and valleys, and traveling a combined distance of more than 18 kilometers.
It is not only the terrain of Mars that can prove perilous to the rovers. Maintaining sufficient power to drive and perform scientific operations is challenging. Both Spirit and Opportunity recently braved a severe global dust storm that reduced the amount of sunlight they received to a tiny fraction of normal for almost two months. Sunlight is used to power the rovers—they were forced to lie dormant and wait out the storm.
Both rovers are situated near the Martian equator, where high and low temperatures can differ by more than 100°C. This is equivalent to going from a beach in Hawaii to the South Pole in midwinter—every day! In spite of all the challenges, both rovers continue to function remarkably well, and mission funding has been extended.
An artist's concept of a rover operating on the surface of Mars. The rovers Spirit and Opportunity have been traversing the terrain of the red planet since January, 2004, far outliving their 90-day design life expectancy. |
Developing the Rovers
The road to the launchpad was nearly as daunting as the journey to Mars. A spacecraft had to be built that could not only make the arduous trip, but could also complete its scientific mission upon arrival—nothing less than exceptional technology and planning was required.
The rovers were developed at the Jet Propulsion Laboratory (JPL), in Pasadena, California, at a cost of approximately $820 million, including launch. The team that sent the two rovers made history, completing in 3.5 years what mission planners usually complete in 5 to 7 years. The team revived an ailing planetary exploration program that NASA had to rebuild following the catastrophic loss of two spacecraft en route to Mars in 1999. The team did this knowing that landing on Mars is always an extreme challenge, particularly with such a short development schedule.
The spectacular scientific and engineering successes of Spirit and Opportunity were aided with mission assurance and project systems engineering expertise managed by Aerospace's NASA/JPL program office, also based in Pasadena. Aerospace provided support to the JPL team in developing the rovers through system-level mission assurance and by providing continuity across systems engineering, mission assurance, and flight project engineering.
Aerospace also wrote the original risk management plan for the project, which documented the approach to capturing, documenting, evaluating, and mitigating project risks. Aerospace conducted requirement audits, performed the linkage between higher- and lower-level requirements, and enforced systems engineering structure and rigor that ensured that key systems engineering requirements, mission assurance, and verification documentation were synchronized.
Aerospace applied failure-mode effects and criticality analysis techniques to interfaces and components of the rovers' flight systems. Aerospace systematically addressed each of the rover and carrier vehicle subsystems, identifying failure modes, classifying root causes, and documenting the criticality to mission success of each potential failure and the likelihood of it occurring during the mission. These data were regularly gathered across all areas of the flight system by interviewing key design engineers, by analyzing design changes, and by documenting the implications of design updates and potential failures on interrelated subsystems.
Reliability Analysis for Selective Redundancy
The Mars rovers were designed to be 30 percent larger than the previous Mars Pathfinder rover and lander, but had to fit within the same volume as their smaller and less capable predecessor. However, volume was not the only constraint facing designers: Mass growth threatened the project throughout its development. In the end, the rover and lander touched down on Mars weighing almost 50 percent more than Pathfinder.
Members of the Aerospace Mars Exploration Rovers engineering team with a replica of the rover. (from left to right:) Eric Breckheimer, David Bearden, Rocky Khullar, Maria Sklar, Matthew Hart, and Abraham Santiago. |
Design information for Mars entry and landing systems is based on experience from the Viking lander missions more than 30 years ago. Engineers decided that if the rovers were too large or too heavy, they would not be able to land safely on the Martian surface.
The mass and volume constraints led to a design philosophy based on high-reliability, mostly single-string subsystems. However, a limited number of subsystems—including the cruise-stage attitude determination and control, power, pyro, and terminal descent subsystems—required some redundancy to maximize reliability and ensure mission success. The questions were: Which subsystems should be made redundant, and how should each be designed?
To answer these, Aerospace offered expert design guidance, evaluating options for diversity in functionality and the use of redundant design elements. Aerospace developed reliability models for alternate designs and collected and applied failure rate data at the component level.
Assuring the Rovers Were Launch Ready
Earth and Mars align for a short time every 26 months, such that a Delta II launch vehicle can be used to carry a spacecraft to Mars. Motion of the planets limited the launch period for the rovers to several weeks. Daily launch opportunities were also constrained to short periods of hours or minutes per day, after which Earth's rotation and the location of the launch site would no longer allow the launch vehicle to reach its target.
The ability to successfully launch on any given day depends on many technical and operational factors, as well as uncontrollable events, such as the weather or errant incursions of aviation or marine vehicles into the keep-out zone surrounding the launch site. Aerospace examined U.S. launch history, focusing on launch delays and root causes to provide an assessment of launch-readiness requirements.
Exploration Continues
Technical specialists from Aerospace were called upon to support major milestone reviews, troubleshoot component-level technical issues, and assist in resolving major risks to the rovers' missions throughout the development of this project. Today, as the rovers continue to send back fascinating and never-before-seen images of Mars, these specialists can be proud of their involvement with, and the remarkable success of, this stunning exploratory project.
