Exploring Mars: Mars Mission Risks
25 Sep 2001
(Source: Jet Propulsion Laboratory)
As the Odyssey spacecraft makes it way toward Mars, mission engineers are working hard to prepare for arrival at Mars at 0230 Universal time Oct. 24 (7:30 p.m. Pacific time Oct. 23). Join Charles Whetsel, chief engineer of the Mars Exploration Program, and Matt Landano, 2001 Mars Odyssey project manager, as they describe the hard yet rewarding road to Mars.
Risky Business: A Mission to Mars
Imagine planning for a long sailing voyage. Your survival depends upon the sturdiness of your craft, planning and skill. Stowed onboard must be all the provisions, tools and hardware you'll need. Your knowledge and judgment about how to navigate through wind, weather and waves will be crucial to staying afloat.
You've learned from the successes and misfortunes of previous voyages. You won't make the same mistakes, but you know you may encounter some new challenges. When something breaks, you will need to be able to work around it. Beneath the surface may lurk something unexpected. Within the bowels of your sailing craft, there may be a weakness or a flaw that won't make itself known until later. And it may get you in the end. Vigilant, wary, you're ready for the best and prepared for the worst, for the things you don't know will happen.
In a way, space engineers say, that's a little of what it's like to work on a mission to Mars.
Earning a Degree from the School of Hard Knocks
Thirty missions to Mars have been mounted by the space agencies of the U.S., Japan, and the former Soviet Union. Of the U.S. missions, 10 out of 15, or two-thirds, have succeeded. Only one of 16 Soviet missions succeeded, and the lone Japanese spacecraft is currently delayed in arrival at Mars due to propulsion problems.
Charles Whetsel, now chief engineer of the Mars Exploration Program, points to the single most instructive lesson of his engineering career. He was two years into his first job out of college. On Aug. 21, 1993, as usual, he showed up for his 6 o'clock, Saturday night shift on the "systems" console for the Mars Observer mission. Only the spacecraft didn't. It disappeared as its propellant tanks were being pressurized in preparation for an engine firing that was to have placed the spacecraft into orbit around Mars. An investigation later determined that warm fuel had condensed in a cold part of the propellant lines too close to the liquid oxidizer, sparking an explosion.
"I'd never even thought about it, that I could come in tomorrow and everything I just invested the last two years in disappears in the blink of an eye," says Whetsel, now 34. "That was my big wake up. I was totally incredulous. Your world changes overnight.
"It affects you on a personal level and it affects everyone you work with. It's equivalent to finding out your company has gone bankrupt or something like that. But it's a little more personal than that. That whole process of losing it, then trying to get it back then working through the whole failure investigation trying to understand what went wrong. We were all working together with the external failure investigation board on that because we wanted to understand what happened."
As graduates of what Whetsel calls "the school of hard knocks," he and his colleagues, veterans of a failed mission and the detailed engineering detective work that followed, became even more valuable contributors to JPL's space exploration enterprise.
Former Soviet space engineer V.G. Perminov, who led design work for the U.S.S.R.'s Mars spacecraft program, wrote a publication called "The Difficult Road to Mars," in which he recounts that country's string of misadventures to the red planet. He points to a Russian proverb to help explain the value of lessons learned from failure: "One beaten person is worth two unbeaten ones."
Whetsel agrees. With a great deal of experience built up through his involvement in earlier missions, now oversees engineering concerns for all Mars missions. "Once you've lost one, it colors the way you look at things. You really push a little harder, you worry a little more."
Mars: So Close, Yet So Far Away
But why have so many Mars missions fallen short of their goal? Is there something special about Mars that leads spacecraft and the teams that guide them like moths to a flame? The answer partly has to do with the sheer number of attempts launched from Earth to the red planet. "We've sent more things to Mars than any other body except the Moon, so we've had more opportunities to fail," says Whetsel.
Some compare the record of Mars exploration to that of the early Moon shots that preceded the astronauts. Many failures occurred early in the program, but the record improved with experience.
In addition, says Whetsel, many people incorrectly assume Mars is an easy target to reach because of its relative proximity and many Earthlike qualities. But just because it's closer to Earth than Jupiter or Saturn doesn't necessarily make Mars a simpler destination.
Lessons Learned: The Silver Lining
A mix of excitement touched with trepidation is building at JPL, where, in about a month, engineers will direct NASA's Mars Odyssey spacecraft to enter orbit around Mars. At the time, Odyssey will be about 150 million kilometers (93 million miles) from Earth. Communicating through its 15-watt radio, the spacecraft will be at one of the most critical junctures of its mission.
It is the first spacecraft to be sent to Mars since the dual loss of the Mars Surveyor orbiter and lander two years ago. Fresh in the collective mind of the space exploration engineering community are recent lessons learned the hard way as the Odyssey team heads toward its orbit insertion maneuver. Those tough lessons, however, are considered by many at JPL and at Odyssey contractor Lockheed Martin Astronautics in Denver, Colo., to be the silver lining ending a cloudy period for Mars exploration in particular and JPL in general.
The team is acutely aware that thousands of engineering details have to proceed correctly for the spacecraft to begin its first successful orbit around Mars. If only one of those details goes wrong, it may unleash a cascade of events that could cause Odyssey to fail its entry into Mars' orbit.
Not that the public should expect anything short of a mission accomplished, says Mars Odyssey Project Manager Matt Landano of JPL:
"I think they ought to be expecting a success. We are doing everything we reasonably could on Odyssey to reduce risk and maximize our prospects for success. We clearly aren't going into this thinking that anything short of success is acceptable. In our minds, it's not ok to fail. We must succeed."
Mars Orbit Insertion: This IS Rocket Science
Following carefully calculated parameters set by navigators and ground controllers, Odyssey's targeting will be fine-tuned with a "trajectory correction maneuver" involving a final whisper of hydrazine gas meted through onboard jets the size of cake-decorating nozzles.
To enter orbit, Odyssey's propellant tanks, the size of big beachballs, must first be pressurized, plumbing lines heated, and the system primed before all 262.8 kilograms of propellant (579.4 pounds) burns in exactly the right direction for 19.7 minutes.
This maneuver, called the Mars orbit insertion, will brake the spacecraft's speed, slowing and curving its trajectory into an egg-shaped elliptical orbit around the planet. In the weeks and months ahead, in a process called aerobraking, the spacecraft will repeatedly brush against the top of the atmosphere to reduce the long, 19-hour elliptical orbit into a shorter, 2-hour circular orbit of approximately 400 kilometers altitude (about 250-miles) desired for the mission's science data collection.
"All of this has to be done by remote control," says Whetsel. During the main engine firing, for instance, "there's no time for the ground (operations team) to interact with the spacecraft." The one-way light time to the spacecraft - the time it takes a radio signal to travel from the communications dishes of the Deep Space Network here on Earth to Odyssey at Mars - will be about 8.5 minutes during the engine firing. "By the time you see whether or not things are going well, in reality, it's essentially already done. So you have to put everything on board the spacecraft ahead of time."
Tapping the Aerobrake
Aerobraking has been used twice before: experimentally at Venus after the Magellan spacecraft's mission was complete in 1994, and with the Mars Global Surveyor spacecraft when it arrived at Mars in 1997. The technique uses the spacecraft's solar panels like wings, or a parachute, to slow it down and lower its orbit, and greatly reduces the need for propellant that would otherwise be needed to place the spacecraft in the desired orbit. Complicating Global Surveyor's aerobraking was a faulty hinge on one of its solar wings. Extra care had to be taken to keep the wing from tearing off the spacecraft, adding a year and a half extra to the aerobraking phase of that ultimately successful mission, which has now returned more data than all other Mars missions combined.
Mars scientists have long desired to have their instruments in a low, circular orbit that affords beneficial lighting conditions and a uniform altitude for photography and other data taking, and Odyssey's aerobraking phase is designed to deliver that. "But at Mars, especially, aerobraking comes with its own set of risks," says Whetsel, who, with his experience on Mars Global Surveyor, may be the most seasoned aerobraking specialist in the solar system. [More information on aerobraking and orbits can be found in "The Basics of Space Flight"]
"Aerobraking relies on a 'Goldilocks' approach," says Whetsel: "Not too big, not too little, not too deep, not too shallow, because if it's too deep you'll damage the spacecraft and if it's too shallow you won't get to the right orbit in time." And if that happens, to borrow from another fairy tale, the big, bad wolf will get you.
"Once you start aerobraking, you're putting the spacecraft through the atmosphere hundreds of times, and the trick is to do that deep enough to shrink the orbit down so you get to a good geometry while the lighting is still good, but not so fast that you overheat the solar arrays," Whetsel said.
That would be easy, says Whetsel, if the Martian atmosphere were just a big unvarying sphere, "but there are real things like weather and dust storms that cause the atmosphere to move up and down."
For weeks, in fact, Mars has been in the midst of a gigantic global dust storm. Scientists on the Mars Global Surveyor team are using their instruments to keep a close watch on the storm. A well-designed drag pass through the atmosphere could doom the mission if, unbeknownst to planners, a dust storm swelled the atmosphere and snagged Odyssey out of the Martian sky. So data from Global Surveyor will be crucial in guiding sibling Odyssey through its aerobraking phase.
"The team has to monitor that process on pretty much a daily basis to see if the atmosphere is growing, how is the spacecraft responding, are the temperatures ok, are we going fast enough, are we keeping the spacecraft healthy," says Whetsel. [See aerobraking interviews and animation at http://www.jpl.nasa.gov/videos/mars/odyssey.html]
Will it be 'Bolero' or Lucy and Ethel in the Chocolate Factory?
The pace for Odyssey's aerobraking at Mars will be brisk. Ultimately, ground controllers will be executing up to 12 drag passes through the atmosphere each day. "Aerobraking is a very judgment-driven sort of thing," says Whetsel. "There's a lot of data that you have to make decisions about. And it's a process that once you've started, you are pretty much committed to seeing it through."
Whetsel compares aerobraking to listening to 'Bolero': "It starts out with these big orbits, and there's a rhythm about when you're going to hit the atmosphere the next time." He drums a desktop to emphasize the beat. "There's a rhythm about all the things you're going to have to get ready for on the next orbit. As the orbit shrinks, the pace picks up and things are happening more rapidly and you either get in the groove or you don't."
On a more comedic note, he says, "It also reminds me of the old classic Lucy Show with the chocolates coming by, and as long as everything's going fine, its going fine, but if you miss one beat, you know you've really gotta scamper to get back into the rhythm as things are going. Because as the orbit shrinks the pace gets faster and faster and faster."
Stick to Your Principles
As the countdown to Odyssey's arrival at Mars continues, and as new Mars missions for 2003, 2005, 2007 and beyond take shape, the recent difficult past is still foremost in the minds of the design, assembly, test and flight teams. JPL's leadership has put new mechanisms in place to capture and institutionalize the wisdom those painful lessons have imparted. Says Landano: "You typically learn a lot more from failure than you do from your successes. Mainly because when you have a failure you dig deeply and widely to uncover the root cause of your shortfall."
In 2000, after the loss of the two Surveyor craft, Landano, veteran of the Mars Viking mission and the Voyager, Galileo and Cassini missions to the outer planets, was asked to formally enumerate the engineering principles that have characterized JPL's space engineering triumphs. He also identified the range of acceptable trade-offs and the consequences when engineering principles were skirted. In the process, he says, "I looked at the things on our past Mars failures that got us, and they are not high-technology things," he says.
"In a complex system like a spacecraft, about a million little things have to happen right. The real complex stuff everyone is watching. Areas that are new technology usually have a hundred eyes looking at them." But he points to overlooked fundamentals, including good communication between team members or a hidden flaw in seemingly minor part as examples of dangerous gremlins. "It's the things that you've done many times that you think you know how to do," Landano says. "Somehow, that's the thing that bites you."
"It's a risky business. And if you let your guard down, if you say 'We know how to do this,' you kind of ease up in your mind or in the way you do it. But when you fail, it forces you to take stock again and say, 'Wow, even though I was successful here, look how slim the margin was. I failed here, and I could have failed over here too.' It provides you this heightened awareness for processes, completeness, penetration and applying rigor to everything you do."
Though not yet etched in stone, "Design, Verification, Validation and Operations Principles for Flight Systems," becoming more popularly known in the space community as "The Landano Principles," are now the engineering and management commandments that JPL and its contractors are following to reduce risk and increase the reliability of missions.
At the heart of JPL's risk reduction efforts, says Landano, is a renewed appreciation for margins that must be built into all elements of a project. "Margins enable a project to make design and operational tradeoffs that can significantly reduce mission risks, " he advises. For example, some extra time built into the schedule allows problems to be solved without the panic that comes from an impossible deadline. Some extra mass set aside for the spacecraft may be useful if something needs to be added later in the design and development process. Provisions for additional skilled personnel, if needed, could speed the project over difficult humps if they arise.
"It's all about margin," he says, "especially when you're trying to do it on a tight schedule, when we're trying to do missions in a one-year to 18-month period shorter than we've typically done projects in the past. Because something's got to give somewhere, right?"
'You Don't Know What You Don't Know'
'Landano's Principles' are seen as one means of imparting the wisdom of experience, success and misfortune to less seasoned engineers as they participate in new missions to Mars and elsewhere. The key to good decision-making in space engineering, he says is not only the knowledge gained in school, but the understanding that comes from the real-life experiences of working day-to-day on a flight mission.
As Whetsel puts it, "There's certain level of healthy paranoia I think it's important to keep. The people you really want on your team are the ones who are really bothered by things like 'Why does that telemetry point always read a little different from those other ones? Is that trying to tell me something?' Or 'What have I not tested. I know they say this will handle these five faults all the way out to this range, but which one breaks first? What if?' Those are the kinds of things you want questioned. But you have to have a balance where you aren't just accepting everything that comes your way or challenging everything, but knowing what to accept and what to challenge," Whetsel says.
"Then," says Landano, "when you get to be an older guy like me and other guys who've been through it two, three or even four or five times, we think we understand it. And then, we find out that in spite of all we think we know there are a whole bunch of things you don't know you don't know. They're the things that can kill you. Now you begin to get what you call wisdom. You've' been through it enough to say, 'You know, I'm not as smart as I think I am. I'd better have a way to deal with the things I don't know I don't know.'"
So how to accommodate for those things you don't know you don't know and still get a spacecraft to Mars? Landano's answer: "Margin, margin, margin."