Alumnus strives to safely land giant rover on Mars

Two years ago, Keith Comeaux (MS 1991, PhD 1995 AA) got a call from NASA’s Jet Propulsion Laboratory (JPL) with an offer he couldn’t refuse: come work on the Mars Science Laboratory (MSL), a next-generation rover headed to the “Red Planet” in 2010. Comeaux is lead verification and validation engineer for entry, descent, and landing, which means he is responsible for making sure that the MSL team has a good plan to ensure a high probability of a successful landing on Mars. It’s a tall order. The craft's method of landing hasn’t been tried before in space and two-thirds of all vehicles sent to Mars have never been heard from again. But nobody has had more success than JPL engineers.

Describe the Mars Science Lab generally.

We’ve got a suite of instruments on the Mars Science Lab rover that will help us look for evidence of habitability — evidence that the climate on Mars was adequate to support life sometime in its past. Some of the instruments will be able to drill into rocks, extract samples from their interior, and crush them into small bits to bring them inside the rover to analyze. Other instruments can then look to see what kind of chemicals are in the rocks, which might give us a clue as to whether the interiors of the rocks are suitable for microorganisms to live. And there’s also a laser on board which can vaporize rocks from a distance that the rover cannot actually reach. Using a telescope on the rover, we will be able to look at the vaporized material that comes off the rocks, so we can determine what kind of chemicals are present.

Your job involves entry and landing. Typically, we have just sort of bounced these things on air bags, is that right?

That was a much easier way to arrive at the surface. We didn’t have to have a control system which precisely guided the craft all the way to a soft landing. With the air bags we can release the rover from the parachute and let it bounce on the ground until it stops. In our case, MSL rover is effectively the size of a small car. When you scale the landed mass to that size, the air bags don’t scale very well with it. It just becomes untenable to try to land a system that big with air bags. So we’ve gone back to powered flight, which was done previously on the Viking missions in the '70s, and of course, on the Moon, with Apollo, and the Surveyor landings in the '60s. We are using descent engines to slow our speed after we drop off the parachute. The large rover also requires the largest capsule that has ever gone to Mars: 4.5 meters across. This time we are automatically guided during the entry. We swing the vehicle back and forth to steer ourselves during the hypersonic entry to Mars, to land precisely at a selected target on the surface of Mars. Once we reach about Mach 2, we deploy the parachute, and that slows us down to subsonic speeds, at which point we drop the heat shield and acquire the surface with radar. So that sequence is pretty conventional with regards to the past, except that we are a guided entry this time, for a precise pinpoint landing. Now the unique feature about our mission (see animations) is that, once we get close to the surface, rather than landing on the surface and having a rover drive off the top of the landing system, we’ve inverted the position of the rover relative to the landing system. The rover is underneath the descent stage, as we call it. The rover flies with the descent stage on its back, if you will, until it gets close to the surface. Then it lowers itself from the descent stage on a bridle, which hovers above the ground, and then it softly touches down on its wheels. The rover is then cut from the descent stage, which flies off, no longer needed. So all we have left after the landing sequence is the rover on the surface of Mars, ready to do science.

So has this idea of hanging from a bridle ever been done before?

That part of it is completely new. There is precedent, however, for this kind of approach. You see, helicopters typically carrying loads flying underneath them. In fact we use the name, Sky Crane, as an homage to the Sky Crane Helicopter, which has been a workhorse of the military for carrying very large loads underneath it. After the rover separates, and you are in Sky Crane mode, the rover is hanging on a 7.5-meter bridle. So that allows the control system to not work as hard in the landing phase of the mission. In addition, the rockets don’t have to get as close to the ground and disrupt the soil and kick up a lot of rocks and dust that might damage the hardware. At the Sky Crane point in the mission we are descending at about three-quarters of a meter per second. And that’s our touchdown speed. We’ll simply place the rover onto the surface at that speed. Two-thirds of all craft that have been sent to Mars have never been heard from again. It is a graveyard for spacecraft. We are very keenly aware of that and part of my job, on the entry, descent, and landing team, is to make sure that we are doing all the right things, both in terms of analysis and testing, to ensure that we have mitigated as much risk as we can with regard to the safety and probability of a successful landing on Mars.

So how do you test this?

We can look at all the pieces, and we test the pieces extensively. We don’t have the luxury of being able to test the full systems here on Earth, because, number one, the gravity is different and, number two, the atmosphere is a hundred times thicker than it is on Mars. So we break apart the system into pieces and we test the functionality of each one of those pieces. For example with the heat shield, we test samples of it in high velocity wind tunnels at NASA Ames and we test the Sky Crane bridle as an individual unit in a test facility here at JPL. From the knowledge that we gain in those tests, we create computer simulation models of our system, and we basically fly the entry, descent, and landing segment in the computer, tens, if not hundreds of thousands of times, to really stress the system as we know it, and ensure that we have a robust performance against things that we actually might encounter as we land on Mars, like dust storms and wind and certain terrain that might otherwise kill us. We put all that into the computer and let it run, and when things don’t work out we go and dig a little deeper to find out why and determine if we need tweak some part of the design of the system to make it better.

How long have you been working on this at JPL?

I’ve been here for two years. Previous to JPL, when I left Stanford, I went to work for Hughes Space and Communications (now part of Boeing) in El Segundo, building communications satellites for companies like DirecTV and XM Radio. And it was there I learned what space systems engineering was all about. I worked on the development of the 702 spacecraft product line. It is a commercial satellite platform. That was one of the first commercial applications of ion propulsion technology in the space industry.

What’s “ion propulsion” about?

Back in the ‘60s, ion propulsion became a newfangled technique for creating velocity changes for spacecraft. And it was based on accelerating heavy molecules of gas using an electrostatic field instead of chemical propulsion. It gets you much more efficient miles per gallon, if you will. It was featured in Star Trek and other science fiction movies but it became a reality in the ‘90s for commercial space applications, and we were among the first to actually deploy it. The spacecraft is designed to last for 15 years in geo-stationary orbit, using ion propulsion as its sole means of control.

How did you get to JPL?

Two years ago, I got a call from JPL. They had an opening and asked me to come join the MSL team and that was just an opportunity that was too good to pass up. Having seen their past successes — Pathfinder, and the Mars Exploration rovers — and, getting back to my background at Stanford, where I studied hypersonic vehicles, together with my system engineering and integration and test experience at Hughes and Boeing, it was a really good fit for me to work on this project.

What was the particular pull to work on the Mars mission?

I was enjoying the work I was doing, but the pull was the opportunity to put hardware on Mars. There are few places in the world like JPL where you can go to do that. And to be able to work in this environment with extremely talented people who have done it before, you know, that was just an opportunity for me to broaden myself and learn about doing things that are almost unimaginable.

What did you study here?

I studied computational fluid analysis, particularly as applied to hypersonic problems. My advisers were Dean Chapman and Bob MacCormack, two giants in the field. They were doing computational fluid dynamics before most people even knew what it meant, back in the ‘60s. For me, that was a real honor, to be working with them, as well as my fellow students, who were among the smartest in the country. I try to continue doing that in my career — to surround myself with good people to challenge myself to become a better engineer. At the time, I worked on a particular hypersonics problem regarding rarified gas dynamics. There exists a set of equations beyond the Navier-Stokes equations that people have used to model rarefiedgas dynamics problems. These extended equations are called the Burnett Equations, which were derived in the 1930s, and are very complex.. I came in at a time when there were more and more apparent successes applying them with computers and my assignment was to push the envelope of applicability. However, there were some peculiarities about the equations that people kept encountering and couldn’t explain. In some cases, they would spontaneously blow up and not provide any real solutions. I was able to put the pieces together, and discovered that the equations don’t actually satisfy the second law of thermodynamics, which is a fundamental requirement of physics. So it turns out the equations were flawed from the very beginning. But people just didn’t understand it because they were so complicated that you couldn’t really analyze them well enough without powerful computers.