Mars Exploration Rovers

Question:
What is the working lifespan of the rovers?

Answer:
The main limiting factor in the rovers' ability to continue functioning is how dust collects on the solar arrays, and reduces the amount of electricity that they can generate.

The expected lifespan of the rovers was 90 sols (1 sol is a Martian day, or 24 hours and 40 minutes in Earth time). They have both so far exceeded expectations, and the team currently expects that they will be able to function well until September (more than 200 sols).

--Eric

Expert:
Eric Rice
Mission Control Team
Mars Exploration Rover Mission
Jet Propulsion Laboratory

Question:
What do you think the major cause for the end of the mission will be?

Answer:
It's very hard to know what may lead to the end of mission.

The common effect that the team has traditionally looked for has been diminishing power return, with dust collecting on the solar arrays. Models of that process predicted a mission life of 90 Sols, and have so far been shown to be inaccurate. As we continue to explore Mars, dust will continue to accumulate, and we will have to operate with less and less power, using less of rovers' functionality as time goes on. While it's possible that power will still end up being the major limiting factor of the mission, it is also important to consider that we are now operating rovers well past their designed lifetime.

If you take a look at many of the deep space orbiters in the past, what has tended to happen is that one system or another will start to fail from the stresses of the harsh environments they're exploring. That system will be turned off, and operations will continue around it, until so many of the spacecraft's systems have failed that the goals of the mission can no longer be pursued.

So, it really is hard to say that there will be one major cause of the end of the mission. If I had to choose one thing to say, though, I would say "environmental stresses."

--Eric

Expert:
Eric Rice
Mission Control Team
Mars Exploration Rover Mission
Jet Propulsion Laboratory

Question:
How long did it take to truck the spacecraft from California to Florida? How many people and what type of special equipment was involved?

Answer:
Thanks for your excellent question! I couldn't find this information on the MER website, so I started emailing some friends working with me on MER. Finally, I heard from the head of ATLO (Assembly, Test, Launch, and Operations) for the spacecraft, including the trips to Florida. It actually took three one-way trips total to get all the MER hardware from California to Florida, a distance of roughly 2600 miles. This took about 53-58 hours per trip, which averages out to a speed of 45-49 mph. I did not find out how many people accompanied the caravan, but I imagine it was a dozen or two. There was special equipment involved in transport, though. One thing typically used for such trips is air conditioning for the spacecraft, above and beyond the normal A/C in your car, for example. I imagine there's some vibration supression as well, though if it can't handle a cross-country trip, it probably can't tolerate the rigors of a Delta II launch! Thanks for a very thought-provoking question.

Todd J. Barber
MER Propulsion Engineer

Expert:
Todd Barber
JPL Staff Propulsion Engineer
Mars Exploration Rover Mission
Jet Propulsion Laboratory

Question:
How high must the internal temperature of the rover be to keep it functioning?

Answer:
Thanks for your excellent question! For most of the electronics in the Warm Electronics Box (WEB), the minimum allowable flight temperature is -40 degrees Celsius. This happens to be the point at which deg. C equals deg. F, so this also corresponds to -40 deg. F. For the battery, the min allowable flight temperature limit is warmer, at -20 deg. C (or -4 deg. F). This is a challenge durning the cold Martian nights (hundreds of degrees below zero!), but our well-insulated WEB allows these internal temperature limits to be met. Thanks for your awesome question!

Todd J. Barber MER Propulsion Engineer

Expert:
Todd Barber
JPL Staff Propulsion Engineer
Mars Exploration Rover Mission
Jet Propulsion Laboratory

Question:
To what extent will each rover operate autonomously compared to commands sent from the JPL mission team?

Mr. McCollum's 8th grade class
Charleston Middle School
Charleston, IL

Answer:
Mr McCollum et al,

There are levels of autonomous operation built into the mission design. At the highest level, everything the rovers do is commanded by JPL, but at lower levels, the rovers can execute a large series of commands, and do some decision making, without human interaction.

The way the rovers operate you can think of in terms of your standard PC. On the rovers, we have "flight software" that handles most of the basic tasks. That's the operating system. Then we send up bundles of commands, or "sequences," that tell the rovers what tasks we want them to carry out, kind of like the program you run in your operating system.

Every day, the science and engineering teams look at the data sent back by the rovers. The science teams take what they are seeing, and plan out what science they would like to do next. The engineering team takes those science goals, and plans out how each rover will take the requested measurements, and how to keep it safe and healthy while doing so. They then build the sequences, and we send those up.

The rovers execute the full sequence of activities without much if any intervention from the operations teams. Most of the day, we're not even watching how things are going, and then we get the data down after everything has been done. The sequences may also call for autonomous decision making built into the flight software, like asking for a drive using auto navigation. The rover will take that and, within the bounds we set for it, do what it needs to do to accomplish the task.

I hope that answers it for you. Thanks for the question!

--Eric

Expert:
Eric Rice
Mission Control Team
Mars Exploration Rover Mission
Jet Propulsion Laboratory

Question:
Do you think that one machine will last more sols than the other?

Answer:
It's likely that one rover will last longer than the other. They are both operating in different environments on Mars, and are subject to somewhat different stresses and wear. They are also performing somewhat different tasks, which will make a difference in the end how quickly parts wear out.

Thanks for the question!

--Eric

Expert:
Eric Rice
Mission Control Team
Mars Exploration Rover Mission
Jet Propulsion Laboratory

Question:
From: Chris Ferguson, 8th grade, Charleston Middle School, Charleston, IL

What were some of the most recent discoveries as Spirit and Opportunity completed their main missions?

Answer:
By looking at the rocks in the craters we have discovered that a shallow salty lake existed there for some time. We are not sure how long it was present. It is believed (but not proven) that the water has all evaporated and escaped from the gravity field of Mars. What is exciting is that the presence of water for some time may mean that primitive bacteria-like life may have been present on Mars. We need to evaluate more rocks from different periods of time to get a better picture of the history of the water. Some of the other, deeper craters that are near Spirit and Opportunity seem to have older rocks exposed that may give more information. The risk of climbing down into those craters is being evaluated.

Expert:
Virginia Ford
Principal Engineer
Jet Propulsion Laboratory

Question:
How much longer do you think the solar panels will last? Why do you think they have lasted this long?

Answer:
The solar panels were designed to last for three months, but we always try to design so that we have margin. When you evaluate the life of a component, you think about what things could happen to make this device fail. In aerospace engineering, we are always trying to work at an extremely high level of reliability. That is because, once a system is launched, we can rarely go fix anything that fails. So we always design so that if the worst case situation we think of occurs we will survive. One of the main reasons that we thought the solar panels would fail is that we thought they might become coated with Mars dust. We didn't know much about how the dust works - what will attract it or what will repel it? is it rough and skratchy so that it would damage the solar cells if we try to brush or blow dust off. We just didn't know what would work best to keep the important surfaces clean. So we decided the best approach would be to do little, with an attempt at avoiding protrusions which would collect dust. So to finally answer your question, the solar panels have lasted this long because the dust isn't as nasty as we thought it might be. They look like they will last quite a lot longer because the power they are supplying is still strong. Of course, we cannot predict when some other event might cause failure. A wire might break, a storm could blow a rock onto a rover, an electrical part might fail. Hopefully, our plan to design to overcome the worst case we can imagine will enable both rovers to continue sending us wonderful data for another three months or longer!

Expert:
Virginia Ford
Principal Engineer
Jet Propulsion Laboratory

Question:
If you were to repeat this mission again, what would you change?

Answer:
I would change the instruments. All the engineering functions have worked perfectly, so I would try to keep them as they are as much as possible. The engineering functions include everything that made landing possible, the communications system and antenna, the power supply (solar cells and batteries), the structure, wheels and motors of the rovers, and the electronics that drive the motion and control the rover functions. Also part of the engineering functions are the six cameras on the rover that provide engineering data - the hazcams and the navcams. The hazard cams look all around the wheels for hazards that might cause a rover to tip over. The navcams look around at the scenery to help decide what to navigate to next. So that leaves the instruments that collect science data. If we were going to repeat this mission, we would want to collect different science data so we could learn more. The process to select the new instruments would be very competitive with scientist teams from all over the world proposing different ideas. It is fun to read all of their ideas to see what neat instruments they would develop to collect data to understand and learn things about Mars. You can tell that I am an engineer because I get excited about the instruments. Scientists get excited about the data.

Expert:
Virginia Ford
Principal Engineer
Jet Propulsion Laboratory

Question:
My kids, Alexandra and Hannah, wanted to know if the Rovers can hear the sounds on Mars. What do the winds sound like? Has there ever been a robot on Mars that could hear sound? Thanks!

Answer:
These rovers cannot hear sounds on Mars, and there haven't been rovers on Mars that could. Sound is caused by the vibration of air, and though the Martian atmosphere is much much thinner than ours on Earth, there would be enough vibration through the air to carry sound waves. To sense this, you would need to develop an instrument that would sense vibration and change that vibration to an electrical signal which could be transmitted back into a sound after being sent home to earth. It would be like a microphone and a speaker that makes sound come out of a radio. All the science instruments included on the rovers were carefully selected to make sure we would collect data that could be used to learn as much as possible about whether Mars once had water on its surface. Perhaps a later mission will use sound sensing to collect important data. It is something to think about!

Expert:
Virginia Ford
Principal Engineer
Jet Propulsion Laboratory

Question:
What are the chances of Spirit rover encountering a dust devil and what would be the result of that?

Answer:
Thanks for your excellent question! We hope the chances are very good for both Spirit and Opportunity encountering a dust devil during the landed mission. In fact, we allow some time during the occasional Martian day ("Sol") to look for dust devils. Mars Pathfinder did see evidence of a dust devil during its three-month mission. The result of encountering a dust devil for Spirit or Opportunity would be two-fold. First off, it would be a scientific boon, helping us to understand these transient weather phenomena in the entirely different environment of the Martian atmosphere. Second, and I think this is the genesis of your question, nothing bad would happen to the rovers at all, even if a dust devil crossed right over the rover! Though Mars has enormous dust devils by Earthly standards, the Mars atmosphere is very thin and tenuous. This means the force of the Martian wind during a dust devil passage is very low, indeed. Thanks again for a wonderful question!

Todd J. Barber
MER Propulsion Engineer

Expert:
Todd Barber
JPL Staff Propulsion Engineer
Mars Exploration Rover Mission
Jet Propulsion Laboratory

Question:
I read that the MER solar cells operate with less than 50% efficiency. Is it possible to develop solar power with a greater efficiency? What is the future for solar energy on Mars? On Earth? Do you anticipate the use of solar power on Mars to be greater than on Earth?

Answer:
The 50% efficiency of the solar panels on the MER rovers right now, is a reduction of 50% from their initial efficiency. Dust is accumulating on the surface of the solar panels, and there is nothing that can be done to remove it. However, the performance will continue to provide us enough power for a while longer. If all goes well, and the rovers continue to work well past their intended lifetimes, we will be able to last the winter when the sun is too low to give enough energy for good power, and be able to do more in the Spring when the sun starts to return to a favorable position in the sky.

Solar power is a wonderful technology, but it is fundamentally limited by the efficiency of the materials used to tap the sun’s energy. Crystalline silicon is the most efficient material we have found so far, and it only captures 15% of the available energy in sunlight. Other materials are being explored for solar technologies, but most of them only capture about 8% of the total solar energy. There is always the possibility that more efficient materials can be found, or that better ways of optimizing the solar sensitive materials can be found.

The future of solar energy on Earth is good. As non-renewable energy sources such as petroleum become more scarce, it is going to get more expensive to use. Right now solar cells are very expensive, but mostly that is because they are not produced in large quantities. Once the demand rises for solar systems, they will become much cheaper to produce. Earth is much closer to the sun than Mars, so solar power is a much more viable alternative for us here. On Mars during the winter the sun is very far away, and the power available is small. NASA is looking into using radioactive power sources for some future missions on the surface of Mars. These power sources were used on the Viking landers on Mars in the 1970s and allowed them to function for many years. Solar power has been used on almost all of the orbiting satellites we send into space, and should continue to be used because solar power is very reliable, and there is little dust or other materials in space to coat the panels and degrade their performance over time.

Expert:
Amy Knudson
Graduate Research Assistant
Arizona State University

Question:
The MER rovers are much larger than the Pathfinder rover, yet they launched on the same Delta II rocket. How did you manage to launch Spirit and Opportunity with the same rocket as Pathfinder?

Answer:
Unfortunately, I did not work on the launching system for the MER program so I don't know all the details of the decisions that were made when chosing the launch vehicle for Spirit and Opportunity. I do know that every mission trajectory is carefully analyzed and evaluated so that just the right amount of force sends the rockets in the correct direction. Also, each launch rocket has a range of mass that it can lift. Many things influence what mass can be lifted by a rocket including, how much fuel is loaded into the rocket, exactly how the rocket needs to leave Earth's gravity - what direction and how fast, and how well the spacecraft is balanced inside the rocket. I also know that Pathfinder went to Mars when Mars was farther away than it was for Spirit and Opportunity. So the same size rocket was probably a good choice, even though Pathfinder wasn't as heavy as either of the MER rovers.

Expert:
Virginia Ford
Principal Engineer
Jet Propulsion Laboratory

Question:
How can the infrared camera tell if a rock has been formed by contact with water?

Answer:
The infrared spectrometer, Mini-TES, tells us the minerals that are present in the rocks. Minerals are the building blocks of rocks, if you pick up a piece of rock sometimes you can see the individual mineral grains. In a granite for instance, you usually see pink, clear, and black crystals, each of these is a different mineral – usually Feldspars are pink, Quartz in the clear, and a whole variety of minerals are black. When we look at the infrared signature of rocks, we see distinctive patterns that tell us which minerals are in a rock. Certain minerals form in volcanic rocks, and others form if that rock has been in contact with water. So what we are looking for are specific minerals that only form in the presence of water.