As we prepare to send our robotic explorer to Moon’s surface, we are fortunate to be able to draw on the experiences of those who have accomplished this feat in the past. Since we are not the first to explore other celestial bodies, history can give us some feedback on what we can expect our rover to be able to do, and what to expect when it gets to its destination. There are two locations in the solar system where humanity has sent mobile rovers to explore. The former Soviet Union sent multiple rovers to the Moon (Lunokhod 1 & 2), and the United States has successfully deployed roving vehicles on Mars (Sojourner, Spirit, Opportunity).
Communication
The most important thing to account for when controlling a distant rover is the communications latency. Since we communicate with the rovers using radio signals, the speed of the signal is limited to the speed of light. Light takes a little over a second to travel between the Earth and the Moon, and anywhere from 4 to 20 minutes to travel from the Earth to Mars. Taking into account the fact that the return signal would take just as long, that means we would not be able to see the results of a command issued from Earth to a rover on the Moon until at least two seconds later. On Mars, we would have to wait for somewhere between 8 and 40 minutes.
Almost real-time
Due to this latency, it is impossible to drive a rover in real-time on Mars, and it can be somewhat difficult to manage such control on the Moon. For Martian rovers, the latency is addressed by sending many commands at once in what’s called a batch. The rover then tries to execute all of the commands and report the results back to Earth. There are, however, many sophisticated programs on the robot that help it to navigate the local terrain, and check to make sure its not doing anything dangerous. If something unexpected happens - like for example a wheel slips on a rock and the rover suddenly slides down - then the rover is programmed to stop what it is doing, report the problem back to Earth, and then wait for further instructions.
Russian Experience
Since we are only going to the Moon, we can learn a lot from how the Soviet Lunokhod (moon walker) missions were conducted. The Lunokhod rovers were controlled from a location on Earth by 2 crews of 5 people each. The crews worked in 2-hour shifts. Each crew had a commander (to manage the team), a driver (to drive the rover), a navigator (to tell the driver where to go), a flight engineer (to monitor the health of the rover), and an antenna operator (to keep the rover’s antenna pointed at the Earth). The rover sent back low quality images to the operators on Earth to help with the navigation. During the Lunokhod 1 mission, these images arrived at a very slow rate. Each image took between 7 and 20 seconds to download. The field of view in these images was also very limited. The driver had to remember what was immediately in front of the rover because the camera could only watch ahead, and there was not way to see what was directly in front of the wheels.
Emergencies
When driving the rover, there are typically only two reasons to stop: when the rover might be getting into a risky situation, or when you want to study something nearby. Usually hazards can be identified in the images sent back from the rover. However, the Lunokhod missions reported two unexpected conditions under which it could be dangerous to drive the rover. The first one is near lunar noon, when the Sun is directly overhead. The second one is when you are driving with the Sun directly behind the rover. The common factor here that poses the danger is the absence of shadows and insufficient contrast on the surface to determine if there is a slope in front of the rover. At lunar noon, there are no shadows at all, and when the rover is driving with the Sun at its back, all shadows are behind the objects casting them. While the Soviet drivers expected the first condition, they did not expect the second. Unfortunately, this resulted in Lunokhod 2 being driven into a crater.
World records
The primary objectives of the Lunokhod missions were to try to avoid obstacles and to travel as far as possible on the Moon’s surface. To this day, Lunokhod 2 still holds the distance record for roving on the lunar surface (37 km). While this was a major accomplishment at the time, this strategy did not bring much scientific value. It is perhaps better to look at the Martian missions to get inspiration for conducting science while roving. In particular, the Spirit and Opportunity rovers can provide us with many ideas on how the rover can be used in innovative ways to get more science out of the mission.
Unexpected discoveries
Although Spirit and Opportunity arrived on Mars at roughly the same time, they landed on opposite sides of the planet. And while Opportunity has been very fortunate in its operations, Spirit has been plagued with mechanical failures and frequent power issues due to dust settling on its solar panels. Spirit is a survivor though. When one of its front wheels stopped working on March 16, 2006, Spirit was still able to make progress by driving backwards, dragging its broken wheel behind. Now, one might think that a rover should only use its wheels to travel, but an unexpected benefit to Spirits wheel failure was that the rover began digging a trench where its wheel was dragging. This led mission scientists to observe some unusual regolith that was being turned up by the wheel. Opportunity also used one of its wheels to dig a trench, though this time it was deliberately instructed to do so. The rover pushed soil forward and backward out of the trench with its right front wheel while other wheels held the rover in place. The rover turned slightly between bouts of digging to widen the hole. The resulting trench is about 50 centimeters long and 10 centimeters deep.
Back to history lessons
We will most likely take lessons from both the lunar and Martian rovers. Our Asimov Jr. rover will be controlled in a manner very similar to the Lunokhod rovers. It would be remote controlled from Earth, with almost real-time data being returned. There will be other processes that will be automated by the programs on the rover, such as keeping the solar arrays pointed at the sun and maintaining a good communications link with the Earth. The rover will also be responsible for monitoring its own vital signs and communicating any problems it detects back to the operators on Earth. Our science objectives are modest at the moment. The primary mission objective is to satisfy the criteria for winning the Google Lunar X-Prize. We are, however, working closely with the DLR to increase the science capabilities of our rover so that we can perform useful data while we are roving across the lunar surface. As with the lunar and Martian rovers that have come before, we expect that there will be much to learn from our rover that we cannot fully anticipate at present. This is what exploration and discovery is all about.
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