The contestants to the Lunar Rover Race will face a number of technological difficulties that will have to be overcome. This section discusses seven important challenges.

Locomotion in Lunar Soil

Robotic rovers will play a key role in preparing human habitats on, for instance, the Moon and Mars, before humans actually arrive there. Locomotion on the Lunar surface is a major issue in robotic rover design.

The sandy composition of the lunar soil will make it difficult for a small wheeled rover to move, much like a normal car which is stuck on loose sand. The principle of "all wheel drive" facilitates efficient movement. Next to powering of all the wheels, wheel size is an important factor in rover locomotion. If the wheels are too small, the size of obstacles will be large in comparison and the rover will not be able to achieve a high speed (in the order of 10 km/h). The Lunar Race Rovers will have sizes roughly equal to that of the Sojourner of the Mars Pathfinder mission, shown in Figure 4-23.

To give an indication of required wheel size: Sojourner was designed to have a top speed of 1 cm/s on the Martian surface, with 13 cm diameter wheels. As the Lunar surface is looser than Mars' surface, the required wheel size may be larger than that of Sojourner. The rule of thumb is: the larger the wheels, the better mobility the rover will have, with the prescribed maximum rover size as constraint. As prediction of locomotion in loose sand is difficult, testing will be necessary to determine an adequate wheel size.

Speed

The LRR is all about speed. Similar to the Paris-Dakar race, the rover that completes the staged track in the least amount of time wins the prestigious gold medal. The created racetrack will then be named by the winner.

To get an indication of current status of rover speed: Mars Pathfinder's Sojourner (Figure 4-23) was capable of reaching a top speed of 1 cm/s. This very successful rover was not designed to reach higher speeds because it was not required for the scientific and technology demonstration objectives of the mission. Another consideration was the required power available at the orbit of Mars. Being at 1.5 astronomical units distance from the Sun, Mars receives less than half the amount of solar power that the Earth and the Moon receive.

The Lunar Race Rovers will be specifically designed for speed and endurance. The speed and endurance requirements are likely to result in completely differently designed rovers than Sojourner.

Technologically, building a rover that moves at high speed for prolonged periods of time in a sandy environment is feasible. However, there are a number of considerations that constrain the maximum speed of the rovers:

1. ON-BOARD POWER: This is limited by allowable rover size and mass. Because the rovers are not allowed to have nuclear power they will most likely rely on solar arrays. From lunar morning to lunar noon (seven Earth days), the solar input gradually increases from 0 W/m2 to approx. 1.36 kW/m2 (the Moon is close to the Earth and therefore, we assume the solar constant is approximately equal to that of the Earth). So in the lunar morning the rovers will receive little power, unless their solar panels can be tilted.
   
2. 2.7S LUNAR TIME-LAG: This will complicate teleoperation. The distance that a rover travels in 2.7 s when it is travelling at 10 km/h is 7.5 m. When viewing an image it must be kept in mind that the actual rover has in real-time already hit an object that is 7.5 m ahead in the image. In order to guide the rover through the landscape safely, the track must at least be visible and navigable to this distance. Allowing for response time of the driver and margins of safety, it will be required to see ahead for approximately 15 m. In order to look ahead that far, the rover's camera will have to be mounted at a considerable height.
    
3. LOW LUNAR GRAVITY LEVEL: This will change the inertial response of the rover. Consider an arbitrary rise on the lunar surface, as shown in Figure 4-24. Because gravitational acceleration is six times lower than on Earth, a high-speed rover will behave very differently on the Moon compared to on Earth. Due to the reduced lunar gravity, the rover will separate from the surface and rise six times further than for a similar hill on Earth. The speed of the rover on a rough course on the Moon will have to be reduced six fold to have it behave in the same manner as it would on a similar test course on the Earth. Although the racing rules do not specify any maximum velocity, a speed of 10 km/h appears to be a feasible guideline.

Teleoperation and Rover Autonomy

As described previously, teleoperation of a high-speed rover in an environment that is delayed by 2.7 s is very complicated. The contestants are free to decide how their rovers should be operated. It is possible to develop or purchase dedicated teleoperation software that takes the time-lag into account by extrapolating the rover's movements and then presents the predicted "actual" position of the rover. These are called predictive displays.

Next to using predictive displays, the time lag problem can also be overcome by having a highly autonomous rover. An autonomous rover derives the optimal path to its goal from sensory inputs and makes its own decisions on long- and short-term navigation and speed:

• NAVIGATION: to determine the location of the target and how to get there in the fastest and safest manner;
   
• OBSTACLE HANDLING STRATEGIES: to decide to go over an obstacle or swerve around it;
   
• SPEED: to determine the maximum allowable safe speed depending on the composition of the surface, size of obstacles, landscape curvature, taking into account the low gravity level.

A highly autonomous rover that travels at high speed through a complicated landscape requires sophisticated, robust control algorithms, presenting major software design challenges. The danger of a high level of autonomy is that the control algorithms can never take every possible situation into account, and rover recovery from an unforeseen incident may be difficult and time consuming.

Rover Stability

The high speed of the LRR, together with the low gravity environment and the existence of bumps and rocks on the lunar surface will require the rovers to be very stable. As shown in the previous paragraphs, the effects of the rover speed are effectively multiplied by six due to the lower gravity. Because rover-ground separation is more likely on the Moon than on Earth, chances of landing on a side or upside-down are generally higher.

Endurance

The LRR will be approximately 400 km long. This long distance will have to be navigated without any hardware maintenance. In addition, software uploads during the race may not be allowed. The rovers must therefore be very reliable and have high endurance.

Thermal Control

The difference in temperature between lunar night and noon is approximately 300 degrees centigrade (-180 degrees C to +120 degrees C). During lunar noon, the power input from the sun is very high and the rovers may need to have a thermal control system to keep excessively high temperatures from damaging the system.

Communication

The rovers will communicate with ground control in a direct manner. The Racing Authority will not provide a relaying service to the contestants. The up and down link will be required for teleoperation of the rover and to send real-time video coverage of the race at a minimum required frame rate. The required video coverage will consume the major part of the bandwidth. With the high-speed objective consuming most of the available power, the challenge will be to transmit the required video coverage with a minimum amount of power. As the frequency for the video coverage is set by the racing authority (K-band, see section 4.6.4), the remaining variables are a) transmitter dish size, and b) transmitter power. The required video transmission power can be decreased by:

• Increasing CPU power to compress the video signal in real-time before transmission, or by
   
• Increasing the transmitter dish size, which increases the overall mass of the transmitter system and decreases maneuverability of the rover.

Therefore, a trade-off must be made between rover mass, broadcasting and processor power, and maneuverability.

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