How Lunar Rovers Work

­It's the early 1970s and an Apollo astronaut is hanging out on the moon with some colleagues. Clad in the requisite bulky space suit, he needs to explore a crater several miles away, so he heads for the rover. He steps up 14 inches (35 centimeters) into the lawn-chair type seat in the center compartment of the aluminum chassis. The rover is about 10 feet long (3 meters), 6 feet wide (nearly 2 meters) and almost 4 feet (1 meter) high. It's roughly the size of a modern Volkswagen Beetle.

His partner joins him in the other seat as the first astronaut surveys the LRV. The communications equipment (high-gain antenna for pictures and data, low-gain antenna for voice and TV camera), power (two 36-volt batteries) and navigation equipment are located in the front compartment. In the center compartment are the two seats, the display unit and the hand controller for driving the LRV. The storage compartment behind them holds scientific and rock sampling gear (tools, bags). Below them the rover's four wheels are each made of two aluminum frames (an inner and outer frame), while the tires themselves are made of galvanized piano wire mesh with titanium chevron treads.

The designated driver looks down at the display console in the center of the LRV crew compartment to get his bearings. The navigation display sits on top with a computer display, a sun compass, speed display (0-12 mph, 0-20 kph), reset buttons and a pitch-angle meter that tracks the slope that the rover's on. On the bottom are the power switches that distribute power from the two batteries, the battery power monitors and the switches that control the electric steering motors and drive motors.

Before the astronaut can start driving, he has to complete the startup checklist, the first step of which is sighting on the sun with the sun compass. Once he gives that reading to the people at mission control, they send back data to program the navigation computer. This reading gives the LRV navigation computer a reference point near the lunar module, the Apollo landing craft that serves as their home base while on the moon. While in operation, the computer keeps track of the rover's bearing with respect to the lunar module by using a gyroscope and by measuring distance (range) through the number of wheel revolutions. A compass on the display shows lunar north.

Once the checklist is completed, it's time to head out.

­The Apollo LRV didn't come with a steering wheel per se. It did, however, have a hand controller located just behind the display console on an armrest, which coordinated the steering, drive motors and brakes. The controller was located in the center of the crew compartment so that either astronaut could drive, although the commander usually did the honors. It also came with a T-handle for easy operation with the suit's bulky gloves.

Each wheel of the LRV could operate independently by an electric motor and steer independently of the other wheels so that the LRV could turn even if one steering linkage failed. Similarly, each wheel also had independent brakes. For NASA, redundancy has always been a priority. In addition, this setup allowed a tight turning radius of 10 feet (3 meters).

The T-handle could pivot left, right, front or back and move forward or backward. It also came with a button that could lock the controller for use in a forward direction, as well as a ring to release the parking brake. The movements of the hand controller guided the LRV like this:

Let's return to our two astronauts journeying outward to explore the crater. The LRV's suspension minimizes the bumps of the uneven terrain, but they're strapped in with ­toeholds, handholds and seat belts anyway. Although the LRV is desi­gned go up a slope as steep as 25 degrees or to travel as far as 40 miles (67 kilometers), they won't travel more than 6 miles (10 kilometers) from the lunar module. If the rover failed, they could still walk back to the module before their life support systems ran out.

And unanticipated problems, mechanical and otherwise, did occur. For example, on the Apollo 17 mission, Commander Gene Cernan broke off a piece of the rover's fender when a hammer in his space suit ­pocket caught it as he passed by. The fender blocked the moon dust kicked up by the rover's mesh wheels. If the astronauts hadn't repaired the fender, the wheels would have covered the astronauts and equipment in moon dust -- a hazard to both the men and the equipment. They fashioned a new fender from a laminated map and duct tape, which allowed them to continue using the vehicle. Pretty ingenious.

What happens once the LRV reaches its destination?

­­Once the astronauts arrive at their destination, they stop and apply the parking brake. After climbing out, they realign both the high- and low-gain antennae to Earth so that they can communicate with mission control. Mission control operates the LRV's TV camera remotely while the astronauts deploy equipment and pick up rock and soil samples, which they place in the back of the LRV.

But how much can they transport in the way of rock samples? Although the LRV itself weighs 460 pounds (209 kilograms) on Earth, it can support 1,080 pounds (490 kilograms) fully loaded. That includes two astronauts in suits and backpacks (800 pounds or 363 kilograms), communications equipment (100 pounds or 45 kilograms), scientific equipment (120 pounds or 54 kilograms) and moon rocks (60 pounds or 27 kilograms) [source: NASA]. That's actually not a big weight allowance for samples if some bigger specimens happened to catch an astronaut's eye.

Once they establish their objectives at the site, the astronauts move on to another site and repeat their work. They visit multiple locations on a single excursion before returning to the lunar module to unload samples, rest and prepare for the next day's moonwalk.

This remarkable vehicle extended our range of lunar exploration. The longest single LRV drive clocked in at 20.5 miles (20.1 kilometers) at a distance of 4.7 miles (7.6 kilometers) from the lunar module during the Apollo 17 mission.

­Now that we've experienced the Apollo LRV, let's look at the much newer lunar rover concepts.

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­While the Apollo LRV was used mainly to extend the exploration capabilities of the astronauts during a short stay on the moon, NASA is planning on building a lunar base for extended missions -- months to years versus Apollo's days. Longer missions require vehicles that are capable of doing heavy work, like construction, digging and hauling loads. To this end, NASA has designed a prototype lunar truck.

The lunar truck is a mobile platform made for traveling on the moon. Like its Apollo forebears, it's not pressurized, so astronauts will have to wear space suits while operating it. The truck is designed to move cargo, and NASA is exploring the possibility of adding other equipment to it, like a backhoe or crane. The truck is intended to carry as many as four astronauts.

The astronaut driver stands at the driver's perch. He or she can look around in any direction to move the truck. The truck has six wheels, and each wheel has two tires. The wheels can be steered independently in a rotation of 360 degrees. This setup gives the truck enormous maneuverability. It can go in any direction: forward, backward, sideways or any combination thereof.

Two electric motors power the truck with a two-speed transmission. The truck can lower to ground level and raise back up with a lifting force of 4,000 pounds (17,800 newtons). It can obtain a maximum speed of 15 mph or 25 kph when unloaded.

The prototype lunar truck was developed at NASA's Johnson Space Center in Houston and tested at the center's lunar simulation area at Moses Lake, Wash., where the sand dunes can simulate the lunar environment.

­Let's look at the new pressurized rover concept.

­Both the Apollo LRV and the space truck were and­ will be operated by astronauts in space suits. That means lunar exploration is limited by the length of life support the suits provide. Another downside of unpressurized rovers is that they don't protect the astronauts from solar flare events, which can potentially expose them to lethal doses of radiation. But a rover with a pressurized environment would allow astronauts to explore more of the moon and offer an emergency shelter from unexpected solar events.

That's the idea behind NASA's small pressurized rover. The SPR consists of a pressurized habitat module mounted on the lunar truck chassis. From the SPR, astronauts could explore the moon's surface from a cockpit with a wide field of view. They could also equip the module as a field science station. In fact, the SPR can go pretty much anywhere the lunar truck goes.

The rover's habitat module (or living environment) would allow two astronauts -- four in emergencies -- to live and work comfortably in a "shirt-sleeve environment" for up to three days. A shirt-sleeve environment just means one in which the astronauts don't have to wear their space su­its. The lunar base is another such environment.

The pressurized module has a small bathroom, a misting showerhead for sponge baths, privacy curtains, cabinets for tools, workbench areas and two crew seats that fold back into beds. The astronauts have to rehydrate food packs because there is no kitchen. All of the features are space-saving. During field tests in Arizona, astronaut Mike Gernhardt reported that it felt comfortable, even like the space shuttle [source: NASA].

Astronauts can enter and exit the module from one shirt-sleeve environment to another by using an airlock docking hatch. They can also exit and enter the rover directly into their space suits through the suitport without having to depressurize the habitat module. That's a feat that Apollo astronauts would envy since they had to depressurize and repressurize the entire lunar module when they exited and re-entered. And unlike Apollo, astronauts wouldn't have to bring their dusty space suits inside, thereby keeping the habitat's inside cleaner. In tests of the suitport, astronauts can don space suits in 10 minutes or less.

­Inside any habitat, like the lunar module or space shuttle, the instruments generate heat. To maintain a constant internal temperature, excess heat must be rejected into outer space. The lunar module rejected heat energy by evaporating water. The space shuttle uses radiators. The SPR habitat module rejects internal heat by melting ice in an ice lock around the suitport, which reduces the amount of water that the rover must carry.

Before the new lunar rover concepts go anywhere near the moon, they will be tested and retested in lunarlike environments. Such environments should have terrain similar to the moon's and ideally experience temperature extremes. NASA has several spots where it likes to try out its concepts.

Desert environments such as the sand dunes of Moses Lake, Wash., and Black Point, Ariz., provide out-of-this-world-type terrain, as well as extreme heat, like the temperatures encountered in direct lunar sunlight. Cold temperatures and lunar landscapes can be found at the Haughton base on Devon Island in the Arctic Circle. Antarctica also provides a similarly well-suited environment for testing lunar rover and lunar base concept technologies.

In a recent three-day test of the SPR at Black Point, a team of astronauts and geologists were charged with learning as much as they could about the lava flows by using the SPR. Astronaut Mike Gernhardt reported that participants spent less time in space suits and that they were more productive. Everyone involved in the program hailed the test as a success. The participants even learned how to change a flat tire while wearing a space suit [source: NASA].

Currently, only China and the United States are actively pursuing a manned lunar program. The Chinese recently unveiled a nuclear-powered robotic lunar rover, but they haven't discussed a manned vehicle. So far, NASA has more experience in placing a man on the moon and in designing and operating lunar rovers.

The lunar truck and SPR represent only two technologies in the NASA Exploration Division's Return to the Moon project. NASA is also developing and testing concepts such as inflatable habitats for a lunar base. Eventually, the launch vehicles Orion CEV and Ares may replace the current space shuttle. With all these technologies in hand, NASA hopes to return men to the moon by 2020.

­Keep reading for more links on articles you might like including lunar landings, Mars rovers and the Google Lunar X Prize.