There can be few people who have not heard that a robotic roving vehicle landed on Mars on July 4th. Aside from the images of the surface, no less impressive is the technology behind the Sojourner, the name given to the Martian roving vehicle. Just the size of a child’s small cart, it sports six wheels and weighs in at about 11kg. Although it is slow moving, at speeds up to 0.6m/min, that speed is fast enough to cover up to 3 m during a day.
One function of the vehicle is to demonstrate that small Rovers can actually operate on Mars. In addition, the Rover is deploying an alpha proton X-Ray spectrometer on rocks and soil and imaging the lander after the landing.
To help cope with the terrain, the vehicle’s wheels and suspension use a `rocker-bogie’ system that is unusual in that it does not use springs. Rather, its joints rotate and conform to the contour of the ground, providing the greatest degree of stability for traversing rocky, uneven surfaces.
A six wheel chassis with 13 cm aluminium wheels was chosen over a four wheel design because it provides greater stability and ability to cross objects. Six wheeled vehicles can overcome obstacles three times larger than those of four wheeled designs. Stainless steel cleats and treads on the wheels provide traction and each wheel can move up and down independent of all the others (Figure 1). Three motion sensors along the frame can detect excessive tilting and stop the Rover before it gets dangerously close to tipping over. On board the vehicle are a pair of front cameras and a rear camera that are responsible for taking pictures of the rocks, soil and terrain around the landing site.
Because the Rover radio transmitter has only a range similar to a `walkie talkie’, it cannot communicate directly with the Earth. Rather, all communications to the Rover from the Earth is done through the aid of the Surface Lander. (Figure 2). The telecommunication system between the Lander and the Rover comprises two UHF radios and two UHF whip antennas that operate at 459.7MHz.
The Rover radio modem is located inside a rugged box to protect it from the extreme cold of the Martian environment. It has a 0.5W heater attached to its metal frame that raises its temperature in the early hours of the Martian morning. The heater was incorporated because the radio modem is not temperature compensated. Without the heater, the frequency of the radio modem would change with temperature.
To help plan for the design of future Rovers, engineers will be able to evaluate the performance of the vehicle while it is on Mars.
Meanwhile, on Earth, the next generation of designs is already underway. NASA’s newest six wheeled prototype Martian Rover, nicknamed Rocky 7, (Figure 3) recently passed its most rigorous field test, travelling over 1000m over Mars-like terrain. It looks very similar to its predecessor, sporting a six wheeled chassis and spring-less rocker-bogie mobility system.
On Rocky 7 is a 1.4m, antenna like mast, which would be deployed once the future Rover was on Mars. The mast has 3i of freedom and can be used to deploy scientific instruments against rocks. Two instruments-a Moessbauer spectrometer and a NMR spectrometer-are mounted on the mast to study surface rocks with different types of coatings which might be found on Mars.
Rocky 7 may be sent to Mars in the years 2001 and 2003. More information about Rover developments for future Mars missions is available at http://robotics.jpl.nasa. gov/tasks/scirover.