On July 27, at the end of a drive of 157 feet (48 meters), the mission odometer on NASA’s Mars rover Opportunity cycled past 25 miles (40.2 kilometers) to become the new reigning champion for miles driven off-Earth, a record previously held for 41 years by the Soviet Union’s Lunokhod 2 rover, successor to Lunokhod 1. Lunokhod 2 landed on our Moon on January 15, 1973 and drove 24.2 miles (39 kilometers) before its mission ended. According to a JPL press release, the Russian rover’s total distance has been verified by images taken of Lunokhod 2′s tracks by NASA’s Lunar Reconnaissance Orbiter (LRO) and vetted by Irina Karachevtseva at Moscow State University of Geodesy and Cartography’s Extraterrestrial Laboratory in Russia, Brad Jolliff of Washington University in St. Louis, Tim Parker of JPL and others using map-based methods for computing distances comparable for Lunokhod 2 and Opportunity.
Cornell University’s Steve Squyres, principal investigator for NASA’s twin Mars rovers, Opportunity and Spirit, described the Lunokhod missions as an inspiration for the Mars rover team. In an homage to the legacy of Russian lunar rover program so many years ago, the Mars rover team chose the name Lunokhod 2 for a crater measuring 20 feet (6 meters) in diameter along the western outer slope of the Martian crater Endeavour earlier this year as Opportunity was gradually approaching the mileage record.
Lunokhod 2 traversed its entire 39 kilometers in less than five months back in 1973. Opportunity, even with superior technology, has taken over ten years to exceed 40 kilometers. The reason is mostly due to operational difficulties related to the round trip light time, a concept very important to telerobotics. Since the Moon is so much closer to human operators on Earth than Mars, the time it takes for a command to leave Earth, travel to the Moon and be confirmed by a lunar rover is less than three seconds. Depending of the relative position of Earth and Mars, the round trip light time for a command to a Mars rover and confirmation of command received is at least seven minutes (best case) and can be as long as forty-one minutes (worst case). Think about it. With lag times measured in tens of minutes, there is no such thing as real time control. Each movement of a Mars rover must be planned in excruciating detail and executed in a series of very specific incremental motions. Then the new position of the rover has to be fully documented (as well as any surrounding hazards) before another set of movements can be planned and executed. This is a slow, repetitive, painstaking and potentially unforgiving process. One wrong move and the entire mission could be jeopardized. Just getting to Mars is hazardous enough. Of 49 missions sent to the red planet, 26 have either failed to launch, failed en route or failed to land.
It should come as no surprise the Mars exploration rovers are not exactly speed demons. Curiosity, the most recent, most advanced and the largest rover on has an average speed of between 0.00018 mph and 0.00073 mph. Initially it was only expected to travel a total distance of 3-12 miles from its landing site during its entire mission. However, Curiosity has already traveled 5.34 miles since landing in 2012.
Numbers three, four and five on our top ten list of off-Earth driving distance rovers are the speed demons of the group. They are the famous “Moon Buggies” of Apollo 17, Apollo 16 and Apollo 15. They racked up total distances driven of 35.74 km, 27.8 km and 27.1 km respectively. Since the predominate exploration modality of the lunar surface by astronauts was geological, it was essential they be able to gather samples of rocks and soil samples from a wide variety of locations for transport back to Earth. This required a whole slew of shovels, tongs, small pick-axes, collection bags, etc. Because of the low gravity of the moon, it was fairly easy for the astronauts to carry the required tools with them. However, transporting tools and samples back to the lander proved to be problematic. It became apparent astronauts had to either abandon their tools at the end of each lunar EVA or harvest fewer samples. Because multiple lunar surface EVAs were performed during each mission, it appeared multiple sets of tools might be required. Apollo 12 astronauts had so much difficulty schlepping hand tools, sample boxes and bags, a stereo camera, and other equipment across the lunar surface, NASA designed a special hand cart, dubbed the MET (Mobile Equipment Transporter) for the Apollo 14 mission. Unfortunately, the MET proved to be a poor solution in the very fine-grained lunar regolith.
Multiple studies had documented the safe distance an astronaut could travel in a bulky spacesuit and still have enough strength, energy and especially life support reserves to make it safely back to the lander. A lunar rover, NASA reasoned, could more than double this distance because if the rover failed, the astronauts could “moon hop” (the term for the half-walk, half-hop method the astronauts found was the the most efficient way to cover territory) back to safety. The contract for the lunar rover was let to the Boeing Company on October 28, 1969. Amazingly the preliminary design review was completed in the first 10 weeks and the final design approval happened with 22 weeks of awarding the contract. A mere 22 months later, despite massive cost overruns, the first fully operational Lunar Roving Vehicle blasted off tucked away in a compartment on the side of the lunar lander on the Apollo 15 mission. On July 30, 1971, astronauts David Scott and Jim Irwin became the first humans to drive across the surface of another celestial body.
The “Moon Buggies” (as they came to be known) were designed to maintain a sustained velocity of 16 kilometers per hour (9.94 mph) on a smooth lunar surface, have sufficient electrical power to operate for a minimum of 78 hours, climb and descend a maximum slope of 25 degrees from horizontal, perform four 30-kilometer (18.6-mile) lunar excursions in a 78 hour period, have no single point failure that would render it inoperable and no second failure that would endanger the crew. They also had to have a turning radius of approximately one vehicle length and be able to travel in reverse.
With their Moon Buggy, Scott and Irwin were able to do accomplish three times the work completed on the three earlier moon landings. The rover functioned flawlessly in temperatures ranging from minus 200 to plus 200 degrees Fahrenheit. Apollo 16 Commander John Young later estimated that without the rover, he and Charlie Duke would have been able to accomplish only about 5 percent of what they did. On Apollo 17, the last lunar mission, Gene Cernan and Harrison Schmitt covered almost 22 miles over three excursions and set a speed record of a little over 11 miles per hour. They brought back nearly 250 pounds of lunar samples. During one of the excursions, Cernan radioed excitedly to Mission Control, “This is quite a machine, I tell you!”
Number six on our list is the Soviet Union’s Lunokhod 1 with a total of 10.5 kilometers on the Moon in 1970-1971.
Number seven is the NASA’s Mars rover Curiosity with total distance of 8.6 kilometers as of July 28, 2014. Curiosity has only been on the Martian surface since 2012. So with a little luck it could climb up the list and perhaps eventually overtake Opportunity as the grand champion.
Number eight is Opportunity’s twin rover, Spirit, with a total of 7.7 kilometers. Spirit was operational and mobile for over 1,942 Earth days before becoming stuck in soft Martian soil on May 1, 2009. Spirit’s last transmission occurred on March 22, 2010.
Number nine is the small NASA Mars rover Sojourner with a total of only 0.1 kilometer, a surprisingly low number considering the massive publicity the little rover generated when it landed in 1997.
Rounding out the top ten is the Chinese lunar rover Yutu with an estimated total distance of slightly less than 0.1 kilometer. Yutu was part of the December 2013 Chinese Chang’e 3 mission to the Moon. It was the first soft landing on the Moon since 1976 and the first functioning rover since Lunokhod 1 ceased operations in May of 1973. Although the rover was mobile the first day, it has not moved since the second day due to an electrical problem. Some of Yutu’s instruments are reported to still be operational.