Launched on November 6, 2011, and alighting on Mars’ surface on August 6 of the following year, the most recent US rover to reach the fourth planet was the compact car-sized Curiosity. Due to its increased mass (compared to previous missions) Curiosity used a landing method that JPL scientists describe as “Seven Minutes of Terror“. The rover, while still aloft, transformed from its stowed flight configuration to a landing configuration while its ferrying spacecraft simultaneously lowered it beneath a descent stage on a 66-foot tether. The rover landed, wheels-down and active, at a spot nicknamed “Yellowknife” in Gale Crater, a site now known as Bradbury Landing; an obvious choice to devotees of the renowned sci-fi writer who know of his long-term fascination with Mars.
Gale Crater is a 3.5 to 3.8 billion-year-old, 100 mile-wide impact crater, first gradually filled in by sediments, possibly until it was completely covered. Wind erosion then scoured out the sediments, leaving the isolated 3.4 mile-high Aeolis Mons (Mount Sharp) at the center of the 96-mile wide crater. Curiosity has since been able to study two billion years of Martian geologic history among the sediments exposed on the mountain. Additionally, the landing site was near an alluvial fan, which is likely the result of ground water flows that occurred either before the sediments were deposited or in relatively recent geologic history.
This Curiosity self-portrait shows the vehicle at a site where it drilled into a target rock nicknamed “Buckskin” on lower Mount Sharp. (The selfie at “Buckskin” depicts only a small fragment of the rover’s robotic arm. Using wrist motions and turret rotations to orient its camera for capturing the multiple shots used to compose the full image, the arm was able to stay mostly out of the frame each time, essentially PhotoShopping itself out of the picture.) The drilling at “Buckskin” was carried out by a percussion driver that carries two spare bits. Since early 2015, the percussive mechanism that helps chisel into rock has had an intermittent electrical short. In December 2016, the drill feed motor caused a malfunction that prevented the rover from moving its robotic arm. Two identical computers power Curiosity, each boasting a 200MHz CPU and 256 MB of RAM.)
During a brief period in 2013 and 2014, Curiosity’s onboard sensors measured a tenfold spike in methane, an organic chemical, in the atmosphere around it and have detected other organic molecules in the rock-powder samples collected by the robotic drill. Martian organics could have either formed on Mars or been delivered to the planet by meteorites. Organic molecules like methane, containing carbon and usually hydrogen, are the chemical building blocks of life, although they can exist without the presence of life.
Shown together on the test fields at JPL’s California headquarters are reserve models of the three types of rover to have operated on Mars. The smallest stands in for Sojourner (1996 to 1997); the rover at left represents the twin craft Spirit (2003 to 2010) and Opportunity (landed 2004 and still functional); while the largest is identical to Curiosity (landed 2011 and still functional). Orbital US missions still active around Mars include MAVEN (since 2014), Mars Reconnaissance Orbiter (since 2005), and Mars Odyssey (since 2001).
The static lander Phoenix reached Mars in 2007 to carry out a series of experiments in organic chemistry, performing the first successful touchdown in a Martian polar region. Another NASA launch, carrying its InSight seismology probe, is set to take place in 2018. Both NASA (Mars 2020) and SpaceX (Red Dragon) have Mars launches planned for 2020. Mars 2020 is another rover, and nations such as China, India, and the EU are planning for more, so wheels will continue to churn the red dust for years to come.
Of course, the first wheeled vehicle to operate on the surface of any interplanetary body was the US Lunar Roving Vehicle carried aboard the Saturn V-boosted Apollo 15 mission.
Note the similarities in the wheel designs between the lunar and Mars rovers.
They are unsealed (to shed dust & debris); lightweight (to allow for more instrumentation payload); rugged (to withstand severe deployment conditions); and flexible (to conform to rough, uneven planetary surfaces). Future missions call for similar wheels to be used on mobile platforms.