What would you say is the greatest off-road vehicle ever built? The humble yet mighty Willys MB? The Cherokee? The Land Rover? The FJ Cruiser? Monster trucks? Baja 1000 rallyers? Dirt bikes? The Thiokol Snowcat?
While all of the above are worthy contenders, I submit for your consideration a quartet of candidates in three model series which were built for the most extreme off-roading ever attempted by the human race. They are also the most advanced driverless vehicles ever built, and have been successful beyond their creators' wildest dreams.
The dream of a manned trip to Mars is far from dead, but it's not going to happen any time soon. Thanks to the peculiarities of orbital mechanics (and the lack of a practical working warp drive) any trip to Mars requires at least 259 days just to get there. That's 259 days outbound, plus that much or more for the return flight, plus another year or so at Mars waiting for the planets to align properly so you can make the return trip. It all adds up to an odyssey of three years just for the whirlwind tour. As good as modern engineering is, we're not quite up to the task of building a closed ecological system that can be depended upon to work for that long, much less one that would fit in a spaceship. That means we have to take along enough of a full range of consumables--food, oxygen, toilet paper, et cetera--to keep the crew alive for the duration, plus propellant for the return trip. Every pound of stuff we pack means we need more propellant to get it all there, which means a bigger spacecraft, which means still more propellant, which means that before long you end up with an immense engineering challenge bearing a politically impossible price tag.
If we can do our exploring with unmanned spaceships, the technical and logistical challenges get a whole lot easier. While electronic and mechanical systems still need to be shielded from the extreme environment of space, they don't eat and don't need oxygen--so we can dispense with the life support apparatus, the well-stocked pantry, and the airtight living space--they're a lot less delecate--so we can subject them to heavier accelerations and a harder landing--and they won't insist on coming back home at the end of the engagement--so we only need enough rocket and fuel for a one-way trip.
In 1966, the Soviet Luna 9 lander became the first spacecraft to land on (rather than smack into) another world. It was followed by several more Luna landers and the American Surveyors. In the 1970s, the two countries first sent landers to the planet Mars, the Soviet Mars probes and the American Vikings.
The one thing all of these landers had in common was that they were stationary. Once they landed, they couldn't move around. They could sample the soil within reach of their robot arms, but if there were any really interesting rocks out of their reach, all they could do was look at them. NASA eventually sent astronauts to the moon who could walk, and later drive, from place to place to do experiments and take samples. The Soviets were hoping to do the same, but their counterpart to the Saturn V, the N-1, blew up every time they launched it.
Though their manned moon exploration program was an epic failure, the Soviets managed to pull off a successful unmanned one. On November 17, 1970, they landed a 1,600-pound, eight-wheeled robot called Lunokhod ("Moon Walker") on the moon's surface. Lunokhod 1 travelled six and a half miles over the next thirteen months and sent back over 20,000 pictures before finally breaking down. Lunokhod 2, landed on January 15, 1973, was even more successful. Though it operated for only four months, it returned over 80,000 pictures and set what is presently the all-time off-world driving distance record, a total of 26.2 miles. Amazing fun fact: Lunokhod 2 is now owned by computer gaming entrepeneur Richard Garriot, alias "Lord British"--he bought it at auction in 1993.
The Lunokhods were "driven" by direct control from Earth, rather than operating autonomously. While there's a slight communications lag between Earth and Moon, it's not enough to prevent the operation of a slow-moving vehicle like Lunokhod in something close to real time. Mars is a whole 'nother story--the communications lag is between seven and forty-two minutes, depending on where the two planets are in their respective orbits, and every so often they're on opposite sides of the sun and there's no communication at all. Any unmanned vehicle that was going to drive around on Mars would have to do its own navigating.
It took a couple of decades before a computer was available that was both compact enough to fit in a practical-sized Mars robot and powerful enough that it could actually find its own way around once it got there. The first rover was carried by the Mars Pathfinder probe, which landed on July 4, 1997.
Valerie Ambroise, a 12-year old girl, wrote the winning entry in a NASA essay contest held to choose the rover's name: "Sojourner," in honor of Sojourner Truth. The rover's namesake was born a slave, escaped to freedom, became a prominent abolitionist, Methodist preacher, and women's rights advocate, and lived to the ripe old age of 86, long enough to see slavery abolished. She was, by all accounts, a formidable woman. Her given name was Isabella Baumfree, but she changed it to "Sojourner Truth" in 1843 to symbolize what she had chosen for her life's purpose: to "travel up and down the land" for truth.
The purpose of the Mars rovers is to undertake a long sojourn up and down the Martian land in search of scientific truth--which is to say, the name Sojourner is perfect for a Mars rover.
Sojourner the rover is a rather small creation, slightly over two feet long, a little over 19" wide, and 11" high (not counting the antenna), weighing in at only 25 pounds in Earth gravity. Powered by a combination of roof-mounted solar panels and a 150 watt-hour nonrechargeable battery, she's equipped with a stereoscopic camera and laser system in front for navigation and an alpha proton x-ray spectrometer for analyzing rocks on the back bumper. Sojourner's designed top speed is 0.4m/minute, which translates to 0.1 miles per hour. The Mars rovers are in no great hurry, and the main thing is to arrive at the next rock in one piece.
Sojourner's on-board computer is roughly equal to a mid-80s IBM PC-XT tricked out with a memory upgrade. That doesn't sound like much, but since it was an "embedded system" running a single program written in machine code, and didn't have to worry about also tending to a monitor, keyboard, mouse, printer, or floppy drive, it was more than equal to the task. It talked to the Pathfinder lander through a 9600-baud wireless modem with a range of 500 meters. Pathfinder then took care of communications with the folks back on Earth.
The most interesting bit of engineering on Sojourner is the suspension and drivetrain. The wheels are mounted on an arrangement called a "rocker bogie," invented by NASA engineer Donald Bickler, which allows it to climb over obstacles twice the size of its wheels while keeping all six of them on the ground. The wheels on the ends can pivot, allowing the rover to turn or even spin around in place. Each wheel has a small electric motor and a set of reduction gears. Since there are no drive shafts or differentials in the center of the rover, it can drive over obstacles without getting high-centered.
Mars has an atmosphere. You really wouldn't want to try breathing it, since it's 95% CO2 and only about one sixteenth as dense as Earth's atmosphere. Still, there's enough air there that any spacecraft entering the Martian atmosphere from space at umpty-thousand feet per second needs a heat shield. Once things have slowed down and cooled off enough that the heat shield is no longer needed, you can pop open a parachute. It'll take off a good bit of your velocity, but it won't encounter enough air resistance to slow you down enough to survive the landing. Because of this, the Vikings carried rocket engines to get themselves down the last few dozen feet safely.
Pathfinder and Sojourner used a completely different approach. Pathfinder folded up into a tetrahedron with Sojourner in the middle. The outer surface of the folded-up Pathfinder was covered in airbags, much like the ones in your car. These inflated while it was hanging from the parachute. At about sixty feet from the ground, Pathfinder cut loose from the parachute and became a giant Martian beach ball. After bouncing and rolling to a stop, Pathfinder deflated her airbags and opened up like a flower--having been cleverly designed so that no matter which way she had landed, everything would end up right side up. A sol (Martian day) later, after verifying that all was in working order, Sojourner rolled off of Pathfinder and got started on the sojourn. Pathfinder carried a stereoscopic camera on a high mast which allowed the scientists to construct a 3D computer model of the landing site, which they then used to give Sojourner her marching orders.
While Sojourner could have been commanded to go to an objective beyond radio range and return, there were enough interesting things close by the lander to keep the scientists busy, and she never got more than 12 meters or so from Pathfinder. The various rocks and other landmarks that she visited were given whimsical names such as "Yogi," "Ender," "Zappa," and "Pop-Tart"--and a distant pair of mountains was christened "Twin Peaks" after the short-lived television drama.
The engineers who built Pathfinder and Sojourner designed them to operate for a minimum of seven days and a maximum of thirty. Pathfinder exceeded this, operating for 83 sols, until September 27, when she stopped communicating due to a battery failure. Sojourner was still in good working order. I remember a newspaper article at the time which gave a rather poignant description of the little rover patiently awaiting new instructions that would never come. We don't know how much longer Sojourner kept running, but she certainly lived up to her name.
The next step in the exploration of Mars was the two Mars Exploration Rovers, Spirit and Opportunity, which arrived in January of 2004. They're bigger than Sojourner--a little over five feet long, seven and a half feet wide, and just short of five feet tall, weighing 480 pounds--and carry more cameras and scientific instruments, but in terms of basic design they're scaled-up Sojourners. They have the same six-wheel drive and rocker-bogie suspension, they get their electricity from solar panels, and they used the same bouncing-ball landing technique.
The biggest difference is that Spirit and Opportunity weren't tied to their landers--they communicate directly with Earth, and they have more powerful computers and better navigational software. They were meant to travel up and down the land as far as their wheels would carry them.
Which turned out to be quite a distance. Designed to operate for only 90 days and 600 meters, Spirit sojourned for 4.8 miles over five years, discovering meteorites and taking video of Martian dust devils, until she became stuck in soft sand in May of 2009. With no roadside assistance available, NASA tried various maneuvers to get her unstuck for eight months before deciding that she would be operated as a stationary science platform. Spirit finally stopped communicating after March 22, 2010.
Meanwhile, Opportunity has become the Energizer Bunny of the Cosmos. Ten years after being launched, and about five months short of the tenth anniversary of landing, she's still going. As of this writing, in a little over 3,400 sols by the Martian calendar, Opportunity has driven 22.5 miles (farther than Apollo 17's manned rover) and is trucking along toward a hill nicknamed "Solander Point" (on the western rim of Endeavour Crater) at up to 100 meters a day. If she can hold together another year or so, she'll have an opportunity to break Lunokhod 2's distance record.
The newest rover, Curiosity, has been on Mars for slightly over a year. Officially known as the "Mars Science Laboratory," Curiosity is much bigger than her predecessors--nine and a half feet long, just short of nine feet wide, and a bit over seven feet tall, tipping the scales at just under two thousand pounds--and is tricked out with seventeen cameras and a full suite of geological and chemical lab instruments, all controlled by two radiation-hardened computers roughly equivalent to Power Mac G3s. That's too much stuff to power with solar panels, so Curiosity uses a radioisotope thermoelectric generator supplemented by two rechargeable batteries. The generator keeps all of the electronics warm as a side benefit, and allows Curiosity to operate at full capacity even at night or during one of Mars' periodic dust storms.
Curiosity is also far too heavy to land using the airbag trick. To get her down safely, NASA came up with a crazy scheme which the engineers nicknamed "Seven Minutes of Terror."
This being the 21st century, there's a Curiosity homepage--two, actually--where NASA posts the latest images and scientific data from the rover, and maps showing where Curiosity has been and where she's going. You can also "like" Curiosity on Facebook or follow her on Twitter, where the NASA public affairs staff posts decidedly non-serious "status updates" written in the first person.
I don't like the notion that one of the most advanced autonomous scientific instruments ever built would pause while exploring another planet to send out hipsterish 140-character quips referencing Casablanca, The Proclaimers, or Van Morrison. (Really, NASA, that's the best you can do?) Nevertheless, it's hard not to think of the rovers as living things of a sort. Maybe it's just because we gave them wonderfully evocative names instead of prosaic alphanumeric designators, or maybe it's more than that. They're plucky, determined, willing to go where few of us would dare, just to show us things none of us have seen before. They are works of genius and expressions of our humanity, and one cannot help but be in awe of them.
--Cookie the Dog's Owner
The panorama of a manned Mars expedition is a still from the alternate history faux-documentary Man Conquers Space, which is a work in progress. All other illustrations are official NASA images.