A portion of a poster titled "Mars Explorers Wanted," one of a series NASA originally commissioned for an exhibit at the Kennedy Space Center Visitor's Complex in 2009. They're all available online for download. Image credit: NASA/KSC
Imagine the surface of Mars on July 20, 1976. It's cold and breezy on the golden plains and sunset is fast approaching. Above, a meteor streaks across the sky, 250 meters per second, a zip of light. Uncommon for a meteor, however, is the parachute that deploys, slowing it in an instant by three-quarters of its speed.
About a mile above the surface, the object resolves into view. It's not smooth like a flying saucer, but rather looks like a machine of some sort, all cylinders, boxes, and cabling. Retrorockets beneath begin to fire, and the whole thing eases to the ground at a few feet per second. It lands, and Mars is again quiet.
As dust begins to settle, the alien spacecraft switches on. An antenna dish rises and orients. It's looking for something. Home. Arms begin to extend, one toward the sky. Viking I, the first scientist to survive the trip to the Martian surface, gets to work.
As NASA celebrates the 40th anniversary of the historic mission, here is a list of every NASA spacecraft to operate successfully on the Martian surface.
1 & 2. VIKING I AND VIKING II
Viking 1's view of Mars. Image credit: NASA
The Viking I lander was looking for life. How little did we know? Carl Sagan lobbied to have a small light installed on the lander's exterior, hoping Martian animals might investigate it during nighttime hours. The lander's feet are lily pad–shaped because scientists thought the surface of Mars might have the consistency of shaving cream. Just to land successfully was an achievement, and everything Viking I and its twin Viking II (which landed two months later) found and didn't find added immeasurably to our then relatively meager understanding of Mars.
The spacecraft collected and analyzed soil samples, returned 360-degree images of the surface, and monitored the weather. Viking I operated for six years—a record unbroken until 2010, by the rover Opportunity. (Viking II operated just over 3.5 years.) Engineers at JPL think the Viking orbiters are still circling Mars, lifeless but speeding along. They've earned their rest, having imaged the entire planet in high resolution, mapped Mars's thermal activity, and studied its atmosphere.
The Sojourner rover checks out (or perhaps bumps into) a boulder. Image credit: NASA
After the Viking landers found no animals, planetary scientists moved on for a while. They explored Venus, and the Voyager probes checked the boxes for the outer planets—those worlds beyond the asteroid belt. Pathfinder was the ultimate test of NASA's "cheaper, faster, better" initiative in the 1990s—and a grand return to the red planet.
Pathfinder was comprised of two elements: the rover Sojourner and a base station, later named the Carl Sagan Memorial Station. The duo neared the surface on July 4, 1997 in a capsule and parachute similar to the Viking lander. When 355 meters above the ground, Pathfinder seemed to burst like a kernel of popcorn, surrounded in a fraction of a second by an inflated shell of giant airbags. Completely encased, when Pathfinder hit the ground, it violently bounced across the Martian surface, the airbags only deflating after the package lulled to a stop.
The rover would have been an astounding success if it operated for a week. It ended up operating for nearly three months. By the numbers, according to NASA, Pathfinder returned: 2.3 billion bits of data; 16,500 images from the base station; 550 images from Sojourner; and data from "15 chemical analyses of rocks and soil and extensive data on winds and other weather factors." Pathfinder data revealed something amazing: Mars was once a warm and wet world.
4 & 5. SPIRIT AND OPPORTUNITY
The Mars rovers Spirit and Opportunity landed in fashions very similar to Pathfinder—airbags, bounces, and all. Spirit first touched Martian soil on January 4, 2004; 24 days and half a planet later, Opportunity landed. The rovers were designed for greater mobility and range than Pathfinder, though they largely studied the same things: rocks, soil, and air. Spectrometers scanned the planet's mineral and chemistry composition. Cameras captured and returned more than 100,000 high-resolution images of terrain and sky (including the first-ever photo of Earth from the ground on another world).
Another similarity to Pathfinder: bang for your buck. Spirit was designed to last 90 days. It ran for six years. In the end, it became stuck in patch of iron sulfate that was concealed by a thin layer of soil. Because of the low cohesion, the rover lost traction. The rover was declared a stationary science platform, and continued to do science for another two months, until low sunlight left its batteries drained. Eventually contact was lost.
Opportunity, meanwhile, refuses to stop working. The closest it came to a demise was in 2005, when the rover drove through a punishing sand trap and was nearly halted. In 2014 (year 10 of its 90-day mission), its flash memory became unreliable to the point that scientists discontinued its use, opting to store data in RAM instead. Last year, it even completed a marathon on Mars, crossing the 26.2-mile mark. Among the findings of the two rovers is a bonanza of water interactions from the Martian past—groundwater, water and magma, frost shaping rock, you name it. Opportunity most notably found evidence that Mars might have been habitable for hundreds of millions of years.
The Mars Reconnaissance Orbiter captured this image of Phoenix descending by parachute to Mars, with a crater in the background. Image credit: NASA/Jet Propulsion Lab-Caltech/University of Arizona
The Mars Polar Lander was intended to be the first spacecraft to set down on a Martian pole. Sadly, a likely crash landing left the spacecraft unresponsive. No one knows for sure what happened, and the Mars Reconnaissance Orbiter has been unable to find its remains.
Out of the missing ashes of MPL rose Phoenix, which carried improved versions of several instruments of its ill-fated progenitor, as well as a lander that had been intended originally for the Mars Surveyor, a canceled mission. Phoenix's landing on May 25, 2008 was notable in that the Mars Reconnaissance Orbiter was able to capture an image (above) of its parachute descent—the first time we've ever seen such a thing. Like Viking I, its final descent used rockets. The mission was a great success, proving the evidence of water ice on Mars just below the surface. It observed falling snow on Mars and, most notably for NASA's "Journey to Mars," found perchlorate, which can be used in the production of both rocket fuel and oxygen—useful items indeed for future Martian colonists. The mission, intended to last three months, went on for five. Poor sunlight and a dust storm interfered with its solar collection, and its batteries were eventually depleted.
Curiosity took this composite image of the higher regions of Mount Sharp on September 9, 2015. Image credit: NASA/JPL-Caltech/MSSS
The rover Curiosity's Rube Goldberg–like landing—which involved rockets, parachutes, sky cranes, tethers and perfect timing—was unlike anything NASA had ever previously attempted. (This landing system will not be a one-off, though. It will be used for the Mars 2020 rover, which launches that year and arrives in 2021.) Curiosity is a habitability study. It is, perhaps, an echo of Viking I, and a reflection of all we've learned. Where scientists in those heady days longed to catch images of Martian animals scurrying up to the spacecraft's nightlight, Curiosity steps back and asks, "Was Mars ever habitable?"
The answer: yes. As it relates to human life on Mars, Curiosity also found radiation levels similar to those on the International Space Station, meaning that colonists might not risk certain death by cancer when they arrive. The mission, now in its fourth year of a two-year mission, could survive well over a decade. Unlike previous Mars rovers, it is nuclear powered and impervious to the whims of dense Martian sandstorms or punishing winters. On the other hand, its wheels have sustained damage, leaving scientists uncertain as to how long the rover can keep moving.
We'll take a deeper look at the Mars 2020 mission later this week. Until then, check out this awesome clip from the 2006 IMAX documentary Roving Mars.