Six Wheels on Martian Regolith: 20 Years Since NASA's Pathfinder Mission Launched to the Red Planet

Having alighted on the Red Planet in July 1997, and having been dug out of Martian regolith by Mark Watney as part of his efforts to achieve salvation, NASA’s Pathfinder mission—which rose from Earth 20 years ago, tonight—has experienced both an exciting past and an excitingly fictitious future. Launched into the night at 1:58 a.m. EST on 4 December 1996, aboard a Delta II booster, Pathfinder went on to become the United States’ first mission to Mars in almost two decades and the first wheeled vehicle to successfully traverse the planet’s ochre-hued surface. In so doing, its six-wheeled Sojourner rover laid the cornerstone for subsequent roving missions, from the Spirit and Opportunity twins to today’s Curiosity and, ahead, to NASA’s in-work Mars 2020.

Original plans saw Pathfinder as part of the expansive Mars Environmental Survey (MESUR) project, which sought to place a network of 16 seismometer-equipped landers in different locations on the surface between 1999 and 2003. Leading this assault on the Red Planet was MESUR-Pathfinder, which would drop a stationary lander and six-wheeled micro-rover onto the surface in July 1997. However, in the aftermath of the loss of NASA’s Mars Observer mission, much of the MESUR infrastructure was shelved, leaving Pathfinder as the only member of the project to bear fruit.

Developed as part of the small-scale, low-cost Discovery program—an effort which also included the Near-Earth Asteroid Rendezvous (NEAR)—Pathfinder was targeted to alight on a rocky plain within Mars’ Ares Vallis, about 530 miles (850 km) southeast of the landing site of NASA’s Viking 1 spacecraft. This place was thought to be the mouth of an ancient outflow channel, with potential evidence of Mars’ watery past, and was described as a “grab-bag” location: in essence, a wide variety of rock and soil types were expected to be within reach of the rover. Furthermore, the solar-powered nature of both lander and rover required maximum levels of sunshine. By July 1997, the Sun would be directly over the 15-degrees-north-latitude region of Mars and the location also afforded the best conditions to open Pathfinder’s parachute and slow the lander to the correct terminal velocity. Due to uncertainties in navigation and atmospheric-entry characteristics, a 60 x 120-mile (100 x 200-km) ellipse within Ares Vallis was selected for the landing attempt.

Pathfinder would be the United States’ third mission to actually touch down on the Martian surface, after the twin Viking landers, but never before had a wheeled vehicle successfully conducted a full program of scientific exploration on the Red Planet. However, unlike Viking’s use of liquid-fueled rockets to alight on the surface at 5 mph (8 km/h), Pathfinder would adopt a quite different Entry, Descent and Landing (EDL) regime. It would employ a 41-foot-diameter (12.7-meter) supersonic parachute, a trio of solid-fueled rockets, and an array of giant, multi-lobed airbags to effect a somewhat harder touchdown at 35 mph (56 km/h). The lander would deploy three ramp-like “petals” and the 25-pound (11.5-kg) rover would drive onto the surface. In June 1995, engineering tests of the parachute system were successfully completed in the desert of Yuma, Ariz., together with evaluations of the solid-fueled motors at the China Lake Naval Weapons Center in Ridgecrest, Calif.

Several weeks later, in mid-July, the tiny rover was named “Sojourner.” A national campaign had been initiated in 1994 by The Planetary Society and the winning essay came from a Bridgeport, Conn., student, named Valerie Ambroise. Her choice was the 19th-century African-American reformist Sojourner Truth. Not only did Truth “travel up and down the land,” advocating the rights of all people to be free and for women to be able to participate fully in society, but the very word “sojourner” alluded to the journey of the traveler. And Truth’s mechanical namesake would certainly be embarking on a historic journey of its own, across one of the most barren wastelands known to humanity.

By the spring of 1996, construction of the 772-pound (351-kg) Pathfinder lander was nearing completion at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., and it was integrated with Sojourner. The spacecraft was then installed inside its atmospheric aeroshell and heatshield, ahead of spin-balance, acoustic and thermal vacuum testing, and shipping to Cape Canaveral in mid-August. For its transportation, Pathfinder was delivered in four parts: the lander, the spacecraft’s “cruise stage,” the aeroshell, and Sojourner itself. Also arriving in Florida at the same time was NASA’s Mars Global Surveyor, which was targeted to launch a month before Pathfinder, but which would follow a longer route and reach the Red Planet a few weeks later, in September 1997.

Together with its upper stage, Pathfinder was delivered to the Cape’s Space Launch Complex (SLC)-17B in the third week of November 1996 for emplacement atop the Delta II booster. Targeted to fly on 2 December, the mission met with two days of delays, before the hands of fortune lined up in the wee hours of the 4th. “And liftoff of the Delta rocket with Mars Pathfinder,” came the announcement, as the highly reliable Delta II roared into the night at 1:58 a.m. EST. Powered by a single RS-27A first-stage engine and nine strap-on boosters, the Delta’s initial push to orbit was supplemented by the Payload Assist Module (PAM) upper stage to deliver Pathfinder onto a course for Mars.

“Overtaking” the slower-moving Global Surveyor in March 1997, Pathfinder’s arrival at the Red Planet on 4 July made headline news all over the world. Plunging through the Martian atmosphere in excess of 17,000 mph (27,000 km/h), its heatshield initially slowed the spacecraft to around 900 mph (1,450 km/h) in under two minutes. This was followed by the deployment of Pathfinder’s supersonic parachute, which arrested the velocity still further to around 160 mph (250 km/h) in the rarefied atmosphere. At 322 feet (98 meters), the gigantic airbags were inflated and the three solid-fueled rockets flared to begin the final descent to a stop about 69 feet (21 meters) above the ochre-hued surface.

At this point, the parachute was jettisoned and the airbag-cocooned lander and rover dropped to the ground, impacting at 46 feet (14 meters) per second, bouncing and rolling at least 15 times until it came to a complete halt. Designed to handle vertical impacts of up to 50 feet (15 meters) per second and “grazing-angle impacts” of up to 92 feet (28 meters) per second, the airbags performed exceptionally and the entire EDL sequence was complete in less than four minutes. Pathfinder had alighted on the surface just 12 miles (19 km) to the southwest of the center of the landing site ellipse.

In the days and weeks which followed, the lander was renamed the “Carl Sagan Memorial Station,” in honor of the celebrated astronomer and cosmologist, who had died the previous year. The Sojourner rover was designed to function on the surface for as little as seven “sols”—equivalent to around a week on Earth—but ultimately continued operations until the end of September 1997, totaling 85 sols. It traveled around 330 feet (100 meters) across the boulder-strewn landscape of Ares Vallis, performing X-ray spectroscopic analysis of several nearby rocks, returning more than 500 images back to investigators on Earth, and examining the chemical composition of the local regolith. Meanwhile, the lander served as an in-situ meteorological station, providing significant insights into the interactions between Mars’ surface and its atmosphere.

Pathfinder had proved an enormous success and would live up to its name as a pathfinder for the landers and rovers which would follow. The twin Mars Exploration Rovers (MERs), dubbed “Spirit” and “Opportunity,” which landed within weeks of each other in January 2004 and touched down in Gusev Crater and Meridiani Planum. Like Sojourner, both of these six-wheeled beasts were brought to the surface via a combination of supersonic parachutes, solid-fueled rockets, and airbags, and like Sojourner both vastly outperformed their projected life spans. Scheduled to operate on Mars for around 90 sols, Spirit completed its 2,000th sol in August 2009, even as it battled technical problems and engineers struggled to extricate it from immobilization within deep sand drifts. Ultimately, by mid-2010, Spirit was declared lost, but Opportunity—with more than 12 years of surface operations—continues to function to this very day.

In fact, as Opportunity began a drive along the Bitterroot Valley portion of Endeavour Crater in October 2016, it was noted that the rover had exceeded its prime-mission duration of just 90 days by a factor of 50. As outlined by AmericaSpace’s Paul Scott Anderson, it had traveled in excess of 27 miles (43 km) and enjoyed a mission which NASA Mars Rovers Project Scientist Matt Golombek described as “pretty much unprecedented.” Opportunity has already exceeded the Apollo 17 lunar rover in holding the record for the farthest-driven U.S. vehicle on the surface of another world.

Of course, Opportunity is not alone on the Red Planet, following Spirit’s demise. Since August 2012—following the skycrane-boosted descent of the Mars Science Laboratory (MSL)—tracks across the ochre regolith are also being made in Gale Crater and the foothills of Mount Sharp by its (exceptionally) bigger brother, Curiosity. The sheer size, weight, and capability of Curiosity is one of the reasons why its EDL regime was quite dissimilar to that of Sojourner, Spirit, and Opportunity.

Airbags were out of the question in bringing Curiosity’s enormous 8,400-pound (3,800-kg) bulk safely through the tenuous Martian atmosphere and precisely setting it down in one piece onto the surface. Rather, the giant rover was placed into a much smaller “landing ellipse” and—although aided by a 52-foot-diameter (16-meter) supersonic parachute—the actual soft-landing of the hardware on the ground was effected by the eight motors of the never-before-used skycrane. This mechanism lowered Curiosity on a 25-foot (7.6-meter) tether to position it “wheels-down” on the surface, concluding NASA’s “Seven Minutes of Terror” by alighting less than 1.5 miles (2.4 km) from its target point. The rover then cut itself free from the skycrane, which performed a flyaway maneuver to crash-land about 2,100 feet (650 meters) away.

With the Mars 2020 rover on the horizon for a July 2020 launch atop an Atlas V booster from Cape Canaveral Air Force Station, NASA’s efforts to plant large vehicles on the Red Planet will mature yet further. Similar in appearance to Curiosity, Mars 2020 will also be delivered to its destination via parachutes and a final skycrane descent. And when one places NASA’s rovers side-by-side for comparison, the leaps in size, weight, and capability are readily apparent. Spirit and Opportunity established the long-duration benchmarks for operating on a world outside the Earth-Moon system, whilst Curiosity and Mars 2020 are shaping up to revolutionize our understanding of the planet upon which water may once have flowed and where microbial life may once have thrived.

And yet, as Sir Isaac Newton once remarked: “If I have seen further, it is by standing on the shoulders of giants.” Giant it may not have been, but the tiny Sojourner rover—which began its long journey to Mars, 20 years ago, today—laid a fundamentally important cornerstone for our continued exploration of this strange world.

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