Twenty years ago, this week, NASA’s Mars Global Surveyor settled into orbit around the Red Planet, to begin what NASA described as “a sustained program of exploration” of our third-closest celestial neighbour after the Moon and Venus. Its arrival came just a few weeks after the arrival of Pathfinder—and the Sojourner rover—and signaled a remarkable shift in U.S. space policy, as an unbroken period of study of Mars study got underway. Two decades later, although Global Surveyor has long since ceased to function, that period of study continued unabated, with missions in orbit and on the ground and many more waiting in the wings for launch in the coming years. Its success today seems all the more remarkable, for the spacecraft suffered a potentially life-limiting series of problems, within hours of launch.
Global Surveyor rose from Earth at the stroke of midday EST on 7 November 1996. Launched from Cape Canaveral Air Force Station, Fla., atop a Delta II booster, it embarked on a 308-day voyage across 466 million miles (750 million kilometers) to the Red Planet and arrived on 12 September 1997. The spacecraft went on to spend more than nine years comprehensively examining Mars from its upper atmosphere to its surface, as well as offering data-relay services for later orbiters and identifying landing sites for other spacecraft. By the time Global Surveyor ended its mission in late 2006, it had spent longer in operational service at the Red Planet than any previous robotic explorer.
Built by Lockheed Martin, it was the United States’ first fully successful venture to Mars since the Viking era in the late 1970s. An earlier mission, Mars Observer, had launched in September 1992 and reached the planet the following August, but contact was lost just days before it was due to enter orbit. The $154 million Global Surveyor carried five instruments, including a camera which was of similar design to the one aboard Mars Observer. However, whereas the doomed spacecraft acquired only three images of the Red Planet, that of Global Surveyor returned no fewer than 243,668 images during its mission.
In addition to the camera, with its narrow-angle and wide-angle components, the spacecraft carried a laser altimeter for precise topographical mapping and passive radiometry, a thermal emission spectrometer—under whose infrared gaze large deposits of the important mineral olivine were discovered in the planet’s Nili Fossae region—and a magnetometer/electron reflectometer and ultrastable oscillator and a Mars Relay Signal Receiver. The latter would play a crucial role in supporting the twin Mars Exploration Rovers (MERs), Spirit and Opportunity.
Having been successfully inserted onto a heliocentric trajectory by its Delta II, Global Surveyor almost immediately ran into problems, when one of its solar arrays failed to deploy properly and was left tilted 20.5 degrees from its fully-open and latched configuration. It was not expected to impose a detrimental effect on the mission, but controllers spent several months of the trans-Mars cruise use electrically-driven positioning actuators in an attempt to gently manipulate it into infurling. By April 1997, little success had been achieved, and it became clear that the “damper arm”—part of the array’s deployment mechanism—had sheared off during Global Surveyor’s first few days in space.
The arrays, each of which included a Kapton flap at its tip for added drag, were expected to slow the spacecraft by 2,684 mph (4,320 km/h) during a four-month “aerobraking” phase to reduce its orbit from a highly elliptical path into a near-circular one. During the transit to Mars, engineers developed a new solar array configuration to complete the maneuver. On 11 September 1997, the spacecraft’s engine fired for 22 minutes to enter a highly elliptical orbit around Mars, with an apoapsis of 33,570 miles (54,026 km) over the southern hemisphere and a periapsis of 163 miles (262 km) over the northern hemisphere.
This initial orbit had a period of about 45 hours, with the expectation that it would be refined into a near-circular mapping orbit by January 1998. However, in early October, the troubled solar array exhibited significant flexion, prompting a decision to adopt a slower, “more moderate” pace for the orbit-shaping regime.
As aerobraking proceeded, the scientific harvest from Mars immediately began to flow. On 16 September, just a handful of days after MOI, scientists were able to provide the first conclusive evidence of a planet-wide magnetic field. According to early data, the field’s maximum strength did not exceed 1/800ths of the field at Earth’s surface, and carried important implications for understanding Mars’ geological history. By the end of March 1998, Global Surveyor’s orbit had been reduced to an apoapsis of 11,100 miles (17,864 km) and a periapsis of 106 miles (170 km), with a period of 11.6 hours. At this stage, aerobraking was suspended for several months, as the remainder of the spacecraft’s scientific instruments were brought online and, during suitable lighting conditions, were brought to bear on examining Viking and Pathfinder landing sites from orbit.
Aerobraking resumed in September 1998 and was completed six months later, by which time Global Surveyor occupied a near-circular orbit at an altitude of 235 miles (378 km), with a period of 117 minutes. This allowed the spacecraft to commence its primary mapping cycle on 1 April 1999, during which it extensively mapped the entire Red Planet. Over the course of the next two years (or one full Martian “year”), evidence was detected of a “layered” crust, requiring the weathering, transportation, and deposition of large quantities of material, as well as a thick mantle coating over all but Mars’ very steepest slopes. Extensive cratering was observed and inferred in the northern and southern hemispheres and Global Surveyor uncovered persuasive evidence—via the presence of hematite-rich deposits—of the existence of liquid water in Mars’ distant past. Traces of planet-wide atmospheric cooling since the Viking missions were identified and, close to the surface, the spacecraft imaged the after-effects of gigantic “dust-devils” and the presence of colossal boulders.
“In some aspects, we now have better maps of Mars than we do of Earth,” exulted Project Scientist Arden Albee of California Institute of Technology in Pasadena, Calif., speaking at the end of the two-year mapping phase in January 2001. By this stage, Global Surveyor had circled Mars 8,505 times and acquired more than 58,000 images, as well as 490 million laser-altimetry readings and 97 million spectral measurements.
A year-long extended mission got underway and was itself extended in February 2002. During this lengthened phase of Martian exploration, the spacecraft routinely performed roll maneuvers to gather targeted images away from its nadir track and photographed the vast monolith on the surface of the planet’s largest satellite, Phobos. Regions of Mars’ surface which were cloaked by dust storms during the initial mapping period were re-mapped, traces of ancient, long-dried-up rivers and possible water-borne sediments were observed and the springtime retreat of the south polar ice-cap was carefully monitored.
Astonishingly, in May 2003, Global Surveyor shuttered the first image of Earth and the Moon, as seen from Mars. From a distance of 86 million miles (139 million km), the Home Planet revealed itself as a -2.5-magnitude object, characterized by a bright patch of cloud cover over the central and eastern regions of North America. And although the far darker Moon was tougher to analyze, Global Surveyor’s images revealed slightly paler tones associated with its bright-rayed crater, Tycho.
During this extended period, NASA’s pledge of “a sustained program of exploration” was steadily realized, with the arrival of the Spirit and Opportunity rovers in the late spring of 2004. And, indeed, Global Surveyor played an integral role in their journeys across the ochre-red terrain. In September of that year, revolutionary image-compensation techniques allowed it to directly observe wheel-tracks left by Spirit at Gusev Crater. This marked the highest resolution ever obtained by a Mars-orbiting spacecraft, within 1.6 feet (0.5 meters) per pixel, far higher than the typical 4.6 feet (1.4 meters) per pixel previously achievable.
The mission was again extended for another two years in October 2004 and supported weather monitoring, which would be continued by NASA’s follow-on Mars Reconnaissance Orbiter (MRO). It also aided the selection of landing sites for the Phoenix mission and for the Curiosity rover. In the late spring of 2005, Global Surveyor became the first spacecraft to directly photograph another Mars orbiter, when it captured images of Europe’s Mars Express from a distance of 155 miles (249 km) and NASA’s Mars Odyssey from a distance of 56 miles (90 km).
Yet another two-year mission extension got underway in October 2006, but sadly Global Surveyor’s days were numbered. A month later, just days before the 10th anniversary of its launch, commands were transmitted to maneuver its solar arrays, but the spacecraft reported an error with one of its motors. On-board software transferred to a backup motor controller, but a two-day break in communication occurred on 3-4 November. Contact was re-established briefly on the 5th, during four different orbits, but no data was returned. Global Surveyor had placed itself into “safe mode” and its survival and resumption of operations depended upon restoring normal communications.
However, no further contact was established with the spacecraft, and in January 2007 NASA declared the mission to be at an end. An internal agency review found that Global Surveyor had most likely succumbed to a battery failure, triggered by a complex sequence of events involving its on-board computer memory and ground-issued commands. It was determined that depleted batteries and improper orientation of its antenna probably led to a loss of orientation control and an inability to communicate with Earth. The loss was a disappointing end to a spectacular mission which had operated longer at Mars than any previous spacecraft and which had continued to return valuable scientific data for more than four times longer than intended.