Twenty-five years ago, today, a spacecraft slipped silently into orbit around Venus to begin an unprecedented mission which would map in excess of 90 percent of the planet’s cloud-obscured surface, using powerful Synthetic Aperture Radar (SAR). As described in a previous pair of AmericaSpace articles—available here and here—the $295 million Magellan mission underwent a lengthy and tortured development process, before it eventually rose from Earth aboard Space Shuttle Atlantis on 4 May 1989. Fifteen months later, on 10 August 1990, following a journey of 1.5 times around the Sun, it became the first U.S. spacecraft to reach another planet in more than a decade and would spend four years acquiring unprecedented radar data of craters, volcanoes, flat plains, hills, ridges, and other geological features on the planet long described as Earth’s “evil twin.” In fact, so impressively comprehensive were Magellan’s results that in those four short years it revealed more about Venus than had ever been attained in centuries of ground-based observations.
Yet Magellan had undergone a difficult metamorphosis from the drawing board to the launch pad to orbit around the planet which, in size and mass, so closely resembles our own world, yet which in so many other respects is a gross perversion of Earth. Since the 1960s, it had been recognized that radar imaging could yield crude maps of Venus’ surface—entirely cloaked from view by noxious clouds of sulphuric acid—and it was these which helped to peg the planet’s sidereal day at 243 Earth-days and ascertained its retrograde rotation. By the end of the following decade, plans to develop a Venus Orbiter Imaging Radar (VOIR) got underway. Had it reached fruition, the VOIR might have been launched by the shuttle as early as December 1984, reaching Venus in May 1985 and mapping up to 50 percent of the surface at resolutions as fine as 1.2 miles (2 km) through the following November. However, VOIR’s hefty price tag caused its launch to be initially postponed until no sooner than 1987 and precipitated its cancelation in 1982. A stripped-down reincarnation of VOIR returned to the fore about a year later, under the new name of Venus Radar Mapper (VRM). Finally, in 1985, the mission was dubbed “Magellan,” in honor of the 16th-century Portuguese explorer Ferdinand Magellan, who mapped and circumnavigated the globe, just as his mechanized namesake would do for Venus.
In the months before the January 1986 destruction of Challenger, Magellan was manifested for a shuttle launch atop General Dynamics’ Centaur-G Prime liquid-fueled booster on Mission 81I in April 1988. According to NASA’s November 1985 shuttle manifest, the mission would have featured a crew of four aboard Atlantis and lasted just two days, delivering Magellan into a low-Earth orbit (LEO) of about 184 miles (296 km). Following deployment and the ignition of its Centaur-G Prime, it would have entered a “Type I Heliocentric Orbit” and been delivered 180 degrees around the Sun to reach Venus about four months later. Insertion of the Martin Marietta-built spacecraft—which comprised a three-axis-stabilized “bus” with twin solar array “paddles,” dominated by a parabolic dish-shaped antenna for high-gain communications and radar-mapping—into Venusian orbit would have been completed by Magellan’s on-board Star-48 solid-fueled rocket motor. However, as outlined in a previous AmericaSpace article, the hazardous Centaur-G Prime was canceled after the loss of Challenger and Magellan found itself baselined instead to fly atop Boeing’s solid-fueled Inertial Upper Stage (IUS).
Far less powerful than the Centaur-G Prime, the use of the IUS required a significantly different trajectory design, and with the resumption of shuttle missions expected in the fall of 1988 the next available “launch window” to reach Venus under the most optimum conditions came in October 1989. That window soon proved untenable, for it was already earmarked for the Galileo mission, whose own trajectory to Jupiter involved a gravity-assisted boost from Venus. Consequently, the Magellan team settled on a four-week window of opportunity which extended from 28 April through 28 May 1989. The trajectory to be employed was known as a “Type IV Heliocentric Orbit,” which required the spacecraft to pass 1.5 times around the Sun and produced a longer journey time of 15 months. On the flip side, however, the Type IV design offered advantages of lower launch energy and Venus approach speeds, as well as permitting Magellan to reach its quarry over the north pole, thus performing mapping swathes in a north-south direction. This was the reverse of what had been planned for the Type I trajectory originally to be followed after a Centaur-G Prime launch.
Meanwhile, in March 1988, the crew of STS-30—Commander Dave Walker, Pilot Ron Grabe, and Mission Specialists Mark Lee, Norm Thagard, and Mary Cleave—were assigned to begin training for the Magellan deployment, to be flown by orbiter Atlantis. As these plans crystallized, the spacecraft which would soon open humanity’s eyes to Venus started to take shape. Following initial tests with a Structural Test Article (STA) in the spring and summer of 1987, Martin Marietta set to work building Magellan itself and successfully tested the interface between the spacecraft and its Hughes Aircraft-built SAR instrument. In April 1988, the SAR was delivered by truck from Los Angeles, Calif., to Martin Marietta’s facility in Denver, Colo., where it was installed aboard the spacecraft for thermal vacuum testing. Six months later, in October, Magellan was delivered to the Kennedy Space Center (KSC) in Florida and transferred to the Spacecraft Assembly and Encapsulation Facility (SAEF)-2 for integration of the high-gain antenna, radar module, and solar arrays. Finally, in February 1989, it was moved to the Vertical Processing Facility (VPF) for attachment to its IUS booster, after which integrated systems testing and a simulated deployment scenario were executed, involving STS-30 astronauts Cleave and Lee. In mid-March, the Magellan/IUS payload was delivered to Pad 39B and loaded aboard Atlantis.
“It was the first time we deployed a spacecraft that was going to another planet from the shuttle,” Cleave later reflected in her NASA oral history. On 28 April 1989, their first launch attempt was scrubbed when a hydrogen recirculation pump developed a short circuit and stalled. The countdown was recycled to track a second opportunity on 4 May, but it seemed that this date was also snakebitten, with dreary, overcast weather and strong winds blowing across the Shuttle Landing Facility (SLF). Finally, 59 minutes into the 64-minute window, the clouds parted, the winds dissipated, and mission controllers took advantage of the break in the weather to send Atlantis on her way.
Six hours later, under the watch of Cleave and Lee, Magellan and its attached IUS were successfully deployed from the shuttle’s payload bay to begin their voyage to Venus. In Cleave’s mind, responsibility passed the Johnson Space Center (JSC) in Houston, Texas, to the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., as soon as Magellan was out of Atlantis’ vicinity. The longer it remained aboard, the more chance existed for problems to evolve. “Get rid of this thing,” she half-jokingly told the NASA oral historian. “First day, it’s outta there!” Ten minutes after departing the shuttle, Magellan’s twin solar array paddles were perfectly unfurled and a pair of IUS burns set the spacecraft on course for its eventual rendezvous with Venus. Over the next year, the spacecraft pulsed its own thrusters—which formed part of a 24-strong set of hydrazine engines for course correction maneuvers, as well as pitch and yaw controllability—to maintain its course for the optimum arrival time at the planet on 10 August 1990.
In general, the trans-Venus cruise ran exceptionally smoothly, although the spacecraft team was faced with a handful of unexpected obstacles. Magellan’s star scanner experienced strange glints of light, called “spurious interrupts,” during its daily calibrations, likely caused by proton bombardment during solar flares or the shedding of small particles from the spacecraft cover as the scanner moved from shade to sunlight. Software patches and spacecraft positioning helped to resolve these problems, but of greater concern were persistent temperature spikes in the Star-48 motor and Magellan’s equipment bays. Although these spikes never grew high enough to trigger “red” alarms, mission managers opted to employ the high-gain antenna to shade the components from the Sun and thereby keep temperatures within the acceptable range.
Late in May 1990, the spacecraft performed three days of radar-taking data, albeit directed into deep space, before turning its high-gain antenna back toward Earth, in order to simulate its forthcoming activities at Venus. Supporting these tests were Magellan’s radar processing and data-management teams, as well as Deep Space Network (DSN) personnel. A final trajectory correction maneuver in late July served to adjust the velocity by 2.3 feet per second (0.7 meters per second). Shortly after noon EDT on 10 August, the 15,000-pound-thrust (6,800-kg) Star-48 motor was fired for 83 seconds as Magellan flew “behind” Venus, as viewed from Earth, with contact lost at 12:41 p.m. EDT and regained at 1:06 p.m. This accomplished a successful insertion into orbit and kicked off a three-week In-Orbit Checkout (IOC) phase. “Real” data was acquired and processed during this phase, but the main purpose of the IOC was to assist the radar team in adjusting their instrument parameters, ahead of the first mapping cycle. The spacecraft’s initial orbit was an elliptical path, lasting 189 minutes, which brought Magellan to a closest point of 183 miles (295 km) and a farthest point of 4,823 miles (7,762 km) from Venus.
However, its first few months proved far from smooth. On 16 August, contact with Magellan was lost for almost 24 hours, and dropped out again a few days later, before the first active radar-mapping campaign—executed by means of an on-board, stored computer sequence—got underway on 15 September, focusing on Venus’ north polar region. “We’ve kicked off radar-mapping,” exulted Project Manager Tony Spear. “We’re acquiring data and everything looks good!” A month later, as Earth and Venus reached “superior conjunction” with the Sun, mapping operations were suspended for several days, after which Magellan suffered a third loss of contact on 15 November. “Occasional minor data losses are expected from time to time when the articulation and attitude-control system halts execution,” NASA reported, but stressed that “on-board systems and protective software have been improved to minimize any data losses.” Eight days later, ground computers were blamed when the spacecraft placed itself into safe mode and four mapping orbits were lost. By the tail end of November, though, Magellan appeared to be moving back onto track, with renewed commands from Earth to update its computer so that the radar-mapping would precisely match the most recent tracking data.
Despite these early difficulties, project managers remained confident that the spacecraft was on target to achieve its target of 70 percent coverage of Venus by the end of the first 243-day mapping cycle. Indeed, by the first week of December, approximately 32.9 percent of the planet’s surface had been imaged, including the large continental area of Ishtar Terra and its 7-mile-high (11.2-km) mountain, Maxwell Montes. The data had specifically identified mountainous slopes dusted with an unidentified metallic substance—hypothesized to be iron pyrite—as well as volcanic dome-like features and vast, horseshoe-shaped geological formations. Of the 473 mapping orbits completed through 3 December, 11.8 orbits of data had been lost, and one of Magellan’s two radar data tape recorders proved troublesome, displaying an increasingly high error rate.
This mixed bag of success and disappointment steadily improved as the spacecraft moved into 1991. Efforts to protect Magellan from the fierce solar heating, by shortening radar-mapping passes and periodically turning the mirror-like solar arrays by 90 degrees to reduce the amount of reflected sunlight onto the spacecraft surfaces, proved successful, and by April a “Two Hide” strategy had been adopted. Under this strategy, part of the spacecraft was kept in the shade of the high-gain antenna twice during each orbit to keep the electronics cool. In the meantime, despite occasional computer troubles, more than 65 percent of Venus had been mapped on at least one occasion and the data offered profound insights into the planet’s surface and atmosphere, with evidence of widespread—and ongoing—volcanism, together with possible tectonic activity. Magellan also confirmed that the number and relative size of impact craters was broadly in agreement with pre-flight predictions, suggesting that the planet’s dense atmosphere had served as a shield against significant micrometeoroid bombardment. Turbulent surface winds and an ancient atmosphere, perhaps 400-800 million years old, or even older, were also hinted at by spacecraft data.
As 1991 wore on, Magellan data hinted at the venting of interior heat, through giant oval hotspots, known as “coronae,” and vast circular structures, called “arachnoids,” together with unusual, petal-shaped lava flows, and its results were employed to examine dust movements to make inferences about wind speeds in the lower atmosphere. By 4 April, the spacecraft reached the end of its first 243-day mapping cycle, successfully hitting its target of imaging 70 percent of the surface and receiving authorization to commence a second cycle on 16 May. By this stage, a spectacular 84 percent of Venus had been mapped—with the remaining 16 percent, including the never-before-imaged south pole—taking priority for the second cycle. Periodic data dropouts and losses of contact continued to trouble the mission, but by late-July more than 90 percent of Venus had been imaged, revealing a vast 4,200-mile-long (6,800-km) channel, longer than the River Nile. Over the following months, global surveys, based upon the first two mapping cycles, revealed that around 85 percent of the planet was covered by volcanic rocks, mostly lava flows which formed Venus’ great plains.
In September 1992, having by this point imaged around 99 percent of the surface, Magellan’s orbit was lowered from 186 miles (300 km) to 111 miles (180 km), in order to begin an entire 243-cycle devoted to global gravity mapping. Six months later, in March 1993, a final map of Venus’ topography was released and presented before the 24th Lunar and Planetary Science Conference in Houston, Texas, depicting the shapes of mountains, canyons and other features at far higher resolutions than had ever been attainable on a global scale. And from 25 May through 6 August 1993, Magellan pioneered a technique known as “aerobraking,” dipping into Venus’ upper atmosphere and employing the effects of drag to reduce its orbit from an elliptical to a circular path, in order to “enhance the scientific return” from what was already being described by Magellan Project Manager Douglas Griffith of JPL as “one of NASA’s most productive space science missions.” It was recognized that surface measurements from the elliptical orbit had been blurred at high altitude, which previously reached a peak of 1,700 miles (2,800 km) at the south pole and 1,300 miles (2,100 km) at the north pole.
The spectacular success of this 70-day-long aerobraking maneuver was detailed by NASA in August. Following its arrival at Venus in 1990, Magellan initially occupied an elliptical orbit with a period of more than three hours, but the aerobraking experiment allowed this to be refined to about 94 minutes, roughly equivalent to that of low-Earth-orbiting shuttle missions. It also enabled the spacecraft to effect a dramatic orbit change, without expending a significant amount of its dwindling attitude-control propellant supply.
However, the end of the mission drew inexorably closer. In early 1994, additional funding was received to complete the gravitational mapping, through September, and this continued period of operation revealed that Venus was still geologically active in places, despite little change on the surface in 500 million years. The data revealed at least two—and probably far more—hotspots, with the Atla Regio and Bell Regio exhibiting clear signatures of “top and bottom loading” elasticity of the surface. By 7 September, the Magellan team effected a week-long “windmill” experiment, turning the spacecraft’s solar array paddles in opposite directions to more carefully determine upper atmosphere molecular pressures.
At the start of October, a series of five Orbit-Trim Maneuvers (OTMs) began to lower the closest point of the orbit to about 86.6 miles (139.7 km), in order to gather aerodynamic data from the sparsely explored upper atmosphere. In so doing, Magellan’s solar array temperatures rose to 126 degrees Celsius (258.8 degrees Fahrenheit) and, remarkably, the spacecraft maintained attitude controllability until the very end. Eventually, the main spacecraft bus voltage reached 24.7 volts and, despite predictions that contact would be lost if it dipped below 24 volts, it was not until 20.4 volts that Magellan’s battery went off-line, due to power starvation. Contact was officially lost at 6:02 a.m. EDT on the 12th, bringing to an end one of the most successful missions ever undertaken in the annals of space science. A total of 98 percent of the surface was mapped at resolutions of better than 1,000 feet (300 meters) and Magellan’s gravity mapping campaign had covered 95 percent of Venus. “The data which streamed back from Magellan’s radar images, its atmospheric studies and its gravity data acquisition maneuvers,” explained Griffith, “have built a vast database of new knowledge about Venus and the formation of the Solar System that will be studied by scientists for decades to come.”