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Immediately after VOIR was canceled, JPL engineers and the scientists involved in Venus radar astronomy set out to investigate how the scientific objectives might be recovered by a less costly mission that would be more politically acceptable. It was estimated that the cost could be halved by deleting all of the instruments apart from the radar, reusing as much hardware as possible and simplifying the mission. The revised proposal became known as the Venus Radar Mapper (VRM).

The cost-saving measures were many. First, VOIR's high-resolution radar mode was deleted, thereby limiting the radar mapping to an equivalent optical resolution of 200-500 meters (nevertheless, this represented a considerable improvement over the Soviet Veneras) and greatly reducing the scientific data rate. Also deleted were all the other scientific objectives involving the atmosphere and ionosphere. The only instruments to be carried were the synthetic-aperture radar and the radar altimeter. The radar altimeter was to provide vertical profiles of areas ranging in size from 2 to 20 km attaining a relative accuracy of 5 meters, but when orbital uncertainties were taken into account the overall result would be a vertical resolution of between 30 and 50 meters. Passive microwave emission observations would also be able to be made by using the radar antenna and receiver as a radiometer. Radio tracking of the spacecraft during limb crossings would provide information on the atmosphere, and long-term tracking would provide data on the planet's gravity field. The use of an elliptical rather than a circular mapping orbit would also reduce costs, because the spacecraft would not require to perform the complex (and essentially untested) aerobraking maneuvers required for circularization, and it would not need to have an orbit-shaping engine. Moreover, whereas the original concept had required two antennas, one to collect radar data and the other to transmit it to Earth in real-time, if the spacecraft used an eccentric orbit a single antenna could collect radar data at periapsis and this would be taped for transmission at apoapsis. Although operating the radar as the spacecraft first approached periapsis and then drew away from the planet would vary both the altitude and speed (as the range varied between 300 and 3,000 km its speed relative to the planet would vary between 8.4 and 6.4 km/s) and result in degraded and varying resolution, new developments in computing meant that advanced digital synthetic-aperture radar processing (in contrast to the analog systems of Seasat and VOIR) would compensate for these effects. Nevertheless, to optimize the performance of the radar to take account of the varying distance and speed, parameters such as the pulse repetition frequency would have to be changed hundreds of times during a periapsis passage. But if the price to be paid to be able to fly the mission was the need to overcome such complications, then so be it. The point was to recover the science.

Although VRM would not be part of either the Planetary Observer or Mariner Mark II programs, it still would be built to their philosophy, in particular the use of hardware developed for other programs. The bus and the dual-role radar and high-gain antenna were spares from Voyager; the power, attitude control and command systems and tape recorders were from Galileo; the medium- and low-gain antennas were from Viking and the Mariners; the thrusters were from Voyager and Skylab; the fuel valves and filters were from Voyager; the radio amplifiers were from Ulysses; a tank from a Space Shuttle auxiliary power unit was used for hydrazine; and so on. By narrowing the scientific focus, switching to an elliptical orbit and by vigorously pursuing the strategy of reusing existing hardware, the estimated price tag of $300 million was about half that of VOIR. The mission was one of the four recommended by the Solar System Exploration Committee, and in 1984 it was approved by NASA - the first new-start planetary mission in many years. Martin Marietta of Denver was awarded a $120 million contract to design, assemble and test the spacecraft, and to assist with launch and in-flight operations. Hughes Space and Communications, having made the synthetic-aperture radar for the Pioneer Venus Orbiter, would provide the new radar system.83,84,85,86

In 1985, in line with NASA's new policy of naming planetary spacecraft after people, VRM was named Magellan in honor of the Portuguese navigator Fernao de Magalhaes, who from 1519 until his death in 1521 led the first circumnavigation of the Earth and provided the first global appreciation of our world, just as the orbital radar would hopefully do in the case of Venus.

Magellan was to be launched in April 1988 by a Shuttle and the escape stage would be a Centaur G, the shorter of the two versions of the Centaur that had been adapted for the Shuttle. It would enter orbit around Venus in October 1988, and its primary mission would last 243 days, which is the length of time the planet takes to rotate once on its axis. The orbit would range between about 250 and 8,000 km, with a period of 189 minutes and an inclination of about 86 degrees to the equator. The periapsis would be in the northern hemisphere in order to obtain excellent data on the high northern latitudes, where some intriguing topographical features were known to be situated. The almost perfect spherical shape of Venus meant that the

The observing geometry of the Magellan synthetic-aperture radar. The resolution on the surface across the orbital ground track was obtained from the time delay or distance coordinate, whilst the resolution along the track came from the Doppler shift coordinate. The radar beam illuminated an area off to one side of the ground track, for otherwise it would be impossible to discriminate between echoes which came from the left and those from the right.

The observing geometry of the Magellan synthetic-aperture radar. The resolution on the surface across the orbital ground track was obtained from the time delay or distance coordinate, whilst the resolution along the track came from the Doppler shift coordinate. The radar beam illuminated an area off to one side of the ground track, for otherwise it would be impossible to discriminate between echoes which came from the left and those from the right.

orientation of the orbit would be so stable that at successive periapses the ground track at the equator would be displaced by only about 20 km, with the result that the 25 x 15,000-km radar swaths (dubbed 'noodles') would conveniently overlap. Moreover, to preclude excessive overlap of swaths at the north pole successive mapping passes would alternate from between the north pole and an intermediate southern latitude, and between a temperate northern latitude and a greater southern latitude. During each swath, the spacecraft would slew around in order to view the surface at a steadily varying angle, while maintaining the antenna 35 degrees off to one side of the ground track (as opposed to 10 degrees in the case of the Veneras) to better distinguish terrains having rough surfaces. Only 37 minutes of each orbit would be devoted to taking radar data. During the remainder, the spacecraft was to turn to point its antenna towards Earth, transmit the recorded data, and undertake engineering tasks such as recalibrating the attitude control system to ensure that successive swaths would be correctly positioned and that the Earth-pointing was accurate in order to enable the data to be transmitted at the highest possible speed. During its primary mission, Magellan was expected to map over 70 per cent of the

Orbiter Structural Spares
When the Magellan spacecraft was at the periapsis of its eccentric orbit of Venus it would alternate between radar swaths covering the high northern or high southern latitudes. The remainder of the time, it would point its antenna at Earth to transmit the radar data.

planet's surface. If the mission were to be extended, it would be able to fill in any gaps (known as 'gores'), image interesting features from different perspectives for stereoscopic analysis, and increase its coverage of the southern hemisphere. At the conclusion of the mission, owing to the fact that our planet is mostly shrouded by water, scientists expected to know more about the surface of Venus than they knew about the Earth.

Although the Challenger disaster affected the Magellan mission, the impact was less traumatic than for the other American planetary missions then in the pipeline. The cancellation of the Centaur G meant that Magellan had to be revised to use the IUS instead. Because it was not known whether the Shuttle would have resumed flying by the April 1988 launch window (it did not) it was decided to postpone the launch. The next window, in November 1989, would have required Magellan to set off within days of Galileo, which was to initiate its long voyage to Jupiter with a flyby of Venus. Rather than rely on back-to-back Shuttle missions, JPL devised an unprecedented 'fourth type' of trajectory in which Magellan would be launched in a 1-month window that extended from late April to late May 1989 and travel more than one and a half times around the Sun to reach Venus, instead of the usual half

Space Solar Array
The Magellan spacecraft.

circuit. Although this trajectory had some advantages in terms of the energy of the departure from Earth and on arrival at Venus, it imposed a 15-month cruise and an 18-day hiatus for solar conjunction midway through the primary mission. If this launch window were to be missed, the next opportunity for a conventional transfer to Venus would not open until May 1991.

The 3,453-kg Magellan spacecraft was dominated by the Voyager antenna that was to be used for both radar data collection and communications. At 3.7 meters in diameter, this spanned the payload bay of the Shuttle. To provide communications at times when neither the high-gain nor the medium-gain antennas could be aimed at Earth, there was a low-gain antenna from Mariner 9 on the feed of the big dish. The high-gain antenna would transmit at either 268.8 kbps or 115 kbps, depending on the size of the Deep Space Network antenna that was used. The radar data from a periapsis pass would be stored on two redundant Galileo-heritage tape recorders, and replayed over two 57-minute sessions when Magellan was near apoapsis. The 1.5-meter-long horn of the radar altimeter was installed alongside the main antenna and offset 25 degrees from its axis so that when the synthetic-aperture radar looked off to the side the other would view the surface more or less vertically. Behind the antennas was a boxy module 1.7 x 1 x 1.3 meters in size which contained all of the radar electronics, some of the communication systems (including the medium-gain antenna protruding from one side, which was to be used primarily during the cruise to Venus and during the orbit-insertion maneuver), reaction wheels for the attitude control system, a star mapper for attitude determination and batteries. The exterior of this equipment module was covered with thermal control louvers to maintain the radar electronics, which consumed 200 W of power, inside its allowed temperature range. Next was the 10-sided Voyager bus, which was 2 meters across and 42.4 cm tall and housed the main computer, tape recorders and a number of other systems. At the center of the bus was a tank for 132.5 kg of hydrazine, which was sufficient for many years of operations in orbit of Venus. On each of two sides of the bus was a solar panel that was hinged against the frame of the spacecraft for launch and in its deployed state could be tilted in order to track the Sun. Each solar panel was 2.5 meters on a side, and at Venus's distance from the Sun would generate 1,200 W of power. At the rear was a propulsion module featuring a cross-shaped truss, at each tip of which was a pod housing a pair of 445-N thrusters, one 22-N thruster and three 0.9-N attitude control thrusters that were also to be used to trim the orbit around Venus. Also mounted on this truss was a small tank of helium to maintain the pressure in the hydrazine tank. The propulsion module also provided attachment points for the structure that would connect the spacecraft with the IUS at launch, and for the 2,146-kg STAR-48B solid-fuel motor that was to be used for the orbit-insertion maneuver at Venus (the same kind of motor as was often used as a kickstage for terrestrial geostationary satellites). Magellan was 4.6 meters from the tip of the feed on the high-gain antenna to the propulsion module, and with the solar panels deployed it spanned 10 meters.87,88

Despite being promoted as a relatively inexpensive mission, by the time that it was ready for launch Magellan's price tag had almost doubled to $550 million; due in part to the delays caused by the Challenger disaster, but also as a result of cost overruns - mostly concerning the development of the radar (over which JPL had at one point to regain control from the contractor) and to redesigns and improvements which made the spacecraft more capable but also more expensive. In fact, as early as 1984 JPL had identified numerous ''half-million-dollar items'' for improving the spacecraft and its performance, some of which were implemented during the years that the Shuttle was grounded. One improvement that was not funded was to put a 30-cm aluminum skirt around the circumference of the antenna in order to boost its radar performance. Another source of overruns was that on the revised schedule Magellan would be launched before Galileo, rather than after it, which meant that some of the spare parts that would otherwise have become available from Galileo had to be retained by that program in case they should be required, with the result that new parts had to be purchased.89 The manufacturer delivered the spacecraft to Cape Canaveral in early October 1988, but on 17 October it suffered an unusual accident when a technician put a connector into the wrong socket of a test battery, causing a short circuit. This started a fire, but it was readily extinguished and the spacecraft suffered only minor damage. At that time, the radar, high-gain antenna and communications electronics had not yet been fitted, and because the 'remove before flight' covers were still in place over most of the installed components they were protected. Nevertheless, it took several days to clean the soot and grease from the spacecraft. Overall, the incident cost the mission $80,000.90,91

The Magellan/IUS stack leaves its cradle in the cargo bay of Space Shuttle Atlantis.

The first attempt to launch Space Shuttle Atlantis on mission STS-30 was made on 28 April 1989, and was aborted 31 seconds before ignition owing to a problem with a pump on the Shuttle. It eventually lifted off on the third attempt, on 4 May, after a 1-hour hold for the weather. It was the first US planetary launch since the Pioneer Venus Multiprobe and International Cometary Explorer were sent aloft in 1978, and also the first planetary mission to be dispatched by the Shuttle, which had imposed such delays on the scientific and planetary exploration programs that most scientists ''[wanted] nothing to do with [it]''.

Five revolutions after launch, Atlantis released the 'stack' comprising the two-stage IUS, the motor for the Venus orbit-insertion maneuver and the spacecraft at an altitude of 296 km above a point 1,000 km southwest of Los Angeles. Shortly thereafter, the astronauts visually confirmed that the solar panels had hinged out correctly. Although the deployed panels would act like flexible appendages during the escape maneuver and introduce anomalous dynamics which the attitude control system of the IUS would have to overcome, it had been decided to deploy them in advance in order to protect them from the efflux from the roll-control thrusters of the IUS, which were close to their ends. Sixty minutes after it was deployed from the Shuttle, the IUS fired its two stages in sequence and injected Magellan into the planned heliocentric orbit ranging between 1.011 x 0.699 AU. Atlantis set down at Edwards Air Force Base in California 4 days later, having conducted a variety of microgravity experiments.92

On 7 October Magellan crossed inside the orbit of Venus and reached its first perihelion, but of course the planet was nowhere near. It then reached aphelion at Earth's orbit in early March 1990, and started back inward. Three corrections were performed during the cruise on 21 May 1989, 13 March and 25 July 1990 to refine the approach to Venus. A number of minor problems were encountered during the cruise, including spurious signals in the star tracker caused by solar protons hitting its detectors. Software patches were written to reject most of the false signals and so restore the tracker's capabilities, which would be essential to ensuring that the attitude of the spacecraft would be able to be updated to within a fraction of a degree on each orbit of Venus. High temperatures were recorded on the main bus, and also on the thrusters - where the heat induced gas bubbles to form in the hydrazine lines. The overheating in the bus was remedied by turning the spacecraft to cast the shadow of the high-gain antenna on the affected bays.93 Two radar rehearsals were also conducted during the cruise: in December 1989 to test its basic working, and then in May 1990 to simulate a full mapping run over an interval equivalent to 20 orbits.94 As Magellan was cruising, two researchers at Brown University, Rhode Island, proposed a controversial theory which argued that Venus had undergone a process of plate tectonics similar to that which occurs on Earth. In particular, they said that in the low-resolution radar imagery from the Pioneer Venus Orbiter the equatorial Aphrodite Terra resembled the mid-Atlantic spreading zone where new lithosphere is continuously created. Moreover, Ovda Regio, a 3,500-km ellipsoid in Aphrodite, bore a striking resemblance to Iceland, which is a plateau on the spreading ridge. The researchers predicted that the higher resolution imagery from Magellan would reveal fine details which would prove their theory.95,96

Once clear of the Shuttle, Magellan deployed its solar panels.

On 10 August 1990, Magellan fired its solid rocket to enter orbit around Venus: the aim point was within 100 km of that planned, and the burn was so precise that an orbit ranging between 289 and 8,458 km with a period of 3.26 hours at an inclination of 85.5 degrees to the equator and a periapsis at 10°N was achieved directly from heliocentric orbit, so no refinements would be required to achieve the mapping orbit. Magellan was only the second US spacecraft to enter orbit around the planet. In fact, the first, the Pioneer Venus Orbiter, which had arrived in 1978, was still operating, and it attempted a fascinating engineering experiment in which its photopolarimeter tried to image the plume from the retrorocket of the new arrival crossing the dark hemisphere, but it was too faint. The perfect orbit was extremely welcome, as the propellant saved would be able to be used to extend the mission to fill gaps in the map and also to change the viewing geometry of the spacecraft and its radar because radar echoes (and the images created from them) depend on the angle at which the beam strikes the surface - features that could not be seen at one angle of incidence may stand out at another, or when the polarization of the beam is changed. Magellan suffered several more glitches at this time: about a minute after the insertion burn one of the four gyroscopes of the attitude control system malfunctioned and was automatically turned off, and a backup memory became corrupted several seconds after the pyrotechnic bolts were detonated to jettison the spent motor casing.97

After several days of checks, the radar was powered up on 15 August and the engineers set about 'tuning' it to obtain the best possible imagery. If all went well, the plan was to make test mapping runs during the next few days and initiate routine mapping on 29 August. But as the scientists were celebrating the ''stunning clarity'' of the images produced from initial test data, the Deep Space Network reported not receiving telemetry from Magellan after it performed the star sightings required to update its attitude at apoapsis. A faint signal was detected 14.5 hours later, only to disappear and recur at intervals of 2 hours, which suggested that the spacecraft was slowly spinning and sweeping the beam of its antenna past Earth once per rotation. Ten hours later, Magellan was instructed to stabilize itself in an attitude that would allow it to point its medium-gain antenna at Earth. When the recorded telemetry was recovered the engineers deduced that the attitude control computer had lost its health-monitoring 'heartbeat', and this had prompted the spacecraft to enter a 'safe mode' that terminated all operations and switched communications to the low-gain and medium-gain systems. In this case, a slew should have been made to align the antenna to Earth and the solar panels to the Sun, but the two reference stars which it was to use to make this maneuver had either been missed or mistaken, and the spacecraft had ended up in an unexpected attitude and unable to communicate with Earth. Some hours later, a series of faults in the attitude control system had caused it to switch over to a simpler 'primitive' orientation control program in a 1 kilobyte Read-Only Memory that used thrusters instead of reaction wheels to maneuver. At that point, as the spacecraft began to cone (as it should) its antenna swept by Earth and control was able to be regained.98

But success in recovering Magellan was short-lived, because a few days later, as the checks were continuing, it again drifted out of attitude and contact was lost.

After 4 hours of waiting for the spacecraft to re-establish contact, the controllers at JPL, fearing either that it might misalign the solar panels and prevent the batteries from charging or that it might attempt unplanned Earth search modes which would waste precious fuel, decided to have the Deep Space Network send commands in the blind. After another 4 hours without a signal, a command was issued to disable the software that monitored the 'heartbeat', and control of the ailing spacecraft was regained. However, the engineers were at odds to explain what was happening to their spacecraft. Although several kilograms of hydrazine had been wasted, enough remained to support many years of orbital operations. While the engineers tried to determine the problem, the scientists presented a preliminary mosaic of the 34-km-diameter crater Golubkina produced from the early test data. It had a resolution as good as 120 meters, which was an order of magnitude better than that provided by the Veneras, and revealed for the first time details of its central peak, terraced wall and surrounding ejecta. Everyone was eager to complete the tests and start routine mapping.

A fortnight after the initial problem, attitude control was returned to the normal system. On 12 September the high-gain antenna was pointed towards Earth for the first time in almost a month, and the resumption of the maximum data rate enabled all the engineering data that had been stored on board since the first anomaly to be downloaded, allowing the engineers to conduct a detailed analysis of the problem. Meanwhile, the spacecraft continued to suffer attitude control problems as a result of taking star sightings at apoapsis.

Nevertheless, the radar was reactivated on 14 September and regular mapping began the following day.99,100 Operations had to be suspended from 26 October to 10 November while Venus was on the far side of the Sun as viewed from Earth, at superior conjunction. The gap in the coverage would be filled in one local day later, in June and July 1991. There continued to be glitches, including overheating of components and excessive vibrations of the solar panels caused by oscillations in the attitude control system. The worst problem occurred in December, 3 months into the mapping mission, when one of the two tape recorders developed a rapidly worsening rate of errors and had to be switched off.101 The first cycle of mapping concluded on 15 May 1991, having covered 83.7 per cent of the surface. Magellan had exceeded by a large margin the data from all the previous planetary missions combined. But as the overheating problem had become progressively worse, more and more of each mapping pass (at worst 55 minutes of it) was devoted to turning the spacecraft so that the high-gain antenna would shade the electronics. This was particularly critical when the orbital geometry was such that the propulsion module would be exposed to the Sun. Moreover, on 10 May Magellan suffered the fifth and longest loss of signal, this time being out of contact for 32 hours. But this was the last such outage, because in July the cause of the problem was finally found. In certain conditions, a software shortcut was putting the attitude control computer in a logical loop. The problem was exacerbated by the section of memory which had been corrupted at orbit-insertion.102

The second mapping cycle followed on immediately after the conclusion of the first, and imaged a swath to the right of the ground track rather than to the left in

A Magellan image of the 340-km-diameter impact crater Golubkina taken while testing the mapping radar, set against a Venera image of the same landscape. The improvement in resolution is striking. (JPL/NASA/Caltech)

order to observe the terrain at a different angle of illumination. This cycle would fill in the gaps left by the communication outages, overheating and tape recorder problems, and expand the coverage to high southern latitudes.

Although Magellan and its radio system were optimized for radar observations, it was possible to 'sound' the atmosphere during radio occultations using the dual-frequency technique by which a signal transmitted from Earth first passed through the planet's atmosphere to reach the spacecraft and was then returned, in this case with the signal being transmitted at both of the spacecraft's operating frequencies. The experiment was conducted only once during the mission, on three consecutive orbits of the second cycle between 5 and 6 October 1991, and the two frequencies allowed the absorptivity of the atmosphere to be probed down to an altitude of 34 km. The polarization of the signal was also measured to detect the effects of clouds and lightning, and although no such effects were seen, the experiment did provide detailed profiles of the abundances of various gases in the planet's atmosphere, in particular of gaseous sulfuric acid.103

On 4 January 1992 the main high-gain antenna transmitter failed, and ceased to return data. Its backup was already known to be defective, since it drew excessive power, overheated and as a consequence introduced 'noise' to the transmission that impaired the data. When it was turned on as a test, it successfully returned data but it overheated and had to be switched off after just 25 minutes. If neither transmitter could be recovered, this would mark the end of the mapping mission, which had by now covered 96 per cent of the planet's surface. Even so, it would still be possible to obtain gravity data using the medium-gain antenna. The solution implemented later that month was to operate the backup transmitter at the lower data rate of 115 kbps to overcome the heating problem. Radar imaging resumed at the start of the third Venusian day, and the objective was to fill in the few remaining gaps. The week in which the radar could not be used was not wasted, because the Doppler-shift on the transmission was monitored in order to collect preliminary data for the gravity survey.104,1°5

A number of special radar experiments were performed during the first cycles. Five orbits on 12 July 1991 were allocated to a high-resolution test which required Magellan to maintain its beam at a constant angle of incidence with respect to the surface of about 25 degrees, rather than allowing it to vary from about 15 degrees over the poles to 45 degrees at periapsis. This effectively doubled the resolution in the 'along the track' direction to 60 meters. And starting on 24 July several orbits were used to collect swaths with the radar looking first to the left of the track and then to the right in order to make stereoscopic views of some southern hemisphere regions. The resulting 'stereo pair' images allowed the heights of single features to be determined to an accuracy of 70-100 meters. This test was so successful that the third cycle was replanned to concentrate on collecting stereoscopic data. On 13 December 1991 several orbits were dedicated to polarimetric observations of Rhea and Theia Mons. The spacecraft was rotated 90 degrees around its roll axis so that vertically polarized radio echoes could be collected to complement the horizontally polarized ones taken earlier. By subtracting the two radar echoes, it was possible to measure the dielectric constant of the surface. Finally, stereoscopic imaging of the Maxwell

A Magellan radar swath showing the complex structure of the Maxwell Montes. The unusual brightness of the elevated terrain is probably due to a surface coating of a material that is strongly reflective at the radar's wavelength. The double-ring of the Cleopatra crater is clearly seen. The black streaks are the result of missing data. (JPL/ NASA/Caltech)

A Magellan radar swath showing the complex structure of the Maxwell Montes. The unusual brightness of the elevated terrain is probably due to a surface coating of a material that is strongly reflective at the radar's wavelength. The double-ring of the Cleopatra crater is clearly seen. The black streaks are the result of missing data. (JPL/ NASA/Caltech)

Montes, Sif Mons and Gula Mons was scheduled between 24 January and 7 February 1992. In particular, scientists hoped to analyze the cliff that forms one side of Maxwell, which was evidently so steep that the altimeter was unable to resolve it. However, the observations could only be made at a time when the spacecraft's apoapsis would be occulted by Venus, which would limit the data that could be returned. Moreover, the opportunity occurred just weeks after the backup transmitter was brought into operation at its lower rate, which further reduced the data return. Although the observation was made, the Maxwell cliff was not able to be imaged owing to the fact that when it was in the field of view the recorder had to change track and reverse the tape direction - the tape had four tracks, alternating between forward- and backward-recorded ones.106

The mapping had to be curtailed when the apoapsis of Magellan's orbit was on the opposite side of Venus, limiting communications, but with the periapsis on the Earth-facing side of the planet conditions were ideal for making the gravity survey. During these orbits, the high-gain antenna was held pointing at Earth in order to receive and return a signal. When the relative motions of Earth and Venus, and of the spacecraft's motion around Venus were subtracted, the residual Doppler shift served to measure the varying gravitational pull from a patch of surface about as wide as the altitude of the spacecraft's orbit. Because an acceleration of less than 1 mm/s2 would impart a measurable Doppler shift, the technique was remarkably sensitive. The first collection of data between 22 April and 16 May 1992 occurred over Artemis Chasma, one of the many rifts of Aphrodite Terra. At the same time,

A hemispherical view of the Magellan data centered at a longitude of 180 degrees. (JPL/ NASA/Caltech)

the venerable Pioneer Venus Orbiter was providing gravity data over the southern hemisphere, although unfortunately in October it entered the atmosphere and was destroyed.107,108

After Magellan's backup transmitter once again overheated in July 1992 it was switched off to save it for the campaign late in the third cycle to fill the last major gap in the mapping coverage. By the conclusion of this cycle on 13 September, the radar had mapped 98 per cent of the surface. Unfortunately, it was not possible to implement an interferometry experiment in which radar data collected at the same geometry on different cycles would allow very fine surface detail to be resolved.109 The fourth and future cycles were to be dedicated to the gravity survey, for which the slow rate over the high-gain antenna was more than adequate.

Each of Magellan's mapping swaths yielded about 100 megabits of imaging and related data - which was almost as much as the combined total from the radars on Veneras 15 and 16. The inflow of data was so prodigious that the computers at JPL used to process it were often running several weeks behind. Another challenge was to provide at least a preliminary inspection of the resulting images; so much so that the project scientists invited colleagues to bring in their post-doctoral and graduate students to assist. The mission also fostered a new era in the relationship between Soviet (later Russian) and American scientists, when three geologists who had 'cut their teeth' interpreting the Venera data at the Vernadsky Institute in Moscow were recruited to assist in analyzing the data.

Magellan confirmed Venus to be a world dominated by volcanism, with related terrains covering at least 80 per cent of its surface. There were structures ranging in size from domes several kilometers across to shields spanning at least 100 km. Almost the entire surface was dotted by thousands of small volcanoes concentrated in clusters. The shields (individually similar to their largest terrestrial counterparts) were located atop large regional rises which might have formed above upwelling plumes deep inside the planet. They could still be erupting. In addition, there were lava plains covering tens of thousands of square kilometers. These lavas appear to have been of low viscosity, and to have flowed in the same manner as the lunar maria. But unlike the lunar maria and the sand deserts of Mars, the plains of Venus had electrical characteristics indicating that, on average, they comprised hard soil. A more viscous and sluggish lava made pancake-like structures in the form of flat circular domes over 50 km across, having steep sides which rose no more than 1 km. These tended to occur more or less in chains of overlapping structures. As lava withdrew from the pancakes, it left troughs and pits on the upper surface. Although similar domes exist on Earth, those on Venus are at least 50 times larger, perhaps because the ambient temperature at the surface of Venus is so great that lava was able to flow further before cooling and solidifying. Remarkably, we may know the composition of a pancake, since there is such a feature within the target ellipse of Venera 8, the lander which performed the first gamma-ray spectrometer analysis of the surface. Another remarkable class of feature were the 'arachnoids' first seen in the Venera radar imagery. These networks of radial ridges in patterns resembling spider's webs (hence the name) are unique to Venus, and were probably formed by a combination of volcanism and tectonism.110

Venus Probe Nasa Magellan
The Magellan view (left) revealed that the multitude of 'spots' on the Yenera image (right) were small volcanoes. (JPL/NASA/Caltech) ^

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