[177] Magellan culminated the shift of radar astronomy toward planetary geology kindled by Apollo and fostered by Viking and Pioneer Venus with the creation of workshops and microsymposia. The workshops attempted to bridge the gap between radar and geologic knowledge among practitioners, while the microsymposia provided annual opportunities for U.S. and Soviet geology and radar scientists interested in Venus to exchange research results. This shifting of the planetary radar paradigm toward geology also manifested itself in articles co-authored with planetary geologists, publication in new journals, especially the Journal of Geophysical Research, and attendance at American Geophysical Union meetings.

Furthermore, the close relationship between NASA missions and ground-based planetary radar astronomy that had developed at Haystack, Arecibo, and Goldstone since 1970 continued with Magellan. The Arecibo and Goldstone radars observed Venus throughout the two decades spanned by Pioneer Venus and Magellan, and their data contributed to the success of those missions. In addition, the range-Doppler images created from that data also drew geologists to planetary radar astronomy.

Magellan, like Pioneer Venus, was not ground-based planetary radar astronomy; it was space exploration. By carrying out imaging from a spacecraft, radar astronomer Gordon Pettengill had erased that distinction. That distinction no longer seemed to describe the field, as Len Tyler and Dick Simpson joined the Magellan radar team. Tyler and Simpson had not abandoned bistatic radar and radio occultation experiments; they had simply added Magellan radar science to their wide range of research interests.

Unlike the Pioneer Venus mission, or the Goldstone and Arecibo facilities, Magellan was not a case of radar astronomy "Little Science" piggybacking onto a Big Science facility. Magellan was Big Science. Moreover, its single scientific instrument was a radar. The Smithsonian push to have Congress fund the NEROC 440-ft (134-meter) dish never reached the floor. With Magellan, then, Congress considered for the first time underwriting construction of a facility dedicated primarily to planetary radar astronomy, albeit one whose lifetime was rather limited. Magellan illustrates the range of factors that influence the scientific conduct and outcome of a Big Science project. The change of administration in 1980, Cold War politics, and the Challenger accident, as well as ongoing and changing budgetary and technological constraints largely shaped the scale and scope of the Magellan mission and its science.

As a mission concept, Magellan began in 1972, when Danny Herman, the head of NASA Advanced Programs, convened an informal meeting of scientists, including Gordon Pettengill, NASA engineers, and representatives of several key aerospace companies at the [178] Denver division of Martin Marietta Aerospace, to discuss putting a synthetic aperture radar on a spacecraft to Venus.¹

Subsequently, two NASA laboratories, Ames Research Center and JPL, organized study groups and began planning the mission and appropriate spacecraft parameters. At Ames, Byron Swenson and John S. McKay put together a study group that worked closely with Martin Marietta Aerospace in planning a Venus SAR mapping mission. They initially proposed a variation on Pioneer Venus with an elliptic orbit. During the period 1972 through 1974, Ames Research Center, Martin Marietta Aerospace, and the Environmental Research Institute of Michigan (ERIM), which had been involved in the development of airborne SAR systems for the military as early as the 1950s, carried out a preliminary evaluation of data handling problems and techniques. The 1974 joint report of Martin Marietta Aerospace and the ERIM defined the project's science requirements and argued in favor of a circular orbit.²

At the same time, a similar study was underway at JPL under Louis D. Friedman. In order to distinguish their Venus project from that of the Ames group, Friedman and Al Laderman named the JPL project the Venus Orbiting Imaging Radar (VOIR). Laderman had played a key role in the development of the SEASAT SAR. He and Friedman intended the acronym to connote the sense of the French verb "voir," meaning to see. VOIR was going to "see" the (optically) unseen surface of Venus. The JPL group included science, mission, and radar people. R. Stephen Saunders was the principal study scientist. Later, he became Magellan Project Scientist, as well as an investigator in the radar group. Saunders had served on the Viking Mission to Mars, before carrying out NASA-funded research in planetary geology and participating on the Shuttle Imaging Radar (SIR-A) project.

The JPL group decided, mainly on the urging of Friedman, to use a circular orbit. A circular orbit would simplify the radar imaging process. The radar system always dealt with the same parameters, because it was always at the same height above the surface. Friedman felt that simplifying the radar versus the increased propulsion required to achieve a circular orbit was a good trade-off. Although the added propulsion needed to achieve a circular orbit would increase the overall cost of the mission, at least it was understood. The synthetic aperture radar was a new technology; an elliptical orbit presented a host of radar and data processing problems. Jim Rose's study group, charged with planning the spacecraft, proposed a vehicle based on the Mariner system. The radar study group specified a radar system compatible with the 3-meter (10-ft) antennas built for the Pioneer missions to the outer solar system. Already, the goal was to economize by using existing technology.³

Many of the initial assumptions concerning look angle, number of looks, various resolution assumptions, number of bits, and other radar system parameters came under criticism by scientists familiar with optical data, but nonetheless responsible for interpreting the radar data. Those criticisms led JPL to redesign away from the Mariner approach and to exploit internal strengths in synthetic aperture radars gained through the SEASAT program. Ultimately, the SEASAT experience gave JPL an edge over its competitors.

[179] SEASAT was an Earth-orbiting satellite equipped with a SAR and designed for oceanographic research. In its 1977 mission and systems study, JPL proposed the SEASAT SAR as the potential design base for the VOIR. JPL argued that SEASAT already had converted the concept of a spacecraft imaging radar into a reality. SEASAT used much of the conceptual and system design contained in the original JPL VOIR study, while later VOIR studies borrowed heavily from the SEASAT experience. JPL also contributed SEASAT staff. John H. Gerpheide, SEASAT Satellite system manager, became VOIR/Magellan project manager. Anthony J. Spear, sensor manager on SEASAT, became VOIR/Magellan deputy manager.⁴

When the Science Working Group convened at NASA Headquarters in November 1977, NASA already had selected the JPL study. NASA charged the Science Working Group with defining the major scientific objectives and rationale for a Venus orbiter equipped with a radar imager, as well as defining other experiments and defining the radar-imaging requirements of the mission, including coverage, resolution, operating wavelength, telemetry data rate, and data processing. The Science Working Group considered the merit of global coverage at medium resolution and imaging selected areas at high resolution.

The composition of the VOIR Science Working Group drew heavily on Pioneer Venus alumni and from both the planetary radar and geology communities (Table 5). The planetary radar members were Don Campbell (Arecibo), Dick Goldstein (JPL), and Gordon Pettengill (MIT), who chaired the Group. Harold Masursky and Gerald Schaber, both astrogeologists from the USGS, Flagstaff, and both participants in Pioneer Venus, also served on the Science Working Group.

The Science Working Group thus became a forum for reinforcing bridges between planetary radar and geology scientists. The geologists were "very helpful in teaching us radar people what it was that turned them on, as it were, while we were helpful to them in terms of optimizing the operation of the radar, so as to provide them with what they wanted," Gordon Pettengill explained. "This interaction shaped the specifications that turned [180] into the VOIR and later the Magellan programs. The process is ongoing. It goes on even today."⁵

NASA was particularly mindful that the Science Working Group "take full account of the anticipated capabilities of Earth-based radar systems as well as the results expected from the Pioneer Venus experiments."⁶ The Committee on Planetary and Lunar Exploration (COMPLEX) of the Space Science Board, and in particular its chairman, Caltech professor of geology and geophysics Gerald J. Wasserburg, was behind that request. The request was logical, Herman judged in retrospect. Given the high cost of VOIR, why should NASA and the Congress commit a large sum of money to a space mission, when Arecibo could acquire the same imaging data for far less money? Having the 1977 VOIR Science Working Group assess the science yield from a large ground-based radar telescope, like Arecibo, compared to the science yield from a spacecraft was, in Herman's words, "very necessary to yield off the devil's advocate question."⁷

Herman already had emphasized to the initial JPL study group the need to consider the capabilities of Arecibo for undertaking ground-based radar observations of Venus. The chief weakness in the development of the Venus radar orbiter concept, he explained, was the belief held by some scientists that upgraded ground facilities could provide data that was almost as good at a far lower cost.

By 1977, range-Doppler imaging of Venus at Goldstone and Arecibo had advanced considerably thanks to the refinement of interferometry techniques and the attainment of finer image resolution. At Goldstone, for example, Dick Goldstein used a radar interferometer, the 400-kilowatt Mars Station linked to a 26-meter Goldstone DSN antenna (DSS-13, known also as the Venus site) located about 22 km to the southeast, to observe and image Venus in 1972 for the first time and subsequently during the winter of 1973-1974 and the summer and fall of 1975. Over that period, image resolution fell from 15 to 10 km, although in some instances Goldstein realized resolutions as low as 5 to 9 km in the East-West direction and 7 to 10.8 km North to South. In 1977, Ray Jurgens and Dick Goldstein organized a three-station interferometer; the Mars Station transmitted, then it and two 26-meter Goldstone DSN antennas (DSS-12 and DSS-13, the Echo and Venus sites, respectively) received. The three-station data yielded image resolutions of 10 and even down to 8 km.⁸

Planetary scientists R. Stephen Saunders and Michael C. Malin of the JPL Planetology and Oceanography Section studied the Goldstone Venus images and concluded that they revealed a complex and varied terrain. They found degraded impact craters and evidence for volcanism. In these radar images, Beta now appeared to be a 700-km diameter region elevated a maximum of about 10 km relative to its surroundings with a 60-by-90-km wide depression at its summit. Saunders and Malin tentatively identified Beta Regio as a shield volcano.⁹

Meanwhile at Arecibo, the radar upgrade from UHF to S-band increased the resolution of Venus radar images abundantly. In 1969, with the old 430-MHz radar operating in an interferometric mode, Campbell, Ray Jurgens, and Rolf Dyce achieved a resolution of [181] only 300 km. An improved line feed brought Venus image resolution down to about 100 km in 1972, the last Venus observations before the S-band upgrade.¹⁰

Concomitant with the S-band radar upgrade, the NAIC constructed a second antenna, a 30-meter equatorially mounted reflector, at a site about 11 km to the North-Northeast of the main 1,000-ft (305-meter) dish. Data taken by Campbell and Dyce in association with Gordon Pettengill during the Venus inferior conjunction of late August and early September 1975 yielded images with surface resolutions approximating those of Goldstone, between 10 and 20 km. Especially interesting was a detailed view of Maxwell.¹¹

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Thanks to hardware improvements, Don Campbell and Barbara Ann Burns, his graduate student, increased the resolution of Venus images to five km during the 1977 inferior conjunction. For her doctoral dissertation, Burns used these radar images to study cratering on the planet. She and Campbell identified over 30 circular features in the images and tentatively classified them as craters, but the level of resolution did not permit them to ascertain whether their origin was volcanic or impact.¹² Also, in conjunction with USGS...

[183] ...geologist Hal Masursky, Don Campbell and Gordon Pettengill studied images of Alpha, Beta, and Maxwell made from combined 1975 and 1977 Arecibo observations.¹³

As Campbell and fellow radar astronomers using the upgraded Arecibo telescope achieved resolutions as fine as five kilometers on Venus during the 1977 inferior conjunction, the high resolution invited comparison with potential space-based radars. In order to evaluate the capabilities of ground-based radars versus orbiting radars, the JPL study group brought in Thomas Thompson. Thompson had conducted lunar radar work at both Arecibo and Haystack for the NASA Apollo program. As a result of Thompson's advice, as well as the counsel of Danny Herman, Friedman's study group framed a radar orbiter mission that complemented, rather than competed with, ground-based radar observations of Venus.¹⁴

Thompson judged that the best ground-based facility would be the upgraded Arecibo telescope. He concluded that the Earth-based radar was a very powerful tool for mapping the surface features of Venus. "We should encourage these efforts with great vigor," he wrote. "It seems certain that the Earth-based mapping will show many features that should be mapped in greater detail with the spacecraft. Also, the spacecraft will be needed to map the hemisphere of Venus which is not pointed toward Earth at each inferior conjunction."¹⁵

The combined revolutions of Venus and Earth around the Sun lead to an interval between inferior conjunctions (known as the synodic period) that nearly matches the spin rate of Venus about its axis, so that Venus presents almost the same hemisphere to Earth observers at inferior conjunction, the only moment when radar astronomers have sufficient signal-to-noise ratio to image the planet.¹⁶ The major argument in favor of a spacecraft imaging mission to Venus was the inability of ground-based radars to image the planet's hidden hemisphere. A major upgrade of the Arecibo (or Goldstone) radar could have enabled it to observe and image Venus at orbital points before and after inferior conjunction. Such an upgrade would have cost less than the Magellan mission, and the improved radar would have been able to carry out radar research on a variety of other solar system targets.

In 1977, NASA asked the VOIR Science Working Group to compare the costs of acquiring the data from a space-based SAR versus from a ground-based radar telescope, like Arecibo. "We knew that NASA did not want to hear that it would be cheaper, even though if you had taken what it actually cost to do Magellan and put it into a ground-based facility, you could have had one beautiful ground-based facility, and you could have endowed a fund to run it for years, forever probably, if you invested the money properly," Gordon Pettengill explained.

Moreover, Pettengill argued, "As an investment in basic research, basic astronomy, a ground-based observatory would be a much wiser investment than sending Magellan out there. But that is not how things work. The money is available for the Space Station, but not available for any ground-based system that perhaps would do some of the same things."¹⁷

Pettengill assigned the tasks of comparing altimetry and radar imaging capabilities of ground-based versus space-based radars to Don Campbell and Dick Goldstein. They [184] completed separate reports, with Goldstein considering altimetry and Campbell appraising imaging capabilities. In each case, they compared a feasible radar design (an array located probably in Puerto Rico to have the planet nearly over head) with the current VOIR design requirements and judged whether the radar could achieve the geologic objectives of the Venus mission as well or better than the VOIR design.

Campbell and Goldstein concluded that the radar array could do the VOIR science (almost). The ground-based radar would not observe Venus at the same angles of incidence as VOIR, yet, because it would be able to observe Venus at a distance of 1.5 astronomical units, it could see the side of Venus hidden at inferior conjunction. The 100-meter resolution attainable from Earth was the same as that set for the VOIR mission. Moreover, the radar array could do the job for less. Pettengill decided to not include their conclusions in the Working Group report "for political reasons." He believed that NASA had no interest in the project, and that the conclusions might be embarrassing.¹⁸

In 1978, VOIR began to come together. The concept and preliminary design studies completed, the time had come to begin implementation studies. Radar development began in 1978 and took place in two stages, called Phase A, lasting from June through August 1978, and Phase B, October 1979 through June 1980. During Phase A, JPL received three proposals to study the VOIR SAR and selected one study contractor, Goodyear Aerospace Corporation. During Phase B, JPL received three proposals and selected two study contractors, Goodyear Aerospace Corporation and Hughes Aircraft Company. Participation in Phase B studies was important for those firms wishing to build the VOIR radar; implementation phase proposals were accepted from only those companies participating in Phase B.¹⁹

The mission, its spacecraft and radar systems, and its science experiments underwent many revisions, and many of the risks foreseen in 1978 materialized before Magellan left Earth. As planned in 1978, the space shuttle would launch the VOIR spacecraft during the period May-June 1983. VOIR would arrive at Venus in November 1983 and spend five months in orbit, reduced from the earlier concept of a 19-month mission. JPL considered the possibility of the launch being delayed until 1984. Such a delay would cause an overlap with Galileo, complicate scheduling the Deep Space Network, and raise costs. A delayed launch also would provide an opportunity for the Soviet Union to obtain Venus SAR images before the United States, thereby making VOIR radar results less interesting, if not inconsequential.

The 1978 version of VOIR also exploited the availability of extant technology. In order to economize and facilitate selling and funding the project, VOIR would use components with proven performance records from other missions. For instance, from Mariner 10 VOIR borrowed its solar array and louvers, from Voyager its spacecraft electronics, from Pioneer Venus its radar altimeter, and from SEASAT its synthetic aperture radar.²⁰

JPL hoped to make VOIR an in-house project. NASA had other ideas. In 1979, NASA stipulated that JPL contract out both the radar and the spacecraft to industry. NASA also [185] specified that the radar would have a single individual, from NASA, shoulder the responsibility of making it work. The NASA Headquarters decision had an immediate impact on VOIR design. The JPL in-house effort, which came to an end in February 1980, had concentrated on using SEASAT technology. Now an industrial design would serve. In order to economize, JPL had proposed using the Galileo circular 4.8-meter antenna for both communications and the SAR. The weight of the Galileo antenna was significantly less than that of a competing antenna design. Instead, JPL now had to undertake a study of the competing and differing antenna patterns proposed by the contractors Goodyear and Hughes.²¹

In planning the VOIR radar mapper, the Science Working Group took into account the resolution of the images sent back by Mariner 9. Those images had revealed for the first time the diversity of Martian geologic structures, including young volcanoes, liquid cut channels, and large canyons of possible tectonic origin and had led to a fundamentally new understanding of the nature of Mars. The VOIR radar mapper had to have comparable or better resolution than Mariner 9. Steve Saunders, project scientist, and Gerry Schubert, a geophysicist in the Department of Earth and Space Sciences, University of California at Los Angeles, originated the high-resolution design requirement.²²

By 1978, the Science Working Group had defined four objectives for the 1984 VOIR mission: 1) images at a resolution and image quality equivalent to the Mariner 9 Mars mission (100 percent of the surface at mapping resolution, 600 meters, and a few percent in a high resolution mode, 100 meters); 2) a global topographic map of the planet; 3) a global map of the gravity field; and 4) new investigations of the atmosphere and exosphere. With surface exploration taking pride of place over atmospheric experiments, VOIR would be an inverse of Pioneer Venus.

In October 1978, NASA dissolved the Science Working Group and issued an Announcement of Opportunity requesting proposals for VOIR science experiments in three categories: 1) surface and interior properties of the planet requiring use of the SAR and altimeter, 2) atmospheric and ionospheric and other geophysical experiments requiring flight instruments other than the SAR and altimeter, and 3) other geophysical, atmospheric, and general relativity experiments using existing spacecraft subsystems.²³

Schooling potential users of Venus radar images became an integral part of the project. Project managers understood the VOIR radar image interpretation community as consisting of 70 investigators plus 130 associates with experience interpreting photographs of the Moon, Mars, and Mercury. In order to inculcate potential users in the interpretation of radar images, JPL planned two radar image interpretation sessions, tentatively scheduled with Goodyear, to take place in 1978 and 1979. NASA and JPL also were to sponsor studies based on the analogy between Venus radar images and radar images from aircraft and Earth satellites.²⁴

After the release of the VOIR Announcement of Opportunity, experiment proposals began to arrive at NASA Headquarters. Gordon Pettengill submitted his proposal to use the synthetic aperture radar to image Venus in February 1979. Pettengill defined his radar experiment in such a way as to dovetail radar and geological science. The proposal focused on "those processes that have shaped the surface of Venus and that have led to the evolution of its distinctive atmosphere. A major intermediary in achieving this goal is the preparation of a global map of the surface morphology in sufficient detail to describe and locate the major geological types and processes exhibited by Venus."

[186] Pettengill's proposal emphasized the general lack of knowledge about the surface features of Venus. Ground-based observations of Venus, Mariners 2, 5, and 10, the Soviet Venera missions, and Pioneer Venus all provided much information about the planet, but the proposal argued, "This knowledge is heavily weighted toward the atmosphere of Venus and its interaction with the solar wind. Comparatively little is known about the solid surface or the interior of the planet."

Pettengill's proposed team of co-investigators followed closely the membership of the disbanded Science Working Group. Apart from Arecibo radar astronomer Don Campbell, most co-investigators came from MIT's Center for Space Research, Pettengill's home organization, JPL, and the U.S. Geological Survey. Representing the USGS were Pioneer Venus veteran Hal Masursky, Gerald Schaber, then assistant chief of the Branch of Astrogeologic Studies, and Laurence A. Soderblum, chief of the USGS Branch of Astrogeologic Studies. Again, radar and planetary geologists associated in a common endeavor.

Among the geologists who ultimately would be the most influential on the VOIR project was James W. Head, III, an associate professor in the Department of Geological Sciences at Brown University. Head had worked at NASA Headquarters for Bell Communications, a telephone company subsidiary that provided systems analysis and support, including geologic work and landing site selection, to NASA on the Apollo missions. His research interests included comparative planetary geology, and he had been active in the geologic interpretation of radar data from the Moon for some years. More importantly, as we shall see, he was a guest investigator on the Soviet Venera 15 and 16 missions.

Pettengill proposed to organize his co-investigators into Task Groups that would participate in and monitor the design and implementation of all aspects of the SAR instrument, its operation during flight, and the reduction of imaging and ancillary radar data, as well as the subsequent geological and geophysical interpretation of the data.²⁵

NASA received several other proposals, but they were not successful for one reason or another. H. MacDonald, a radar geologist at the University of Arkansas, proposed interpreting VOIR data in the form of a radar landform atlas of Venus. The project largely duplicated the mapping contemplated in the Pettengill proposal. Another unsuccessful proposal came from Charles A. Barth, at the Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, to act as Principal Investigator on the airglow photometer experiment.²⁶ The airglow photometer was to measure the horizontal and temporal characteristics of the nightside thermospheric circulation. That proposal failed for reasons external to VOIR, as we shall see.

The Stanford Center for Radar Astronomy also submitted a proposal; it succeeded. Proposing radio and radar experiments on NASA space missions was their normal mode of conducting scientific research. Len Tyler, Dick Simpson, and John F. Vesecky proposed to study radar backscatter from the surface of Venus, in order to infer the small-scale physical texture of the surface, and to relate that texture to the large-scale formations visible in the VOIR images. Rather than create a separate investigative group, the Stanford researchers proposed that they participate in the radar group with Pettengill.²⁷

Congress already had voted VOIR a new start in the NASA budget, when Ronald Reagan became president in January 1981. As a result of decisions reached in the early months of the new administration, the problems foreseen in 1978 - overlap with Galileo, complication of DSN scheduling, escalated costs, and an opportunity for the Soviet Union to obtain Venus SAR images before the United States - all came true. National politics now took its turn in shaping the VOIR mission. Early in the new Republican administration, as a political signal that the new president was serious about cutting the budget, or at least the civilian portion of the budget, the budget czar David Stockman pressured NASA to sacrifice a major project. NASA chose VOIR.²⁸

Failure to fund the project until fiscal 1984, when VOIR became a new NASA start, led to a postponement of the launch schedule to April 1988. This postponement provided the Soviet Union an opportunity to obtain the first SAR images of Venus. In this case, the Cold War rhetoric of the White House did not have its equivalent in the Space Race. The Space Race was dead. Starting in the early 1970s, as the United States withdrew from the war in Vietnam and the Apollo program's objective had been met several times over, a period of détente started. The U.S. and U.S.S.R. signed an accord in 1972 to allow the exchange of scholars between the two countries. A decade later, however, the United States let the accord lapse in protest over the Soviet imposition of martial law in Poland. Nonetheless, many U.S. and Soviet scientists sought to collaborate, not compete, and they did so with the tacit approval of their governments. Cold War rivalry and competition no longer held sway.²⁹

The justification for canceling VOIR was its high cost. The project, conflated into an exploration of the surface, interior, atmosphere, and ionosphere of Venus, carried a total price tag of $680 million. NASA and JPL sought ways to slash that price tag to $200 to $300 million.³⁰ In the opinion of Gordon Pettengill, the project "was climbing a cliff. The project people at NASA Headquarters were told that if they could cut the cost in half, they could have their project. In other words, they had to do it for $300 million instead of $600 million. So an ad hoc group of JPL and NASA Headquarters people was put together to study ways of cutting costs."³¹

NASA renamed the low-cost reduced mission the Venus Radar Mapper (VRM).³² The trick was to lower the price tag, while still getting the science done. A number of approaches were suggested and taken, not all of which were technological, such as the reduction of personnel levels. Many of the cost-cutting decisions directly reduced the scientific scope of the mission. For example, one of the earliest decisions was to jettison all scientific experiments that did not use the radar. Only the altimetry and imaging experiments, which used the radar instrument, and the gravity experiment, which was carried out by the Deep Space Network, remained. Among the rejected experiments was the airglow photometer.³³ As Pettengill pointed out, however, "they saved $150 million by [188] getting rid of the four non-radar experiments that originally were intended for the mission."³⁴

Throughout various iterations of the project, the dimensions of the high and low resolution radar images vacillated. In fact, for a while, the high resolution detailed images of selected surface features disappeared entirely. In an early 1981 iteration, the VRM was to map at least 70 percent of Venus with a resolution of 600 meters and take high resolution (150-meter) data over about one percent of the planet. As described at a January 1982 briefing at NASA Headquarters, however, the VRM was to have no high resolution capability and would image only 70 percent of the planet with a resolution of better than one km. At a February 1982 conference held at JPL for the selected contractors, Hughes (SAR) and Martin Marietta (spacecraft), the SAR performance parameters called for coverage of 90 percent of the planet with a single resolution of 300 meters. By 1984, though, when the VRM became a NASA new start, the baseline performance had been raised to resolutions of 215 meters by 150 meters and 480 meters by 250 meters.³⁵

The resolution, and consequently the science that the VRM would achieve, was a trade-off against the cost of the project. Only by lowering overall costs did JPL and NASA manage to put together a mission capable of high resolution. One of the key cost-reduction approaches was to "maximize inheritance," a term that meant to borrow as much technology from other projects as possible. Magellan was to be pieced together from other NASA projects.

Among the projects from which the VRM borrowed, or considered borrowing, were Viking, Voyager, Galileo, and ISPM (International Solar Polar Mission). The VRM proposed borrowing such hardware items as the Voyager 3.7-meter dish antenna for its synthetic aperture radar, Galileo's tape recorder, and Viking's S-band low-gain antenna. Also, JPL suggested using NASA standard equipment as well as various SEASAT parts, such as sun sensor and solar array drive electronics and the solar array actuators.³⁶

In order to improve the VRM's data handling capabilities, JPL modified the radar guidelines in order to use the Galileo Golay code, rather than the Golay code planned by Hughes (contractor for the SAR). The Galileo Golay code and a restructuring of the radar burst header format (for more efficient handling by the Deep Space Network) resulted in a considerable saving in ground software costs.³⁷

Another key decision was the switch from a circular to an elliptical orbit. With an elliptical orbit, the parameters of the radar varied as a function of the spacecraft's altitude above the planet's surface. Mapping from an elliptical orbit eliminated the need for aerobraking. Aerobraking is a technique for trimming a spacecraft's orbit by having it pass repeatedly through a planetary atmosphere. Its use would reduce the amount of propulsion needed for initial orbit insertion. Aerobraking offered a low-cost, low-risk option that would both save fuel, and therefore mission weight, and lower mission costs.

Using digital processing to simplify the electronics was a significant saver of money. Original VOIR planning centered on analog processing for the radar, but by 1981 it had become clear that using digital circuitry was the preferred technology. The parameters of an analog system could not change during flight; so, aerobraking and a circular orbit were necessities. Digital processing allowed the radar parameters to change during flight, thereby tolerating the variations of a less expensive elliptical orbit.³⁸

The decisions to change the orbital geometry, use digital processing, and borrow technology from other projects lowered project costs to the point where VRM became a new NASA start in 1984. As a result of the postponed launch of VRM, Soviet scientists gained an important scientific opportunity to image Venus. When it appeared that the United States would launch VOIR on schedule, Soviet scientists decided to launch their own Venus imaging mission only if the United States did not send a Venus radar mapper before 1984. Once NASA delayed launch of the VRM beyond 1984, Soviet scientists had to move forward their own Venus radar mapper very quickly in order to seize the opportunity.

In June 1983, the Soviet Union flew two spacecraft equipped with radar mappers to Venus; they arrived four months later. Venera 15 and 16 covered the same polar region of Venus (30° North to the pole), probably on the assumption that one of the spacecraft might fail. Their goal was to map that region at a resolution of one to two km in daily, 150 by 7,000 km strips 10° to the side of the orbital track, covering a total area of 115 million square kilometers by the time the main mission ended in July 1984.³⁹

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[190] Interpreting the images from the Venera 15 and 16 mission required more information about Venus surface features than the Soviet Union had available. Previous Soviet missions had landed only in limited areas of the planet. Soviet scientists, desperately in need of information, turned to their American colleagues to exchange Venus data.

Since the Apollo era, several American scientists had made frequent trips to Moscow and to international meetings where they met Soviet planetary scientists. Of those American scientists, two of the most important ones for the Magellan mission were Jim Head of Brown University and Hal Masursky (USGS), a member of the Pioneer Venus science team. As Venera 15 and 16 data became available, its value to future American exploration of Venus, especially the VRM mission, was apparent, and a parallel American interest in collaboration developed.

On 25 March 1984, Alexander Basilevsky, a geologist and chief of the Vernadsky Institute Planetology Laboratory, and Valery L. Barsukov, director of the Vernadsky Institute (the Soviet equivalent of the USGS), presented Venera 15 and 16 results at the Lunar and Planetary Science Conference held in Houston. United States scientists appreciated that the Venera 15 and 16 SARs had yielded mosaickable images of a large part of the northern hemisphere.

COMPLEX, the Committee on Planetary and Lunar Exploration of the Space Science Board, requested that VRM scientists present an assessment of the Venera 15 and 16 accomplishments, as well as a summary of VRM capabilities and, if deemed desirable, ways of improving VRM. Gordon Pettengill presented what was known about the Soviet Venera mission, including SAR characteristics, range of resolution, and coverage, and he compared Venera results with Arecibo high resolution range-Doppler images.⁴⁰ Having reliable images of Venus was vital to the planning of the VRM mission. Although VRM scientists already had data with which to plan the mission, the Venera 15 and 16 data would have added important information on the northern hemisphere. Only two other sources of images of Cytherean surface features were available.

One source was Pioneer Venus. Its radar altimeter measured the height of about 90 percent of the surface at roughly 75 km intervals, while the mapper mode furnished low (20 to 40 km) resolution radar images of only the equatorial region. Pioneer Venus had not covered the northern polar region, unlike Venera 15 and 16. Higher resolution imaging was available from Arecibo, the second source of Venus surface images. Arecibo covered about 25 to 30 percent of the planet at resolutions around 2 to 4 km. However, Arecibo could image well only the hemisphere of Venus facing Earth at inferior conjunction.⁴¹

If they could be had on magnetic tape in a digital format, the Venera 15 and 16 data would have assisted VRM planning significantly. The data did become available, but not through any political maneuvering by the corresponding state departments or other high-level official channels. The exchange of scientific results between Soviet and U.S. scientists interested in the surface features of Venus came about as the result of an arrangement made among the scientists themselves and their parent institutions.

The 11 March 1985 session on Venus at the Lunar and Planetary Science Conference featured Soviet presentations of their recent interpretations of Venera 15 and 16 results by Alexander Basilevsky, Valery L. Barsukov, and two others from the Vernadsky Institute. Subsequently, on 19-20 March 1985, the first microsymposium took place at Brown University. The four Soviet scientists reviewed recent results of Venera 15 and 16, Arecibo, Pioneer Venus, as well as future Venus missions and Venus science in general. Among those attending were geologists James Head and Harold Masursky and radar astronomers Gordon Pettengill and Don Campbell of Arecibo.

[191] It was at the March 1985 microsymposium that James Head reported that the Soviets appeared to be receptive to the idea of providing some of their data. Preliminary results indicated that the Venera SAR radar parameters would not be a major obstacle to their use by American scientists. Moreover, both Soviet and American investigators had reached a preliminary agreement on the choice of a particular small feature for the definition of the Venus prime meridian. The features had appeared in both the Arecibo range-Doppler images and the Venera 15 and 16 SAR images. Establishment of a coordinate system was important to the planned VRM cartography efforts.

In November 1985, Vladimir Kotelnikov, the leader of Soviet ground-based radar astronomy research, then head of Interkosmos, delivered to Jim Head a tape with one strip of digital Venera image data with accompanying altimetry. Head distributed the tape to Saunders, Pettengill, Campbell, and Masursky for analysis. They had no difficulty in displaying the image using conventional American image processing techniques.

The Soviet-American agreement to exchange Venus data was underway. The agreement materialized as a protocol signed in 1982 between the Governor of Rhode Island (the location of Brown University) and the Soviet Academy of Sciences. Under the agreement, one microsymposium per year was to take place in each country. Traditionally, the American microsymposium has been held in March or April at Brown University; while the Soviet meeting takes place at the Vernadsky Institute in Moscow in August. James Head organized the Brown University group, and Valery Barsukov, director of the Vernadsky Institute, organized the Soviet group. The creation of the microsymposia owed much to the fact that Head was a guest investigator on Venera 15 and 16.⁴²

The Soviet data delivered over the following years at subsequent microsymposia played an important role in the creation of planning maps for the VRM/Magellan mission. The microsymposia were but one forum within which geology and radar communities worked together. The VRM Radar Investigation Group (in charge of the radar science) was another forum that brought the two communities together in a common effort. The Radar Investigation Group (RADIG) was a large and multifaceted organization typical of Big Science. In order to more effectively coordinate and carry out VRM and Magellan science, Pettengill divided the group into smaller subgroups (Table 6).

The VRM (and later Magellan) Radar Investigation Group combined the former Synthetic Aperture Radar Group and the Altimetry Investigation Group of the VOIR project. Gordon Pettengill headed the Radar Investigation Group (RADIG). Three RADIG subgroups dealt with mission design, while three other subgroups concerned themselves with scientific interpretation. These last three subgroups treated cartography and geodesy, surface electrical properties, and geology and geophysics. Geology and geophysics, the largest and most complex area of scientific interpretation, consisted of even smaller groups dealing with volcanic and tectonic processes; impact processes; erosional, depositional, and chemical processes; and isostatic and convective processes.⁴³

Not only did the RADIG bring together planetary radar and geology communities, but it illustrated how flight science groups organized Little Science to function as Big Science, if only on a temporary basis within ephemeral organizations. Ordinarily, in a way characteristic of Little Science, scientists work alone at a university or technical school with a small budget and modest laboratory equipment. NASA flight missions bring these individual scientists together and make them function in ways customarily associated with Big Science, mainly as part of a large group. Any given scientist works as a member of two groups, one defined by a flight instrument and the other by the scientist's discipline or...

...subdiscipline. Grouped together around a common instrument, scientists jointly design the instrument that will generate their data. Grouped together around a common scientific interest, such as magnetospheres or geology, scientists jointly utilize data derived from the operation of all flight instruments. However much these scientists function within a Big Science organization, the organization itself is defined by the temporary lifetime of the project. In the end, they are once more Little Science.

In December 1985, NASA Headquarters notified JPL that the VRM had a new name, Magellan. The name reflected NASA's general plan of naming major planetary missions after famous scientists and explorers (Galileo, Magellan, Cassini).⁴⁴ Ferdinand Magellan had been a Portuguese navigator and explorer who led an expedition into the Pacific Ocean under the Spanish flag.

By the end of 1985, construction of the Magellan radar instrument was underway. After Hughes Aircraft Company and Goodyear Aerospace Corporation completed Phase B studies of the project in June 1980, JPL issued a Request for Proposals for the synthetic aperture radar system, including the antenna design, in April 1981. The selection of the SAR and spacecraft contractors were separate processes.⁴⁵

[193] Hughes had hoped to turn its experience with the Pioneer Venus orbiter mapper into an advantage, while Goodyear had been one of the first firms to commercialize aircraft SAR systems to study the Earth. In 1983, NASA and JPL signed contracts with Hughes and Martin Marietta for the SAR and spacecraft. Hughes signed the definitive radar contract on 24 January 1984, and the contract was executed 27 January 1984. Throughout 1985 and 1986, Hughes increased the number of employees working on the Magellan radar. The project had the second highest priority within the Hughes Space and Communication Group, behind a smaller classified project.⁴⁶ Its Pioneer Venus gambit had paid off for Hughes.

Magellan was on schedule and under budget when the Space Shuttle Challenger blew up on 28 January 1986. The tragedy caused a serious delay in the Magellan launch schedule. In fact, the disaster adversely affected all Shuttle flights. The Shuttle would not fly until the cause of the Challenger accident was determined and corrective solutions found to prevent future repetitions of the accident. Only then would a new Shuttle flight schedule be drawn up.

In February 1986, Magellan mission personnel began to appraise probable launch dates. Realizing the uncertainties of the Shuttle launch schedule, they investigated two launch windows that followed the approved launch period in April 1988. One was between 28 October and 16 November 1989, the other between 25 May and 13 June 1991. In each case, Magellan would spend eight months in orbit performing its prime mission, and the mission would end at superior conjunction, in November 1990 or in June 1992, depending on the launch window.⁴⁷

A delayed launch also raised the likelihood of conflicts with the Galileo launch. If Magellan held to its approved launch schedule in April 1988, and Galileo delayed 13 months, then coverage conflicts on the Deep Space Network eased considerably. Whatever launch window Magellan eventually had, conflict with the Galileo launch and scheduling of the Deep Space Network would have to be taken into consideration. Further complicating the launch schedule was the cancellation in June 1986 of the Shuttle/Centaur, which was to launch Magellan. After a study of alternate launch vehicles, in October 1986 NASA settled on a combination of the Shuttle and a launcher known as an Inertial Upper Stage (IUS) and assigned Magellan a position on the Shuttle manifest for April 1989.⁴⁸

The change required reduction of the spacecraft mass, as well as new structural loads analyses. In order to undertake the analyses, a second spacecraft structure was needed for static load tests. The only one available was on the Voyager spacecraft hanging in the Smithsonian Air and Space Museum in Washington. NASA made arrangements to borrow the Voyager bus from the museum and conducted the tests.⁴⁹

The Challenger accident also affected Magellan's use of Galileo technology. Because Magellan launched before Galileo, the extra Galileo components were not available. Ground support equipment to be borrowed from Galileo were unavailable. Now the "spare parts" Magellan was to borrow from Galileo had to be returned to Galileo and purchased new for Magellan.

The delay of Magellan also raised the cost of the project. The total dollar impact, including the cost of hardware, mission design, and mission operations, was estimated to be about $150 million. Gordon Pettengill summed up the situation: "That disaster need not have happened, but it did; it was just one of those things. Magellan would not have been as expensive, if we had launched when we were originally planned to launch."⁵⁰

[194] JPL received unofficial notification in May 1986 from NASA Headquarters that Magellan had slipped to the October-November 1989 launch window, but no official launch date had yet been established. Nonetheless, the Magellan project proceeded on the assumption of that launch window.⁵¹

Meanwhile, the collection and exchange of radar data for the assembling of maps to be used in planning the mission proceeded. The Brown University-Vernadsky Institute microsymposia continued to play a vital role in the exchange of scientific information between American and Soviet scientists. In April 1986, the third international microsymposium on Venus took place at Brown University. Valery Barsukov, Alexander Basilevsky, and four other Soviet scientists presented preliminary scientific results of the Venera 15 and 16 missions and a description of the radar system.

The Soviet scientists presented the Magellan project with three Venera data tapes consisting of unpublished SAR digital data. They stipulated that the data be used strictly for planning the Magellan project; it was not for scientific publication or distribution, until the Soviet scientists had published the information. The request was reasonable; it protected their priority of discovery. In exchange, the Soviet scientists received high resolution digital data from the Viking mission to assist them in planning their Phobos mission to Mars's moon.⁵²

The following year, Magellan investigators James Head, Steve Saunders, Hal Masursky, Gerald Schaber, and Don Campbell attended a microsymposium held 11 to 15 August 1986 at the Vernadsky Institute in Moscow. They and their Moscow colleagues exchanged views on the interpretation of Venus data from Venera 15 and 16 and Arecibo. The Soviet investigators presented the Magellan scientists with eight tapes of Venera 15 and 16 digital radar images and altimetry profiles for use by the Magellan project for planning purposes.⁵³

At the following microsymposium held at Brown University in March 1987, scientists debated the origin and evolution of volcanic structures and deposits, domes, parquet terrain, impact craters, ridge and linear mountain belts, and plate tectonics. Only slight consensus over the interpretation of features emerged, because the resolution of features in Pioneer Venus images (25 km) and Venera 15 and 16 images (1-3 km) was sufficiently coarse to give rise to ambiguities in interpretation. Magellan's higher global resolution (about 300 meters) promised to resolve many questions of geologic interpretation. Soviet scientists provided the Magellan project with additional Venera 15 and 16 digital tapes; in return they received more high-resolution Viking imaging data of Phobos and the surface of Mars.⁵⁴

The microsymposia demonstrated the fruitful cross-fertilization of planetary geology and radar. In order to facilitate the use of radar data by geologists, Magellan Project Manager John Gerpheide, Program Scientist Joseph Boyce, Principal Investigator Gordon Pettengill, Project Scientist Steve Saunders, and Science and Mission Design Manager Saterios Sam Dallas formulated preliminary plans in July 1986 for various radar workshops. The first, to be held in 1987, was to cover radar operation and processing, the second the interactions between radar waves and planetary surfaces, and the third interpretation of SAR images. The second and third workshops were held in 1988 and 1989, respectively. The sessions were open to Magellan scientists and to the Planetary Geology and Geophysics Program investigators. In addition, they planned one-day Venus science symposia to be held in conjunction with other project meetings for each year between 1987 and 1989.⁵⁵

[195] In 1987, 32 scientists and project personnel participated in the field trip to various sites in the Mojave Desert and Death Valley. The goal was to compare a variety of geologic features with SAR images of the areas. Steve Wall, Magellan Radar Experiment representative, organized the field trip, which Tom G. Farr of JPL's Geology and Planetology Section led. Gerald Schaber of the USGS contributed to the technical presentation by sharing his knowledge of Death Valley.⁵⁶

In May 1988, the USGS Flagstaff hosted another field trip, which was incorporated as part of the quarterly meeting of Magellan scientists and project staff. The major objective was to familiarize participants with specific radar geology targets in a semi-arid, relatively vegetation-free environment. The trip also entailed comparing geologic features with X-band and L-band SAR images. The field exercise was planned and led by Gerald Schaber, Richard Kozak, and George Billingsley, all three with the USGS Flagstaff.⁵⁷

These field trips helped to introduce geologists to the interpretation of radar data. Geologists learn from "hands-on" experience, but that kind of experience is impossible when dealing with the geology of Venus. Radar images, moreover, are not created by the reflection of light, but by the scattering and reflection of electromagnetic waves. They cannot be read like photographs, and radar maps cannot be read like ordinary geological maps.

In order to fill in that gap, data to create a series of S-band radar images of the lunar surface were collected at the Arecibo Observatory between 1982 and 1992. The images were made at various angles of incidence at a number of known lunar locations, such as the Apollo 15 and 17 landing sites, Mare Imbrium, and craters Copernicus and Tycho, in order to provide experience in interpreting surface geology in radar images. Don Campbell, assisted by Peter Ford of MIT and later by Cornell graduate student Nick Stacy, made the observations and images in collaboration with Jim Head of Brown University. While initial image resolutions ranged from 200 to 300 meters, Nick Stacy brought image resolution down to 25 meters beginning in 1990. Elaborate data processing techniques attempted to replicate the synthetic aperture radar techniques used from spacecraft and aircraft.⁵⁸

As Gordon Pettengill pointed out, the workshops were not the main path for geologists to learn about radar. "The people who attended those made up a small fraction of the overall community. That route is an exception to what I would call the more general experience. Generally, people become part of a team, and they work with radar people, like myself, who then, by a process I would call osmosis, pass along the mystique of what is going on, when you see these structures on a radar image, how to interpret them, and what to look out for, so you don't make errors."

This process of osmosis, Pettengill explained, "is the best way to go. A formal course is difficult. They call them workshops. They are useful. But you need both. You need the workshop as well as years of working with other people and growing used to what you are seeing."⁵⁹

That process of osmosis was most evident at the Arecibo Observatory, where Don Campbell and his graduate students Barbara Burns and Nick Stacy and Research Associate John K. Harmon, collaborated with Jim Head and other geologists at Brown University through an informal accord between the NAIC and Brown University beginning around 1980. The heart of the accord was a cooperative effort to analyze Arecibo Venus imagery. [196] As a result of the arrangement, a number of Brown students, such as Richard W. Vorder Brueggie and David A. Senske, became involved in the analysis of Arecibo radar range-Doppler imagery and wrote their theses from the data.

"The effort was not under any formal agreement between the NAIC and Brown University," Don Campbell explained. "We badly needed the backing of a planetary geology group. We were into geology at this point. We were down to a few kilometers of resolution, and they were extremely enthusiastic. Jim Head was very enthusiastic and had a lot of students. They were very intent on getting ready for the Magellan mission and spent a lot of effort on both the Pioneer Venus and Venera data sets."⁶⁰

The Arecibo-Brown arrangement thus fostered the geological interpretation of Venus radar images well before Magellan began its mapping mission. A major area of interest was in identifying and explaining tectonic activity on the planet. Some of the 1979 high-resolution Arecibo radar images suggested Earth-like tectonic features, such as folds and faults, while 1983 Arecibo radar images confirmed the presence of rifting in the southern Ishtar Terra and surrounding plains and general tectonic activity in Maxwell Montes.⁶¹ Later studies examined evidence for tectonic activity in Beta Regio, Guinevere Planitia, Sedna Planitia, and western Eistla Regio in the planet's equatorial region, as well as in the southern latitudes around Themis Regio, Lavinia Planitia, Alpha Regio, and Lada Terra.⁶²

Don Campbell also collaborated with Jim Head's group in searching for evidence of volcanism. Arecibo radar images of southern Ishtar Terra and the surrounding plans revealed significant details of volcanic activity. Images made from data gathered at Arecibo during the summer of 1988 of the area extending from Beta Regio to the western Eistla Regio furnished strong evidence that the mountains in Beta and Eistla Regiones, as well as the plains in and adjacent to Guinevere Planitia, were of volcanic origin. Arecibo radar images of the southern latitudes showed additional evidence for past volcanic activity on Venus.⁶³

The study of cratering on Venus started by Barbara Burns for her doctoral thesis continued at Arecibo, too. She based her initial analysis on data collected in 1977 and 1979. As of 1985, Burns was able to identify only two features that exhibited unambiguous radar characteristics that could tentatively distinguish them as either volcanic (Colette) or impact (Meitner) in origin. Don Campbell, with Jim Head and John Harmon, continued [197] Burns's crater studies. Images made from the data collected during the inferior conjunction of 1988 of the area from Beta Regio to western Eistla Regio revealed a low density of impact craters greater than 15 km in diameter in that region compared to the average density for the higher northern latitudes. These crater densities suggested that the plains were geologically younger than the northern regions.⁶⁴

Campbell, with graduate student Nick Stacy and computer software manager and part-time radar astronomer Alice Hine, made a further analysis of cratering by looking at diameter-frequency distributions in the low northern latitudes and the southern hemisphere. The Arecibo investigators found that the average crater density for all craters in the northernmost quarter, using Venera 15 and 16 data, was 1.27 per million square kilometers, while the average for the southern hemisphere (as imaged by the Arecibo radar) was 0.95 per million square kilometers. The different crater densities suggested that the southern latitudes were geologically younger than the low northern latitudes imaged by Venera 15 and 16.⁶⁵

Don Campbell also participated in the microsymposia organized by Brown University and the Vernadsky Institute. As a result, he also came to collaborate with Alexander Basilevsky and other Soviet geologists on the interpretation of Venera 15 and 16 results, and that collaboration led to co-authorship of a paper with combined Vernadsky Institute and Brown University authors.⁶⁶

Don Campbell's osmotic infiltration of the scientific community interested in Venus typified the shifting paradigm of ground-based planetary radar astronomy toward geology. Further facilitating that shift was the availability of techniques, hardware, and software at Arecibo that yielded high-resolution range-Doppler images and topographical data. Image resolution improved to one to three km in 1983 and to 1.5 km in 1988, the last observations made before the arrival of Magellan at Venus.

Because Magellan used a frequency close to that of the Arecibo radar, there was some concern that the Arecibo radar might contaminate the Magellan data or endanger the spacecraft, so Don Campbell did not pursue Venus mapping after 1988.⁶⁷ Nonetheless, the participation of Arecibo ground-based investigators in Venus radar geology illustrated that the marriage of radar and geology was not limited to Magellan and space-based radars.

Throughout 1987 and into 1988, assembly of the Magellan spacecraft and final testing of the radar proceeded. Hardware, testing, and integration costs, coupled with an overall tight NASA budget, necessitated cutbacks and deferrals from Magellan's fiscal 1988 budget to later years. Some of the top staff transferred to other projects. Magellan Science Manager Neil Nickle, for instance, stepped down, and Thomas Thompson replaced him. Thompson had carried out lunar radar research at Arecibo and Haystack as early as the 1960s, and he was still making lunar observations with the Arecibo UHF radar as late as 1987. Also, he had been on the SEASAT radar team in the 1970s and more recently had made radar observations of Mars with the Goldstone Mars Station.⁶⁸

[200] In September 1988, a month ahead of schedule, the completed craft was shipped to Kennedy Space Center, where final assembly and testing took place. The Magellan launch date was moved up on the Shuttle manifest from October-November 1989 to April-May 1989 to accommodate the launch of Galileo, which needed to go to Venus for a gravity boost. The next launch window, June 1991, would have brought Magellan to Venus nearly a year later than the April-May 1989 opportunity. Launching six months earlier also meant that Magellan would have to circle the Sun one and a half times, rather than the usual one-half circuit, before encountering Venus. Although this trajectory took Magellan almost a year longer to reach Venus than the October-November 1989 opportunity, it still saved a year over the June 1991 trajectory. On 4 May 1989, after trouble with software, a hydrogen pump, and the weather, the Shuttle Atlantis carried Magellan aloft from Kennedy Space Center. Magellan became the first planetary mission launched by the Space Shuttle. More problems, including several losses of signal, plagued Magellan's mission.⁶⁹

Magellan entered orbit around Venus on 10 August 1990, 15 months after launch. On 15 August, the radar sensor was turned on and powered up in preparation for the first in-orbit radar test. The next day, during the radar test, the spacecraft lost its "heartbeat" and protected itself by invoking on-board fault-protection routines. Ground control noted this immediately by the terrifying loss of signal. Communications were re-established, then lost a few days later. After a shaky start, the radar began mapping on 15 September 1990.

Mission personnel arranged the first images into mosaics. The mosaics covered about 500 km segments of 30 or more individual image strips. One of the first mosaics was centered at 27° South latitude and 339° longitude in the Lavinia region of Venus. It showed three large impact craters, with diameters ranging from 37 to 50 km. The craters showed many features typical of meteorite impact, including rough, radar-bright ejecta, terraced inner walls, and central peaks. Numerous domes of probable volcanic origin were visible in the southeastern corner of the mosaic. The domes ranged in diameter from 1 to 12 km; some had central pits typical of volcanic shields or cones.⁷⁰

During its 243-day prime mission, Magellan amassed more imaging data than all previous U.S. planetary missions combined.⁷¹ Magellan mapped over 90 percent of the planet's surface, covering regions from 68° South latitude to the North pole. The images were to have a resolution of about 120 meters near the equator, degrading slightly to about 190 meters near the poles because of the elliptical nature of the orbit. Although budgetary cuts had threatened to lower the resolution of Magellan radar images, the application of advanced digital electronic circuitry had restored the mission's high resolution capability.

SAR data from each orbit was to be processed to make image strips about 350 pixels wide in the across-track dimension by 220,000 pixels in the along-track direction. Some 1,852 such SAR image strips were to be generated by JPL's Multimission SAR Processing Laboratory during the primary mission. These strips were to be sufficient in number and coverage to encircle the planet, with overlap of adjacent strips even in lower latitudes. Image element widths were 75 meters to properly preserve both the along and cross-track spatial resolutions.

Each strip is called a Full-Resolution Basic Image Data Record or F-BIDR. In total, the 1,852 F-BIDR SAR image strips formed a data set in excess of 100 billion bytes. The large volume and the unwieldy width-to-length ratios for the data made them unsuitable for general use. Thus, further processing was necessary to produce mosaicked images (Mosaicked Image Data Records or MIDRs) that could be more readily used in photo-[201] interpretative studies and in comparisons with the other Magellan data. Generating full-resolution mosaics for the 90 percent of the planet covered by F-BIDRs created an enormous data set, severely taxing available processing facilities. To streamline processing and to focus efforts toward production of sets of mosaics that could be used for a variety of studies, a decision was made to compile and distribute global mosaics from compressed F-BIDR data.⁷²

The USGS converted the data into a set of 62 maps in the standard 1:5,000,000 USGS planetary series. The maps showed SAR data at a resolution of about one km, and they were to contain altitude contours. In addition, a set of about 200 photomosaics were to show the entire mapped area of the planet at a resolution of 225 meters, and an additional set of about 250 photomosaics at the highest resolution, about 100 meters, were to be prepared for selected sections of the planets. Complementary data products were to include a topographic map at about 10-km surface resolution with a height accuracy of better than 50 meters, as well as special products displaying surface roughness, reflectivity, brightness temperature, and emissivity. Today, the radar data is also available in annotated digital form on CD-ROMs.⁷³

Key to creating these and other Venus images was an accurate knowledge of the planet's pole position and spin vector. An analysis by Irwin Shapiro and John Chandler of 1988 Arecibo radar data supplied by Don Campbell, Alice Hine, and Nick Stacy provided a new pole position, accurate to better than 3 km, and a more accurate measurement of the planet's rotational period.⁷⁴ Such participation in NASA space missions by radar astronomers as "mission support" already had been the norm for two decades.

Don Campbell and Gordon Pettengill also worked closely with Stanford scientists Len Tyler and Dick Simpson, who participated on the science team. Tyler chaired the Surface Electrical Properties (SEP) Team, composed of Tyler, Campbell, and Gerald Schaber (USGS). Tyler, Simpson, and John Vesecky used the altimeter function of Magellan's radar to look at dielectric constants and roughness, to study the top meter of Venus's surface, and to relate its structure to its interaction with radar waves. They transferred their data to a CD, with the intention of sending copies to scientists with whom they...

...collaborated, such as Don Campbell, Peter Ford, and Gordon Pettengill, as well as interested geologists.⁷⁵

Typical of Big Science projects, Magellan thus became a meeting ground for different scientific disciplines and subdisciplines. Its broad tent covered traditional ground-based radar astronomy and Stanford bistatic radar astronomy, as well as planetary geology. Magellan accelerated cross-fertilization between planetary geology and radar that [203] made radar results (mainly range-Doppler images and topography) more accessible to a larger community of investigators. As Don Campbell reflected: "We are suddenly much more respectable than we used to be! I don't want to characterize what people thought of us, but to some degree I suspect that we were regarded as a little bit of the fringe. Radar astronomy was regarded as a messy and expensive occupation. We came up with good stuff, but how we did it was not all clear!"⁷⁶

As radar astronomers grew closer to planetary geology, they sought out their new audience in new scientific settings. Radar astronomers still discussed their findings at meetings of the IAU, the AAS Division for Planetary Science, and URSI, but also at American Geophysical Union (AGU) meetings. General science and astronomy journals, such as Science and The Astronomical Journal, and even more so the specialized planetary science journals, such as Icarus and Earth, Moon, and Planets, remained forums for publication. In addition, because they had added the planetary geology community to their audience, radar astronomers now published in the Journal of Geophysical Research and Geophysical Research Letters.

The new audience also shaped radar astronomy funding, although less so at the Arecibo Observatory, where the NSF-NASA agreement assured an annual budget for radar astronomy research. Researchers elsewhere seeking NASA money for planetary surface studies faced the demands of the NASA planetary geology program. When Dick Simpson or Len Tyler, for instance, applied for geology program funds to study planetary surfaces, geologists reviewed their proposals. One of the frequent comments by those reviewers was that the proposal should include a geologist on the science team. As a result, Dick Simpson approached USGS Menlo Park geologist Henry Moore to collaborate with him.⁷⁷ Through their role as proposal reviewers, then, planetary geologists began to shape radar astronomy research proposals.

Throughout the 1970s, as planning for Magellan and the flight of Pioneer Venus took place, the field of radar astronomy, measured in terms of active practitioners and telescopes, grew smaller. In 1980, the Arecibo Observatory was essentially the sole active telescope; it supported four active investigators. In contrast to this Little Science reality stood the Big Science of Magellan. Around a single radar instrument, the big-budget, multi-year mission organized individual scientists into groups that crossed turf boundaries (radar astronomy versus Stanford "space exploration") and that fostered common interests among fields (planetary radar and geology scientists).

Although the exploration of planetary surfaces with space-based radars seemed to invigorate radar astronomy, the space-based approach has its limits in an era of budgetary limits. Cassini probably will be the last mission to carry a radar experiment into space. As currently conceived, Cassini will explore Saturn's cloud-covered moon, Titan, with a SAR. No other solar system bodies have impenetrable atmospheres that lend themselves to radar investigation. The problem of transmitting data back to Earth at distances beyond the orbit of Saturn is a major, though not insurmountable obstacle (as Voyager has shown). The use of laser rather than radar altimeters on future missions means that modifying the altimeter to carry out imaging, as was done on Pioneer Venus, has reached its technological limit (although military research may well yield a laser altimeter capable of imaging).

However, the most formidable barrier to any future mission is the shrinking space and national budgets. The Voyager, Galileo, and Magellan spacecraft were expensive, costing $2-3 billion, huge, standing seven meters high, as tall as most homes, and heavy, weighing several tons. Galileo, for example, weighed three tons. In order to accommodate a future of smaller budgets, NASA has initiated the Discovery program, in which low-cost ($150 million limit) small, lightweight spacecraft with limited scientific objectives carry [204] out solar system exploration. One problem with this approach is that missions to Jupiter and Saturn or beyond simply cost too much to fit the budgetary limits set for Discovery missions.⁷⁸ Such is the price of practicing science on a large scale.

Magellan also effectively ended ground-based radar observations of Venus. Although a few experiments were still possible, for example, the detection of rain on Venus with an X-band radar or polarization studies of surface scattering properties,⁷⁹ they likely will not achieve prominence. Indeed, Don Campbell, who has spent his scientific career doing radar studies of Venus, volunteered to Nick Renzetti of JPL at the Lunar and Planetary Conference at Houston in 1985 that he was not likely to do any more Venus observations; instead, he planned to concentrate on asteroid and comet experiments.⁸⁰

Campbell typified the new direction that planetary radar astronomy began taking after 1975, when the Arecibo and Goldstone upgraded radars became available. Technology still drove planetary radar astronomy. New and better instruments and innovative techniques allowed radar astronomers to solve problems previously unsolvable and to detect and study solar system objects never before explorable with radar. The exploration of those objects in turn presented unusual radar characteristics that led radar astronomers to solve new scientific problems. The dynamic resonance between radar techniques (epistemological issues) and problem solving (scientific questions) thus remained at the heart of planetary radar astronomy. Nonetheless, despite a short spurt of growth following the inauguration of the upgraded Arecibo and Goldstone radars, by 1980 the planetary radar literature had reached a plateau of activity; the field had reached the limits to its growth.

1. Herman, telephone conversation, 20 May 1994.

2. Memorandum, Louis D. Friedman to J. C. Beckman, "VOIR, Archeology, 10/79," Box 14 [hereafter Friedman-Beckman Memorandum]; VOIR Historical Perspective, "VOIR, VOIR Mission, Briefing to NASA Code S, 5/78," Box 8; "VOIR, A Study of an Orbital Radar Mapping Mission to Venus, Vol. 1, 9/73," Box 10; "VOIR, Report, A Study of an Orbital Radar Mapping Mission to Venus, Vol. 2, 9/73," Box 14; "VOIR, Report, A Study of an Orbital Radar Mapping Mission to Venus, Vol. 3, 9/73," Box 14; and "VOIR, (NASA) Correspondence VOIR Mission Study Books, 10/77," Box 10, JPLMM.

3. Friedman-Beckman Memorandum; VOIR Historical Perspective; "VOIR, Report, A Study of an Orbital Radar Mapping Mission to Venus, Vol. 2, 9/73," and "VOIR, Report, A Study of an Orbital Radar Mapping Mission to Venus, Vol. 3, 9/73," Box 14, JPLMM; V-Gram no. 9 (October 1986): 3; Campbell 8/12/93.

4. Friedman-Beckman Memorandum; VOIR Historical Perspective; "VOIR, (NASA) Correspondence VOIR Mission Study Books, 10/77," Box 10, JPLMM; Robert C. Beal, Venus Orbiter Imaging Radar FY77 Study Report Radar Studies, Report 660-60 (Pasadena: JPL, 2 May 1977), pp. 5-1 through 5-18; V-Gram no. 9 (October 1986): 2. On VOIR's SEASAT legacy, see also Murray, pp. 127-129.

5. Pettengill 29/9/93.

6. "VOIR, (NASA) Correspondence, VOIR Mission Study Books, 11/78," Box 13, JPLMM.

7. Herman, telephone conversation, 20 May 1994.

8. Herman, telephone conversation, 20 May 1994; Rumsey, Morris, R. Green, and Goldstein, "A Radar Brightness and Altitude Image of a Portion of Venus," Icarus 23 (1974): 1-7; Goldstein, R. Green, and Rumsey, "Venus Radar Images," Journal of Geophysical Research vol. 81, no. 26 (10 September 1976): 4807-4817; Goldstein, R. Green, and Rumsey, "Venus Radar Brightness and Altitude Images," Icarus 36 (1978): 334-352; Jurgens, Goldstein, Rumsey, and R. Green, "Images of Venus by Three-Station Radar Interferometry-1977 Results," Journal of Geophysical research vol. 85, no. A13 (30 December 1980): 8282-8294.

9. Saunders and Michael C. Malin, "Surface of Venus: Evidence of Diverse Landforms from Radar Observations," Science 196 (1977): 987-990; ibid., "Geologic Interpretation of New Observations of the Surface of Venus," Geophysical Research Letters 4 (1977): 547-550.

10. Campbell, Jurgens, Dyce, Harris, and Pettengill, "Radar Interferometric Observations of Venus at 70-Centimeter Wavelength," Science 170 (1970): 1090-1092; NAIC QR Q2/1972, pp. 3-4, and Q3/1972, pp. 3-4.

11. Campbell, Dyce, and Pettengill, "New Radar Image of Venus," Science 193 (1976): 1123-1124.

12. Campbell and Barbara Ann Burns, "Earth-based Radar Imagery of Venus," Journal of Geophysical Research vol. 85, no. A13 (30 December 1980): 8271-8281; Burns, "Cratering Analysis of the Surface of Venus as Mapped by 12.6-cm Radar," Ph.D. diss., Cornell University, January 1982.

13. Pettengill, Campbell, and Masursky, "The Surface of Venus," Scientific American 243 (August 1980): 54-65.

14. Thompson 29/11/94; Friedman-Beckman Memorandum.

15. Venus Orbiting Imaging Radar Study Team Report (Preliminary Draft (Pasadena: JPL, 31 August 1972), pp. 22-28, and Friedman and J. R. Rose, Final Report of a Venus Orbital Imaging Radar (VOIR) Study 760-89 (Pasadena: JPL, 30 November 1973), Pettengill materials.

16. For an explanation of the relationship between Venus's spin and rotational rates, see Goldreich and Peale, "The Dynamics of Planetary Rotations," Annual Review of Astronomy and Astrophysics 6 (1968): 287-320.

17. Pettengill 28/9/93.

18. Pettengill 3/10/93.

19. "VOIR, Venus Orbital Imaging Cost Review, 6/78," Box 5; "VOIR, Venus Orbiting Imaging Radar Review, 4/80," Box 10; and "VOIR, VOIR 88, Viewgraph Presentation to NASA Administrator, 11/87," Box 10, JPLMM.

20. "VOIR, Status Briefing to Committee on Planetary and Lunar Exploration, NASA Headquarters, 6/78," and "VOIR, VOIR 84, Delayed Launch Option, 6/78," Box 3, JPLMM.

21. Pettengill 28/9/93; "VOIR, Venus Orbiting Imaging Radar Review, 4/80," and "VOIR, Venus Orbiting Imaging Radar Review, 4/80," Box 10, JPLMM.

22. A. Gustaferro to W. B. Hanson, 8 May 1979, "Magellan Documentation," NHO; Friedman-Beckman Memorandum; VOIR Historical Perspective.

23. "VOIR, (NASA) Correspondence, VOIR Mission Study Books, 11/78," Box 13; "VOIR, Status Briefing to Committee on Planetary and Lunar Exploration, NASA Headquarters, 6/78," Box 3; and NASA, Announcement of Opportunity no. OSS-5-78, 12 October 1978, Box 13, JPLMM.

24. "VOIR, Venus Orbital Imaging Cost Review, 6/78," Box 3, JPLMM.

25. "VOIR, Scientific Investigation and Technical Plan, Proposal to NASA, 2/79," Box 13, JPLMM; V-Gram no. 11 (April 1987): 16; V-Gram no. 13 (October 1987): 14; and V-Gram no. 11 (April 1987): 11.

26. "VOIR, A Proposal to NASA, Submitted by University of Arkansas, 7/79;" "VOIR, Contract Request for Proposal (APE) Airglow Photometer Experiment, 5/81, 12/81;" and "VOIR, Proposal to NASA, for Airglow Photometer Experiment for the VOIR Mission," Box 13, JPLMM.

27. "VOIR, Proposal to the NASA Management Section, 2/79," Box 13, JPLMM.

28. Pettengill 28/9/93; Saunders, Pettengill, Arvidson, William L. Sjogren, William T. K. Johnson, and L. Pieri, "The Magellan Venus Radar Mapping Mission," Journal of Geophysical Research vol. 95, no. B6 (1990): 8339; Waff, Jovian Odyssey: A History of NASA's Project Galileo, chapter "Surviving the Reagan Revolution," pp. 8-10, Waff materials.

29. Henry S. F. Cooper, Jr., "A Reporter at Large: Explorers," The New Yorker 64 (7 March 1988): 50.

30. "VOIR, Venus Mapper New Start Plans, 3/82," and "VOIR, Venus Radar Mapper, A Proposed Planetary Program for 1988," Box 10, JPLMM.

31. Pettengill 28/9/93.

32. For a brief period in 1981 and 1982, project documents used the name Venus Mapping Mission (VMM).

33. "VOIR, Project Management, Venus Orbiting Imaging Radar, 1981-82," Box 14; "VOIR, Venus Mapper Briefing to NASA Headquarters, 1/82," Box 10; and "VOIR, Request for Proposal for VOIR Synthetic Aperture Radar, 7/81, 3/3," Box 13, JPLMM.

34. Pettengill 28/9/93.

35. "VOIR, Request for Proposal for VOIR Synthetic Aperture Radar, 7/81, 3/3," Box 13; "VOIR, Venus Mapper Briefing to NASA Headquarters, 1/82," Box 10; "VOIR, Venus Mapper Conference w/Hughes and MMC, 2/82," Box 10; and "VOIR, Project Management Report, 1984, 1/2," Box 14, JPLMM.

36. "VOIR, Venus Mapper Briefing to NASA Headquarters, 1/82," Box 10; "VOIR, Venus Mapper New Start Plans, 3/82," Box 10; and "VOIR, Venus Radar Mapper, A Proposed Planetary Program for 1988," Box 10, JPLMM.

37. "VOIR, Project Management Report, 1984, 1/2," Box 14, JPLMM.

38. Pettengill 28/9/93.

39. Andrew Wilson, Solar System Log (London: Jane's, 1987), pp. 112-113.

40. "VOIR, Project Management Report, 1984, 1/2," Box 14, JPLMM.

41. Campbell 8/12/93.

42. Cooper, "A Reporter," p. 50; Ford 3/10/94; Campbell 8/12/93; "VOIR, Report Project Management, 1985," Box 14, JPLMM. The August 1991 microsymposium was delayed until November because of the putsch.

43. V-Gram no. 8 (24 March 1986): 2-4.

44. "VOIR, Report Project Management, 1985," Box 14, JPLMM.

45. "VOIR, Venus Orbiting Imaging Radar Review, 4/80," Box 10, and "VOIR, Request for Proposal for VOIR Synthetic Aperture Radar, 7/81, 1/3," Box 13, JPLMM.

46. Various documents, Box 6, and "VOIR, Report Project Management, 1986, 1/2," Box 14, JPLMM.

47. "VOIR, Report Project Management, 1986, 1/2," Box 14, JPLMM.

48. V-Gram no. 10 (January 1987): 1 & 4.

49. V-Gram no. 10 (January 1987): 1.

50. Pettengill 28/9/93; V-Gram no. 10 (January 1987): 1.

51. "VOIR, Report Project Management, 1986, 1/2" and "VOIR, Report Project Management, 1986, 2/2," Box 14, JPLMM.

52. "VOIR, Report Project Management, 1986, 1/2," Box 14, JPLMM.

53. "VOIR, Report Project Management, 1986, 2/2," Box 14, JPLMM.

54. V-Gram no. 12 (July 1987): 1.

55. "VOIR, Report Project Management, 1986, 2/2," Box 14, JPLMM.

56. V-Gram no. 11 (April 1987): 1.

57. V-Gram no. 15 (January 1989): 15.

58. Ford 3/10/94; Campbell 8/12/93; Nicholas John Sholto Stacy, "High-Resolution Synthetic Aperture Radar Observations of the Moon," Ph.D. diss, Cornell University, May 1993; NAIC QR Q1/1982, Q4/1986, Q2/1990, Q4/1990, and Q3/1992.

59. Pettengill 29/9/93.

60. Campbell 8/12/93.

61. Campbell, Head, John K. Harmon, and Alice A. Hine, "Venus: Identification of Banded Terrain in the Mountains of Ishtar Terra," Science 221 (1983): 644-647; L. S. Crumpler, Head, and Campbell, "Orogenic Belts on Venus," Geology 14 (1986): 1031-1034; Stofan, Head, and Campbell, "Geology of the Southern Ishtar Terra/Guinevere and Sedna Planitae Region on Venus," Earth, Moon, and Planets 38 (1987): 183-207; R. W. Vorder Brueggie, Head, and Campbell, "Orogeny and Large-Scale Strike-Slip Faulting on Venus: Tectonic Evolution of Maxwell Montes," Journal of Geophysical Research vol. 95, no. B6 (1990): 8357-8381.

62. David A. Senske, Campbell, Stofan, Paul C. Fisher, Head, Stacy, J. C. Aubele, Hine, and Harmon, "Geology and Tectonics of Beta Regio, Guinevere Planitia, Sedna Planitia, and Western Eistla Regio, Venus: Results from Arecibo Image Data," Earth, Moon, and Planets 55 (1991): 163-214; Bruce A. Campbell and Campbell, "Western Eistla Regio, Venus: Radar Properties of Volcanic Deposits," Geophysical Research Letters vol. 17, no. 9 (1990): 1353-1356; Senske, Campbell, Head, Fisher, Hine, A. de Charon, S. L. Frank, S. T. Keddie, K. M. Roberts, Stofan, Aubele, Crumpler, and Stacy, "Geology and Tectonics of the Themis Regio-Lavinia Planitia-Alpha Regio-Lada Terra Area, Venus: Results from Arecibo Image Data," Earth, Moon, and Planets 55 (1991): 97-161.

63. Stofan, Head, and Campbell, "Geology of the Southern Ishtar Terra/Guinevere and Sedna Planitae Region on Venus," Earth, Moon, and Planets 38 (1987): 183-207; Campbell, Head, Harmon, and Hine, "Venus: Volcanism and Rift Formation in Beta Regio," Science 226 (1984): 167-170; Campbell, Head, Hine, Harmon, Senske, and Fisher, "Styles of Volcanism on Venus: New Arecibo High Resolution Radar Data," Science 246 (1989): 373-377; Campbell, Senske, Head, Hine, and Fisher, "Venus Southern Hemisphere: Geologic Character and Age of Terrains in the Themis-Alpha-Lada Region," Science 251 (1991): 180-183; Senske, Campbell, Head, Fisher, Hine, de Charon, S. L. Frank, S. T. Keddie, K. M. Roberts, Stofan, Aubele, Crumpler, and Stacy, "Geology and Tectonics of the Themis Regio-Lavinia Planitia-Alpha Regio-Lada Terra Area, Venus: Results from Arecibo Image Data," Earth, Moon, and Planets 55 (1991): 97-161.

64. Burns and Campbell, "Radar Evidence for Cratering on Venus," Journal of Geophysical Research vol. 90, no. B4 (1985): 3037-3047; Campbell, Head, Hine, Harmon, Senske, and Fisher, "Styles of Volcanism on Venus: New Arecibo High Resolution Radar Data," Science 246 (1989): 373-377.

65. Campbell, Stacy, and Hine, "Venus: Crater Distributions at Low Northern Latitudes and in the Southern Hemisphere from New Arecibo Observations," Geophysical Research Letters vol. 17, no. 9 (1990): 1389-1392.

66. A. T. Basilevsky, B. A. Ivanov, G. A. Burba, L. M. Chernaya, V. P. Kryuchkov, O. V. Nikolaeva, Campbell, and L. B. Ronca, "Impact Craters on Venus: A Continuation of the Analysis of Data from the Venera 15 and 16 Spacecraft," Journal of Geophysical Research vol. 92, no. B12 (1987): 12,869-12,901; Stofan, Head, Campbell, Zisk, A. F. Bogomolov, Rzhiga, Basilevsky, and N. Armand, "Geology of a Rift Zone on Venus: Beta Regio and Devana Chasma," Geological Society of America Bulletin 101 (1989): 143-156.

67. Campbell 8/12/93; Burns, "Cratering Analysis of the Surface of Venus," p. 1; Stofan, Head, and Campbell, "Geology of the Southern Ishtar Terra/Guinevere and Sedna Planitae Region on Venus," Earth, Moon, and Planets 38 (1987): 183-207; Richard W. Vorder Brueggie, Head, and Campbell, "Orogeny and Large-Scale Strike-Slip Faulting on Venus: Tectonic Evolution of Maxwell Montes," Journal of Geophysical Research vol. 95, no. B6 (1990): 8357-8381.

68. Thompson 29/11/94; V-Gram no. 15 (January 1989): 16; V-Gram no. 14 (May 1988): 2; NAIC QR, Q2/1987.

69. V-Gram no. 15 (January 1989): 1; V-Gram no. 16 (August 1989): 1.

70. V-Gram no. 18 (October 1990): 1-2.

71. V-Gram no. 13 (October 1987): 1.

72. V-Gram no. 10 (January 1987): 9-10.

73. V-Gram no. 8 (24 March 1986): 2-3.

74. Magellan Final Science Reports, Report D-11092 (Pasadena: JPL, 22 October 1993), p. 25; Shapiro, Chandler, Campbell, Hine, and Stacy, "The Spin Vector of Venus," The Astronomical Journal 100 (1990): 1363-1368. See also the analysis done at Goldstone: Slade, Zohar, and Jurgens, "Venus: Improved Spin Vector from Goldstone Radar Observations," The Astronomical Journal 100 (1990): 1369-1374.

75. Simpson 10/5/94; Simpson and Tyler, "Venus Surface Properties from Magellan Radio and Radar Data," V-Gram 18 (October 1990): 12-18. For the results, see Tyler, Ford, Campbell, Charles Elachi, Pettengill, and Simpson, "Magellan: Electrical and Physical Properties of Venus' Surface," Science 252 (1991): 265-270; Tyler, Simpson, Michael J. Maurer, and Edgar Holmann, "Scattering Properties of the Venusian Surface: Preliminary Results from Magellan," Journal of Geophysical Research 97 (1992): 13,115-13,139. Pettengill and Ford also produced dielectric-constant and roughness maps to accompany the global topography and emissivity data they produced. The Stanford investigators used different, but complementary, algorithms that combined the altimetry and imaging SAR data to obtain estimates of surface roughness and dielectric constant. Both data sets were made available on CD-ROMs.

76. Campbell 9/12/93.

77. Simpson 10/5/94.

78. Richard A. Kerr, "Scaling Down Planetary Science," Science 264 (1994): 1244-1246.

79. Goldstein 14/9/93; Pettengill 4/5/94. Bill Smith tried to look for rain in Venus' atmosphere at the Haystack Observatory in the 1960s. Smith 29/9/93.

80. GSSR Min. 28/3/1985.

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