Blueprint for 1970s planetary exploration (1968)

In August 1967, Congress refused to support NASA's plans for the 1970s. Citing fiscal restraint, it rejected piloted Mars/Venus flyby missions in 1975 and 1977 and canceled the Voyager Mars/Venus program, NASA's only robotic program planned for the decade. The Apollo Applications Program, which had been tapped as the agency's main 1970s piloted program, suffered a cut of half a billion dollars.

This assault on NASA's future was partly the result of the deadly Apollo 1 fire (January 1967), which undermined confidence in the U.S. civilian space agency. A growing Federal budget deficit fueled by the escalating war in Indochina also played a role.

NASA's detractors argued that piloted flybys, Voyager, and AAP were stealthy steps toward an early commitment to costly piloted Mars landing missions. Others complained that NASA's program lacked "balance." This criticism meant different things coming from different people. For some, it meant that NASA gave to astronauts tasks that robots could perform more cheaply and with less risk; for others, it meant that NASA placed too much emphasis on the moon and Mars and not enough on the rest of the Solar System.

NASA officials met with Congressional leaders in late September 1967 to try to negotiate a replacement for Voyager. NASA Administrator James Webb and others reminded them that, with Voyager gone, the U.S. would have no robotic planetary program after the Mariner 1969 Mars flyby missions, leaving to the Soviet Union the prestige benefits of Solar System exploration. Congress relented partially, agreeing to initiate funding in Fiscal Year 1969 for a pair of Mariner 1971 Mars orbiters and a pair of Mariner-based Mars orbiter/lander missions in 1973.

This concession, combined with the successful first unmanned flight of the Apollo Saturn V rocket (Apollo 4) in early November 1967, encouraged some within NASA to look for ways of accommodating the detractors while continuing planning for piloted Mars missions. In late November-early December 1967, NASA's Office of Manned Space Flight asked J. Downs and W. Thompson of Bellcomm, NASA's Apollo planning contractor, to develop a plan for a feasible "balanced manned and unmanned planetary program through 1980." Their blueprint, completed in late February 1968, included Mariner-based robotic Mars and Venus spacecraft as precursors to piloted Mars and Venus flybys and robotic pure science missions to Mercury, Jupiter, Saturn, and beyond.

Downs and Thompson kicked off their program with a Mariner Venus flyby in 1970. The spacecraft, which would be built from "spare parts" left over from Mariner Mars 1969, might use Venus's gravity to speed it toward a flyby of the planet Mercury. The next year, NASA would launch the Mariner Mars orbiters it had discussed with Congress. The Bellcomm engineers called for them to be launched on Titan III-C rockets (bottom image below) so that they could each carry to Mars a 350-pound rough-landing probe bearing 13 pounds of instrumentation. The probes would begin the in-situ search for life on Mars.

Next up, in 1972, a Titan III-C would launch a Venus orbiter with an atmosphere probe. In keeping with NASA's agreement with Congress, two more Titan III-C rockets would launch one Mars orbiter with probe each in 1973. Downs and Thompson expected that the 1971 landing probe would have found life on Mars, so the instruments on the twin 1973 probes could focus on learning about that life. In addition to Mars, the orbiters would image Phobos and Deimos, the two small martian moons.

The year 1973 would also see a Mariner spacecraft fly past Venus and release a 600-pound probe designed to survive landing on the cloudy planet's harsh surface. With help from Venus's gravity, the Mariner would then fly past Mercury. Downs and Thompson noted that placing a spacecraft into orbit around Mercury would demand a great deal of energy (hence propellant), and advised that the decision about whether to fly a Mercury orbiter should be postponed until after the 1973 flyby. They also noted that the next Venus-Mercury flyby opportunity would not occur until 1982.

In 1974, NASA would expand its horizons to the stars by launching a 600-pound "Galactic Jupiter Probe" on an Atlas rocket with a Centaur upper stage. As envisioned by engineers at NASA's Goddard Space Flight Center in Maryland, the Galactic Jupiter probe would explore Jupiter and use a gravity assist from that giant planet to gain speed and bend its course. The spacecraft would climb above the plane of the ecliptic to explore interplanetary particles and fields and, ultimately, escape the Solar System entirely to wander derelict among the stars.

In the Downs-Thompson blueprint, 1975 was a busy year. A Mars orbiter more sophisticated than any launched before would dispatch a heavy probe to a site scientists had identified as exobiologically interesting based on Mariner Mars 1971 and 1973 data. A second Galactic Jupiter Probe would begin its journey to Jupiter and beyond, and NASA would launch two Venus orbiters, each bearing two rough-landing probes.

The year 1976 would see the first of four NASA missions to non-planetary Solar System bodies: an Atlas-Centaur would launch a Mariner past short-period Comet d'Arrest. In 1978, a Mariner would fly past the asteroid Icarus, and asteroid Eros would receive a Mariner in 1979. Finally, a Mariner launched on a Titan III-C/Centaur would fly past Comet Encke in 1980.

In 1977, NASA would launch a Venus orbiter with a high-resolution cloud-piercing radar and multiple atmosphere probes. The new-design Venus orbiter used in 1975 and 1977 would need a launch vehicle more powerful than the Titan III-C - possibly a reduced-capability Saturn V, Downs and Thompson wrote. The 1975 and 1977 Mars missions would also need this powerful rocket.

The 1977 Mars flight would serve as a dedicated precursor for the piloted Mars/Venus flyby mission scheduled for launch in 1978. Its landing probe would, for example, provide data on the topography of a landing site chosen for one of the piloted flyby spacecraft's large Mars Surface Sample Return (MSSR) probes.

The year 1977 would also see the first "Grand Tour" spacecraft leave Earth on a Titan III-C with a Centaur upper stage. The new-design 1000-pound spacecraft would fly past Jupiter and receive a gravity-assist "kick" to Saturn. The gravity-assist it would receive while exploring Saturn would speed it onward to mysterious Uranus, where a third gravity-assist would send it on to Neptune. The spacecraft would fly past the Solar System's most distant gas giant planet nine years after departing Earth. A second Grand Tour spacecraft would leave Earth in 1978.

Also in 1978, NASA would launch the first of two piloted Mars/Venus flyby missions. Downs and Thompson wrote that the two piloted flyby missions would serve as precursors for a piloted Mars landing mission in 1984. The 1978 mission would fly past Venus in 1979, where the crew would release weather balloons and surface impactors. Later in the year, it would fly past Mars, releasing a small swarm of MSSR probes. These would land, collect Mars samples, and return them to the astronauts on the flyby spacecraft for immediate analysis. In 1981, the astronauts would fly past Venus a second time and return to Earth. The second piloted Venus/Mars/Venus flyby mission would depart Earth in 1981 and return home in 1983.

Minimum-energy launch opportunities are what they are, so it is not too surprising that NASA carried out missions resembling those in the Downs-Thompson blueprint. The 1971 Mariner Mars orbiters, for example, corresponded to the Mariner 9 mission, though the latter included no landing probe. (Mariner 8, the first of the intended pair of 1971 Mars orbiters, crashed in the Atlantic after its Atlas-Centaur launch vehicle failed.) The 1973 Mariner-based Mars orbiters and landers were named Viking, then funding cuts pushed their launch to 1975. NASA missed the 1970 Venus-Mercury opportunity, but launched Mariner 10 in 1973 (middle image above). It flew past Venus in February 1974, then past Mercury in March 1974, September 1974, and March 1975.

NASA launched its first Galactic Jupiter Probe two years early; Pioneer 10 left Earth in March 1972 and flew past Jupiter in December 1973 (bottom image above). Its twin, Pioneer 11, left Earth in April 1973, flew past Jupiter in December 1974, and flew past Saturn in September 1979. NASA cancelled the Grand Tour in 1972, but launched the Mariner-based Voyager 1 and 2 spacecraft in September 1977 and August 1977, respectively. Voyager 1 flew past Jupiter in March 1979 and Saturn in November 1979 (top image above). Voyager 2 flew past Jupiter in July 1979, Saturn in August 1981, Uranus in January 1986, and Neptune in August 1989.

NASA launched no piloted flyby in 1978; in fact, when that launch opportunity came and went no American astronauts had reached space since July 1975 (and none would again until April 1981). Instead, it launched the first U.S. Venus orbiter, Pioneer Venus 1 (May 1978), and Pioneer Venus 2 (August 1978), which carried a cluster of four Venus atmosphere entry probes (top image below). Budget cuts and Space Shuttle problems meant that Pioneer Venus 2 was the last U.S. planetary probe to leave Earth for nearly 11 years.

A Feasible Planetary Exploration Program Through 1980 - Case 710, J. P. Downs and W. B. Thompson, Bellcomm, February 29, 1968.

Asteroid Belt fly-through/Jupiter flyby (1965)

On January 1, 1801, the first day of the 19th century, astronomer-monk Giuseppe Piazzi discovered a new world between the orbits of Mars and Jupiter. The object, which he named Ceres, was hailed as a new planet - the first discovered since William Herschel found Uranus in 1781. The following year, Heinrich Olbers discovered Pallas in the same region. Olbers told Herschel that he believed Ceres and Pallas to be fragments of a destroyed planet. Herschel suggested that they and any other bodies found between Mars and Jupiter should be considered members of a new category of Solar System bodies, not planets. He suggested that they be called asteroids.

Juno and Vesta were discovered in 1804 and 1807, respectively, but then the discoveries stopped. As a result, Ceres, Pallas, Juno, and Vesta retained a tenuous grip on their planet status. In 1845, however, Astraea became the fifth world found between Jupiter and Mars. It was followed close on by Hebe, Iris, and Flora in 1847, Metis in 1848, Hygeia in 1849, Victoria, Parthenope, and Egeria in 1850, Irene and Eunomia in 1851, and Psyche, Thetis, Melpomene, Fortuna, Massalia, Lutetia, Kalliope, and Thalia in 1852. By the mid-1850s, Herschel's designation for these bodies had won wide acceptance.

As the number of asteroids discovered climbed toward 100, the region in which they orbit became known as the Asteroid Belt. By the centenary of Piazzi's discovery, more than 400 asteroids had been charted. Most follow orbits that keep them always within the Belt, but some - for example, Eros, discovered in 1898 - cross the orbit of Mars and approach Earth. Others - for example, Achilles, found in 1904 - reside at Jupiter's trojan points, 60° ahead or behind the planet along its orbit about the Sun.

By February 1965, when Lockheed Missiles and Space Company submitted the results of a study of robotic Asteroid Belt and Jupiter missions, more than 2700 asteroids were known. Lockheed's study, conducted between July and December 1964 on contract to the Jet Propulsion Laboratory in Pasadena, California, aimed to determine the feasibility of three classes of asteroid missions and to use the asteroid missions as the basis for planning Jupiter flybys.

Lockheed proposed that all of its missions employ a "universal space bus" to which mission-specific components could be added. The company assumed that its spacecraft would rely for electricity on Radioisotope Thermoelectric Generators (RTGs) rather than the solar panels that powered Mariner Venus and Mars flyby spacecraft. RTGs were, it noted, less susceptible to meteoroid damage than the large panels that would be required to generate adequate power beyond Mars. It noted, however, that nuclear fuel for RTGs would be scarce and costly until the 1970s.

The first mission class on Lockheed's list took in Asteroid Belt "minimum flythrough" density missions. These would seek to determine the density of meteoroids in the Asteroid Belt so that engineers could design subsequent Belt-crossing spacecraft with adequate shielding. The first flythrough mission by a 346-pound spin-stabilized spacecraft with six pounds of science instruments might launch as early as 1967, Lockheed estimated. The company acknowledged, however, that no scientific instruments suitable for the mission yet existed; meteoroid detectors designed for use near Earth had low reliability, so were unlikely to function for long enough to reach the Asteroid Belt.

Missions in the second class would be outwardly similar to those in the first. Their primary scientific objective would, however, be to build on the results of the density missions to determine the composition of meteoroids in the Asteroid Belt.

An Atlas rocket with an Agena D upper stage could boost a minimum flythrough mission into a Sun-centered orbit with an aphelion (farthest point from the Sun) at twice Earth's average solar distance, Lockheed estimated, while an Atlas/Centaur would permit a spacecraft to plumb the Asteroid Belt out to 2.25 times Earth's distance. They would thus restrict exploration the the Asteroid Belt's inner edge. An Atlas/Agena D with a High-Energy Kick Stage (HEKS), on the other hand, could boost a flythrough spacecraft to the outermost edge of the Asteroid Belt.

Lockheed's third mission class would see spacecraft fly past Ceres and Vesta at a distance of 1000 kilometers. An Atlas/Agena D/HEKS rocket could launch a 1049-pound spacecraft past either asteroid in any launch opportunity between 1969 and 1980, the company found. With a mind toward ensuring reliability, Lockheed favored launch opportunities that enabled short-duration voyages to Ceres and Vesta. The shortest Ceres mission (360 days) could launch during a 30-day window in 1970, while an opportunity for a 240-day Vesta mission would occur in 1978. For comparison, the longest Ceres mission, launched in 1971, would need 690 days to reach its target, and the longest flight to Vesta (in 1969) would need 550 days.

The Jupiter flybys would build on the asteroid missions. A 1000-pound Jupiter flyby spacecraft could launch in 1971 or 1975 on an Atlas/Centaur/HEKS, but would need about 700 days to reach its target. Lockheed thus advocated the use of more powerful Saturn IB and Titan IIIC rockets. A Titan IIIC/HEKS could boost a 1289-pound flyby spacecraft to Jupiter in only 500 days, while a Saturn IB/Centaur/HEKS combination would permit an even shorter trip time with a more massive spacecraft.

Jupiter orbits the Sun at about 5.2 times the Earth-Sun distance, so a flyby mission to the Solar System's largest planet would have little option but to rely on RTGs, Lockheed found. For the Jupiter flyby, the universal space bus would have plugged into it a seven-foot-diameter dish-shaped high-gain antenna for reliably transmitting data across the enormous distance separating Earth and Jupiter. The "flex-rib" antenna would open like an umbrella near Earth after the spacecraft separated from its booster rocket.

The spacecraft would zip past Jupiter's cloud tops at a distance of about 70,000 kilometers bearing 150 pounds of science instruments. Lockheed noted that some scientists had questioned the need for a camera on the Jupiter flyby; they argued that the planet's "cloud covered surface" would likely be so exotic as to defy interpretation even if it could be glimpsed.

In July 1972, Pioneer 10 became the first spacecraft to enter the Asteroid Belt. During the six-month crossing, the 258-kilogram RTG-powered spacecraft detected far fewer dust particles and meteoroids than expected. It departed the Asteroid Belt undamaged in February 1973, and flew past Jupiter at a distance of 130,000 kilometers on December 5, 1973 (top image above). Having braved the imagined perils of the Asteroid Belt, the intrepid robot explorer suffered damage in Jupiter's radiation belts, scrubbing a planned television survey of Io, the planet's innermost large moon.

The Jupiter-bound Galileo spacecraft was the fifth spacecraft to enter the Asteroid Belt. On October 29, 1991, it became the first to fly past an asteroid (Gaspra, the 951st asteroid discovered, at a distance of 1604 kilometers). Galileo zipped past Ida, the 243rd asteroid discovered, at a distance of 2410 kilometers on August 28, 1993. Ida was found to have a kilometer-wide moon, which scientists named Dactyl. By then, controllers on Earth had largely abandoned their efforts to unfurl Galileo's umbrella-like high-gain antenna, several ribs of which had jammed during deployment on April 11, 1991.

The Dawn spacecraft left Earth on a Delta II 7925H rocket on September 27, 2007. The solar-electric propulsion spacecraft performed a gravity-assist flyby of Mars on February 17, 2009, and arrived in orbit around grooved, cratered Vesta on July 16, 2011 (images below), making it the first spacecraft to orbit a Main Belt asteroid. Dawn is scheduled to depart 530-kilometer-diameter Vesta in mid-2012 bound for Ceres. If all goes as planned, it will orbit spherical, 950-kilometer-diameter Ceres - the only dwarf planet inside the orbit of Neptune - in February 2015.

Asteroid Belt and Jupiter Flyby Mission Study: Final report, Lockheed Missiles & Space Company, February 28, 1965.