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.

Giotto II (1985)

On the overcast morning of July 2, 1985, the eleventh Ariane 1 rocket launch took place at the Centre Spatial Guyanais in Kourou, French Guiana, an outpost of the European Community located a few degrees north of the equator on the northeast coast of South America (top image above). The last Ariane 1 to fly, it bore aloft Giotto, the first European Space Agency (ESA) interplanetary spacecraft (middle image above). Giotto's destination was Comet Halley.

A "dirty snowball" containing materials left over from the birth of the Solar System 4.6 billion years ago, Halley needs about 76 years to revolve around the Sun once. Its elongated orbit takes it as near to the Sun as the space between the orbits of Venus and Mercury and as far from the Sun as the cold emptiness beyond the orbit of Uranus.

Comet Halley has passed through the inner Solar System 42 times since its first recorded apparition. In 837, it passed just 5.1 million kilometers from Earth; during that apparition, its dust tail must have spanned nearly half the sky, and its bright coma - the roughly spherical dust and gas cloud surrounding its icy nucleus - may have been as large as the full moon. During its apparition of 1301, Italian artist Giotto di Bondone painted Comet Halley. The Giotto spacecraft was named for him.

The Ariane 1's third stage injected 980-kilogram Giotto into a 198.5-by-36,000-kilometer orbit about the Earth. Thirty-two hours after launch, as it completed its third orbit, flight controllers in Darmstadt in the Federal Republic of Germany commanded Giotto to ignite its French-built Mage solid-propellant rocket motor. The aft-pointing motor burned 374 kilograms of propellant in 55 seconds to inject the spinning 2.85-meter-tall, 1.85-meter-diameter spacecraft into orbit about the Sun.

Two months before Giotto's launch, Americans P. Tsou (Jet Propulsion Laboratory), D. Brownlee (University of Washington), and A. Albee (Caltech) proposed in a paper in the Journal of the British Interplanetary Society that a second Giotto mission be dispatched to fly close by one of 13 candidate comets between 1988 and 1994. The spacecraft, which they dubbed Giotto II, might launch on an Ariane 3 or in the payload bay of a Space Shuttle. Giotto II's "free-return" trajectory would take it as close as 80 kilometers from the target comet's nucleus, then would return it to Earth. Near the comet, Giotto II would expose sample collectors to the dusty cometary environment. Near Earth, it would eject a sample-return capsule based on the proven General Electric (GE) Satellite Recovery Vehicle (SRV) design. The capsule would enter Earth's atmosphere to deliver its precious cargo of comet dust to eager scientists.

Tsou, Brownlee, and Albee pointed out that the Mage solid-propellant motor was not required to boost Giotto into interplanetary space; that is, that the Ariane 1 could do the job itself. Giotto was, however, based on a British Aerospace-built Geos magnetospheric satellite design, which included the Mage motor. Re-testing the design without the motor would have cost time and money, so ESA elected to retain it for Giotto. After noting that the GE SRV could fit comfortably in the space reserved for the Mage, they proposed that the reentry capsule replace the motor in Giotto II.

Giotto included a "whipple bumper" on its aft end to protect it from hypervelocity dust impacts. During approach to Comet Halley, the spacecraft would turn the bumper in its direction of flight. The bumper comprised a one-millimeter-thick aluminum shield plate designed to break up, vaporize, and slow impactors, a 25-centimeter empty space, and a 12-millimeter-thick Kevlar sheet to halt the partially vaporized, partially fragmented impactors that penetrated the aluminum shield.

In the case of Comet Halley, dust would impact the bumper at up to 68 kilometers per second. Tsou, Brownlee, and Albee noted that the 13 candidate Giotto II comets were all less dusty and would have lower dust impact velocities than Halley. Because of this, Giotto II would need less shielding than Giotto.

Impacting dust would nonetheless create challenges for Giotto II. Tsou, Brownlee, and Albee devoted much attention in their paper to how the spacecraft might successfully capture dust for return to Earth. One capture system, a variant of the whipple bumper, would use a shield made from ultrapure material to vaporize and slow impacting dust particles. The vapor from the impactor and the impacted part of the bumper would then be captured as it condensed. Scientists would disregard the bumper material when they analyzed the condensate.

Tsou, Brownlee, and Albee then noted that thermal blankets returned from the Earth-orbiting Solar Maximum Mission satellite had shown that intact capture of high-velocity particles was possible. The multilayer Kapton/Mylar blankets collected hundreds of intact meteoroids and human-made orbital debris particles. They described preliminary experiments in which "underdense materials" (polymer foams and fiber felts) were subjected to high-velocity impacts by meteoroid and glass fragments fired from gas guns. The experiments suggested that such materials could indeed capture at least partially intact dust particles.

Giotto flew past the Comet Halley nucleus (top image below) at a distance of 596 kilometers on March 13-14, 1986. The comet's 15-by-eight-kilometer heart was extremely dark, with powerful jets of dust and gas blasting into space.

The intrepid probe suffered damage from dust impacts - for example, one large particle sheered off more than half a kilogram of its structure - but most of its instruments continued to operate. ESA thus decided to steer it toward another comet. On July 2, 1990, five years to the day after its launch, Giotto flew past Earth at a distance of 16,300 kilometers, becoming the first interplanetary spacecraft to receive a gravity-assist boost from its homeworld. The gravity assist put it on course for Comet Grigg-Skjellurup, which it flew past at a distance of 200 kilometers on July 10, 1992. After determining that Giotto had less than seven kilograms of hydrazine propellant left on board, ESA turned it off on July 23, 1992. The inert spacecraft flew past Earth a second time at a distance of 219,000 kilometers on July 1, 1999.

By that time, a comet coma sample return mission was under way with two of the Giotto II proposers playing central roles. In late 1995, Stardust became the fourth mission selected for NASA's Discovery Program. Brownlee was Stardust Principal Investigator and Tsou, Stardust Deputy Principal Investigator, designed the mission's sample capture system. The 380-kilogram Stardust spacecraft (top image below) left Earth on a free-return trajectory on February 7, 1999 and flew past Comet Wild 2 (one of the 13 Giotto II candidates) at a distance of about 200 kilometers on January 2, 2004. Stardust captured dust particles in aerogel, a silica-based material of extremely low density that was not available when Tsou, Brownlee, and Albee proposed Giotto II.

Stardust returned to Earth on January 15, 2006. Its sample capsule streaked through the pre-dawn sky over the U.S. West Coast before parachuting to a landing on a salt pan in Utah (bottom image below). When opened on January 17, 2006 at NASA's Johnson Space Center, Stardust's 132 aerogel capture cells contained thousands of intact dust grains captured from Wild 2. Subsequent analysis indicated that some probably formed close to other stars before the Solar System was born.

"Comet Coma Sample Return via Giotto II," P. Tsou, D. Brownlee, and A. Albee, Journal of the British Interplanetary Society, Volume 38, May 1985, pp. 232-239.

Optical astronomy from a manned planetary flyby spacecraft (1968)

Hubble Space Telescope/Spitzer Space Telescope composite image of the Sombrero Galaxy in visible and infrared light.

Hubble Space Telescope image of the Whirlpool Galaxy, a face-on spiral similar to our own Milky Way.

One criticism often leveled against the manned Mars/Venus flyby mission concepts NASA studied in the 1960s was that the flyby astronauts would be inactive during most of their mission. They would, the argument ran, justify their presence on board only during the planetary flyby or flybys, which would account for only a small portion of the total mission duration. The 1975 Mars flyby mission the NASA Planetary Joint Action Group proposed in 1966, for example, would last for 667 days, but the flyby spacecraft would spend only about 20 days within two million kilometers of Mars (top link below).



This argument could only be persuasive, however, if one assumed that the sole purpose of a manned flyby mission was to accomplish its flyby or flybys. In fact, NASA planners expected that the flyby astronauts would perform a wide range of experiments and observations throughout their mission. A manned flyby spacecraft would amount to a multi-purpose space station in orbit about the Sun instead of the Earth. Manned Mars/Venus flybys would also serve as experience-building steps toward manned Mars landing missions, much as Project Gemini had been for the Apollo Program.



In August 1966, H. London, an engineer with Bellcomm, NASA's advanced planning contractor, described how the 1975 manned Mars flyby spacecraft could explore asteroids in the Main Belt between Mars and Jupiter (middle link below). Its crew would, he explained, have at their disposal a 40-inch reflecting telescope intended primarily for high-resolution Mars photography during the flyby. Following its Mars flyby, the spacecraft would reach aphelion (its farthest point from the Sun) within the Main Belt. This would bring it to within 20 million miles of at least two major Main Belt asteroids.

Piloted flyby spacecraft over Venus with one-meter telescope in action.

In a February 1968 follow-on to the 1966 memorandum, Bellcomm engineer W. Grobman estimated that a one-meter (39.6-inch) reflecting telescope on board a flyby spacecraft would be capable of photographing objects as faint as 27.4 magnitude. For comparison, the 200-inch reflector on southern California's Mt. Palomar could photograph no object fainter than about 23.5 magnitude.



Grobman cited two factors that accounted for the flyby spacecraft telescope's superlative performance. First, the instrument would operate beyond Earth's obscuring atmosphere, well away from moonlight, aurorae, humidity, clouds, and city lights. Second, it would be capable of very long exposures.



A telescope in low-Earth orbit would circle the planet in about 90 minutes, so even with carefully controlled slewing could in most cases observe an astronomical target for no more than 45 minutes before losing sight of it behind the Earth. The flyby spacecraft telescope, on the other hand, would not follow a fast planet-centered orbit, and would spend most of its time far from any object that could block its view of the universe.



According to Grobman, the only obvious limit on exposure duration would be the ability of astronomical film to record arriving photons. He calculated that scientifically useful exposures lasting as long as 40 hours might be possible with existing films. He noted in passing, however, that radiation in interplanetary space might darken film. One solution, he wrote, would be to replace film with an unspecified electronic imaging system.

Spitzer Space Telescope infrared image of Messier 78, a nebula in the constellation Orion.

Hubble Space Telescope deep-field image of distant galaxies.

The flyby spacecraft telescope would be capable of recording spectra of very faint objects. Recording the spectra of distant galaxies would, Grobman explained, help to improve understanding of the relationship between galactic distance and speed of recession first noted by Edwin Hubble in 1929. Because the flyby spacecraft telescope would operate outside of Earth's atmosphere, the spectra it recorded could include regions of the electromagnetic spectrum invisible to Earth-bound telescopes, he added.



Grobman cited as another class of potential targets objects that change brightness over periods of an hour or less, such as the mysterious distant quasars (now known to be giant black holes in galaxy centers). The flyby spacecraft telescope, he wrote, would be capable of detecting subtle and rapid variations in the brightness of even very faint objects.



"Targets of opportunity" - that is, objects that appeared unexpectedly during the manned flyby mission - would be fair game for observation, Grobman added. Such targets might include supernovae and novae within or beyond our Milky Way Galaxy and comets newly arrived in the inner Solar System.

Hubble Space Telescope image of a Supernova 1994D (lower left) in Galaxy NGC 4526.

Spitzer Space Telescope infrared image of Comet Encke.

Grobman completed his memorandum after August 1967, when the U.S. Congress specifically forbade further NASA work on manned planetary flybys. Though no manned Mars/Venus flyby astronauts ever pointed a telescope at a distant galaxy or nearby asteroid, the Hubble Space Telescope in low-Earth orbit, the Spitzer Space Telescope in Earth-trailing solar orbit, and other automated instruments have decisively demonstrated the capabilities of space-based telescopes.



In the past decade and a half, NASA has worked toward placing large automated observatories into halo orbits around the Sun-Earth libration (L) points. Astronauts might voyage to the Sun-Earth L points to assist in deployment of new observatories and to upgrade and perform repairs on existing ones. Manned telescope servicing missions to the Sun-Earth L points could also serve as experience-building steps toward manned Mars missions (bottom link below).

Hubble Space Telescope image of red supergiant star V838 Moncerotis and "light echoes" on nearby gas and dust.

Hubble Space Telescope image of stars and planetary systems forming in the Carina Nebula.

Optical Astronomy on a Manned Planetary Flyby Mission - Case 710, W. D. Grobman, Bellcomm, Inc., February 8, 1968.



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