A Robotic and Piloted Planetary Exploration Program for the 1970s and Early 1980s (1968)

Leaving home: Earth as viewed from Apollo 4 on 9 November 1967. Image credit: NASA.

It was the best of times. It was the worst of times. (With apologies to Charles Dickens.)

For NASA, the year 1967 began with the promise of a bold start for the Apollo Applications Program (AAP), the planned successor to the Apollo lunar program, which would see space station missions in low-Earth orbit and advanced lunar exploration missions. Top NASA officials briefed the press on their ambitious AAP plans on 26 January 1967 (see "More Information" below). 

Barely a day later, fire raged through the crew cabin of the Apollo 1 Command and Service Module (CSM) spacecraft during a test on the launch pad, killing astronauts Gus Grissom, Ed White, and Roger Chaffee. The resulting investigation angered Congress — NASA had failed to report persistent problems in its relations with North American Aviation (NAA), the CSM prime contractor. Affronted legislators, already eager to cut government expenditures because of the soaring cost of U.S. military involvement in Indochina, responded in August-September 1967 by slashing President Lyndon Baines Johnson's Fiscal Year (FY) 1968 NASA budget request by nearly half a billion dollars. 

The cuts mostly affected projects aimed at giving NASA a post-Apollo future; AAP, of course, but also the Voyager robotic Venus/Mars exploration program (see "More Information" below) and advance planning for piloted missions beyond the Moon, including piloted Mars/Venus flybys. Members of the NASA Office of Manned Space Flight (OMSF) Planetary Joint Action Group (JAG) had hoped that major funding for piloted flybys could begin in FY 1969, with the first in a series of piloted flybys — a Mars flyby with sample return — leaving Earth in late 1975 (see "More Information" below).

Even as OMSF had sought piloted flybys, the scientific community had continued its perennial quest for an expanded robotic program. In a February 1967 report to the Johnson White House, the President's Science Advisory Council (PSAC) disparaged piloted flybys and urged a 1970s program that would see robotic spacecraft begin a wide-ranging reconnaissance of the entire Solar System. Scientists were outraged when instead the FY 1968 budget cuts threatened to end U.S. robotic exploration entirely after the twin Mariner '69 Mars flybys.

In October and November 1967, NASA Administrator James Webb spoke out in favor of new robotic planetary missions in the 1970s. He urged members of Congress to take note of Soviet plans for robotic exploration beyond the Moon. Talks began with White House budget officials and Congressional leaders aimed at salvaging a 1970s planetary program from the wreckage of the FY 1968 budget process.

Meanwhile, in Florida, components of AS-501, the first flight-ready Saturn V rocket, came together with an Apollo CSM in the giant Vertical Assembly Building (VAB) at NASA Kennedy Space Center (KSC). Without the three-stage behemoth an Apollo Moon landing was impossible.  The testing and assembly process had begun months before the Apollo 1 fire with the aim of a launch in the first quarter of 1967, but preparation for the automated test mission — which NASA designated Apollo 4 — hit one snag after another. 

Following the fire, NASA subjected the CSM NAA had delivered to KSC for the Apollo 4 mission to enhanced scrutiny. The spacecraft, designated CSM-017, was found to contain more than 1400 wiring errors. Fixing them required months. Welding errors in the NAA-built Saturn V S-II second stage also needed correction. 

Troubled assembly: the Apollo 4 CSM and Saturn V rocket in the Vertical Assembly Building at NASA Kennedy Space Center, Florida. Image credit: NASA.

The giant rocket was at last rolled out to Launch Pad 39A on 26 August 1967, but its troubles were not over, for Apollo 4 was also a test of launch pad hardware and pre-launch procedures. As the launch team struggled to make pad and rocket function together, the press, the public, and the Congress became increasingly impatient.

Apollo 4 lifted off at last on 9 November 1967. Rocket, spacecraft, launch facilities, and world-wide tracking & communications network operated together almost flawlessly.

The Apollo 4 Saturn V and CSM climb toward orbit. Image credit: NASA.

About three hours after insertion into a 190-kilometer-high (118-mile-high) low-Earth orbit, the AS-501 Saturn V S-IVB third stage restarted to boost CSM-017 into an elliptical orbit. It was the first orbital restart of the stage, which would boost Apollo missions out of Earth orbit to the Moon. 

Near orbital apogee CSM-017 separated from the S-IVB. The spacecraft fired its Service Propulsion System (SPS) main engine to increase its altitude to 18,092 kilometers (11,242 miles), then fired it again for 4 minutes and 30 seconds to hurl itself at Earth at a lunar-return speed of 24,911 miles (40,090 kilometers) per hour. 

CSM-017 split into its component modules — Command Module (CM) and Service Module (SM) — then the former reoriented itself with its bowl-shaped heat shield forward so that it could withstand fiery atmosphere reentry. The SM burned up as planned. The CM's heat shield, meanwhile, reached a temperature of nearly 2760 C (5000° F). Crew cabin temperature did not exceed 21 C (70° F). Just eight and a half hours after liftoff, the Apollo 4 CM deployed three parachutes and lowered to a splashdown in the Pacific. 

The unmanned Apollo 4 Command Module (right) bobs in the Pacific Ocean near Hawaii at the end of its eight-and-a-half-hour test flight. One of its three main parachutes remains attached; it would be retrieved for analysis along with the spacecraft. Image credit: NASA.

The trade magazine Aviation Week & Space Technology reported that, ironically, on the very day of NASA's Apollo 4 triumph, NASA Marshall Space Flight Center (MSFC) in Huntsville, Alabama, had laid off workers as a result of the FY 1968 budget cuts. NASA MSFC was the home of the Saturn family of rockets. 

On 12 December 1967, a little more than a month after Apollo 4, President Lyndon Baines Johnson toured NASA's Michoud Assembly Facility near New Orleans, Louisiana, where Saturn rockets were assembled and tested. His visit was meant to reassure local and state officials and to raise worker morale. Whether he succeeded is open to interpretation. Standing before a partially complete Saturn V S-IC first stage, Johnson told the workers

. . .that man will make space his domain is inevitable. Whether America will lead mankind to that destiny does not depend on your ability, but depends on our vision, our willingness, and our national will and determination. This great pilgrimage of man — like all his adventures — costs money. Christopher Columbus spent more years trying to find money for his voyage than he spent discovering the New World. In the modern world, we can no longer depend on Queen Isabella pawning her jewels. We have to depend on taxes. We must have revenues that only Congress can grant. . . So we will advance in space to the extent that our people and their representatives are prepared for us to advance and are prepared to pay the cost of that advance. We may not always proceed at the pace we desire. I regret — I deeply regret — that there have been reductions and there will be more. There have been interruptions. . . But I do have faith and confidence in the American people.

This background may help to explain why two engineers at Bellcomm, NASA's Washington, DC-based advance planning contractor, responded as they did when NASA invited them in late November-early December 1967, to state their opinions on the course U.S. planetary exploration should take in the 1970s and early 1980s. In a report completed and distributed to relevant NASA facilities on 26 February 1968, J. P. Downs and W. B. Thompson were cautiously optimistic. 

Downs and Thompson explained that their report reflected "the authors' thinking at. . . [a] particular time" and that it was "a reflection of a long term point of view." They assumed that the deep FY 1968 budget cuts were a short-term, temporary setback, not a sign of a long-term trend. In fact, they anticipated an annual NASA budget of between $5 billion and $6 billion by FY 1971 or FY 1972, when, they expected, NASA would start development of a piloted planetary program.

At the same time, the Bellcomm engineers cautioned that "[a]s more information becomes available on technical details and resources, the program may change." They added, however, that "the rationale expressed. . . is expected to remain much as it is now."

Downs and Thompson described a NASA planetary program containing 21 missions to 11 Solar System bodies between the years 1969 and 1984. Missions would occur in three "branches." The first branch would comprise missions to Venus and Mars that would serve as precursors to at least three piloted Mars and Venus missions. Missions in the second branch would explore Mercury, Jupiter, and the other "major planets" (Saturn, Uranus, and Neptune), a task they called "the major challenge to the unmanned program." The third branch would include missions to explore two comets and two asteroids. 

Their program would begin with the twin Mariner '69 Mars flybys already on NASA's schedule and continue in 1970 with a Mariner Venus/Mercury dual flyby mission launched on an Atlas/Centaur rocket. The Atlas/Centaur was already in early 1968 the workhorse of the NASA robotic lunar and planetary program. 

The Venus/Mercury mission, which would form part of both the first and second of Downs and Thompson's three branches, would seek gaps in Venus's cloud cover in the hope of glimpsing its mysterious surface. In addition, as the spacecraft flew past the planet, it would transmit radio signals to Earth through the Venusian atmosphere in an attempt to chart its structure.

Mariner Mars '69 engineering model. Note the large steerable camera "pod" mounted below the hexagonal bus body, the high-gain dish antenna on top, and the four solar arrays. Image credit: NASA.

Space workhorse: an Atlas-Centaur rocket launches the Surveyor 1 lunar lander on 30 May 1966. Image credit: NASA.

During the flyby, Venus would give the spacecraft a gravity assist that would reduce by between 50% and 75% the amount of propulsive energy it would need to reach Mercury. Downs and Thompson explained that the innermost planet is, by dint of its proximity to the Sun, often lost in glare when viewed from Earth and hence mysterious; orbiting close to the Sun also means that its orbital speed is high, making it difficult for spacecraft to reach.

In 1971, NASA would launch on a Titan III-C rocket its first new-design Mars orbiter and surface probe. Downs and Thompson suggested that the new orbiter might be based on the Boeing Lunar Orbiter design. The Titan III-C, a U.S. Air Force rocket, was meant to replace the Saturn IB-Centaur rocket formerly emphasized in NASA planetary mission plans. Use of the Titan III-C in the Downs and Thompson program was a response to a statement by NASA Administrator James Webb that the Saturn IB would be phased out to save money. 

18 June 1965: the first Titan III-C rocket stands on the pad at Launch Complex 40, Cape Canaveral Air Force Station, Florida. Image credit: U.S. Air Force.

Boeing-built Lunar Orbiter spacecraft. Image credit: NASA.

The 159-kilogram (350-pound) battery-powered survivable surface impactor probe would include an atmosphere entry shell, a parachute, a protective impact shell carved from soft, lightweight balsa wood, and 13 pounds of science instruments. These might include a life detection device. Instruments on the entry shell would  chart atmospheric structure as it plummeted toward the surface after separation from the impactor. These data would enable engineers to design heavier, more sophisticated Mars landers. 

NASA would launch in 1972 its first new-design Venus orbiter and atmospheric probe on a Titan III-C. In addition to "a concentrated search over the entire planet for visible access to the surface," the orbiter would employ an imaging radar to chart surface topography. The probe would measure the thermodynamic properties of the atmosphere to enable design of meteorological balloon probes suited to Venusian conditions.

In 1973, NASA would ramp up the pace by launching on three Titan III-Cs a pair of Mars orbiter/impactor probe missions and a second Mariner-derived Venus/Mercury flyby spacecraft. The latter would resemble that launched in 1970 but would add a Venus survivable surface impactor probe. The prime objective of the Mars impactor probes would be to search for life. 

The 600-pound Venus impactor probe would attempt to return data on the planet's harsh surface conditions for at least an hour. The dense Venusian atmosphere would, Downs and Thompson wrote, enable a survivable landing without a parachute.

The following year, NASA would launch its first flyby mission to Jupiter on a Titan III-C augmented with a Centaur upper stage. Dubbed a "galactic Jupiter probe," it would be the first NASA spacecraft designed for an operational lifetime of up to 10 years. It would survey interplanetary particles and fields and aid future spacecraft designers by surveying the interplanetary meteoroid environment with particular emphasis on the Asteroid Belt between Mars and Jupiter. A Jupiter gravity-assist would make it the first spacecraft to escape the gravitational grip of the Sun.

NASA would ramp up the planetary exploration pace in 1975 by launching four rockets — probably Titan III-Cs with Centaur upper stages. An orbiter and surface probe would leave Earth for Mars. Two orbiters with impact lander probes would launch to Venus. The space agency would also launch a clone of the 1974 galactic Jupiter probe mission.

The year 1976 would see NASA's first mission to a comet. After launch on an Atlas/Centaur, a Mariner-derived spacecraft would race past Comet d'Arrest. Downs and Thompson explained that the small size of the comet nucleus and the rapid speed of the flyby would require NASA to develop a sophisticated new tracking system for its comet spacecraft cameras.

In 1977, the first Mariner-derived "Grand Tour" spacecraft would depart Earth on a Titan III-C/Centaur. A series of gravity-assist flybys would speed it across the outer Solar System, enabling it to explore all four planets beyond the Asteroid Belt in the space of a decade. That same year, NASA would launch on two Titan III-C/Centaur rockets a Mars orbiter with an impactor and a Venus orbiter with a pair of impactors. The Venus impactors might be targeted to land on high-elevation surface features; these might, Downs and Thompson suggested, have cooler temperatures than lower elevations, and thus be more likely to support life.

The year 1978 would see launch of NASA's first asteroid mission (a flyby of asteroid Icarus using a Mariner-derived spacecraft launched on a Atlas/Centaur) and the second "Grand Tour" mission (a clone of the 1977 mission). It would also see an significant shift in the character of the U.S. planetary program as astronauts joined the action. 

Thompson was a veteran of the NASA OMSF Planetary JAG piloted flyby studies. The NASA budget seemed unlikely to stretch far enough to support development in time to carry out the Planetary JAG's 1975 piloted Mars flyby mission, so the Bellcomm engineers opted instead to take advantage of an opportunity to launch a piloted Venus/Mars/Venus flyby mission in late 1978. 

The piloted flyby spacecraft and its Earth-orbit departure booster stack would be assembled in Earth orbit using components launched on two-stage Saturn V rockets. After leaving Earth orbit and discarding its boosters, it would follow a free-return heliocentric path that would end at Earth. Only minor course corrections would be required after Earth-orbit departure.

In 1979, the crew of the piloted flyby spacecraft would deploy automated meteorological balloons and impactor probes as they passed Venus for the first time and automated sample returners as they passed Mars. The balloons would drift the Venusian atmosphere for a long period. They would seek evidence of life in cool atmosphere layers. 

Astronauts would examine in a sealed lab the Mars dirt and air the sample returners launched to the flyby spacecraft to determine whether they could be safely returned to laboratories on Earth. The following year (1980) would see the mission carry out its second Venus flyby — a clone of the first — followed a few months later by a direct Earth-atmosphere reentry.

The years 1979 and 1980 would also see the last two Mariner-derived comet/asteroid flyby missions on the Downs and Thompson schedule. The first, the last mission launched on an Atlas/Centaur, would visit asteroid Eros, while the second, launched on a Titan III-C/Centaur, would race past Comet Encke.

A second piloted flyby mission would depart Earth in 1981. During its Venus flybys in that year and in 1983 it would deploy a pair of balloon-borne "several thousand pound" Buoyant Venus Stations of a type proposed by the Martin Company in 1967, as well as an unspecified number of long-duration Venus landers. All would look for life. The Mars flyby in 1982 would see more surface sample collection and observations tailored toward selecting sites for eventual piloted Mars landings.

Downs and Thompson expected that their 1984 piloted planetary mission, the last on their schedule, would probably take the form of a Venus orbiter. A piloted Venus mission would, they wrote, "serve to pace the development of a high energy space storable propulsion system." After proving that it could slow the piloted Venus spacecraft so that Venus's gravity could capture it into orbit and accelerate it out of Venus orbit back toward Earth, the compact, powerful, long-lived rocket stage would propel piloted Mars orbiter and landing missions and boost out of Earth orbit large new-design robotic outer planet and "deep space" spacecraft.

The Bellcomm engineers' report landed on desks across NASA in late February. Their timing could have been better — barely a month ahead of its distribution North Vietnam attacked South Vietnam on the eve of Tet, the Chinese New Year, leading to greatly expanded U.S. involvement in the Vietnam War. The Tet Offensive created new pressure on the Federal purse, helping to ensure (among other things) that NASA's budget slide would continue in FY 1969 and beyond.  

Despite the war and other national challenges, in the period covered by the Downs and Thompson plan NASA managed to fly a dozen planetary missions, of which 11 reached their targets. In large part, these were justified in terms of heading off new Soviet space victories and providing an avenue for the development of new technology with defense implications. 

All the flown missions were directed toward major planets; none would visit asteroids or comets and (of course) none would include astronauts. Italicized initial dates given below are launch years.

• 1969: The Mariner '69 Mars flyby spacecraft were designated Mariner 6 and Mariner 7 after launch; they left Earth atop Atlas/Centaur rockets.
• 1971: The Mariner '71 Mars orbiter spacecraft were designated Mariner 8 and Mariner 9 after launch; Mariner 8's Atlas/Centaur rocket malfunctioned but Mariner 9, the first planetary orbiter, was a great success, mapping all of Mars until late 1972.
• 1972: Pioneer 10, launched on an Atlas/Centaur rocket with a solid-propellant kick stage, became the first spacecraft to traverse the Asteroid Belt;  in 1973, it became the first spacecraft to fly past Jupiter. The gravity-assist kick it received made it the first spacecraft placed on a path to escape the Solar System.
• 1973: Pioneer 11 followed Pioneer 10 through the Asteroid Belt to Jupiter; in 1979 it became the first spacecraft to fly past Saturn.
• 1973: Mariner 10 left Earth on an Atlas/Centaur rocket and flew past Venus in early 1974; later that year it became the first spacecraft to fly past Mercury. It flew past Mercury twice more in 1974-1975.
• 1975: Viking 1 and Viking 2, each of which comprised a lander and a Mariner-derived orbiter, launched atop Titan III-E rockets, arriving in Mars orbit in June 1976 and August 1976, respectively. Viking 1, which touched down on 20 July 1976, was the first successful Mars lander; Viking 2 landed successfully on 3 September 1976. Their life detection experiments yielded equivocal results.
• 1977: The Mariner Jupiter-Saturn '77 spacecraft were renamed Voyager 1 and Voyager 2. They left Earth atop Titan III-E rockets. Voyager 1 flew past Jupiter in 1979 and Saturn in 1980; Voyager 2 flew past Jupiter in 1979, Saturn in 1981, Uranus in 1986, and Neptune in 1989.
• 1978: Pioneer Venus Orbiter and Pioneer Venus Multiprobe (PVM) launched atop Atlas/Centaur rockets. Though not designed to survive landing, one PVM small probe continued to operate after striking the surface, becoming the first (so far only) successful U.S. Venus lander.

The Pioneer Venus Multiprobe bus (lower right) is shown deploying three small probes (center) and one large probe (upper left). In reality the large probe was deployed on 16 November 1978 and the small probes were deployed on 20 November 1978. The bus and probes entered the Venusian atmosphere on 9 December 1978. Image credit: NASA.

In their report, Downs and Thompson anticipated that NASA would be given the go-ahead to start a new piloted planetary program in FY 1971 or  FY 1972, and after a fashion they were correct. In January 1972, President Richard Nixon called on Congress to fund the winged Earth-orbital Space Shuttle. 

Originally proposed as a low-cost fully reusable Space Station crew rotation and resupply vehicle, the Shuttle became instead a multi-purpose spacecraft after Nixon refused to fund a Space Station. It would be only semi-reusable, which lowered its development cost but dramatically increased its operations cost. Among its goals was to launch all U.S. robotic planetary spacecraft.

Downs and Thompson's NASA budget prediction — $5-6 billion annually by about FY 1972 — entirely missed the mark. In terms of buying power in an inflationary time, NASA's budget remained at about half that amount throughout the 1970s and early-to-mid 1980s. Funding scarcity adversely impacted both Shuttle development and planetary exploration. 

Shuttle development problems traceable to funding shortfalls, lack of successful new Soviet planetary missions, tight planetary science budgets, and the Challenger accident (28 January 1986) came together to create an 11-year hiatus in new U.S. planetary launches following the 1978 Pioneer launches. The stoppage ended at last with the launch of the Magellan Venus radar mapper on board the Shuttle Orbiter Atlantis on 4 May 1989. 

By the time Magellan flew, NASA had announced that it would cease Shuttle planetary launches after it launched the Galileo Jupiter orbiter and probe and Europe's Ulysses solar polar orbiter in favor of resuming planetary launches on expendable rockets. Galileo launched on board the Orbiter Atlantis on 18 October 1989 and Ulysses launched on board the Orbiter Discovery on 6 October 1990. 

Sources

The first two sentences of this post are based on the first sentence of Charles Dickens' 1859 novel A Tale of Two Cities.

The Space Program in the Post-Apollo Period: A Report of the President's Science Advisory Committee, "Prepared by the Joint Space Panels," The White House, February 1967.

"Science Advisers Urge Balanced Program," Aviation Week & Space Technology, 6 March 1967, pp. 133-137.

"Orbiters Studied for Planetary Missions," W. J. Normyle, Aviation Week & Space Technology, 23 October 1967, pp. 30-32.

"Washington Roundup: NASA Thanks You," Aviation Week & Space Technology, 20 November 1967, p. 25.

"Apollo 4 Closes Gaps to Lunar Mission," W. J. Normyle, Aviation Week & Space Technology, 20 November 1967, p. 26-27.

"NASA Pushes Planetary Program," W. J. Normyle, Aviation Week & Space Technology, 27 November 1967, pp. 16-17.

"Remarks Following an Inspection of NASA's Michoud Assembly Facility Near New Orleans," President Lyndon Baines Johnson, 12 December 1967 (https://www.presidency.ucsb.edu/documents/remarks-following-inspection-nasas-michoud-assembly-facility-near-new-orleans — accessed 30 August 2022).

"A Feasible Planetary Exploration Program Through 1980 — Case 710," J. P. Downs and W. B. Thompson, Bellcomm, Inc., 26 February 1968.

Astronautics & Aeronautics 1967, NASA SP-4008, 1968, pp. 43-45, 246, 248, 255-256, 282-284, 295-296, 314, 320, 323-324, 333, 336-343, 352-353, 373-375.

Stages to Saturn: A Technological History of the Apollo/Saturn Launch Vehicles, NASA SP-4206, Roger E. Bilstein, NASA, 1980, pp. 351-360.

More Information

"Essential Data": A 1963 Pitch to Expand NASA's Robotic Exploration Programs

NASA's Planetary Joint Action Group Piloted Mars Flyby Study (1966)

Missions to Comet d'Arrest & Asteroid Eros in the 1970s (1966)

"Assuming That Everything Goes Perfectly Well in the Apollo Program. . ." (1967)

The First Voyager (1967)

Triple Flyby: Venus-Mars-Venus Piloted Missions in the Late 1970s/Early 1980s (1967)

Things to Do During a Venus-Mars-Venus Piloted Flyby Mission (1968)

An Unfortunate Condition: A 1967-1968 Pitch to Launch a Comet Halley Rendezvous Mission in the Late 1970s

Comet Halley's last visit before the space age: a photographic plate captured at Yerkes Observatory on 6 June 1910. Image credit: Yerkes Observatory.

Herman Michielsen was a Senior Staff Scientist at Lockheed Missiles & Space Company's Palo Alto Research Laboratory in California in August 1967, when he presented a paper on possible missions to Comet Halley to an American Institute of Aeronautics and Astronautics (AIAA) conference in Huntsville, Alabama. His paper was the earliest oft-cited work describing options for exploring Comet Halley using spacecraft during its 1985-1986 apparition, the first that would take place since the advent of spaceflight in 1957. 

Lockheed funded Michielsen's Comet Halley research under its Independent Research Program, which gave its scientific staff opportunities to perform studies on company time outside their normal range of work. At the time he presented his Comet Halley paper, much of Michielsen's work had focused on calculating lunar and planetary ephemerides using advanced computers and on Earth satellite tracking. He was an important figure in the Independent Tracking Coordination Program, which aimed to supplement the limited number of professional Earth satellite visual observations with those of skilled amateurs around the world. 

Comet Halley requires little introduction; it is the one recurrent comet the name of which is widely known to non-astronomers. Observations of Comet Halley were recorded in China as early as 240 BC. Not until the 18th century, however, was it understood that Comet Halley follows an elliptical Sun-centered path that brings it to a perihelion (closest point in its orbit about the Sun) between the orbits of Venus and Mercury about every 76 years. 

The comet is named for Edmond Halley, the English astronomer who wrote in 1705 that comets observed in 1531, 1607, and 1682 were in fact a single comet. Halley successfully predicted that the comet would return in 1758, though he did not live to see its return.

Michielsen noted that short-period comets — that is, any comet with a period of 200 years or less — are typically visible only using telescopes and barely show a tail. Comet Halley is a short-period comet but bucks this tendency, making it an object of interest for future exploration using robot probes. The Lockheed scientist predicted that its return in 1985-1986 would become "a culmination point in the field of cometary probes."

Comet Halley is, however, not an ideal target for a spacecraft because it follows a retrograde path around the Sun. The great majority of Solar System bodies orbit their primary — the Sun, a planet, or any of the various categories of small body — in a prograde direction, which is to say counterclockwise. For its part, Comet Halley orbits the Sun clockwise. Michielsen called this "an unfortunate condition."

Michielsen calculated that spacecraft on a prograde intercept path would encounter Comet Halley at Earth's distance from the Sun (one Astronomical Unit, or AU) moving at about 60 kilometers per second (km/sec) relative to the comet; at Comet Halley's perihelion distance, 0.59 AU from the Sun, the relative intercept speed would exceed 90 km/sec. High encounter speeds near and at perihelion would mean that a probe could view the comet's nucleus, which was expected to measure at most a few tens of kilometers across, for only a very short time, making impossible any in-depth observations when the comet was most active.

At the time Michielsen presented his work, most comet scientists favored astronomer Fred Whipple's "dirty snowball" model of the structure of the comet nucleus. It should be noted, however, that in 1967-1968 rival models had supporters. Confirming the nature of the nucleus was among the most important justifications for comet exploration until the 1980s.

Michielsen proposed that an effort be made in time for the 1985-1986 apparition to place a robot probe into a retrograde Sun-centered orbit that would enable it to rendezvous with and travel beside Comet Halley for weeks or months. He wrote that a rendezvous mission would permit "a return of useful data many orders of magnitude greater than that from even a number of high-speed intercepts." A rendezvous would, however, be extremely challenging in terms of propulsive energy required.

A Comet Halley rendezvous might approach feasibility, he wrote, if the rendezvous probe were first launched into an elliptical Sun-centered orbit with an aphelion (farthest point in its orbit about the Sun) at about seven AU (that is, between the orbits of Jupiter and Saturn, which orbit the Sun at 5.2 AU and 9.5 AU, respectively). He proposed a launch in 1978, with the probe approaching aphelion in 1982. 

Near aphelion, the spacecraft would move relatively slowly, so could place itself into a retrograde orbit using a propulsive maneuver (an "aphelion pulse") that changed its speed by only about 9.3 km/sec. Combined with Earth-departure and fine-targeting maneuvers, the total propulsive velocity change required to carry out a Comet Halley rendezvous in 1985 would amount to about 31 km/sec.

Diagram of Comet Halley and rendezvous spacecraft paths during Michielsen's aphelion-pulse mission. Please click on image to enlarge. Image credit: DSFPortree.

Other options would enable a Halley rendezvous with even less propulsive velocity change, Michielsen added. Departing Earth in 1973 would, for example, trim the aphelion pulse velocity change by 2.5 km/sec. The 12-year flight time from Earth launch to Halley rendezvous might, however, be seen as excessive.

In the early-to-mid-1960s, many planners considered the possibilities of propellant-saving gravity-assist maneuvers. Michielsen explained that a spacecraft launched on 13 September 1977 that passed in front of Jupiter on 16 September 1978 would be slowed and its course bent onto a retrograde path that would permit a rendezvous with Comet Halley on 27 May 1985, 254 days before its predicted perihelion on 5 February 1986. He also described a mission launched from Earth on 16 October 1978 that would encounter Jupiter on 14 October 1979 and rendezvous with Comet Halley on 10 September 1985, 148 days ahead of predicted perihelion. 

Jupiter would be better positioned for the gravity-assist flyby in the 1977 opportunity, Michielsen added, thus reducing the required Earth-departure velocity and the velocity at which the spacecraft would approach Comet Halley. The propulsive velocity change from Earth departure through Halley rendezvous would total 24.6 km/sec for the mission launched in 1977 and 25.6 km/sec for the 1978 mission. 

Michielsen then briefly explored the possibility of a Saturn gravity-assist flyby, which he said was suggested at the August 1967 AIAA meeting by Maxwell Hunter, who was a National Space Council member from 1962 until he joined Lockheed in 1965. A Saturn flyby Comet Halley rendezvous mission launched from Earth on 30 August 1973 would require a total propulsive velocity change of 22.2 km/sec; one launched on 14 September 1974 would need 22.9 km/sec. Saturn flyby would occur on 19 January 1976 for the 1973 launch and on 14 January 1977 for the 1974 launch; Comet Halley rendezvous would take place on 18 April 1985 or 21 June 1985, respectively.

In the second half of his paper, Michielsen gave close attention to the problem of precise prediction of Comet Halley's return, and it is in this context that his work is most often cited today. He noted that digital computers had enabled researchers to confirm that the gravity of the planets — in particular, Jupiter, Earth, and Venus — had caused Comet Halley's orbital period to vary by up to 1000 days over the centuries. In addition, a non-gravitational effect — the explanation of which he declared was beyond the scope of his paper — caused a shift in the perihelion date of about four days during each of the six apparitions spanning the period from 1456 to 1835. 

The non-gravitational effect Michielsen was loath to explain had been attributed to jets of gas and dust that form when a comet nucleus is heated by the Sun. These jets would, it was believed, behave like natural rocket motors. This hypothesis would eventually be confirmed, but the Lockheed scientist was probably wise to treat the potentially controversial problem as an unnecessary distraction when he presented his study of Comet Halley rendezvous methods.

The shift in perihelion date meant that a Comet Halley probe launched in the late 1970s would need to perform additional propulsive maneuvers to ensure a close rendezvous. The magnitude of the maneuvers required would begin to become apparent, he predicted, in November 1983, when Earth's largest telescopes would begin to photograph Comet Halley between the orbits of Saturn and Jupiter at a distance of 8.5 AU from the Sun. Michielsen expected that, if reacquisition took place at that time, then a sufficient number of observations could occur to ensure that maneuvers requiring a total propulsive velocity change of just 1.2 kilometers per second would yield a "worthwhile rendezvous mission." Later reacquisition might demand a greater propulsive velocity change.

As it turned out, the advent of CCD technology enabled reacquisition of Comet Halley more than a year ahead of Michielsen's predicted date. On 16 October 1982, observers using the 200-inch Hale Telescope at Mount Palomar in California became the first humans to glimpse Comet Halley since 1911. The comet, which had yet to show a tail, lay beyond the orbit of Saturn when it was reacquired.

Advances in astronomy technology mean that Comet Halley has remained visible since its 16 October 1982 reacquisition. When it reaches perihelion in July 2061, it will have been visually tracked for 79 years.

This post is the first in a new series called "Preparing for Halley." It aims to describe U.S. efforts to launch a spacecraft to Comet Halley in 1985-1986. The series is timed to coincide with Comet Halley's aphelion passage late in 2023, after which it will be inbound for its 2061 apparition. Other posts on comet exploration relevant to Comet Halley missions in 1985-1986 can be found by following the "More Information" links below. 

Comet Halley reacquired: CCD image captured at Palomar Observatory on 16 October 1982. The circle was added to make faint Comet Halley stand out among the background stars. Image credit: D. Jewitt & D. Edward Danielson, California Institute of Technology.

Source

"A Rendezvous with Halley's Comet in 1985-1986," H. F. Michielsen, Journal of Spacecraft and Rockets, Volume 5, Number 3, March 1968, pp. 328-334; paper presented at the AIAA Guidance, Control, and Flight Dynamics Conference in Huntsville, Alabama, 14 August 1967.

More Information

Missions to Comet d'Arrest & Asteroid Eros in the 1970s (1966)

Cometary Explorer (1973)

A 1974 Plan for a Slow Flyby of Comet Encke

Catching Some Comet Dust: Giotto II (1985)

The Challenge of the Planets, Part Three: Gravity

Catching Some Comet Dust: Giotto II (1985)

Giotto 1 liftoff. Image credit: European Space Agency.

On the overcast morning of 2 July 1985, the eleventh Ariane 1 rocket to fly lifted off from 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. The last Ariane 1 to fly, it bore aloft Giotto, the first European Space Agency (ESA) interplanetary spacecraft. 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 elliptical orbit takes it from the cold emptiness beyond Neptune to the space between the orbits of Venus and Mercury. Halley travels around the Sun in a retrograde orbit, meaning that it orbits "backwards" relative to the eight planets and most other objects making up the Solar System.

Comet Halley has passed through the inner Solar System 30 times since its first verified recorded apparition in 240 B.C. In 837 A.D., 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 appeared as large as the full Moon.

Shortly after its bright apparition in the year 1301, Italian artist Giotto di Bondone was inspired to add Comet Halley to his painting The Adoration of the Magi. The Giotto spacecraft was named for him.

Comet Halley appears near the top of Giotto di Bondone's The Adoration of the Magi. 

Throughout most of its known apparitions, Comet Halley was not understood to be one comet repeatedly passing through the inner Solar System. Not until 1705 did English polymath Edmond Halley determine that comets seen in 1531, 1607, and 1682 were probably one comet orbiting the Sun. He predicted that, if his hypothesis was correct, the comet should reappear in 1758 (which it subsequently did).

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 drum-shaped Giotto to ignite its French-built Mage solid-propellant rocket motor. The motor burned 374 kilograms of propellant in 55 seconds to inject the spinning 2.85-meter-tall, 1.85-meter-diameter spacecraft into an orbit about the Sun.
 
Two months before Giotto's launch, Americans P. Tsou (Jet Propulsion Laboratory), D. Brownlee (University of Washington), and A. Albee (California Institute of Tech) proposed in a paper in the Journal of the British Interplanetary Society that a second Giotto mission be launched to fly close by one of 13 candidate comets between 1988 and 1994. They proposed that the new 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 had not been required to boost Giotto into interplanetary space; that is, that the Ariane 1 could have done 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, in Giotto II, the reentry capsule should replace the motor.

Giotto included on its aft end a "Whipple bumper" — named for its inventor, planetary astronomer Fred Whipple — to protect it from hypervelocity dust impacts. During approach to Comet Halley, the spacecraft turned the bumper toward 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 was expected to impact the bumper at up to 68 kilometers per second. Tsou, Brownlee, and Albee noted that the 13 candidate Giotto II target 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.

Comet dust would, nevertheless, create challenges for Giotto II. Tsou, Brownlee, and Albee devoted much of their paper to a description of how the spacecraft might successfully capture dust for return to Earth. One proposed capture system, based on the Whipple bumper design, 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 also noted that thermal blankets from the Solar Maximum Mission (SMM) satellite, launched into Earth orbit on 14 February 1980, had demonstrated that intact capture of high-velocity particles was possible. The multilayer Kapton/Mylar blankets, which were returned to Earth on board the Space Shuttle Orbiter Challenger at the end of mission STS 41-C (6-13 April 1984), were found to have collected hundreds of intact meteoroids and human-made orbital debris particles.

The three scientists described preliminary experiments in which gas guns were used to fire meteoroid and glass fragments at "underdense materials," such as polymer foams and fiber felts. The experiments suggested that such materials could capture at least partially intact comet dust particles.

Giotto's encounter with Comet Halley spanned 13-14 March 1986. At closest approach the spacecraft passed just 596 kilometers from Halley's nucleus. The comet's 15-by-eight-by-eight-kilometer heart turned out to be extremely dark, with powerful jets of dust and gas blasting outward into space.

Artist's concept: Giotto at Halley. Image credit: European Space Agency.

Halley's hot heart as imaged by ESA's Giotto spacecraft. 

The intrepid probe suffered damage from dust impacts — one large particle sheered off more than half a kilogram of its structure — but most of its instruments continued to operate after the Comet Halley flyby. ESA thus decided to steer Giotto toward another comet.

On 2 July 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 flyby put it on course for Comet Grigg-Skjellurup, which it passed at a distance of 200 kilometers on 10 July 1992.
 
After determining that Giotto had less than seven kilograms of hydrazine propellant left on board, ESA turned it off on 23 July 1992. The inert spacecraft flew past Earth a second time at a distance of 219,000 kilometers on 1 July 1999.
 
By that time, a comet coma sample return mission was underway with two of the Giotto II proposers playing central roles. In late 1995, Stardust became the fourth mission selected for NASA's Discovery Program of low-cost robotic missions. Brownlee and Tsou, respectively Stardust Principal Investigator and Deputy Principal Investigator, designed the mission's sample capture system.

Artist's concept of the NASA Stardust spacecraft at Wild 2. Image credit: NASA.

The 380-kilogram Stardust spacecraft left Earth on a free-return trajectory on 7 February 1999, and flew past Comet Wild 2 (one of the 13 Giotto II candidates) at a distance of about 200 kilometers on 2 January 2004. Stardust captured dust particles in aerogel, a silica-based material of extremely low density that was invented in the 1930s. Tsou, Brownlee, and Albee had apparently been unaware of aerogel when they proposed Giotto II in 1985.

Stardust returned to Earth on 15 January 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.

When opened on 17 January 2006 at NASA Johnson Space Center, in the same lab that received the Apollo Moon rocks, Stardust's 132 aerogel capture cells were found to contain thousands of intact dust grains captured from Wild 2. Subsequent analysis indicated that some probably formed close to other stars before our Solar System was born.
 
Sources

"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.

"ESA Remembers the Night of the Comet," European Space Agency, 11 March 2011 (https://www.esa.int/Science_Exploration/Space_Science/Rosetta/ESA_remembers_the_night_of_the_comet — accessed 26 August 2016).

"Stardust: NASA's Comet Sample Return Mission," NASA Jet Propulsion Laboratory (https://stardust.jpl.nasa.gov/home/index.html — accessed 26 August 2016).

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A 1974 Plan for a Slow Flyby of Comet Encke

Cometary Explorer (1973)

Missions to Comet d'Arrest and Asteroid Eros in the 1970s (1966)

A 1974 Plan for a Slow Flyby of Comet Encke

So close: the CONTOUR spacecraft. Image credit: NASA.

Comet Halley is often called "Humankind's Comet" because it has appeared throughout much of recorded human history and because its orbital period of about 76 years is roughly equivalent to a human lifespan. Given the often frustrating nature of spaceflight planning, Comet Encke could be nicknamed "Spaceflight's Comet."

It has made the short list of targets for comet-exploring spacecraft for half a century. With one of the shortest orbital periods of any comet — just 3.3 years — and an inclination relative to the plane of the Solar System of only about 10°, Encke is among the comets most easily accessible to spacecraft. Yet despite being named the target of many proposed comet missions, Encke has never received a visitor from Earth.

Humans came closest to exploring Comet Encke nearly two decades ago. Following its launch on 3 July 2002, NASA's 775-kilogram COmet Nucleus TOUR (CONTOUR) spacecraft moved through a series of elliptical phasing orbits about the Earth designed to position it for a solid-propellant rocket motor burn on 15 August 2002. The burn would have launched it into solar orbit near the Earth. CONTOUR would then have re-encountered Earth in August 2003. The gravity-assist kick it was meant to receive from our planet would have put it on course for a Comet Encke close flyby on 12 November 2003.

Instead, the CONTOUR spacecraft disintegrated during its Earth-departure burn. Observers visually tracked three objects where there should have been one CONTOUR.

The CONTOUR Mishap Investigation Board determined that the most likely cause of the failure was an obvious-seeming design flaw: that the spacecraft's solid-propellant rocket motor, embedded at its center, produced enough heat that it weakened CONTOUR's structure, causing the spacecraft to break apart under acceleration. The Board cautioned, however, that lack of telemetry during the Earth-departure burn left open the possibility of several other causes, including rocket motor casing rupture, meteoroid or human-made space debris collision, or attitude-control failure leading to a destructive tumble.

If engineers and scientists at NASA Goddard Space Flight Center (GSFC) had gotten their way, Comet Encke would have received its first visitor as early as 3 December 1980. In fact, it would have received two visitors at the same time, for they envisioned launching two spacecraft to Comet Encke on a single rocket. The Encke probes, near twins, would have flown by the comet at a relatively slow speed compared with other proposed comet spacecraft; hence, in the November 1974 NASA Technical Note they wrote to describe it, they dubbed their mission a "ballistic slow flyby."

The twin Comet Encke ballistic slow flyby spacecraft stacked within their streamlined Centaur launch shroud. The adapter would join with the top of the Centaur upper stage. Image credit: NASA.

The Comet Encke probes were meant to depart Earth between 16 and 30 August 1980 atop a Titan rocket with a Centaur upper stage. Ironically, given CONTOUR's fate, the NASA GSFC team rejected an additional solid-propellant "kick" rocket motor as too risky. The probes would travel on a curving ballistic path directly from Earth to Encke; hence the term "ballistic" in the mission's description.

Robert Farquhar led the four-person GSFC team. In 1972-1973, he had participated in NASA GSFC's 35-member Cometary Explorer Study Group, which aimed to explore Comet Grigg-Skjellerup in April 1977 and Comet Giacobini-Zinner in February 1979 using a single 450-kilogram spinning spacecraft. The NASA-appointed Comet and Asteroid Science Advisory Committee had endorsed Cometary Explorer as the first step in a logical program of comet exploration leading to a NASA Comet Halley mission in 1985-1986.

Unfortunately, the U.S. civilian space agency, faced with rapidly declining budgets and bearing the heavy burden of Space Shuttle development, had been unable to fund Cometary Explorer. The 1980 Encke slow flyby mission would, it was hoped, put NASA comet exploration back on track to Halley.

Technicians at Cape Canaveral lower the launch shroud over the West German-U.S. Helios B solar probe spacecraft. Image credit: NASA.

Farquhar's team modeled its Comet Encke mission on the German-U.S. Helios A/Helios B Sun probe missions. Helios A left Earth in late 1974 (about a month after the NASA GSFC group published its Technical Note, in fact). The Helios probes were designed to survive an approach to 0.3 times the Earth-Sun distance, which is inside the orbit of the planet Mercury. The Encke probes, for their part, would pass their cometary target as it neared perihelion (the point in its orbit where it was nearest the Sun) at 0.34 times the Earth-Sun distance. The Helios probes would orbit in the plane of the Solar System; the Encke probes would match their target's modest orbital tilt.

The NASA GSFC team's Encke probes, which would spin to create gyroscopic stability, would move apart immediately after they separated from their launch vehicle's Centaur stage. Farquhar's team dubbed them the "tail probe" and the "coma probe." Each would resemble the lower half of a hourglass-shaped Helios spacecraft. Solar cells on their sides would power spacecraft systems and a suite of science instruments.

If necessary, a course-correction rocket burn would take place 10 days after launch. A second burn 50 days after launch would aim the tail probe at a point in the Comet Encke's wan tail about 10,000 kilometers behind the nucleus and would aim the coma probe at a point immediately in front of the nucleus. A third, very modest, course-correction burn was scheduled for Launch +85 days. The two spacecraft would encounter Comet Encke at about Launch +102 days.

Depending on their launch date, the Comet Encke spacecraft would reach their target between 3 December and 8 December 1980 moving at between 7.6 and 9.03 kilometers per second. Comet Encke would reach perihelion on 6 December. The Encke flybys would occur at around 1000 hours Greenwich Mean Time on all days of their arrival window so that the 100-meter dish-shaped antenna at Effelsberg, West Germany — the same antenna used to communicate with the Helios probes — could receive data for as long as possible before the twin probes set below the local horizon.

Image credit: NASA.

Farquhar and his colleagues envisioned that their two probes would carry slightly different science payloads. The 375-kilogram coma probe, which would linger within 1000 kilometers of the sunlit side of the nucleus for nearly 42 minutes, would include a despun platform bearing its radio dish antenna, TV camera, neutral mass spectrometer, UV spectrometer, and Lyman-alpha spectrometer. The 325-kilogram tail probe would include a despun antenna, but would lack the coma probe's despun platform with its four instruments. Both probes would include on their spinning main sections an ion mass spectrometer, a DC magnetometer, an AC magnetometer, an electron analyzer, a plasma analyzer, an electric field detector, a dust detector, and a dust composition instrument.

The NASA GSFC team was not the only group in 1974 that planned a 1980 Comet Encke mission. The NASA GSFC scientists and engineers made a point of comparing their mission plan with its main rivals. They explained that, in their comparison, "the primary evaluation criteria [would] be the science value and realism of attaining mission objectives."

Their plan's leading rival, a mission design advocated mainly by the Jet Propulsion Laboratory and its contractors, was based on solar-electric propulsion. Launch would take place on 17 December 1978 and a Comet Encke flyby would occur on 6 November 1980. The NASA GSFC team noted that the mission's 30-centimeter-diameter solar-electric (ion) propulsion thruster had yet to be developed, let alone tested; nevertheless, it would be expected to operate flawlessly for 690 days.

In addition, the thruster would interfere with the spacecraft's particle-and-fields instruments. Interference would not cease when the thruster was switched off.

Assuming that its untried thruster functioned as hoped, however, the solar-electric spacecraft would pass Comet Encke moving at only four kilometers per second, which constituted an advantage over NASA GSFC's ballistic slow flyby. It would do so, however, more than a month before perihelion, when Comet Encke was still about 0.5 times the Earth-Sun distance from perihelion. At that point in its orbit, the nucleus would be relatively inactive: if past observations were any guide, Comet Encke would have almost no tail.

The ballistic slow flyby's lesser rival was a ballistic fast flyby advocated mainly by NASA Ames Research Center and its contractors. A spin-stabilized spacecraft similar to the Pioneer 10 and Pioneer 11 outer Solar System spacecraft would launch on 18 August 1980 atop a relatively cheap Atlas/Centaur rocket with a solid-propellant kick stage. After a voyage of just 92 days, the spacecraft would whiz past Comet Encke on 18 November 1980 at a blistering 20.1 kilometers per second.

Farquhar's group noted that high-speed impacts with Comet Encke dust particles could easily destroy the ballistic fast flyby spacecraft, and that its camera would likely return only motion-blurred images (assuming that it had time to locate the nucleus or any other important comet features). It would remain within 1000 kilometers of the nucleus for a mere nine minutes.

The NASA GSFC team concluded that, compared with the solar-electric and ballistic fast flybys, the ballistic slow flyby was "superior in every respect." This assertion may well have been correct; the rivalry between the slow flyby, solar-electric, and fast flyby groups split the small community of comet exploration advocates, however, helping to ensure that no spacecraft explored Comet Encke in 1980.

Comet Encke as observed by the MESSENGER Mercury orbiter on 17 November 2013. Encke passed the planet Mercury at a distance of just 3.7 million kilometers and reached perihelion four days later. Image credit: NASA/JHUAPL/Carnegie Institution of Washington.

Sources

Mission Design for a Ballistic Slow Flyby of Comet Encke 1980, NASA Technical Note D-7726, R. Farquhar, D. McCarthy, D. Muhonen, and D. Yeomans, NASA Goddard Space Flight Center, November 1974.

Comet Nucleus Tour CONTOUR Mishap Investigation Board Report, NASA, 31 May 2003.

More Information

Cometary Explorer (1973)

Missions to Comet d'Arrest and Asteroid Eros in the 1970s (1966)

Cometary Explorer (1973)

Explorer 50/Interplanetary Monitoring Platform-J (IMP-J) satellite. Image credit: NASA.

The world's longest-running spacecraft series is the Explorer series, which began with the launch of Explorer 1, the first U.S. Earth satellite, on 31 January 1958. The U.S. Army carried out Explorer missions until NASA opened its doors on 1 October 1958. NASA Headquarters tapped NASA Goddard Space Flight Center (GSFC) in Greenbelt, Maryland, to manage the Explorer Program. Explorer 6, launched 7 August 1959, was the first in the series to reach Earth orbit under NASA auspices.

The NASA Explorers were envisioned as low-cost science satellites. Explorer 6, a 142-pound spheroid with four paddle-like solar panels, carried lightweight, relatively simple radiation and micrometeoroid detectors. Simple did not, however, mean insignificant: Explorer 6 conducted the first detailed survey of the Van Allen radiation belts, which contain solar radiation particles trapped by Earth's magnetic field. Subsequent Explorers took many forms, but Sun-Earth interactions and the interplanetary environment remained major Explorer Program areas of interest.

NASA launched Explorer 50/Interplanetary Monitoring Platform-J (IMP-J) on 26 October 1973. The following month, a 35-member NASA GSFC team, the Cometary Explorer Study Group, completed a report for the Greenbelt center's Cometary Study Office which laid out a design for a low-cost dual-comet Explorer mission.

One goal of the proposed mission, which aimed to carry out ballistic (unpowered) intercepts of Comet Grigg-Skjellerup and Comet Giacobini-Zinner in 1977 and 1979, respectively, was to gain experience ahead of a Comet Halley mission. Halley, well known to the public and of significant scientific interest, was due to return to the inner Solar System in 1985-1986.

That NASA GSFC should seek a leading role in comet exploration is not surprising. Comets interact profoundly with the Sun and the interplanetary environment. The Greenbelt center staked its claim to comets as early as March/April 1970, when the NASA GSFC-managed Orbiting Astronomical Observatory-2 (OAO-2) spacecraft turned its ultraviolet telescopes toward Comet Bennett, a long-period comet discovered in December 1969.

OAO-2 revealed a large "halo" of hydrogen gas surrounding the comet, which implied that it had a nucleus made up at least partly of water ice. This helped to lend support to astronomer Fred Whipple's "dirty snowball" comet model.

Cometary Explorer spacecraft. A = solid-propellant kick motor; B = hydrazine propellant tank (one of eight); C = science instrument ring; D = solar cell ring (one of three); E = hydrazine thruster (one of six); F = main antenna reflector spin motor; G = main antenna reflector. The right side of the image shows a cutaway of the spacecraft; the left side shows its exterior appearance. Image credit: David S. F. Portree/NASA.

The Cometary Explorer Study Group based its 450-kilogram spacecraft on the drum-shaped Explorer 50/IMP-J design, which was very similar to Explorer 43/IMP-H (launched 13 March 1971) and Explorer 47/IMP-I (launched 23 September 1972). The spacecraft's 1.4-meter-wide, 1.8-meter-tall structure would be made up of four stacked 16-sided "rings." Of these, three rings would carry on their outer surfaces solar cells which together would generate 162 watts of electricity.

Within the fourth ring would be mounted most of Cometary Explorer's dozen science instruments. The instrument ring would have attached to it six appendages: four evenly spaced, 61-meter-long cable antennae for measuring interplanetary electric fields and a pair of instrument booms, each about three meters long.

Top view of Cometary Explorer spacecraft. A = cable antenna (one of four); B = instrument boom (one of two); C = counter-rotating high-gain radio antenna. Image credit: David S. F. Portree/NASA.

For stability, Cometary Explorer would spin about its long axis at least 15 times per minute. Most of its equipment would not be affected by its spin or would be aided by it; for example, acceleration ("artificial gravity") the spin would create within the spacecraft would help to move hydrazine propellant from eight small pressurized tanks in the lower solar-cell ring to six thrusters spaced around the spacecraft.

Cometary Explorer's top-mounted high-gain antenna, on the other hand, would become useless if it spun with the spacecraft. An electric motor in the antenna base would thus turn the antenna against the spin so that it would remain stationary relative to the rest of the spacecraft. This would help to keep it fixed on Earth throughout Cometary Explorer's two-and-a-half-year mission.

Cometary Explorer would lift off from Cape Canaveral, Florida, on 4 November 1976, at the start of a 10-day launch opportunity. A Delta rocket would place the spacecraft and a solid-propellant upper stage into Earth parking orbit. At the appropriate time, the solid-propellant motor would ignite to place Cometary Explorer on course for Comet Grigg-Skjellerup. Its job complete, the spent upper stage would separate; the spacecraft's small hydrazine thrusters would then tweak its Sun-centered orbital path to ensure a successful comet intercept.

Cometary Explorer inside its 2.44-meter-diameter streamlined launch shroud. A = outline of launch shroud; B = Cometary Explorer spacecraft; C = solid-propellant upper-stage motor. Image credit: David S. F. Portree/NASA.

At the time the Cometary Explorer Study Group prepared its report, Comet Grigg-Skjellerup orbited the Sun once every 5.1 years. Its elliptical orbit had a perihelion (point closest to the Sun) of 0.99 Astronomical Units (AU), and an aphelion (point farthest from the Sun) of 4.93 AU. An AU, incidentally, is equal to the mean Earth-Sun distance (149,597,871 kilometers).

Grigg-Skjellerup's orbital elements were the result of a close (0.33 AU) pass by Jupiter in early 1964; prior to that encounter, its orbital period had been 4.9 years and its perihelion distance 0.86 AU. Though Grigg-Skjellerup's orbit had been precisely determined following the Jupiter encounter, the Group advised that observatories on Earth should locate and track the comet before Cometary Explorer's launch to help to ensure a successful intercept.

Cometary Explorer would pass about 1000 kilometers from the Sun-facing side of Grigg-Skjellerup on 11 April 1977, traveling at 15.2 kilometers per second relative to its target. At time of intercept, comet and spacecraft would orbit the Sun only 0.2 AU from Earth. In addition to collecting data on the comet's interactions with the Sun and interplanetary space and the composition of its gas and dust, scientists would attempt to image Grigg-Skjellerup's nucleus.

Departure from Grigg-Skjellerup would mark the start of Cometary Explorer's "extended mission," which would last nearly two years. The spacecraft would for a time follow the initial orbit that had taken it past Grigg-Skjellerup; then, on 26 October 1977, nearly a year after its launch, it would return to Earth to perform a gravity-assist flyby at a distance of about 42,000 kilometers.

Before and after its Earth flyby, Cometary Explorer would pass through and attempt to define the limits of Earth's magnetotail, the part of its magnetosphere pushed outward by the solar wind. During the flyby, the spacecraft would ignite the solid-propellant kick motor embedded in the "thrust tube" at the center of its lower solar-cell ring. This, combined with Earth's gravity, would bend its course toward its second target, Comet Giacobini-Zinner. As Earth grew small behind it, flight controllers would use Cometary Explorer's hydrazine thrusters to refine its trajectory.

The Giacobini-Zinner intercept would take place 1.83 AU from Earth on 19 February 1979. Relative to the comet, the spacecraft would zip along at a speed of 20.8 kilometers per second.

The Cometary Explorer Study Group explained that the Grigg-Skjellerup and Giacobini-Zinner encounters would occur in "the proper order," meaning that the least perilous comet intercept would occur first. Grigg-Skjellerup, Cometary Explorer's primary target, was not a dusty comet, so the group felt that the spacecraft would not suffer crippling damage as it flew past. Giacobini-Zinner, on the other hand, was a dusty comet, so was more likely to damage or destroy Cometary Explorer.

In the foreword to its report, the Cometary Explorer Study Group warned readers that NASA had already rejected its proposed mission. The space agency had cited its rapidly shrinking budget when it turned down the NASA GSFC plan.

The Group argued, however, that its report was still worthy of publication because it had "established the framework for investigating future ballistic intercept missions to comets." In the decade that followed the Cometary Explorer study, several of the Group's members — but most notably Robert Farquhar, Mission Definition Manager for the study — would continue to plan inexpensive, pioneering missions to comets. More often than not, these would aim to prepare NASA to explore Comet Halley in 1985-1986. Several of these proposed missions will be described in future posts.

Sources

System Definition for "Cometary Explorer": A Mission to Intercept the Comets Grigg-Skjellerup (1977) and Giacobini-Zinner (1979), NASA TM X-70561, NASA Goddard Space Flight Center, November 1973.

Encyclopedia: Satellites and Sounding Rockets of Goddard Space Flight Center - 1959-1969, NASA, no date (1970).

"NASA Facts: Explorer Satellites," E-10-62, NASA, 1962.

More Information

A 1974 Plan for a Slow Flyby of Comet Encke

Missions to Comet d'Arrest and Asteroid Eros in the 1970s (1966)

Missions to Comet d'Arrest & Asteroid Eros in the 1970s (1966)

Comet Ikeya-Seki lights the pre-dawn sky at Kitt Peak on 29 October 1965. Image credit: Roger Lynds/NOAO/AURA/NSF.

The comet now designated C/1965 S1 — better known as Comet Ikeya-Seki, for its discoverers, Kaoru Ikeya and Tsutomu Seki — was perhaps the intrinsically brightest comet of the past 1000 years. Ikeya and Seki independently discovered the inbound "dirty snowball" on 18 September 1965. As it neared the Sun in mid-October 1965, it blazed in daylight skies as magnitude -10 (that is, it was 100 times brighter than the planet Venus, which is typically the brightest object in earthly skies after the Sun and Moon).

Shortly before it became lost in the Sun's glare, Comet Ikeya-Seki was observed to have broken into three major pieces. On 21 October 1965, the three parts of the comet passed a mere 450,000 kilometers (km) from the Sun. The fractured comet then emerged from the glare into dawn skies sporting a very bright tail, possibly because its fragmentation had exposed fresh ice. The fragments remained close together as Comet Ikeya-Seki retreated from the Sun. By early 1966, it was lost to sight. Its fragments might reenter the Inner Solar System as separate comets in about 900 years.

Soon after Comet Ikeya-Seki left the Inner Solar System, Tim Kreiter, with NASA Lewis Research Center in Cleveland, Ohio, prepared a brief report in which he outlined what might have become NASA's first missions to a comet and an asteroid. Kreiter sought to demonstrate that the space agency could aim for targets other than the Moon, Venus, and Mars — all of which had been reached by mid-1965 — using existing or near-term spacecraft technology and "intermediate-sized" Atlas-derived launch vehicles.

Kreiter's target comet was 6P/d'Arrest, named for its 19th-century discoverer, German astronomer Heinrich Louis d'Arrest. Kreiter cited a NASA-funded March 1965 Illinois Institute of Technology Research Institute (IITRI) study that singled out the 1976 d'Arrest opportunity as the most favorable of any comet mission opportunity between 1965 and 1986.

Comet d'Arrest, a relatively faint comet, follows an elliptical Sun-centered orbit with an aphelion (point farthest from the Sun) just beyond the orbit of Jupiter and a perihelion (point nearest the Sun) just beyond Earth's orbit. As the comet neared perihelion, solar heating would cause its surface to become active, making it an interesting target for spacecraft exploration.

A flyby spacecraft that intercepted Comet d'Arrest at the time of its 1976 perihelion would do so less that 0.16 Astronomical Units (that is, 0.16 times the Earth-Sun distance — about 23.9 million km) from Earth. Relatively close proximity to Earth would permit a typical low-power spacecraft transmitter to return data at a relatively high rate and enable astronomers using Earth-based telescopes to closely monitor Comet d'Arrest during the flyby.

Jupiter's gravity often reshapes comet orbits. Because of this, Kreiter wrote, astronomers would need to track Comet d'Arrest for at least two months before spacecraft launch to precisely determine its orbit. Again citing the 1965 IITRI study, he explained that Comet d'Arrest would need to attain a visual magnitude of at least 20 before it could be spotted using Earth's largest telescopes. This would, he wrote, occur seven or eight months before planned launch of the comet probe, providing space navigators with ample time to plot the comet's precise orbit.

Kreiter considered Atlas rocket/upper stage combinations designated SLV-3A/Agena, SLV-3C/Kick, and SLV-3C/Centaur. These could in 1976 launch to Comet d'Arrest robotic spacecraft of considerable mass: about 430 kilograms (kg), 700 kg, and 1050 kg, respectively. For comparison, Mariner 4, which left Earth on a Atlas SLV-3/Agena on 28 November 1964 and flew past Mars on 15 July 1965, was limited to a mass of about 260 kilograms.

Kreiter had his Comet d'Arrest flyby spacecraft leave Earth between 22 March and 21 April 1976. Daily launch windows lasting about 2.5 hours would occur throughout the planned launch period. The Atlas would boost the Agena, Kick, or Centaur upper stage and attached spacecraft into a 185-km-high parking orbit about the Earth. The upper stage and spacecraft would then loiter for from 10 to 20 minutes before the former ignited to place the latter on course for Comet d'Arrest.

Travel time to Comet d'Arrest would range from 115 to 145 days, with the precise duration and flyby date determined by the date of launch from Earth. The spacecraft would zoom past the comet moving at about 12.8 km per second, Kreiter calculated, so would spend only about 4.4 hours within 100,000 km of its target. This might mean, he wrote, that some desirable scientific investigations — for example, imaging the comet's mysterious heart, a feat which would demand precise camera aiming — would probably not be feasible.

30 May 1966: An Atlas-Centaur rocket launches Surveyor 1, the first lunar soft-lander. Kreiter's 1974 Eros and 1976 Comet d'Arrest missions might have launched on a similar rocket. Image credit: NASA.

Kreiter's flyby mission to Near-Earth Asteroid 433 Eros would launch between 9 August and 8 September 1974, about 18 months ahead of his Comet d'Arrest mission. He estimated that an Atlas SLV-3A/Agena could launch to Eros a spacecraft with a mass of about 470 kg; the corresponding masses for the SLV-3C/Kick and SLV-3C/Centaur launches were, respectively, 740 kg and 1050 kg. The Eros flyby spacecraft and its upper stage would orbit Earth for from 20 to 40 minutes before the latter ignited to send the former on its way.

The Eros spacecraft would fly past its target between 140 and 170 days after launch, with the precise encounter date being dependent on its launch date. As with the Comet d'Arrest mission, the flyby would occur near the target body's perihelion, when Eros and the Eros spacecraft would pass no more than 0.13 Astronomical Units from Earth. Moving at about 7.2 km per second, the spacecraft would spend about 7.9 hours within 100,000 km of Eros.

American space scientists would have to wait many years for NASA comet and asteroid missions. In the interim, they made do. The International Comet Explorer (ICE) — originally designated International Sun-Earth Explorer-3 — was launched in 1978, repurposed and renamed in 1982, and flown through 21P/Giacobinni-Zinner's tail on 11 September 1985. The Galileo Jupiter orbiter flew past Main Belt asteroid 951 Gaspra on 29 October 1991 on its way to its second gravity-assist Earth flyby.

The first successful NASA spacecraft designed especially to explore an asteroid and a comet did not leave Earth until 1996 and 1999, respectively. The former was the Near-Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft, which entered orbit around Eros on 14 February 2000, and the latter was Stardust, which flew by 81P/Wild 2 on 24 September 2003.

Both missions were far more ambitious that Kreiter's simple flybys. After orbiting Eros for almost a year, NEAR Shoemaker landed on the asteroid on 12 February 2001 and operated for about two weeks before draining its batteries. Stardust completed its primary mission by dropping off at Earth a reentry capsule containing Comet Wild 2 dust samples on 15 January 2006; in its extended mission, it flew past 9P/Tempel 1 on 15 February 2011, where it imaged a crater blasted by the Deep Impact spacecraft on 4 July 2005.

Sources

"Splendor in the Night," M. Jewell, Time, 22 October 1965, p. 90.

"The Great Comet of 1965," B. Marsden, Sky & Telescope, December 1965, pp. 332-337.

"Reports on Comet Ikeya-Seki (1965f)," L. Robinson, Sky & Telescope, January 1966, pp. 52-55.

Intercept Missions to Asteroid Eros in 1974 and Comet d'Arrest in 1976, NASA Technical Memorandum X-1288, T. Kreiter, October 1966.

More Information

Cometary Explorer (1973)

A 1974 Plan for a Slow Flyby of Comet Encke

Catching Some Comet Dust: Giotto II (1985)