Recently I re-read the 1980 revised edition of Robert Powers’ book Planetary Encounters: The Future of Unmanned Spaceflight which I had bought just a couple of weeks after I had graduated high school in 1980. Included in the book was information on a long list of potential planetary missions that were hoped to be flown in the 1980s and beyond in a continuation of the Golden Age of planetary exploration in which I had grown up.

Unfortunately, the reality was that the increasingly ambitious goals of the planetary science community ran headlong into escalating costs and schedule delays of the Space Shuttle program as well as the new fiscal realities of the Reagan Administration with its sharply reduced budgets. The result was the “Great Hiatus” in planetary exploration of the 1980s where not a single American planetary mission was launched between 1978 and 1989. And the only American planetary encounters during the 1980s were those involving missions that had been launched before the hiatus began.

In the first part of this essay, the predictions of 1980 for the exploration of the inner solar system were compared to how things actually unfolded (see “The Future That Never Came: Planetary Missions of the 1980s”). With a third of a century of hindsight, we now review how the predictions of 1980 for the exploration of the outer solar system and icy bodies fared.

Jupiter

Mission Launch Date Encounter Date Galileo Orbiter/Probe Jan 1982 Dec 1984 “Galileo 2” Orbiter/Ganymede Lander 1987 1990 “Galileo 3” Orbiter/Lander 1989 1992

The revised edition of Powers’ book was released in the wake of the Voyager 1 and 2 encounters with Jupiter on March 5 and July 9, 1979, respectively. Already in the pipeline was the follow on to the Voyager mission called Galileo. According to Powers’ book, Galileo was scheduled to be launched in January 1982 from the Space Shuttle using a Centaur upper stage to send the spacecraft directly to Jupiter. Once there, an entry probe would perform in situ measurements of Jupiter’s atmosphere and an orbiter that would continue the exploration of the Jovian system started by the Pioneer and Voyager flybys. The general belief at the time was that this would be the first in a series of missions to Jupiter that could eventually include simple landers to explore first Ganymede and then the other Jovian moons into the 1990s and beyond.

Unfortunately, delays with Galileo’s development and problems with the Space Shuttle exacerbated by tightening budgets eventually pushed the expected launch date out to May 1986. But even this launch date would slip in the wake of the Challenger disaster in January 1986. With the hydrogen-fueled Centaur deemed too dangerous for use on the Space Shuttle, Galileo was switched to the less capable IUS (Interim Upper Stage) and was forced to fly a longer, indirect path to Jupiter involving one Venus and two Earth flybys to provide the needed gravity assist. Galileo was finally deployed from the Space Shuttle Atlantis on STS-34 in October 1989. While Galileo finally reached Jupiter in December 1995 (11 years later than was scheduled in 1980), a problem with the deployment of the high gain antenna resulting from years of unplanned storage and extra cross-country trips severely limited the flow of data during its eight-year mission at Jupiter.

As it turned out, no Galileo-class follow on missions have ever been funded. NASA’s only post-Galileo mission dedicated to Jupiter is the comparatively modest Juno which was launched in August 2011 and is scheduled to enter orbit in July 2016. Its mission is simply to characterize Jupiter’s gravity field and magnetosphere from an extended polar orbit. ESA currently has JUICE (Jupiter Icy Moon Explorer) under development for launch in 2022. It will arrive at Jupiter in 2030 and flyby Europa and Callisto before finally settling into orbit around Ganymede. NASA is currently studying proposals for missions meant to explore Europa for launch in the 2020s but the fate of this mission is far from certain given NASA’s current budget realities. The aggressive exploration of Jupiter and its system of moons as envisioned in 1980 will take many decades longer than originally hoped.

Saturn

Mission Launch Date Encounter Date Voyager 1 Saturn/Titan Flyby Sep 5, 1977 Nov 1980 Voyager 2 Saturn Flyby Aug 20, 1977 Aug 1981 “Huygens” Orbiter/Titan Lander 1988 1992 Orbiter/Saturn Probe 1989 1999

The situation for the exploration of Saturn proved little better than it did for Jupiter. When Powers revised book was published, Pioneer 11 had just made the first close flyby of Saturn in September 1979 opening the way for the Voyagers. The Voyager 1 spacecraft successfully made its close encounter with Titan (one of the primary science objectives of the Voyager program) and Saturn in November 1980 (see “Voyager 1: The First Close Encounter with Titan”). This freed Voyager 2 to follow a path past Saturn in August 1981 that complimented the encounter of its predecessor and allowed it to reach Uranus and Neptune later in the 1980s.

In 1980, it was naturally assumed that the Voyager mission would be followed by a more advanced orbiter just as Galileo was expected to do with Jupiter. One concept, fancifully named “Huygens” would have been launched in 1988 and include a Titan lander whose design would be based on what was learned by Voyager 1. A Galileo-like mission with an orbiter and an Saturn atmospheric probe was also considered. Unfortunately, none of these proposals gained any traction during the 1980s.

Eventually, the Mariner Mark II concept was developed. It would be used to fly what we today call flagship-class missions that would complement the less expensive and more narrowly focused Observer missions (the Discovery-class missions of today). Years of studies, development and proposals eventually led to the joint NASA/ESA Cassini-Huygens mission launched in October 1997 (the only mission of this series to actually fly). These spacecraft reached Saturn in July 2004 with Cassini entering orbit to study Saturn and its moons while Huygens entered the atmosphere of Titan, landed and operated briefly on its surface in January 2005 (only a dozen years behind the optimistic schedule of 1980). With the end of the Cassini mission in 2017, no other missions to Saturn are currently being considered.

Uranus & Neptune

Mission Launch Date Encounter Date Voyager 2 Uranus Flyby Aug 20, 1977 Jan 1986 Voyager 2 Neptune Flyby Aug 20, 1977 Aug 1989 “Herschel” Uranus Orbiter/Probe 1990 1996

As with Saturn, the only encounters with Uranus and Neptune planned for the 1980s were from Voyager 2 launched in 1977. Since Voyager 1 had successfully completed its objectives at Titan and Saturn, this freed Voyager 2 to follow a trajectory past Saturn that allowed a flyby of Uranus in January 1986 (see “Voyager 2: The First Uranus Flyby”). Despite the problems with its only working receiver and the actuators used to point its cameras, Voyager 2 successfully returned data from Uranus and flew on to Neptune in August 1989 exceeding the wildest hopes of its builders (and space enthusiasts) in the process (see “Finishing the Grand Tour: Voyager 2 at Neptune”).

While the expectation in 1980 was that Voyager 2 would be followed up with orbiter missions to Uranus and Neptune with fanciful names like “Herschel” and “Leverrier”, respectively, in the 1990s, in reality these mission never materialized. Despite the fact that Neptune-size extrasolar planets decades later are now recognized as an important class of objects and the exploration of Uranus has been given a high priority for exploration, no missions to these worlds are currently being given serious consideration.

There had been some proposals for a Uranus mission for launch in the early 2020s to take advantage of an alignment with Jupiter for a gravity assist that would help decrease launch energy requirements, but these proposals never received the needed support. There are some mission concepts still being examined like ESA’s ODNUIS that would send orbiters to Uranus and Neptune but such missions are now looking at launch dates in the 2030s with encounters taking place in the 2040s or even later. It now seems that the missions to Uranus and Neptune envisioned in 1980 will happen a half a century later than thought back in those optimistic times.

Pluto

Mission Launch Date Encounter Date “Percival Lowell 1” Flyby 1980-1985 2000-2020

The only body defined as a “planet” in 1980 that had not been explored or had the possibility of being reached by a mission already launched was Pluto which today has been recategorized as a dwarf planet. While a direct flight from Earth to Pluto could have been launched during the early 1980s, it would not reach Pluto until the early decades of this century depending on the details of its trajectory (Powers appeared to be unaware of potential Jupiter gravity assist opportunities that would exist around 1992 that could have shaved a decade or more off of the travel times). After years of study and proposals, NASA’s New Horizons mission was finally launched to Pluto in January 2006 to reach its target in July 2015. Given how long it took to get this first mission to Pluto, it will likely be at least another quarter of a century before another mission will reach this distant world (see “The Next Mission to Pluto”).

Comets & Icy Bodies

Mission Launch Date Encounter Date Comet D’Arrest Flyby 1980 1982 Comet Kopf Flyby 1983 1983 Comet Halley Rendezvous 1982 1986 Comet Giacobini-Zinner Flyby 1985 1987 Comet Encke Sample Return 1990-1994 1992-1996 2060 Chiron Flyby 1993 1996

By far the biggest disappointment for space enthusiasts 34 years ago was Comet Halley. During the 1970s, NASA mission planners had convinced themselves that a rendezvous with Comet Halley was the only mission type that could return scientifically useful results. Even with the expected advances in spacecraft design and navigation, NASA was convinced that a flyby using a simple ballistic trajectory with its encounter velocity in excess of 60 kilometers per second was not a viable option. In order to decrease the encounter velocity and ideally rendezvous with Comet Halley, a whole range of potential options were examined including solar sails and ion propulsion. Unfortunately, all of these options involved the development of new technologies and would prove to be very expensive. By around the time Powers’ book had already been published, NASA had decided not to send a mission to Comet Halley. While NASA passed on Comet Halley, the Soviet Union, ESA and Japan sent a total of five spacecraft to this famous target and in March 1986 returned mountains of scientifically useful information despite the high encounter velocities (see “The Missions to Comet Halley“).

While NASA did not launch a dedicated mission to Comet Halley, in 1982 they did redirect ISEE 3 (International Sun-Earth Explorer 3) towards Comet Giacobini-Zinner. ISEE 3 had finished its mission at the L1 Sun-Earth Lagrange point 1.5 million kilometers from Earth and used its modest propulsion system and five lunar flybys to send it on its way towards the comet. ICE (International Cometary Explorer), as it was renamed, reached Comet Giacobini-Zinner in September 1985 – a half a year before the international armada of spacecraft reached Comet Halley making this the first comet encounter (see “ICE: The First Comet Flyby“).

It would be the late 1990s and long after the prediction in Powers’ book before NASA would start launching comet missions. NASA’s next mission to encounter a comet was the Deep Space 1 which passed by Comet Borrelly in September 2001 as part of a technology demonstration mission which included a prototype ion propulsion system. NASA’s first dedicated comet mission, Stardust, was launched in February 1999 and reached Comet Wild 2 in September 2003. Stardust used an array of aerogel cells to capture cometary dust at an encounter velocity of 6.1 kilometers per second for eventual return and study back on Earth (an innovative sample return method that had yet to be invented in 1980). Despite the slow start, comet exploration continues today with ESA’s Rosetta mission and its Philae lander.

As for encounters with other icy bodies, the only large ones known in 1980 aside from Pluto and its then recently discovered moon was the Centaur-class asteroid 2060 Chiron orbiting between Saturn and Uranus. While no mission was ever seriously considered for this far off world, the much larger number of icy objects discovered over the last couple of decades beyond Pluto in what has been called the Kuiper Belt has garnered much attention. New Horizons will flyby a recently discovered Kuiper Belt object, temporarily designated “2014 MU69” in January 2019 to start exploring the small icy bodies of this realm of the solar system only a couple of decades behind the optimistic schedule of 1980.

Postscript

This review of the predictions for future planetary missions from Robert Powers’ 1980 book, Planetary Encounters: The Future of Unmanned Spaceflight, for me was a nostalgic look at what young space enthusiasts like myself were hoping for at what turned out to be the end of the first Golden Age of planetary exploration. Hopefully the lessons we learned a third of a century ago during the subsequent Great Hiatus will help us avoid the mistakes that were made in the 1980s as we stand at the threshold of the end of the second Golden Age with a threatened repeat of ever tightening budgets.

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Related Reading

“The Future That Never Came: Planetary Missions of the 1980s”, Drew Ex Machina, November 27, 2014 [Post]

“Voyager 1: The First Close Encounter with Titan”, Drew Ex Machina, November 12, 2015 [Post]

“Voyager 2: The First Uranus Flyby”, Drew Ex Machina, January 24, 2016 [Post]

“Finishing the Grand Tour: Voyager 2 at Neptune”, Drew Ex Machina, August 25, 2019 [Post]

“The Missions to Comet Halley”, Drew Ex Machina, March 6, 2016 [Post]

“ICE: The First Comet Flyby”, Drew Ex Machina, September 11, 2015 [Post]

“The Next Mission to Pluto”, Drew Ex Machina, July 15, 2014 [Post]

General References

Robert M. Powers, Planetary Encounters: The Future of Unmanned Spaceflight (Revised Edition), Warner Books, 1980

Paolo Ulivi with David M. Harland, Robotic Exploration of the Solar System Part 1: The Golden Age 1957-1982, Springer-Praxis, 2007

Paolo Ulivi with David M. Harland, Robotic Exploration of the Solar System Part 2: Hiatus and Renewal 1983–1996, Springer-Praxis, 2009