The history of planetary exploration is filled with proposals that never flew. While there are some that, at best, had dubious chances at success, there are many others that could have added greatly to our current knowledge of the Solar System. Among those good ideas that never flew is a largely forgotten proposal from the mid-1970s to adapt the design of the Pioneer spacecraft that made the first flights to the outer planets in the 1970s into carriers for atmospheric probes destined for Saturn and Uranus. I recall first reading a brief mention about this proposal decades ago when I was a student but recently stumbled upon some four-decade old NASA technical reports about the concept that rekindled my interest in this old proposal. Considering that we have yet to probe the atmosphere of Saturn even to this day or send a follow up mission to Uranus almost three decades after the historic Voyager 2 encounter, I figured it might be time to share the story of the proposed Pioneer Saturn/Uranus probe missions.

Adapting to a New Mission

By the 1970s, NASA was flush with ideas on exploring the outer planets of the Solar System especially Jupiter, the largest and easiest to reach of these worlds. Unfortunately, NASA-sponsored studies for Jupiter atmospheric probes performed in 1971 concluded that the high entry speeds for such missions were beyond the state of heat shield technology of that time. Saturn and Uranus, with their smaller masses, would have lower entry speeds that would make constructing an atmospheric probe using existing technology more practical despite their greater distances and longer transit times. With this in mind, the Outer Planet Science Advisory Group of NASA recommended that the feasibility of atmospheric probes directed towards Saturn and Uranus be examined. During 1972-73, a team at NASA Ames Research Center worked with contractors to determine if it would be possible to modify the proven Pioneer F/G spacecraft design to carry atmospheric probes based on existing technology to these outer planet targets.

The Pioneer F/G spacecraft, known as Pioneer 10 and 11 after their launch on March 3, 1972 and April 6, 1973, respectively, were spin-stabilized spacecraft with masses of 258 kilograms built by TRW. Launched using Atlas-Centaur rockets fitted with TE-M-364-4 solid motor kick stages to boost their injection velocities to reach Jupiter, Pioneer 10 and 11 were simple yet robust spacecraft tasked with performing an initial reconnaissance of Jupiter and its environment ahead of the more complex Voyager missions set for launch in the summer of 1977. Depending on what Pioneer 10 found during its encounter with Jupiter on December 5, 1973, Pioneer 11 would be directed to make a much closer flyby that would send it on a new trajectory high above the ecliptic plane and across the Solar System to reach Saturn almost five years later and perform an initial exploration of that system over a year ahead of the first Voyager.

These Pioneer spacecraft used Plutonium-fueled RTGs for power and carried a suite of instruments with a total mass of 30 kilograms to measure radiation, magnetic fields and micrometeoroids as well as a simple two-color imaging photopolarimeter. Using the spin of the spacecraft to scan the scene to create crude images, this instrument secured views of Jupiter and Saturn with much better resolution than was available from ground-based telescopes of the day. With the addition of hardware to carry an atmospheric probe along with some probe communications equipment and other minor modifications, it was found the Pioneer F/G spacecraft could easily act as an atmospheric probe carrier.

The carrier would also be equipped a mix of instruments old and new in order to make useful observations of the target planet during its flyby. Among the proposed instruments would be a new solid state line scan imager with twice the resolution of Pioneer’s older imaging photopolarimeter that would also provide optical navigation data vital for mission success.

The atmospheric probe design, which would be identical for Saturn and Uranus, was based in large part on development work already well underway in support of the Pioneer Venus mission which was eventually launched in 1978 (see “NASA’s Unintentional Venus Lander“). The final configuration of the probe had a mass of about 92 kilograms (about the same as the eventual Pioneer-Venus “Small Probe” design) and would be constructed to survive atmospheric pressures of up to 10 bars. Because of the limitations of the communication link with the carrier spacecraft, which would relay recorded and real-time data from the probe back to Earth during its flyby of the target planet, the final probe configuration did not include a parachute. This would allow the probe to penetrate as deeply and as quickly as possible into the atmosphere before the carrier moved out of range.

Each probe would carry five instruments. Temperature and pressure gauges would provide direct information on these atmospheric properties. Accelerometers would provide information that would allow the probe’s trajectory to be reconstructed as well as provide data on atmospheric density during descent. A nephelometer would characterize particles in the various cloud layers surrounding Saturn and Uranus. The probes’ instrument suite would be rounded out by a neutral mass spectrometer which would allow the composition of the atmosphere to be measured.

The proposed Pioneer probe mission set examined consisted of a single launch directly to Saturn in November 1979 and a pair of launches in December 1980 to Saturn with a subsequent flyby of Uranus. During this period, there were no energetically favorable Jupiter gravity assist options to Saturn that would decrease the mission length. However, in 1980 there existed an alternate Jupiter-Uranus mission profile for one of the two probes. The addition of this option had the advantage of increasing the 24-day launch window for the Saturn-Uranus option by another two weeks and could be used if the second probe could not be launched in time for the preferred Saturn-Uranus window for some reason. The Jupiter-Uranus option also had the advantages of avoiding any hazards associated with the close passage of the rings of Saturn and it would reach Uranus 1½ years earlier than the Saturn-Uranus option.

The capability to target a probe to Saturn with a 1979 and 1980 launch would be maintained in order to provide redundancy in case the first probe failed to meet its objectives. All three spacecraft, with an estimated mass of about 480 kilograms each, would require the use of a Titan IIIE-Centaur fitted with a TE-364-4 solid rocket kick stage like that used to launch the Voyagers. A timeline of launch and encounter dates for these three proposed Pioneer probe missions in relation to the Pioneer 10 and 11 as well as Voyager missions that were actually flown is shown in Table I below.

Table I: Timeline of Proposed Pioneer Saturn/Uranus Missions

Spacecraft Launch Date Jupiter Encounter Saturn Encounter Uranus Encounter Pioneer 10 Mar 3, 1972 Dec 5, 1973 – – Pioneer 11 Apr 6, 1973 Dec 3, 1974 Sep 1, 1979 – Voyager 1 Sep 5, 1977 Mar 5, 1979 Nov 12, 1980 – Voyager 2 Aug 20, 1977 Jul 9, 1979 Aug 26, 1981 Jan 24, 1986 Pioneer-Saturn ’79 (one spacecraft) Nov 1979 – Apr 15, 1983 Sep 1987 (Optional) Pioneer-Saturn/Uranus ’80 (two spacecraft) Dec 1980 – Jan 4, 1984 Nov 1987 Pioneer-Jupiter/Uranus ’80 (alternate) Dec 1980 May 1982 – Apr 1986

Pioneer Saturn ’79 Mission

The first of the trio of new Pioneer missions would have been launched in November of 1979 on a direct trajectory to Saturn. In order to ease mission planning and maximize the science return of the Pioneer Saturn ’79 mission, a fixed arrival date of April 15, 1983 was chosen for a transit time to target of 3.4 years. This arrival would have been almost 20 months after the eventual Voyager 2 encounter with Saturn.

The nominal mission design called for the atmospheric probe to be released 35.7 days before the encounter and targeted to enter Saturn’s atmosphere in the daylight hemisphere at middle northern latitudes while at the same time avoiding a passage through the hazardous rings. Shortly after deploying the probe, the carrier would perform a deflection maneuver with a delta-v of about 70 meters/second at a range of 30 million kilometers from Saturn so that it could safely flyby the ringed planet while making its own observations as well as relay data from the probe.

During its approach to Saturn, the Pioneer carrier would pass as close as 150,000 kilometers of Titan allowing scientifically useful observations to be made of Saturn’s largest moon. The atmospheric probe would enter Saturn’s atmosphere at a speed of 9.2 kilometers/second and start relaying data to the Pioneer carrier flying overhead about a minute later. The probe would continue to transmit data with the range to the carrier decreasing from 100,000 to 80,000 kilometers until the atmospheric pressure reached 10 bars about 28 to 63 minutes after entry, depending on the assumed properties of Saturn’s atmosphere.

In the mean time, the carrier would follow a trajectory that would pass no closer that 2 Saturn-radii (R S ) from the planet. This trajectory would ensure that the carrier spacecraft would pass no closer that 2.4 R S and well beyond the visible rings during the ring plane crossing. In addition, the carrier would have made its approach observations and completed relaying the data from the probe before the ring plane crossing just in case the passage proved to be more dangerous than expected and the carrier was destroyed. Assuming the carrier survived its passage by the rings, its trajectory would permit an occultation of the planet as viewed from the Earth allowing its radio beacon to probe the rings and atmosphere.

There also existed the option where a properly targeted flyby of Saturn at a distance of 2.2 R S over 30.5° south latitude would allow the Pioneer Saturn ’79 carrier to continue on towards Uranus with a 4.4 year transit time. If this option were exercised, the carrier would flyby Uranus in September 1987 – about 20 months after the eventual Voyager 2 encounter with Uranus and a couple of months before the Pioneer Saturn/Uranus ’80 mission would reach its target.

Pioneer Saturn/Uranus ’80 Mission

The next phase of the proposed mission would include the launch of a pair of spacecraft towards Saturn in late November or early December 1980. Once again, in order to maximize the science return and ease planning for the Pioneer Saturn/Uranus ’80 mission, a fixed encounter date of January 4, 1984 with a 3.1 year transit time was adopted – 8½ months after the scheduled encounter date of the Pioneer Saturn ’79 mission. If both spacecraft could not be launched in the allowed window or if the passage near Saturn’s rings proved to be too hazardous, there was the alternate trajectory option of launching a probe towards Uranus using a Jupiter gravity assist later in the month. Depending on the exact launch date, this latter option would require a distant flyby of some 10 to 20 Jupiter-radii about 17 months after launch that would be far removed from the worst of the giant planet’s radiation hazards. The distant encounter would also allow Jupiter to be observed for the first time in the almost three years since the eventual flyby of Voyager 2.

Since the atmospheric probes of these spacecraft were of identical design, the option existed for one of them to enter Saturn’s atmosphere in case the Pioneer Saturn ’79 mission had failed to meet its objectives. The encounter with Saturn and the events surrounding a possible Saturn entry in January 1984 would have been similar to those of the earlier mission. One major difference would be that the potential encounter geometry with Titan would not be nearly as good as it had been during the Pioneer Saturn ’79 mission. The Pioneer Saturn/Uranus spacecraft would pass no closer than 320,000 kilometers of Saturn’s largest moon with doubtful prospects of returning scientifically useful data.

If the spacecraft were to continue on to Uranus after its encounter with Saturn (with or without its atmospheric probe depending on the decision made), a trajectory with a 2.73 R S closest approach point would be followed. In addition to allowing the Uranus option to be exercised, this trajectory would also allow the atmosphere and rings of Saturn to be probed by the carrier craft’s radio beacon as would have been done during the Pioneer Saturn ’79 mission.

With the pole of Uranus pointed more or less towards the Sun and with no further encounters planned afterwards, there were a much larger range of options available for the Uranus encounter. About 20.8 days before reaching Uranus in November 1987, the atmospheric probe would be deployed followed by the carrier making a deflection maneuver 25 million kilometers from Uranus with a delta-v of 61 meters/second. The probe would enter the Uranian atmosphere at a speed of 13.8 kilometers/second and transmit its findings back to Earth via its carrier spacecraft. Depending on the properties of the atmosphere, the probe was expected to survive for 29 to 74 minutes before it reached the depth in the Uranian atmosphere where the pressure exceeded 10 bars. The carrier spacecraft would flyby Uranus at a distance of about 3 planet radii (well beyond the rings of Uranus which were not even discovered until 1977) or even closer as it relayed the probe data and made its own observations. The sequence of events for a spacecraft that followed the Jupiter-Uranus trajectory option with its April 1986 arrival date (less than three months after the eventual Voyager 2 encounter with Uranus), would have been similar.

A Mission Not Flown

So what happened to these proposed Pioneer missions to Saturn and Uranus? Obviously, NASA opted not to fly them and quietly shelved the idea in 1974 largely for budgetary reasons. At this time, NASA’s post-Apollo era budget was shrinking at the same time development of the new Space Shuttle was ramping up. In addition, NASA already had two expensive planetary programs in the works. The Viking program to land and search for life on Mars was experiencing cost overruns as it pushed towards a 1975 launch. There was also the Mariner Jupiter-Saturn (later known as Voyager) program that was under development. This pair of spacecraft was much more advanced than the Pioneers and there was the option of modifying this design to be a probe carrier for a possible Mariner Jupiter-Uranus mission for launch in 1979 that could return far more data than a Pioneer-based spacecraft. There was also a reluctance in NASA to purchase any more expensive Titan IIIE-Centaur rockets through the US Air Force after Voyager as the agency was planning to make its transition to the Space Shuttle.

In this climate, NASA was simply unwilling to commit to the Pioneer missions to Saturn and Uranus described here. And it was only with great difficulty that NASA managed to get funding for its more modest Pioneer Venus mission approved by Congress in 1975. No one knew it at the time, but the foundations were beginning to be laid for what would become the Great Hiatus – the period from 1978 to 1989 when there were no American planetary missions launched (see “The Future That Never Came: Planetary Missions of the 1980s Part I” and “Part II”). Although these Pioneer missions would have returned exciting new data on the outer planets on the heels of the Voyager encounters in the 1980s, at very least these design studies did lay the groundwork for other missions including the eventual Galileo probe design that was used to plumb the depths of the Jovian atmosphere in December 1995. And maybe these modest designs may inspire present-day planners for long overdue missions to explore the atmospheres of Saturn and especially Uranus using more modern and capable technology.

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Related Reading

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

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

“The Future That Never Came: The Planetary Missions of the 1980s – II”, Drew Ex Machina, December 1, 2014 [Post]

“NASA’s Unintentional Venus Lander”, Drew Ex Machina, June 13, 2016 [Post]

General References

Richard O. Fimmel, James Van Allen and Eric Burgess, Pioneer: First to Jupiter, Saturn and Beyond, NASA SP-446, 1980

Richard O. Fimmel, Lawrence Colin and Eric Burgess, Pioneer Venus, NASA SP-461, 1983

Byron L. Swenson, Edward L. Tindle and Larry A. Manning, “Mission Planning for Pioneer Saturn/Uranus Atmospheric Probe Mission”, NASA Technical Memorandum TM X-2824, September 1973

“Outer Planet Entry Probe System Study – Final Report: Volume IV Common Saturn/Uranus Probe Studies”, Martin Marietta, January 1973

“Saturn Uranus Atmospheric Entry Probe Mission Spacecraft System Definition Study”, NASA CR-137648 (Prepared by TRW Systems Group), July 15, 1973

“Study of the Adaptability of Existing Hardware Designs to a Pioneer Saturn/Uranus Probe – Final Report”, NASA CR-137650 (prepared by Martin Marietta Corp.), October 1973