The Voyager spacecraft – NASA’s longest operational interplanetary probes – are about to enter their 34th year of operation, as they continue their sail out of the confines of our solar system toward the ever-present void of interstellar space. And as controllers on Earth manage the gradual and progressive power-down of the two spacecraft, the Voyagers continue to beam back invaluable and never-before collected data on the calm and turbulent outer-most edges of our sun’s immediate zone of influence in local space.
The Voyager concept – A Grand Tour of the outer planets:
The mission of the Voyagers was simple enough on the surface: visit the solar system’s gas and ice giant planets in a few short years.
However, this ‘simple’ concept would, under ‘normal’ circumstances, have been extremely difficult to achieve in an ~10-year period given the vast and often incomprehensible distances between the orbital paths of the outer planets – not to mention that these outer giants of our solar system rarely “align” with one another.
But in the late 1970s, that very elusive “alignment” of Jupiter, Saturn, Uranus, and Neptune (an event that occurs only once every 175 years) became a reality and gave NASA the opportunity to do something that had never before (or since) been attempted: send two probes to visit all four of the outer giant planets within an ~10-year span of time.
Though the development period of the two probes that would eventually become the most well-known unmanned space probes in history was fraught with political, financial, and technical difficulties, the triumphs of the spacecraft, and the people who built and operated them, would clearly establish the Voyager project as the gold standard in planetary exploration.
Trials and tribulations – The long road through mission development:
Following the identification of the alignment of the outer planets and the continued research into gravitational assist flybys of planets, NASA developed the Grand Tour proposal in the 1960s to send a total of four probes to, at the time, all five of the outer planets (Jupiter, Saturn, Uranus, Neptune, and Pluto).
In 1965, Gary Flandro, a Caltech graduate student working at the Jet Propulsion Laboratory (JPL), was assigned the task of coming up with feasible trajectories for missions to the outer planets.
The challenges were tremendous, ranging from guidance, communications, power, and the simple fact that no spacecraft had ever been designed with such a long lifetime in mind. Nonetheless, Flandro found trajectories with launches between 1975 and 1981 that would allow visits to various combinations of Jupiter, Saturn, Uranus, Neptune, and even Pluto.
These trajectories would require mission lifetimes of 30 years, though individual planetary encounters would occur at intervals of several years. Despite all the obvious difficulties, the possibilities were very attractive to NASA.
By 1970, planetary scientists were cautiously supportive of the Grand Tour, and President Nixon added his support, saying that preparations would begin in 1972. Committees and working groups then began the long task of sorting through scientific objectives as well as instrument packages that could accomplish them.
However, it quickly became clear that this project would not be achieved cheaply, and estimates in 1970 were nearing $1 billion (about $6 billion in 2011 dollars). Therefore, significant tradeoffs were going to be needed.
Echoing today’s situation, a highly constrained NASA budget with many expensive priorities nearly killed the Grand Tour outer planets missions.
In the early 1970s, NASA was faced with the expense of the final Apollo missions, the Viking program, Skylab, and the beginnings of the Space Shuttle Program.
By late 1971, the Grand Tour program was unsustainable, and NASA Administrator James Fletcher decided to terminate the mission.
However, all was not lost. Responding to the cancelation of the Grand Tour mission, JPL created a scaled-down program based on the successful Mariner spacecraft. The program was called Mariner-Jupiter-Saturn ’77 (MJS’77).
The MJS’77 plan called for the launch two identical spacecraft derived from the Mariner series. The MJS’77 mission, the two spacecraft carried designations of Mariner 11 and Mariner 12 and would use Titan boosters outfitted with large solid motors and the high-energy oxygen/hydrogen Centaur upper stage to propel them into an Earth-escape trajectory.
A final kick from a smaller solid rocket would finally propel the Mariners onto course for dual encounters with Jupiter and Saturn.
By this point in the project, the two spacecraft were being designed to last through Saturn flybys only.
As the design of the MJS’77 spacecraft matured, the extent of the innovations needed to carry off a mission of this mission quickly became clear.
A nuclear power source would be needed, as the distance from the Sun was far too great for the solar photovoltaic panels of the era to power the spacecraft. Likewise, the new spacecraft would need computers capable of autonomously operating the spacecraft’s engineering systems and instruments during planetary flybys.
The computers would also taking care of routine housekeeping tasks and beam back “life sign” information to Earth. In turn, the computers would receive updated command loads periodically, with more updates near critical events prior to and during planetary encounters.
Moreover, radiation hardened electronics had to be quickly added to the spacecraft late in the design phase due to the Pioneer 10 and 11 discoveries of a very intense radiation environment near Jupiter.
Furthermore, to guard against the possibility of loss of mission due to the failure of the dual high bandwidth X-band transmitters, a Reed-Solomon data compression encoder was added for use by the S-band low-gain transmitters.
In 1976, led by the dramatic improvements over the basic Mariner design and a much bolder mission then originally conceived, the MJS’77 project manager began a campaign to change the name of the project from MJS’77 to Voyager.
By 1977, NASA was referring to the mission as Voyager instead of MJS and was even considering the possibility that one of the spacecraft (now referred to as the the Voyagers) could continue on to Uranus if possible. Until the point, open discussions on the extension of the missions beyond Saturn was highly taboo given the extreme budget pressure at NASA.
The first Voyager (officially known at the time as VGR77-1), was a non-flight, proof of concept test model that contained only a partial set of electronics designed to allow engineers to work with the hardware and practice methods of construction.
The remaining two flight spacecraft, VGR77-2 and VGR77-3, soon followed in production, with VGR77-3 receiving the highest performing power unit.
Soon, VGR77-3 was renamed Voyager 2 and VGR77-2 became Voyager 1. By this point, launch of the two space-worthy Voyager probes was set for August and September 1977.
The Voyagers’ missions – visits to the giants of our solar system:
Launching first, Voyager 2 was lofted into space on August 20, 1977 at 10:29.00 EDT from Space Launch Complex 41 at the Cape Canaveral Air Force Station, FL by a Titan IIIE/Centaur rocket.
Despite being launched second, Voyager 1 quickly overtook Voyager 2 and reached both Jupiter and Saturn before its sister probe.
Specifically for the Jupiter-Saturn mission, the original trajectories of both Voyager 1 and Voyager 2 ensured encounters with Jupiter and Saturn for both probes.
Voyager 1 experienced closest approach with Jupiter on March 5, 1979 and Saturn on November 13, 1980. Conversely, Voyager 2 made its closest approach with Jupiter on July 9, 1979 and Saturn on August 26, 1981.
During Voyager 1’s approach to Saturn, the probe was sent into a close pass by Titan – resulting in a huge gravity boost and acceleration out of the ecliptic of the solar system and well off course from any of the remaining planets in the solar system.
Satisfaction of this mission goal eliminated the possibility of sending Voyager 1 to Pluto.
Nonetheless, the successful flyby of Titan satisfied a key Voyager mission objective and allowed NASA to proceed with formal plans to use Saturn’s gravity to alter Voyager 2’s trajectory to ensure an encounter with Uranus.
Voyager 2’s targeting at Saturn was perfect, and the spacecraft’s trajectory was adequately altered to send the spacecraft to Uranus.
Closest approach to Uranus occurred on January 24, 1986 and gave Voyager 2 the honor of being the first, and to date, only spacecraft to explore a planetary body outside the orbital distance of Saturn.
Moreover, it marked a huge milestone for Voyager 2 – which was only designed to operate through its encounter with Saturn five years prior.
Time of closest approach to Neptune occurred on August 25, 1989 – exactly 12 years 5 days after the probe’s launch. The encounter with Neptune ended with the decision to swing Voyager 2 around Neptune and perform a very close flyby of its moon Triton.
This move swung Voyager 2 out of the elliptic plane of the solar system at a more severe angle than that of Voyager 1’s.
Moreover, the encounter with Triton shot Voyager 2 to the “south” of the elliptic plane. Voyager 1’s exit from the elliptic plane was to the “north” of the elliptic.
Barely managing to survive the difficult early years, the Voyagers ended the Neptune flyby as the scientific equivalent of rock stars, with many public television stations around the United States carrying live broadcasts as science images of Neptune and its moon Triton were returned to Earth during the night.
But the Voyagers still weren’t done.
Lovingly called the “Family Portrait,” the image shows six of the solar system’s eight planets. Mercury (due to its proximity to the sun) and Mars (due to light scattering) were the only two planets not visible in the produced photo.
At the time, Pluto was still considered a planet; however, Pluto was not included in the family portrait due to its low reflectivity and distance from the sun.
The “family portrait” is the source of “The Pale Blue Dot” photograph of Earth and stands today as the most-distant photograph ever taken of the solar system – and the last photographs ever taken by the Voyagers.
The Voyager Interstellar Mission: To the edge of the solar system… and beyond!
But even after 1990, the Voyager spacecraft did not stop and were not retired. Shattering all estimates as to their life expectancy post-Saturn, both Voyager 1 and Voyager 2 continue to operate and transmit invaluable scientific data to Earth to this day.
The VIM will carry the spacecraft through until at least 2020 with the goal to explore the space environment at unprecedented distances from the Sun, searching for the boundary between the solar system and interstellar space by examining the charged particle environment, magnetic fields, and plasma waves present at the edge of the solar system.
At present, Voyager 1 is 117.8 AU distance from the Sun. Voyager 2 is almost 96.04 AU distance. The great distances mean that commanding the spacecraft must be done with care. With round-trip light travel times of more than 32 hours 32 minutes for Voyager 1 and 26 hours 24 minutes for Voyager 2, the concept of “real-time commanding” assumes a whole new meaning.
Barring a catastrophic failure on one of the spacecraft, it is the gradual loss of electrical power that seems most likely to be the ultimate cause of the end of the Voyager mission.
Even though much of the instrument complement was turned off years ago, power margins on the spacecraft are steadily dropping; they currently have about 25 watts of margin on average, though configuration changes, such as turning on the gyros, can result in the margins dropping to near 10 watts.
Thus, with good luck and careful management, the twin Voyagers will give humanity the first measurements taken from interstellar space.
Jeff Goldader holds a BSc in Physics and Astronomy from the University of Washington, and a PhD in astronomy from the University of Hawaii. For three years, he held a research fellowship at the Space Telescope Science Institute in Maryland, and he was a Lecturer at the University of Pennsylvania for five years. He is currently a high school physics teacher in Pennsylvania.
(Images via NASA JPL, NASA, L2 Historical)