Personal Interest - Voyager Space Program
The twin spacecraft Voyager 1 and Voyager 2 were launched by NASA in separate months in the summer of 1977 from Cape Canaveral, Florida. As originally designed, the Voyagers were to conduct closeup studies of Jupiter and Saturn, Saturn's rings, and the larger moons of the two planets.
To accomplish their two-planet mission, the spacecraft were built to last five years. But as the mission went on, and with the successful achievement of all its objectives, the additional flybys of the two outermost giant planets, Uranus and Neptune, proved possible -- and irresistible to mission scientists and engineers at the Voyagers' home at the Jet Propulsion Laboratory in Pasadena, California.
As the spacecraft flew across the solar system, remote-control reprogramming was used to endow the Voyagers with greater capabilities than they possessed when they left the Earth. Their two-planet mission became four. Their five-year lifetimes stretched to 12 and is now near forty-three years.
Eventually, between them, Voyager 1 and 2 would explore all the giant outer planets of our solar system, 48 of their moons, and the unique systems of rings and magnetic fields those planets possess.
Had the Voyager mission ended after the Jupiter and Saturn flybys alone, it still would have provided the material to rewrite astronomy textbooks. But having doubled their already ambitious itineraries, the Voyagers returned to Earth information over the years that has revolutionised the science of planetary astronomy, helping to resolve key questions while raising intriguing new ones about the origin and evolution of the planets in our solar system.
The command computer subsystem (CCS) provides sequencing and control functions The CCS contains fixed routines such as command decoding and fault detection and corrective routines, antenna pointing information, and spacecraft sequencing information.
The Attitude and Articulation Control Subsystem (AACS) controls spacecraft orientation, maintains the pointing of the high gain antenna towards Earth, controls attitude maneuvers, and positions the scan platform.
Uplink communications is via S-band (16-bits/sec command rate) while an X-band transmitter provides downlink telemetry at 160 bits/sec normally and 1.4 kbps for playback of high-rate plasma wave data. All data are transmitted from and received at the spacecraft via the 3.7 meter high-gain antenna (HGA).
Electrical power is supplied by three Radioisotope Thermoelectric Generators (RTGs). The current power levels are about 249 watts for each spacecraft. As the electrical power decreases, power loads on the spacecraft must be turned off in order to avoid having demand exceed supply. As loads are turned off, some spacecraft capabilities are eliminated.
The Voyager Interstellar Mission has the potential for obtaining useful interplanetary, and possibly interstellar, fields, particles, and waves science data until around the year 2025 when the spacecraft's ability to generate adequate electrical power for continued science instrument operation will come to an end.
Pioneer 10 made its closest encounter to Jupiter on 3 December 1973, passing within 81,000 miles of the cloudtops. This historic event marked humans' first approach to Jupiter and opened the way for exploration of the outer solar system - for Voyager to tour the outer planets, for Ulysses to break out of the ecliptic, for Galileo to investigate Jupiter and its satellites, and for Cassini to go to Saturn and probe Titan. During its Jupiter encounter, Pioneer 10 imaged the planet and its moons, and took measurements of Jupiter's magnetosphere, radiation belts, magnetic field, atmosphere, and interior. These measurements of the intense radiation environment near Jupiter were crucial in designing the Voyager and Galileo spacecraft.
Pioneer 10 made valuable scientific investigations in the outer regions of our solar system until the end of its science mission on 31 March 1997. The Pioneer 10 weak signal continued to be tracked by the DSN as part of an advanced concept study of communication technology in support of NASA's future interstellar probe mission. The power source on Pioneer 10 finally degraded to the point where the signal to Earth dropped below the threshold for detection in 2003. Pioneer 10 will continue to coast silently as a ghost ship through deep space into interstellar space, heading generally for the red star Aldebaran, which forms the eye of Taurus (The Bull). Aldebaran is about 68 light years away and it will take Pioneer over 2 million years to reach it.
Launched on 5 April 1973, Pioneer 11 followed its sister ship to Jupiter (1974), made the first direct observations of Saturn (1979) and studied energetic particles in the outer heliosphere. The Pioneer 11 Mission ended on 30 September 1995, when the last transmission from the spacecraft was received. There have been no communications with Pioneer 11 since. The Earth's motion has carried it out of the view of the spacecraft antenna. The spacecraft cannot be maneuvered to point back at the Earth. It is not known whether the spacecraft is still transmitting a signal. No further tracks of Pioneer 11 are scheduled. The spacecraft is headed toward the constellation of Aquila (The Eagle), Northwest of the constellation of Sagittarius. Pioneer 11 will pass near one of the stars in the constellation in about 4 million years.
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| Left: Illustration of the spacecraft in space: Pioneer 10 Leaving the Planets Behind, by Don Davis. Right: Copy of the plaque carried by Pioneer 10, with information about its creators, in case the spacecraft is found by an intelligent alien civilisation. |
Voyagers 1 and 2 are identical spacecraft. Each was launched with instruments to conduct 10 different investigations. The instruments included television cameras, infrared and ultraviolet sensors, magnetometers, plasma detectors, and cosmic-ray and charged-particle sensors. Four of these instrument suites — the cameras, photopolarimeter and infrared and ultraviolet sensors — operated from a scan platform that enabled them to be pointed with an accuracy better than one-tenth of a degree. In addition, the spacecraft radio was used to conduct investigations.
The Voyagers travel too far from the sun to use solar panels; instead, they were equipped with power sources called radioisotope thermoelectric generators (RTGs). These devices, used on other deep space missions, convert the heat produced from the natural radioactive decay of plutonium into electricity to power the spacecraft instruments, computers, radio and other systems.
The spacecraft are controlled and their data returned through the Deep Space Network (DSN), a global spacecraft tracking system operated by JPL for NASA. DSN antenna complexes are located in California’s Mojave Desert; near Madrid, Spain; and in Tidbinbilla, near Canberra, Australia.
Jupiter
Voyager 1 made its closest approach to Jupiter on March 5, 1979, and Voyager 2 followed with its closest approach occurring on July 9, 1979. The first spacecraft flew within 128,400 miles (206,700 kilometres of the planet’s cloud tops, and Voyager 2 came within 350,000 miles (570,000 kilometres).
Voyager visited eight of Jupiter’s moons and discovered three. Discovery of active volcanism on the satellite Io was easily the greatest unexpected discovery at Jupiter. It was the first time active volcanoes had been seen on another body in the solar system. Together, the Voyagers observed the eruption of nine volcanoes on Io, and there is evidence that other eruptions occurred between the Voyager encounters.
Saturn
The Voyager 1 and 2 Saturn flybys occurred nine months apart, with the closest approaches falling on November 12 and August 25, 1981. Voyager 1 flew within 40,000 miles (64,200 kilometres) of the cloud tops, while Voyager 2 came within 26,000 miles (41,000 kilometres).
The two Voyager spacecraft took images of 17 of Saturn’s moons, including four that they discovered. The irregular shapes of Saturn’s eight smallest moons indicate that they, too, are frag- ments of larger bodies. Unexpected structure such as kinks and spokes were found in addition to thin rings and broad, diffuse rings not observed from Earth. Much of the elaborate structure of some of the rings is due to the gravitational effects of nearby satellites. This phenomenon is most obviously demonstrated
by the relationship between the F-ring and two small moons that “shepherd” the ring material. The variation in the distances between the moons and the ring may explain the ring’s kinked appearance. Shepherding moons were also found by Voyager 2 at Uranus.
Uranus
In its first solo planetary flyby, Voyager 2 made its closest approach to Uranus on January 24, 1986, coming within 50,600 miles (81,500 kilometres of the planet’s cloud tops.
Voyager found 11 new moons at Uranus and visited 16. Most of the moons discovered by Voyager 2 are small, with the largest measuring about 90 miles (150 kilometres) in diameter. The moon Miranda, innermost of the five large moons, was revealed to be one of the strangest bodies yet seen in the solar system. Detailed images from Voyager’s flyby of the moon showed huge fault canyons as deep as 12 miles (20 kilometres), terraced layers, and a mixture of old and young surfaces. One theory holds that Miranda may be a reaggregration of material from an earlier time when the moon was fractured by an violent impact.
The five large moons appear to be ice–rock conglomerates like the satellites of Saturn. Titania is marked by huge fault systems and canyons indicating some degree of geologic, probably tectonic, activity in its history. Ariel has the brightest and possibly youngest surface of all the Uranian moons and also appears to have undergone geologic activity that led to many fault valleys and what seem to be extensive flows of icy material. Little geo- logic activity has occurred on Umbriel or Oberon, judging by their old and dark surfaces.
All nine previously known rings were studied by the spacecraft and showed the Uranian rings to be distinctly different from those at Jupiter and Saturn. The ring system may be relatively young and did not form at the same time as Uranus.
Neptune
Voyager flew within 3,000 miles (5,000 kilometers) of Neptune on August 25, 1989. Neptune orbits the sun every 165 years. It is the smallest of our solar system’s gas giants. The length of a Neptunian day has been determined to be 16 hours, 6.7 minutes. Voyager imaged eight of Neptune’s moons, discovering five of them. Triton, the largest of the moons of Neptune, was shown to be not only the most intriguing satellite of the Neptunian system, but one of the most interesting in all the solar system. It shows evidence of a remarkable geologic history, and Voyager
2 images showed active geyser-like eruptions spewing invisible nitrogen gas and dark dust particles several miles (kilometers) into the tenuous atmosphere. Triton’s relatively high density and retrograde orbit offer strong evidence that Triton is not an original member of Neptune’s family but is a captured object. If that is the case, tidal heating could have melted Triton in its originally eccentric orbit, and the moon might even have been liquid for as long as one billion years after its capture by Neptune. Voyager 2 solved many of the questions scientists had about Neptune’s rings. Searches for “ring arcs,” or partial rings, showed that Neptune’s rings actually are complete, but are so diffuse and the material in them so fine that they could not be fully resolved from Earth. From the outermost in the rings have been designated Adams, Plateau, Le Verrier and Galle.
Intersteller Mission
Voyager 1 first crossed the boundary into interstellar space during 2012 while its twin Voyager 2 crossed in 2018. During their journey through and out of the solar systems they provided evidence through data gathered from their instruments about the nature how our Sun's influence defines the boundaries of the solar system.
Termination Shock:Blowing outward billions of kilometers from the Sun is the solar wind, a thin stream of electrically charged gas. This wind travels at an average speed ranging from 300 to 700 kilometers per second (700,000 - 1,500,000 miles per hour) until it reaches the termination shock. At this point, the speed of the solar wind drops abruptly as it begins to feel the effects of interstellar wind.
Heliosphere:
The solar wind, emanating from the Sun, creates a bubble that extends far past the orbits of the planets. This bubble is the heliosphere, shaped like a long wind sock as it moves with the Sun through interstellar space.
Heliosheath:
The heliosheath is the outer region of the heliosphere, just beyond the termination shock, the point where the solar wind slows abruptly, becoming denser and hotter. The solar wind piles up as it presses outward against the approaching wind in interstellar space.
Heliopause:
The boundary between solar wind and interstellar wind is the heliopause, where the pressure of the two winds are in balance. This balance in pressure causes the solar wind to turn back and flow down the tail of the heliosphere.
Bow shock:
While the probes have left the heliosphere, Voyager 1 and Voyager 2 have not yet left the influence of our solar system and the sun, and won't be leaving anytime soon. The boundary of the solar system is considered to be beyond the outer edge of the Oort Cloud, a collection of small objects that are still under the influence of the Sun's gravity. The width of the Oort Cloud is not known precisely, but it is estimated to begin at about 1,000 astronomical units (AU) from the Sun and to extend to about 100,000 AU. One AU is the distance from the Sun to Earth. It will take about 300 years for Voyager 2 to reach the inner edge of the Oort Cloud and possibly 30,000 years to fly beyond it.
On February 14, 1990, Voyager 1 took the last pictures of the Voyager mission. Beyond the outermost planet in our solar system, at a distance of about 3.7 billion miles (6 billion kilome- ters), Voyager 1 turned its camera inward to snap a series of final images that became its parting valentine to the string of planets it called home.
Mercury was too close to the sun to see, Mars showed only a thin crescent of sunlight and Pluto was too dim, but Voyager was able to capture cameos of Neptune, Uranus, Saturn, Jupiter, Earth and Venus from its unique vantage point. These images, later arranged in a large-scale mosaic, make up the only family portrait of our planets arrayed about the sun.
It was an image from this set that inspired Carl Sagan, the Voyager imaging team member who had suggested taking this portrait, to call our home planet “a pale blue dot.”
After that set of portraits, the cameras on Voyager 1 and 2 were switched off and the software controlling them removed from the spacecraft. There was very little for the cameras to see in the vast, dark emptiness of space. Mission managers needed to make space in the memory and conserve power for other instruments that would be able to detect the changes in the charged particles. It would be those changes that would describe what the far reaches of the solar system were like. This milestone marked the end of the Grand Tour mission and the beginning of Voyager’s Interstellar Mission.
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| Solar System Portrait - 60 Frame Mosaic |
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