The Grand Tour
ODYSSEY TO THE BOUNDARIES OF THE SOLAR SYSTEM: THE GRAND TOUR
The unmanned Voyager 1 and Voyager 2 spacecraft have a special place in History as the most successful interplanetary missions to date. In epic journeys lasting already more than three decades, they easily doubled the knowledge we had about the Solar System. They continue in the forefront as the farthermost-from-Earth human artifacts, and continue flying away in the cosmic ocean.
At the end of the 60s technicians at the Jet Propulsion Laboratory of the California Institute of Technology, under contract with NASA, realized that there would be a very rare alignment of planets by 1980. They thought they could take the opportunity to launch a pair of unmanned probes to explore the outer planets of the Solar System, as the alignment would make possible to fly the spacecraft by all these planets in one pass, one after another. These missions was informally called The Grand Tour.
Actually the technique was a bit more complex than what it seems: the first problem is that while the spacecraft, after launch, no longer need engines beyond the initial impulse (because in space there is no air to brake them and they can continue to travel simply by inertia), the rockets available could not provide enough momentum to overcome the gravity of the Sun and send them beyond Jupiter. But they realized that they could exploit the movement of Jupiter itself and accelerate the spacecraft, like a Formula 1 car placed in the vacuum suction generated right behind another car does in order to gain speed and overtake it. The "vacuum suction" in this case would be the very gravity of Jupiter. All you have to do is to place the spacecraft in the right place and at the right time, behind Jupiter, for this to trail it with is gravity and so the spacecraft ends speeding up. This could also be done with Saturn, in order to reach Uranus, and with Uranus to reach Neptune.
The other problem is that the spacecraft should last for about 15 years, "without ever stepping into a garage, not even for an oil change." They should be highly sophisticated and robust, what put them beyond the budget. In the end the money available allowed to build two spacecraft with an useful-lifespan guarantee of 5 years, i.e., up to Saturn.
The concept was tested with the latests spacecraft of the Pioneer series, the 10 and 11, launched in 1972 and 1974 to Jupiter. As they were designed primarily to explore the space environment between the planets, their cameras were not sophisticated enough to send great news. Eventually the 11 survived to reach Saturn.
THE VOYAGER SPACECRAFT
Both Voyager spacecraft, with a total mass of 800 kg, were identical: a small octagonal structure with computers and a bit of maneuvering fuel, sporting an attached dish antenna, 3,7 m in diameter, to communicate with Earth. Because of the distance that they must travel, it would be hours between the sending of a radio signal (traveling at the same speed as the speed of light) and the reception of the reply. It was necessary to provide some autonomy to the spacecraft; that is why these were among the first which had some degree of artificial intelligence, at least to detect faults and try to correct them before they become serious problems. The fault protection program could interrupt the work, pointed the cameras away from the Sun to protect them and pointed the antenna toward Earth, awaiting for instructions.
As they would travel so far from the Sun it was not possible to use photoelectric panels; batteries would have to be very long lasting, and the only option was plutonium nuclear batteries, installed in a beam away from the main spacecraft body. Another beam, longer still, held a magnetometer, away from the other circuits of the spacecraft, to measure magnetic fields in space and those of the planets. Two long wire whips antennae would measure electric fields and collisions with dust particles. Across the spacecraft central body another additional beam held various instruments to detect protons, electrons and ions.
A rotary platform hold five telescopes: one of 6 cm in diameter, coupled to an ultraviolet spectrometer, which through a very sophisticated analysis of their colors could study the atmospheres and fogs; another of 20 cm, coupled to a photopolarimeter to analyze how the light reflected from surfaces, providing information on their texture (i.e., smooth or rough); another, a 50 cm telescope, was coupled to infrared sensors to measure temperatures and chemical compositions using spectrometry; and two visible-light cameras: one coupled to a 6 cm telescope, for panoramic photos; and another attached to the fifth telescope, 18 cm in diameter, for high-resolution, detailed photographs.
When the spacecraft is passing behind a planet, or satellite, or ring, fluctuations in the radio signal could be measured, and that could be used to indicate the composition of the object of study. By calculating the changes in velocity and in position of the spacecraft an idea of the force of gravity of a planet or satellite could be obtained, therefore its mass, and by the sizes measured by the cameras, its density, and thereafter their possible internal constituting materials could be inferred.
Voyager 2 was launched first, on 20 August 1977, from Cape Canaveral, Florida, USA, on the tip of a 633-ton Titan 3 rocket. Once accelerated to more than 45 000 km per hour in less than 15 minutes of operation, inertia would do the rest. Voyager 1 followed on 5 September 1977, when the Earth was a little closer to Jupiter. The plan was to follow a large curved trajectory, bent by the Sun's gravity, flying by Jupiter in 1979 and by Saturn in 1980-81. The 1 should pass right near the largest satellite of this, Titan, so that its trajectory would curve upward, away from the other planets. But optimism was great, and just in case, if Voyager 2 reached Saturn in good health, technicians could position it so that by virtue of its trajectory it could be catapulted towards Uranus, speculating that it can survive up to a flyby of this planet in 1986. And if things continued better than expected and the spacecraft continue functioning, it could be speculated to try to get to Neptune, much later in 1989.
But the funds authorized by NASA initially were good enough only through Saturn.
To communicate with the spacecraft three of the world's largest parabolic antennas, 66 meters in diameter each, would be used: one in Australia, one in Spain and one in California, to provide coverage regardless of the rotation of the Earth.
One of the calibration photographs by Voyager 1, weeks after the departure, was shot looking back to its starting planet: it was the first photograph showing the Earth and the Moon together in space.
In April 1978, through a combination of human and technical errors, Voyager 2 changed several times from the primary radio to the backup, with the end result that the backup broke and the primary was left with frequency fluctuations at the slightest temperature change. The mission was saved only because the technicians were able to calculate the heat generated at each time by each part of the spacecraft, so they can predict the frequencies to talk with the Voyager 2.
THE ENCOUNTERS WITH JUPITER
Ninety days before reaching Jupiter, Voyager's photos already exceeded the best photos taken with telescopes from Earth. At sixty days before the flyby the planet was photographed every two hours, to observe the movements of the giant clouds. At thirty days before the encounter the photos were already better than the Pioneers'. The computers were ready and programed for the rapid sequence of activities set for the day of the encounter. On 5 March 1979, Voyager 1 passed by Jupiter at a distance equivalent to the distance from the Earth to the Moon, crossing the orbit of its major moons. Voyager 2 followed on 9 July 1979.
Jupiter, a giant ball of gas, has whitish and reddish bands moving around the planet at different speeds, creating all sorts of swirls and turbulence. The heat emanating from the interior is larger than what it receives from the Sun, and drives the giant meteorological machine, including spectacular hurricanes such as the Great Red Spot, larger than planet Earth.
The four largest satellites of Jupiter were discovered by Galileo Galilei in 1609 with the help of the newly invented telescope. Through the centuries these tools have improved fantastically, but even so, due to the distance, they have not been able to improve the vision of these objects beyond small disks. There were no details, no relief, no scenery. There were no maps.
Now, thanks to the Voyagers, Ganymede, Callisto, Europa and Io are "places". The first one, almost as big as planet Mars, has craters and other regions where the surface has been coated over by apparent thawing. This is a type of terrain never seen before. The second one, frozen to the point that the ice is as hard as rock, preserves much of all the Solar System's known craters, recording the conditions during the asteroid and comet bombardment of 4 billion years ago, when the planets formed. The third, is the opposite: it has the smoothest surface in the Solar System, where fractures in the surface ice indicates the latter is floating over a warmer interior, maybe even liquid. The fourth has no visible craters, covered by multicolored, sulfur, lava snow. For the first time, active volcanoes were discovered elsewhere than Earth: about ten of them turn on and off, indicating that Io is the most active body in the Solar System.
The photos raised the total count of known satellites of Jupiter from 13 to 16.
The Voyagers penetrated through Jupiter's magnetic field, so intense that it traps high-speed atom particles that can kill a human being in minutes. One of the spacecraft even has its three internal clocks out of synchrony due to the intense radiation.
Passing behind Jupiter, the Voyagers managed to photograph auroras, lightning, and in backlight, thin dust rings encircling the largest planet. Saturn, the second largest, is not alone in this.
THE ENCOUNTERS WITH SATURN
By July 1980 Voyager 1 was so far from Earth that their small telescopes were already getting better images of Saturn that the biggest ground-based ones. On 25 August the systematic-imaging phase began. This study allowed to observe the movements of the beige-colored clouds and to measure winds over 1800 km/h.
New moons were discovered. Some were within the rings, acting as "shepherds moons" that maintain by their force of gravity the ice particles going around the planet together.
On 12 November 1980 Voyager 1 crossed between the icy moons of Saturn. Bodies made of rocks or of gas have already been studied, but these, made of almost pure water ice, were entirely new. Besides signs of terrible impacts, large parts of their surfaces have been shaped by fluid flooding, probably due to some internal heat. Titan, almost as big as Mars and the only satellite in the Solar System with a dense atmosphere, has an orange haze that did not allow the surface to be seen. But the chemicals in its air are those believed to have been the components of the atmosphere of the Earth when life arose here. The intense cold prevents liquid water from exist, but it did not rule out that there are lakes of other substances, such as certain hydrocarbons.
On 26 August 1981 it was time for the 2, looking at targets not analyzed by the 1. Together they discovered that Saturn has much more than the five rings known at the time, but rather a complex of thousands of rings, with innumerable details, like dusty "bicycle spokes", "braids" and deformations from the circular shape.
Voyager 2 continued to resist, so the technicians made it fly by the planet at the right angle so that gravity and the movement of Saturn launched the spacecraft on its way to Uranus. NASA provided funding for a few more years. But when this spacecraft were just passing behind Saturn, its rotary platform with the telescopes and cameras jammed: lubricant was already very slim. If they failed to move it, the spacecraft would be blind at reaching Uranus. But by applying heat and cold in calculated points, technicians succeeded in freeing it. From then on they should be very careful.
THE ENCOUNTER WITH URANUS
The maneuver at flying by Saturn was so precise that years later the spacecraft was insignificantly 200 km from planned. On 4 November 1985 computers started again to execute commands received from Earth, 3000 million kilometers away, and began to photograph Uranus systematically. The count of known satellites quickly rose from 5 to 15.
This gaseous world, four times larger than Earth and of a greenish-blue color due to methane, is lying on its side, and the orbits of its dark dust rings and satellites are vertical. But surprisingly the magnetic field axis does not goes through the poles, but out of both sides at the equator.
Uranus is so far away from the Sun that the lighting is just 1% of what is on Earth, so low that upon arrival on 24 January 1986, the cameras of Voyager 2 had difficulty capturing images. The only solution was to leave them open for a long period of time collecting light, while slowly and carefully rotating the spacecraft to keep them aimed at the target to avoid blurring the photos. This improvised technique was so successful that when rushing by the satellite Miranda, Voyager 2 got the most detailed photos of the entire mission.
Unlike other icy satellites, Miranda, barely 500 km wide, seemed a puzzle assembled with the wrong parts, remains perhaps of other satellite or of it itself rejoined after a catastrophic collision.
It will be several decades before another mission will go that far away and supersede these data.
THE ENCOUNTER WITH NEPTUNE
Twelve years after its launch, Voyager 2 was so distant that enlarging the three Earth-based communication antennas to 70 meters in diameter and installing new receivers were not enough: there was necessary to ask for help to two extra radio telescopes, a second in Australia and one in Japan, so that together they can hear the data that the spacecraft would send from Neptune. The distance was so great that, even though the signal was traveling at the speed of light, after sending a command technicians could leave and then come back nearly 10 hours later, before receiving the spacecraft reply.
The photographs of Neptune showed a blue world with white clouds, reminiscent of Earth, although made of gas, with storms as violent as those of Jupiter. Voyager 2 confirmed the dark rings of dust and added six new natural satellites to the list of the two known back then. The largest of these, Triton, was barely a blip in the biggest ground-based telescopes, but on the postcards sent by Voyager 2 it appears almost as big as the Moon, but made of strange pink and green ices of nitrogen and methane. The high-resolution cameras showed that among the ices, mysterious geysers spurt black vapors containing molecules similar to those that exist in living organisms. Triton turned out to be an active world and another piece in the puzzle to understand the origin of life.
Moving away from Neptune, Voyager 2 easily photographed its night hemisphere, which we can not see from Earth... evidence that it had come to a place where humans had never been before. Only spacecraft allow us to do that.
FAMILY PORTRAIT OF THE SOLAR SYSTEM
But the last pictures of the Voyager mission would be taken on 14 February 1990.
Since the beginning of Civilization planets were mere dots in the sky. For several centuries telescopes have shown them to us the size of coins, although their satellites remained little more than points. Voyager 1 and Voyager 2 have brought us maps of distant lands and visions of bizarre events, like the stories told by sailors who ventured centuries ago to places at the corners of the Earth. Now it was time to complete the opposite view: technicians asked Voyager 1 to show us the Earth as a simple point in space, as it is seen from beyond Neptune. It appears as a pale blue dot, reminding us that our civilization is the owner of just a tiny speck in the cold vastness of the Universe.
THE INTERSTELLAR MISSION
After the early Pioneer 10 and 11, Voyager 1 and 2 are the first spacecraft to escape from the Solar System. In fact, as these travel faster, at this time they are further away than the Pioneers, and unlike their forerunners, they continue to transmit useful information on the characteristics of the space environment that far away from the Sun. Our star continues to dominate with its various radiations, but the Voyagers are already feeling the influence of other stars. They will be the first spacecraft to go from the "fresh water" realm of the Solar System and enter the interstellar ocean of the Milky Way.
Both Voyager each carry a sophisticated gold disc record (with a guarantee period of 1 million years) with messages from Earth, in sounds on one side and electronic images on the other. They are like bottles thrown to the sea, maybe not so much in the hope of being found by a superior interstellar civilization, but rather as a testimony that one day we were here, and we built these wonderful craft with the most elevated of our intentions.
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Based on a lecture given at USP on 22 July 2000. An abridged version of this article was published in ABC Color on 17 September 2006. Photograph: Even with the best telescopes, Neptune appears only as a small, blurry circle. Voyager 2 traveled for 12 years through the cold, black interplanetary space to obtain this fantastic photograph, which shows this distant planet as it really is. Credit: NASA / JPL-Caltech.