In the year 1977, we launched two unmanned spacecraft off towards the end of our solar system. Voyager 2, oddly, was launched first, on August 20th; Voyager 1 was launched a little over two weeks later, on September 5th. They were launched specifically at these times to take advantage of an unusual planetary alignment that occurs once every 176 years. This alignment allowed the probes to utilize the gravity of the planets that they approach to accelerate, thus drastically decreasing the time necessary for the crafts to approach the outer planets. It is estimated that, without this alignment, it would take 30 years for them to reach Neptune; using it, the trip took only 12.
The scientific goal of these two probes was, in general, to send a man-made object out to the outer planets in our solar system, to gather data so that we can better understand these objects. The specific goals of the project were not really set in stone because no one really knew if the equipment would function as planned, or if new information would be discovered that would lead scientists to look into new ideas, so any plans for the mission would have to be adaptable based on new discoveries and information. But the scientists working on the project were pretty confident that the probes could easily reach Jupiter and Saturn, so the exploration of these planets was the initial goal of the project. After reaching Saturn, Voyager 1 was deemed unfit for continued exploration, so it was placed on a trajectory at a sharp angle to the planetary orbital plane, out into deep space. Voyager 2's mission, on the other hand, was extended; it continued on to Uranus and Neptune. Both probes are in significantly better condition than was originally projected; we still communicate with them on a regular basis, and we plan to continue to do so until around the year 2020, when each of their nuclear power supplies will cease to provide enough fuel for them to function.
In all cases, the primary scientific goal was discovery; we wanted to gather as much data about these planetary systems as possible, for the purpose of better understanding them. At Jupiter, for example, more than 33,000 pictures were taken and transmitted back to Earth. Io, one of Jupiter's moons, was discovered to have active volcanoes, something that was not previously thought to be true. This makes Io the only body in our solar system other than Earth itself that has volcanic activity. The volcanoes are now thought to be caused by intense tidal forces caused by interactions between the gravitational fields of Jupiter and two of its large moons, Ganymede and Europa. These volcanoes were not discovered by either of the Pioneer craft that flew by Io, so it appears that the volcanoes are either intermittent or a recent phenomenon. These volcanoes will be the objects of future study; their study awaits the sending of another probe that passes close enough to Io to observe them.
Jupiter itself was the object of much study; its atmosphere was the focus of this study because it is hard to observe much else about Jupiter from space. By observing atmospheric disturbances, it was discovered that they were caused by the movement of large amounts of mass, as opposed to energy. Observations also disproved the theory that many convection currents existed above 45 degrees north and south, by latitude. Convection currents could be seen in pictures as bright spots in the clouds or upwellings of cloud material in the otherwise linear, east-to-west-running cloud systems. Neither of these were observed in unusual quantities in any pictures, and there were plenty of pictures taken that showed Jupiter up to the 60 degree, and even 75 degree, north and south latitude lines. The Voyager space probes also observed temperature changes in Jupiter's atmosphere; most notably, they saw that, while Jupiter's outer atmosphere has a temperature of -112 degrees Celsius, it has an inner temperature that peaks at 830 degrees. This high inner temperature was not observed by the earlier Pioneer spacecraft, so Jupiter is clearly undergoing dramatic internal changes.
There was not a great deal of new information gathered about Jupiter's other satellites. Europa and Ganymede, Jupiter's two largest satellites, showed physical features indicative of tectonic action; unlike Earth, however, these tectonic plates likely slide not on a layer of molten rock, but on a layer of molten ice. More precise measurements of Ganymede were taken, showing that, in fact, it is the largest satellite in this solar system, discounting atmospheres. It was discovered that Jupiter, in fact, does have a 'ring', like that of Saturn. Several new moons were discovered in orbit around Jupiter; two smaller ones with diameters of 40 kilometers were in orbit just outside Jupiter's ring, and one 80-km one was between the orbits of Amalthea and Io. A 5 million amp current was discovered to flow between Jupiter and Io; this current was predicted, but it was predicted to only be one million amps. There were many other discoveries around Jupiter, but few of them called into doubt present knowledge; most of them just verified what we already predicted or knew.
After this, the two Voyager probes proceeded on to Saturn. At this point, they were moving at different-enough speeds that one, Voyager 1, was significantly ahead of the other, but for clarity their observations will continue to be lumped together. Through the Voyager probes, we discovered at least as much about Saturn as we did about Jupiter, if not more. Saturn's atmosphere was known beforehand to be primarily hydrogen and helium; the Voyager probes showed, however, that it had less helium in the upper atmosphere; this implied to many scientists that the helium was sinking down into the lower parts of the atmosphere. As an interesting side note, Saturn is the least dense of all of the planets; if a big enough lake could be found, it would float in water.
Saturn was found to have a very violent atmosphere. Winds on it were measured at 500 meters per second, for example; it's genuinely hard to get anything at all moving at 500 meters per second here on Earth. Towards the Equator, winds seemed to go mostly in an easterly direction; higher up in latitude, winds go in both easterly and westerly directions. Saturn's temperature was also measured; it ranged from 80 to 140 Kelvins; that's below -200 degrees Fahrenheit. Saturn was also show to rotate fairly quickly relative to Earth; its day was measured at roughly 10 and 2/3 Earth hours.
Saturn's rings, its most famous features, were also an object of notable study. Many partial and incomplete parts of rings were discovered, and holes were found in the rings that do circle Saturn completely. The reason for these holes is unknown; measurements were taken to test the theory that they were formed by satellites picking up dust particles in that region, but no such satellites could be found, so this hypothesis was effectively disproven. Other than these holes, though, Saturn's rings were discovered to be remarkably even; their only clear distortion is caused by interfering gravitational waves from Saturn and its moon. Saturn's rings were also verified as having remarkably odd dimensions; they follow elliptical paths around Saturn, but the paths do not follow Kepler's laws. Saturn is at the center of the ellipse, not at a focus. Understanding the reasons for this is a major goal for scientists studying Saturn.
Saturn's satellites were also the target of much attention. Titan, Saturn's largest satellite, was precisely measured; previous measurements were based on appearance, and so could only measure Titan's atmosphere's size, but measurements made by the Voyager probes checked the actual size of the solid portion of Titan. It was shown to actually be smaller than Ganymede this way. Titan is very cold, at 289 degrees Fahrenheit. It's not quite cold enough to have lakes of methane, as has been theorized, but lakes of ethane are likely. The chemical composition of Titan is right for the formation of hydrogen cyanide, a very early building block of life. This is why scientists watch Titan with such interest; it represents, to an extent, early Earth.
There were also discoveries about Saturn's other moons. Just for starters, six new moons were found, including Atlas, Saturn's innermost moon, bringing the total up to 17. Three additional moons were glimpsed by the Voyager probes, but not enough data was gathered about them to verify their existence. Pictures of many satellites were taken, and most of them were shown to have many crater impacts, implying that they are quite old. Phoebe is the 'oddball' moon in orbit around Saturn; it orbits more in the ecliptic than any of the other moons, and its composition and color is different from the other moons. Scientists think that it may be a captured asteroid; if so, it is the best view of an asteroid that we so far have gotten, and it should provide much encouragement for continuing study.
After this, Voyager 1 and Voyager 2 split up. Voyager 1 headed out into deep space; Voyager 2 continued on to Uranus. Little was known about Uranus before the Voyager mission there; it is comparatively small, and its great distance from Earth makes it difficult to observe. In order to receive radio signals from Voyager 2 at this great distance, many massive Earth-based radio receivers were linked together to amplify the signal, thus providing a unique use for radio astronomy.
Ten new moons were discovered to be orbiting Uranus, bringing the total up to 15. The moons appear to be primarily composed of ice and rock; many are essentially inert, but some show signs of geologic activity. Uranus's rings were found to be composed mostly of very fine dust, with an outer border composed of rocks a foot or so in diameter. Uranus's atmosphere was found to be mostly hydrogen and helium, like Saturn and Jupiter before it, but with a higher concentration of hydrocarbons. Its temperature is only 60 Kelvins above absolute zero, so it is quite frigid. Uranus's magnetic field is skewed at 60 degrees from its axis of rotation, much greater than Earth's 12 degrees, not to mention the 1 degree of Saturn. Less was learned about Uranus than about the previous planets because only one probe was able to visit it, but still, there was no shortage of data.
After a fair bit of course correction, Voyager 2 was able to go on towards Neptune. At Neptune, the atmosphere was the target of the most interest. It is primarily composed of methane; when methane is hit by sunlight, it decomposes and falls into the lower atmosphere, where it recombines back into methane. This ensures a dynamic atmosphere, even this far from the sun. Jupiter has its Great Red Spot; Neptune was found to have a similar Great Dark Spot. Both are continuing atmospheric storms, and they are each bigger than Earth. Neptune has a very cold upper atmosphere, at 55 Kelvins, but its inner atmosphere is actually very hot, at 480 degrees Celsius. The methane atmosphere, and the effect described above with it breaking down and reforming, are thought to cause this heat effect; the methane insulates the interior of the planet, and the convection flows of decomposed and recomposed methane heat the planet. Neptune's magnetic field was found to be at an odd angle, 55 degrees, to its axis of rotation, and it was also found to be centered over halfway from the center to the surface of the planet. These are both unusual phenomena, and they both warrant much further study.
Triton, one of Neptune's eight moons, was also examined. It appears to be a captured object; it has too high a density to be a former part of Neptune. Volcanic eruptions of liquid methane and nitrogen have given it an unusual geologic past; given that its surface temperature is 38 Kelvins, molten nitrogen is not really that abstract an idea. Triton had been discovered ahead of time, but Voyager did discover six other moons orbiting Neptune.
Neptune's rings were previously thought to be partial rings; scientists were spending a fair amount of time trying to understand how partial rings could exist there before the arrival of Voyager 2. When it did arrive, however, it discovered that the 'partial' rings, in fact, fully encircled Neptune, rendering all Neptune-based partial-ring hypotheses irrelevant. A number of distinct ring layers were discovered; most ring planets seem to have several ring layers, but the specific layers for Neptune had not previously been identified.
After Neptune, Voyager 2 was launched down from the ecliptic, so that it was going down while Voyager 1 was going up. They are headed out into deep space, towards the heliopause, the boundary of the Sun's influence. Researchers have hopes that they will continue to work for several more decades so that they can continue to measure information about deep space.
Showing and explaining all of the data gathered by the mission would be a near-impossible task; it has taken several groups of scientists decades to analyze it all. These past few pages have just been a brief summary of the kinds of information gathered; included with this essay are some of the pictures taken by the Voyager probes.
This is a picture of one of the Voyager space probes. They both look essentially the same; in fact, they are both essentially the same, with only some minor differences in specific instrumentation. The Voyagers used gyroscopes and star-based navigation systems to keep themselves pointed directly back at Earth, for the purpose of transmitting and receiving signals. There are seven main instruments on the Voyager probes, each with a short acronym: the PLS, the LECP, the CRS, the MAG, the PWS, the PRA, and the UVS. Teams are presently working here on Earth to analyze the data from the first five of these sensors; data from the last two is being stored for future use.
The PLS, or Plasma Science, sensor, measures ion concentrations characteristic of the solar wind. The PLS on Voyager 1 no longer functions correctly; it broke, and we don't know exactly why, but we still seem to get good data from Voyager 2. The LECP measures Low-Energy Charged Particles, particles with charges in the 10 to 10,000 kilo-electron volt range. The CRS is the Cosmic Ray Subsystem; it, logically, measures cosmic rays. The MAG is a magnetometer; it measures, as one might guess, magnetic fields and their intensities. The PWS, or Plasma Wave Subsystem, measures the frequency of plasma waves. The PRA is the Planetary Radio Astronomy subsystem; the UVS is the Ultraviolet Subsystem. Both Voyagers have an onboard optical camera; the camera on Voyager 2, however, has a higher resolution than that of Voyager 1.
The power supplies used by the Voyager probes are possibly the most advanced elements of their design. Power is generated through the radioactive decay of plutonium. This provides a very dense, long-lasting power supply. Originally, each of the power supplies on each probe could provide 470 watts of power, but because much of the plutonium has decayed at this point, the output of each supply is now 315 watts. Leaders of the mission have already started to shut down system components so that there is enough power available for the probes to continue to function. The UV scanner on both probes has been turned off; in another decade or so, there are plans to disable the gyros, leaving the probes unable to reorient themselves. A decade after that, it is estimated that there will no longer be enough energy to power the transmitter on the probes, so the mission will then be effectively over.
As the purpose of this mission was primarily exploration, relatively little science was involved in the mission itself. The data gathered by the probes, however, was and is used extensively for scientific purposes; it is, for the most part, the only data of its kind, so anyone wanting to study the outer planets stands to benefit greatly from using it.
As mentioned before, there are five different groups working here on Earth to process data from five of the different instruments on Voyager. One of them, the MIT Space Plasma Group, has made their findings available and comprehensible to the general public. They are the group that monitors the PLS sensor; their goal is to measure and plot the speed of the solar winds at different points in space. They have created a record of the speed of the solar wind every hour since 1977, and they have made many different graphs of this data. Here is an example of such a graph, of the average solar wind speeds over the past 500 days:
(Click here for an enlarged view of the graph
The MIT Space Plasma group also tracks irregularities in the solar wind. The most recent such irregularity was caused by a massive solar flare on July 14, 2000. This solar flare produced a shock wave in the solar wind; this shock wave was detected here on Earth and, later, by the Voyager spacecraft. Based on observations here on Earth, a prediction was made as to the characteristics of the shock wave when it reached the Voyager probes. The predictions were similar to the actual results, but they were not exactly the same; scientists are studying the situation to try to account for the discrepancies. Here is a graph of the predictions compared to the real data:
The data gathered by this project is used for many different purposes. One of these purposes is to predict the motion of the heliosphere, the front border of the heliopause. The heliopause is the border where outgoing solar wind and incoming interstellar plasma meet; it is, hopefully, the next destination for the Voyager probes. Using data gathered close to Earth in combination with data from the Voyager satellites, a rough model has been created of the shape of the heliosphere; an animation of that model can be found here.
The Voyager missions so far have been a great success. They have gathered much useful data, and given us much to think about and study. Amazingly, they continue to function and to transmit back to Earth, even though they are now well past Pluto and rapidly approaching the heliopause. Voyager has been an excellent experiment and adventure, and it will continue to be so well into the future.