Into the unknown

Science celebrates 30 years of the Voyager space probes whose discoveries opened up new frontiers in the study of space.

Voyager 2 used

IN an auditorium at the Jet Propulsion Laboratory (JPL) of the National Aeronautics and Space Administration (NASA) in California occupying pride of place is a full-size replica of the two Voyager spacecraft, launched 30 years ago. Why, after all this time, are the Voyager probes still remembered so fondly? Quite simply, they opened up whole new vistas of the solar system, made many new discoveries that have influenced every space mission since and, despite being the most distant human-built objects ever, are still conducting valuable science.

Sent on two different trajectories, Voyager 1 completed its primary mission in 1981 after it encountered Saturn, while Voyager 2 continued onwards to visit Uranus and Neptune. Both are still going strong, voyaging outwards from the sun into realms unknown and unexplored on the edge of the solar system and beyond. To celebrate the anniversary of these remarkable emissaries from earth, let us take a look at their goal and objectives already achieved.

Voyager 2 was launched on a Titan-Centaur rocket from the Kennedy Space Centre on August 20, 1977. Voyager 1 blasted off two weeks later, on September 5, 1977. The reason for the discrepancy in the launch dates was to allow Voyager 2 the option of continuing on to Uranus and Neptune. So, while Voyager 1 was launched second on a shorter, faster trajectory that would take it to Jupiter and Saturn, Voyager 2 was launched to make use of the Grand Tour concept and set on a trajectory that would allow it to use the gravity of the planets it visited to slingshot it towards the outer planets, Uranus and Neptune.

The multi-planet mission would have been much more difficult had it not been possible to make use of what is usually called the gravity assist technique, which was discovered by scientists Michael Minovitch and Gary Flandro of the United States in 1965.

This technique can be explained in a simple way. As Voyager neared Jupiter, for example, it came under the influence of the planets gravity. This affected Voyagers flight path in two ways. First, as Jupiters gravity drew the spacecraft closer to the planet, Voyagers velocity increased. Jupiter also pulled the spacecraft off its relatively straight course so that it flew in a curve around Jupiter towards a rendezvous with Saturn.

In this way, the orbital motion of one planet is used to increase the spacecrafts velocity and the gravitational field of the planet is used to redirect the spacecrafts course to the next planet. This technique allowed Voyager 2 to reach Neptune in just 12 years instead of 30.

But for a spacecraft to avail itself of this gravity assist technique, all four of the outer gas giants (namely Jupiter, Saturn, Uranus and Neptune) have to form a pattern so that the spacecraft may fly past them. Such a fortuitous arrangement of the four planets occurs once in 175 years and when the opportunity came in 1977 NASA launched the Voyagers.

Voyagers 1 and 2 are the most sophisticated robotic spacecraft ever flown. Unlike earlier spacecraft, they were programmed to make independent decisions that safeguard both the spacecraft and their ability to communicate with earth. The two spacecraft have been found to be extremely adaptable and this has allowed engineers to give Voyager 2, in particular, new capabilities as it flew from planet to planet.

Voyager 2 has been heavily reprogrammed during its flight, and its six onboard computers have been continually given newly developed and more expedient methods of processing and packaging data for return to earth. Voyager 2 carries instruments to conduct 11 investigations. Among them are television cameras, infrared and ultraviolet detectors and a communications system that doubles as a radio experiment. Three sets of twin computers control the spacecrafts stability and govern its complex activities.

Communicating between the outer solar system and earth is not as simple as just phoning home. Signals from the Voyagers are very faint, so a big ear is needed to hear them squawk. To this end, NASA put together the Deep Space Network three large radio-dish facilities spread around the world, 120 apart (in Madrid, Spain; Canberra, Australia; and in the Mojave Desert in California) to ensure that there are no blind spots.

The Voyagers return real-time data at a rate of just 160 bits per second. By the end of the Neptune encounter (Voyager 2s closest approach to Neptune occurred on August 25, 1989), a combined total of five trillion bits of data had been returned to earth by the spacecraft, enough for 6,000 sets of the Encyclopaedia Britannica.

The total cost of the Voyager mission up to the Neptune encounter was $865 million, which worked out at about 20 cents a person a year. A further $30 million has been allocated since then for the extended mission. Considering what Voyager has taught us about the solar system, it is fair to say it has been money well spent.

The weight of each Voyager craft, including all equipment and their hydrazine fuel supply, is 815 kilogram. The spacecraft run on nuclear power thanks to the small quantity of plutonium-238 each carries on board in its radioisotope thermal generator (RTG). In 1977 they provided 470 watts of 30-volt Direct Current (DC) power thanks to the radioactive decay of plutonium. However, by 2001 it had dropped to below 320 watts for both spacecraft, which is actually much better than what had been predicted.

However, it does mean that neither can continue indefinitely as power levels continue to drop. The cameras have been switched off, but the ultraviolet detectors still function. Nevertheless, instruments will gradually be switched off and it is expected that there will not be enough power left to run any instrument sometime after 2020. Voyager 1 is speeding away at 17 km a second and Voyager 2 is slightly slower, at 15 km a second.

If any aliens (or indeed humans in the future) chance upon either Voyager craft, they will be treated to a gold-plated 12-inch phonographic record containing sights and sounds from earth. The disc was the brainchild of Carl Sagan and includes greetings spoken in 55 languages, 115 images of earth, various sounds distinct to our own planet and music from all over the world.

The records cover gives details on how to play it, plus the information on the location of the solar system. There is also an ultrapure source of uranium-238 attached to the cover so that anyone who finds Voyager will be able to ascertain how long ago it was launched, thanks to the decay rate of the uranium. Of course, whether anyone will find it is completely unknown.

Voyagers greatest discovery is the erupting volcanoes on Io, a satellite of Jupiter, because this was very much unexpected. Studying images just downloaded from Voyager 1 in 1979, navigation engineer Linda Morabito noticed something odd on the limb of Io. It was a plume of sulphur, belched 30 times higher than Mount Everest by a volcano called Pele. This was an incredible discovery the first example of active volcanism on a body other than earth. Io has since been found to be covered in volcanoes, powered by Jupiters gravitational tides flexing the innards of Io, keeping the moon hot and volcanically active. When Voyager 2 flew past a few months later, it found most of the volcanoes were still active.

Voyager 1 flew past Saturn on November 12, 1981, coming within 64,200 km, while Voyager 2 got there on August 25, 1981, at a distance of 41,000 km. They found Saturns rings to be a place of incredible beauty with tens of thousand of tiny icy, dusty ringlets, with small shepherd moons orbiting between them to keep them in line, occasionally kicking up kinks and clumps and braids in the rings, particularly in the F-ring. Spokes, dusty radial features that shimmer outwards from the planet along the rings, were discovered and are now thought to be clouds of dust held aloft above the B-ring by electrostatic forces. Intriguing moons were observed and we got our first glimpse of the likes of Titan, Iapetus, Phoebe, Dione, and Mimas with its huge crater Herschel and icy Enceladus.

Voyager 2 is the first and only craft to visit Uranus and Neptune. There were some big surprises. For example, the magnetic pole for all the planets which Voyagers had visited up until then Jupiter, Saturn, earth itself of course, and Mercury is very close to the rotational axis of those planets, which makes them good for compasses. So it was a great surprise to find that the magnetic poles of both Uranus and Neptune were nearer the equator than the pole. This was unexpected.

Another surprise was that Uranus small moon Miranda, which is only about 500 km across, a tiny little world, has one of the most complex geologic surfaces ever seen. Another very interesting thing Voyager found was that there was a great dark spot on Neptune. Another surprising fact was that Neptune had some of the fastest winds in the solar system.

Voyagers have concluded their planetary missions, it does not mean they are wandering idly through space. They are still kept busy collecting scientific data on the outer edge of the solar system, searching for the location of the heliopause, where the solar wind slows before coming into contact with the interstellar gas beyond the solar system. Both Voyagers are expected to cross the heliopause in the next few years.

The fate of the Voyager probes is to wander the Milky Way galaxy forever, journeying to other stars and generally living up to their name as they voyage through interstellar space. In 40,000 years Voyager 1 will pass within 1.6 light years of the red dwarf star AC+ 79 3888 in the constellation Camelopardalis. In 296,000 years Voyager 2 will fly past Sirius at a distance of 4.3 light years. In a real sense, the Voyager probes are wonderful creations of the scientists, engineers and technologists of NASA, working as a team.

Sending spacecraft to another world is very expensive, and it may seem pointless when that world seems totally hostile to human life. What practical value is there in sending a space probe to Jupiter or Saturn? To resolve that question we need to consider the distinction between science, technology and engineering.

Science is nothing more than the logical study of nature and the goal of science is a better understanding of how nature works. Technology, in contrast, is the practical application of scientific knowledge to solve a specific problem. Engineering is the most practical form of technology. An engineer is likely to use well-understood technology to find a practical solution to a problem.

We might describe science that has no known practical value as basic science or basic research. Our exploration of worlds such as Jupiter or Saturn would be called basic science and it is easy to argue that basic science is not worth the effort and expense because it has no known practical use.

Of course, we have no way of knowing what knowledge will be of use until we acquire that knowledge. In the middle of the 19th century, Queen Victoria is supposed to have asked physicist Michael Faraday what good his experiments with electricity and magnetism were. He answered, Madam, what good is a baby? Of course, Faradays experiments were the beginning of the electronic age.

Many of the practical uses of scientific knowledge that fill our world transistors, vaccines, plastics began as basic research. Basic scientific research provides the raw materials that technology and engineering use to solve problems.

Science is the study of nature and as we learn more about how nature works we learn more about what our existence in this universe means for us. The seemingly impractical knowledge we gain from space probes tells us about our planet and our own role in the scheme of nature. Science tells us where we are and what we are, and that knowledge is beyond value.

Professor Amalendu Bandyopadhyay is a senior scientist at the M.P. Birla Institute of Fundamental Research, M.P. Birla Planetarium, Kolkata.