Artist's impression of one of the Voyager probes travelling through deep space.
Orwell Astronomical Society (Ipswich)
Looking Forward To The
Voyager Project, 1977 Onwards
In 1977, NASA launched two Voyager spacecraft to explore the outer Solar System. The launch
date afforded an unique alignment of the outer planets, enabling the spacecraft to utilise the gravity of Jupiter and Saturn to deliver a "sling shot" to reach the more distant planets. NASA launched both probes from Cape Canaveral, Voyager 2 on 20 August 1977 and Voyager 1 on 05 September 1977.
Artist's impression of one of the Voyager probes travelling through deep space.
There were problems with the launch of Voyager 2. The Voyagers essentially consist of two modules, the main module and the science module, connected by a boom (see diagram below). The main module contains rocket motors, the radio transmitter and receiver, aerials, navigations systems, etc. The science module contains a TV camera and other scientific equipment. The boom was folded at launch, and the flight plan called for it to extend once Voyager 2 was in space. Unfortunately, a faulty sensor failed to inform mission control that the boom had extended, causing some concern. However, mission control was able to confirm that the boom had indeed extended as planned by using Voyager 2's TV camera, which provided a view of the main module as seen from the end of the boom, showing that all was well. As a double check, mission control then commanded the TV camera to photograph a star field. Sensors in the camera told which way it was positioned relative to the boom, and Sun/star sensors on the main module provided information on the orientation of the main module in space. Mission controllers compared images of the star field with those expected with a fully extended boom and confirmed that the boom was indeed fully extended. Incidentally, the images of the star field were much better than anticipated, which augurs well for close-up pictures of Jupiter in early 1979.
Shortly after launch, there was great expectation of the Voyager probes, illustrated by the following statement by Carl Sagan about their forthcoming encounters with Jupiter and Saturn [1]:
They are not moons like our Moon, they're astonishingly different. There's Europa, there's Ganymede, there's Callisto. There's Iapetus, which is six times brighter in one hemisphere than in the other. How does that come about? What's that do? There's Io, an orange-coloured moon that leaves a great, donut-shaped cloud of gas around Jupiter. The position of Io somehow governs when it will make radio bursts towards the Earth. How does that come about? We're gonna fly right through that stuff and we'll know...... We'll put a dozen pictures inside the Great Red Spot of Jupiter. Well, no-one has any idea what's in there. We'll put a dozen pictures along the width of the rings of Saturn. What are we going to see? Some totally new phenomenon? Nobody has the foggiest idea! There's gonna just be discoveries droppin' out every day!
If all goes well, Voyager 1 will flyby Jupiter, Saturn and Saturn's largest moon, Titan. If Voyager 1 is successful in its flyby of Titan, Voyager 2 will flyby Saturn in such a way as to continue its journey to Uranus and perhaps even to Neptune. Unfortunately, it is not possible for either Voyager to flyby close to Titan and continue to Uranus, as the orbital planes of the bodies are too dissimilar.
The two spacecraft will will send useful information to Earth long before they reach Jupiter. Both carry ultra-violet spectrometers, which will make observations of the inter-planetary medium (among other observing targets). The composition of the inter-planetary medium is similar to that of the Sun, and observations of it will prove useful to cosmologists interested in the ratio of helium to hydrogen in the Universe.
The Voyagers will start photographing Jupiter while on their approach to the planet, some 80 days distant. They will also search for hydrogen clouds surrounding the planet and its system of satellites. The images should contain considerable detail, better than the best images returned by Pioneers 10 and 11 in 1973 and 1974. Approximately eight days before closest approach to Jupiter, the spacecraft will use their wide field cameras to photograph the entire planet and narrow-angle cameras to record detailed images of specific features such as the Great Red Spot.
Voyager 1 will return the first photographs in December 1978. Once at Jupiter, it will fly within 4.9 planetary radii (circa 350,000 km) of the centre of the planet at 12:49 UT on 05 March 1979. It will pass within 415,000 km of Amalthea, Jupiter's small, innermost satellite, which is almost impossible to observe from Earth. It will then pass a mere 22,000 km from Io, innermost of the large, Galilean satellites. It will fly almost parallel to and slightly below the orbit of Io for approximately five hours, imaging the south polar region of the satellite. Shortly thereafter, it will pass behind Jupiter then, after emerging from occultation by the planet, pass within 700,000 km of Europa, execute a close pass of Ganymede and then one of Callisto. It will then depart the Jovian system and speed en route to Saturn, but will continue to observe Jupiter for approximately a month, almost until Voyager 2 begins its observations of the planet.
Voyager 2 will start photographing Jupiter in April 1979. At Jupiter, it has a slightly different trajectory to Voyager 1, as it needs to fly further from the planet in order to preserve the option to fly to Uranus. Its closest approach to the planet is scheduled for 23:00 UT on 09 July 1979 at a distance of 10 planetary radii. It will then pass Callisto, make a very close approach (55,000 km) of Ganymede and finally flyby Europa. It passes behind Jupiter on its path out of the Jovian system.
Together, images from both Voyagers should provide almost complete coverage of Ganymede and Callisto.
Voyager 1 is scheduled to reach Saturn on 12 November 1980. It will first make a close approach to Titan and 16 hours later will make its closest approach to Saturn itself. At 01:00 UT on 13 November, it will pass 3.3 planetary radii from the centre of the planet and should capture magnificent views of its south polar regions and of the ring system. Then it will make close approaches of the satellites Mimas, Enceladus, Dione and Rhea providing the first view of the moons as anything other than tiny, fuzzy points of light seen in Earth-based telescopes. Voyager 1 will then pass behind Saturn and through its shadow.
Assuming that Voyager 2 follows a trajectory for Uranus, it will reach Saturn on 27 August 1981. At Saturn, it will first encounter Titan at a distance of 353,000 km, then Rhea and then Tethys at about 170,000 km. As it swings behind the planet it will pass within 150,000 km of Enceladus, make a very close approach of Mimas and finally pass Dione. It will then pass behind the planet.
Both Saturn flybys will take place outside the rings but will nevertheless give good views of them. Voyager 1 will be placed to obtain better images of the rings than Voyager 2. However, Voyager 2 will obtain an unique view of the dark side of the rings at 13:00 UT on 27 August 1981 when it is at a distance of 2.7 radii from the centre of the planet.
On leaving Saturn, Voyager 1 will head for interstellar space, while (assuming that it has enjoyed a successful flyby of Saturn and Titan) Voyager 2 will head for Uranus. Voyager 2 will continue imaging Saturn until the end of September 1981, then power down non-essential equipment. As it approaches Uranus it will re-activate all equipment and arrive at the planet on 30 January 1986. Details of its trajectory at Uranus remain to be worked out, and will need to take account of the unusual axial inclination of the planet. Uranus has an axial inclination of 98°. It has five satellites, all of which orbit in the equatorial plane , so that the system of moons will appear face-on to Voyager 2. The flyby could take place when all satellites are on one side of the planet but, if Voyager 2 is to pass close to the planet itself, it can pass close to at most one satellite. However, it is hoped to photograph all the satellites well enough to reveal surface details.
There is even a possibility that Voyager 2 may be sent onwards from Uranus to Neptune, arriving in 1990, after which it will leave the Solar System and head into deep space. During the potential 13 year flight of Voyager 2 to the edge of the Solar System, it will cover a distance of over 30 AU (4500 million km). It should continue travelling indefinitely; the Voyagers have been built to last for at least 40,000 years...
The NASA JPL (Jet Propulsion Laboratory) manages the Voyager Project and is responsible for building the two spacecraft and conducting tracking, communications and mission operations. The launch vehicles are the responsibility of NASA's Lewis Research Centre. There are 11 science teams comprising 85 scientists concentrating on different aspects of the mission.
The Voyagers are bigger and heavier (by some 80 kg) than previous missions. Among the science instruments which they carry are very advanced TV cameras, instruments to investigate the meteorology of Jupiter and the Great Red Spot and instruments to investigate why Jupiter appears to be shrinking.
Ground controllers can control the equipment aboard the Voyagers from Earth, a distance of thousands of millions of kilometres. This provides a powerful capability to explore many different planets on a single mission. Each Voyager is controlled by a computer which ground controllers can re-programme while the craft is in flight, enabling optimisation for each planetary encounter, and providing the ability to overcome any on-board failures caused by the intense radiation around Jupiter and Saturn.
The Voyagers carry 11 scientific instruments and also a gramophone record made from copper with recordings of a baby crying, a speech by US President Carter, samples of over 60 languages recorded at the United Nations, plus many other sounds. However, the Voyagers do not carry a gramophone to play the record because of the need to minimise the weight of the craft. As one scientist put it: any alien intelligent enough to intercept the Voyagers in space would have sufficient intelligence to work out how to play the record and quickly manufacture a machine to do so!
Voyager is the most far-reaching spaceshot launched from planet Earth. The spacecraft will travel some 2.5 billion km to Uranus and, possibly, a further two billion km to Neptune, and will then continue travelling indefinitely away from the Solar System. The spacecraft, since they will travel so far from the Sun, differ from the Mariner probes in that they have no solar panels to turn sunlight into electrical energy. This is because at great distances from the Sun the amount of sunlight received is negligible. Instead, the Voyagers use radio-isotope thermoelectric generators (RTGs), which generate energy through the radioactive decay of plutonium. The RTG is mounted on a beam extended from the spacecraft, thus minimising the chance of stray radiation interfering with the scientific instruments. The RTG develops a total of about 400 W, of which 100 W is used to communicate with Earth (at the distance of Saturn). The science experiments consume 108 W (bear in mind the wattage of the average everyday electric light-bulb, and think of all the scientific experiments that must operate using this power!)
Voyager's antennae also differ from those of the Mariner spacecraft. To send information to Earth with a given amount of transmitter power from the distance of the outer planets requires a much larger antenna than is needed at the distance of the inner planets. The antenna on Voyager is a 3.7 m diameter dish. The antenna is, of course, precision built to minimise wasteful scattering of radio waves. Together with an improved transmitter and the large receiving aerials of the Deep Space Network around the Earth, Voyager is able to transmit 115,000 bits/sec of information from Jupiter and 44,000 bits/sec from Saturn. (To gain an appreciation of these bit-rates, note that you can only send information over the telephone at the rate of approximately 100 bits per second, however fast you talk.) The large, high-gain antenna is kept pointing constantly towards Earth by what can simply be described as "electronic eyes". There is a low gain antenna in front of the large antenna so that if contact with Earth is lost for any reason, a low bit-rate channel is maintained until the onboard computer can re-orientate the spacecraft and re-establish communications via the large antenna.
The spacecraft can transmit to Earth on two frequencies. During the long cruises between the planets, the lower frequency, known as S-band, will be used to send data to Earth at a relatively low rate. This is adequate for interplanetary science information, which can be received on a 26-metre antennae, releasing the larger antennae of the Deep Space Network for other uses. For the encounters with planets when a very large amount of data has to be transmitted, the higher frequency X-band is used. The X-band transmitter power outputs are 21 or 12 watts, whereas those of the S-band are 28, 20 or 10 watts. Both transmitters have a standby unit on board in case the primary fails.
Each Voyager, like the earlier Mariners, is built around its electronics package, the latter being surrounded by the antennae, with a small propellant tank in the centre of the electronics compartments. Unlike earlier spacecraft, each Voyager does not use one large engine for propulsion. Instead it uses hydrazine fuel (which doesn't have to be ignited) to control its attitude through 16 small thrusters situated all around the space vehicle. Two guidance systems send command messages to the thrusters. The first is the familiar Canopus star tracker system, which uses a "fix" on the Sun and the bright star Canopus to determine its position. The second is a complicated system of internal gyroscopes, called the Internal Reference Unit. For some of the more lengthy manoeuvres, thrusters must be switched on for as much as an hour.
To add to the final velocity required at launch to escape the gravity of the Earth and to set out on a trajectory to Jupiter, each Voyager has a solid rocket system weighing 1210 kg, delivering a thrust of 71,000 Newtons. The rocket system is discarded from the spacecraft after it has been fired.
The main body of the Voyager spacecraft, termed the mission module, weighs 810 kg, of which the scientific instruments make up only 105 kg. As with the transmitters, much of the electronics aboard the spacecraft is duplicated in case of damage by interplanetary high energy particles and other causes due to a long duration spaceflight. The spacecraft has, of course, been designed so that the instruments are affected as little as possible by the magnetic field that they themselves create.
Originally, the Voyager mission was concerned with Jupiter and Saturn. However, interest grew in the satellites of the planets and so the mission was extended to include them. The mission is now scoped to investigate five bodies roughly the size of our Moon: the four Galilean satellites of Jupiter together with Saturn's larget moon, Titan. Titan is a particularly interesting object for investigation as it has an atmosphere, thought to consist mainly of methane. (However, it does not hold onto its atmosphere very tightly, and if its temperature were raised by as little as 38°C its atmosphere would escape.) The science instruments aboard the Voyager probes are quite varied:
Voyager's cameras will photograph the planets to a detail never before possible, using the latest cameras and techniques. Other instruments, for example, the plasma experiment, will measure levels of plasma. The cameras and spectrometers are mounted on a science boom, away from the large, high gain antenna, to facilitate the greatest possible area of clear vision. A narrow-angle camera system of 1500 mm focal length acts like a telephoto lens to capture fine detail.
NASA developed a completely new wide-angle camera system for the mission. This has a small telescope of 200 mm focal length, giving a wide field of view. Both cameras have eight colour filters enabling Voyager to take several pictures of an object, using one filter at a time; subsequent superposition of the images in processing laboratories on Earth can generate a colour image.
Also on the moveable platform of the science boom are the ultra-violet and infrared spectrometers, a radiometer, and a photopolarimeter. Experiments for measuring interplanetary particles are also on the boom, though not on the moveable platform. Magnetic fields are measured from another boom 13 m long. Two long antennae detect radio waves emitted from the planets as well as the extremely rarified gases in interplanetary space.
The infrared spectrometer will measure temperatures at various depths within the atmosphere of a planet, giving information about gas flow and general chemical compositions. The UV spectrometer should also be useful in providing information about planetary atmospheres and should yield information about the lightest elements in the atmosphere, hydrogen and helium. The polarimeters will provide useful information about the chemical group called aerosols and should reveal characteristics of the surfaces of the moons. NASA will employ several detectors to provide information about the type and flow of charged particles (protons, electrons, etc) in interplanetary space and in the proximity of the giant planets. A cosmic ray telescope will be used to detect high-energy particles in interplanetary space. Measurements of charged particles and magnetic fields in the neighbourhood of Jupiter will provide an improved understanding of the processes that enable the planet to accelerate electrons to high velocity. Measurements of the radiation belts around Jupiter and Saturn will provide a better understanding of their composition and mechanisms. Measurements of planetary magnetic fields will enable scientists to construct better models of the internal structure of the planets.
The composition of a planetary atmosphere reads like a fingerprint of the planet's past temperature, atmospheric composition etc. Knowledge of planetary atmospheres will, once information is available on most of the planets in the Solar System, enable scientists to build a picture of the evolution of the planets and of the Solar System in general. The Voyager probes should provide new insights into the atmospheres of the giant planets and scientists are keen to obtain in particular measurements of the temperature, pressure, density, gaseous and particulate compositions of the atmospheres.
It has been found that heat from inside Jupiter plays by far the largest part in the circulation of the planet's atmosphere. The atmospheres of Saturn, Uranus and Neptune may behave in the same way. As the Voyagers approach Jupiter, Saturn, Uranus and possibly Neptune they will photograph the fast-rotating atmospheres. (This may be difficult for Neptune where, at such a large distance from the Sun, light is not abundant and therefore pictures require a lengthy exposure.) The Voyagers' cameras will work some of the time jointly with radio science experiments to study the atmospheres of the planets.
The Voyagers will also study the rings of the giant planets. They will look for evidence of water ice, ammonia and silicates in Saturn's rings. The amount by which the rings scatter sunlight will give a measure of the size of particles of which they are composed. They will also observe Uranus' recently discovered rings.
Far encounter pictures of the satellites of the planets are expected to reveal details between five and 15 km across. The spacecraft are expected to approach close enough to three satellites to observe details as small as one kilometre across. Their cameras will, of course, look for the usual geological features such as craters, plains, scarps, mountains and polar caps. Wide-angle pictures may reveal the global distribution of geological areas and perhaps show why there are variations in colour and albedo on the satellites. Sizes and shapes of features will be measured to an accuracy of 0.1% to 1%.
The satellites of Jupiter and Saturn offer barriers to charged particles that rotate with the satellites in their orbits, much as the planets do to the solar wind (the stream of charged particles emanating from the Sun). The Voyager probes should analyse charged particles in Jupiter's magnetosphere. Close approaches of the Voyager probes to Jupiter's satellites Io, Ganymede and Callisto will enable the instruments to detect possible "wakes" in the stream of charged particles in the lee of the moons (like the wake of a ship in the sea on Earth), and estimate the rate at which the moons encounter charged particles and the speed at which their wakes close.
Titan, the largest of Saturn's satellites, possesses an appreciable atmosphere. It has a diameter of 5000 km, making it larger than the planet Mercury. Because of its large size and much greater distance from the Sun, Titan has retained an atmosphere whereas Mercury has not; in fact, Titan is the only satellite in the Solar System to have an appreciable atmosphere. The atmosphere leaks away into space at a minute but non-negligible rate: it is thus thought to sustain a toroidal concentration of atmospheric material in orbit around Saturn. If this torus is outside Saturn's magnetosphere and interacts with the solar wind, it might give rise to a detectable bow shock. This would allow the Voyagers to observe the interaction of a gas cloud with the solar wind.
Each Voyager probe will use the 10 m long whip-like antenna of its planetary astronomy experiment to detect the emission of radio waves by Jupiter. The Voyagers will re-transmit radio emissions at Jupiter and Saturn, enabling scientists on Earth to compare the characteristics of the received radio waves and deduce information about the media in the vicinity of the Voyager probe (a similar principle to radar). Transmissions from the Voyager probes should provide information on the sizes of the planets, the sizes of their satellites, the composition of Saturn's rings together with the orbital motions and position of the planets and satellites which the probes flyby.
At Saturn, the Voyagers should determine the presence or otherwise of a magnetosphere, and thus a magnetic field. Some earlier experiments with an Earth-orbiting satellite suggest that Saturn does possess a magnetic field, but it is very difficult to measure this from Earth. The Voyager probes can look for the shock wave caused by the solar wind hitting Saturn's magnetosphere and the effect that Saturn's rings have on it: if Saturn has a magnetic field, the rings may have a major impact on charged particles in the vicinity of Saturn. The Voyager probes will also try to determine whether Saturn's magnetic axis is tilted with respect to its rotational axis.
Whether or not Uranus and Neptune have magnetospheres we cannot say for certain at present because at the distance from Earth they are not detectable. However, the probability is that the two planets have extensive magnetic fields; if Voyager 2 reaches them, it will be able to detect even the weakest of magnetic fields.
Configuration of the Voyager probes.
[1]
Newsweek, 05 September 1977
Roy Cheesman, Simon Harvey and Mark Howe