Tours, Grand and Otherwise: 1975 to 1992
In the sixth chapter of The Next Fifty Years in Space Patrick Moore predicts what the following seventeen years hold in store regarding the unmanned exploration of the outer Solar System. Looking back on these predictions from our vantage point in 2026, we can see that for the most part they have stood the test of time. In 1975, when Moore wrote this chapter, NASA already had plans in hand to take advantage of a fortuitous alignment of planets to shorten the time it would take a probe to reach the outer planets:
By a lucky chance, Nature was―for once―co-operative. It was found that in the late 1970s the giant planets would be so arranged that the gravitational field of one could be used to swing a probe outward toward another. This led to the concept of the Grand Tour ... Jupiter is the key to the whole situation, because it is the senior member of the Sun’s family, and is more massive than all the other planets combined ... A probe approaching it would be accelerated by the tremendous gravitational pull, and both the direction and the velocity of the vehicle would be changed; although it would lose some of its new-found speed as it drew away from Jupiter, it would have picked up extra impetus. By the time that this had been lost, Saturn would be within range, so that the process could be repeated. ―Moore 79–80
This game of “interplanetary billiards” enables probes to reach the outer planets in much shorter time frames than would be otherwise possible. Budgetary constraints forced NASA to cancel the original and most ambitious version of the Grand Tour in December 1971, and substitute in its place the slightly more cost-effective Voyager Program. The two Voyager craft were preceded by Pioneer 10 and 11, which had both completed their flybys of Jupiter when Moore sat down to write this chapter. Pioneer 11 made its flyby of Saturn in September 1979.
Curiously, Moore sounds somewhat downbeat when he looks forward to the Voyager missions. In 1975 the two probes were still being referred to as Mariner 11 and Mariner 12, though Moore only refers to a single Mariner:
Whether or not Pioneer 11 will survive as an operational unit until it encounters Saturn remains to be seen, but we must remember that it was not originally designed for any such mission, so that failure would not be too disappointing. More will be expected from the Jupiter-Saturn Mariner probe, which is scheduled for launching some time between mid-August and mid-September 1977. It will by-pass Jupiter in the summer of 1979, and will also survey the satellite system, with encounters with Io and also the outermost of the major moons, Callisto, which seems to be ice-covered.
The Mariner should reach the neighbourhood of Saturn in spring 1981, and will make fairly close approaches to two of the satellites: Iapetus, which is remarkable because of its variations in brilliancy―either it is irregular in shape, or has one bright and one darker hemisphere―and, of course, Titan. There will also be a near approach to the rings before the probe begins to recede, beginning its never-ending journey into interstellar space. By 1992 it should have reached a distance from the Sun equal to that of Neptune, though we can scarcely dare to hope that its transmitters will still be operating. ―Moore 88
Moore closes this section with a prediction that did come to pass, though not as soon as he hoped:
Other projects are very much in the planners’ minds. There may, for instance, be a Jupiter orbiter―that is to say, a probe which will approach the giant planet and then enter a closed path round it ... An orbiter may be achieved in 1981 or thereabouts. Next may come a probe designed to go right down on to―or, rather, into―Jupiter itself ―Moore 89–90
NASA’s Galileo spacecraft was the first probe to orbit Jupiter. It spent nearly eight years between 1995 and 2003 in the Jovian system, and deployed a probe that entered Jupiter’s atmosphere. It was followed by Juno, which entered orbit in 2016 and is still active in the Jovian system.
Uranus, Neptune, and Beyond
Moore was confident that what he calls the “billiards” technique would allow probes to reach Uranus by 1985 and Neptune & Pluto before 1992. For this he scores two out of three: launched in 1977, Voyager 2 made its flyby of Uranus in January 1986 and of Neptune in August 1989, but New Horizons did not reach Pluto until July 2015. Moore also speculates about the possibility of a tenth planet―or Planet Nine, now that Pluto has been downgraded to minor-planet status―beyond the orbits of Neptune and Pluto:
There may also be a tenth planet, far beyond either Neptune or Pluto ... and sporadic attempts have been made to calculate its position. Up to now the planet has not been located, and we are by no means certain that it exists at all. To carry out a full-scale hunt would occupy the time of a very powerful telescope for some years, because Planet Ten is bound to be very faint; it will look just like a star (as, indeed, Pluto also does), so that only its movement from one night to another will betray its nature. Since we have no real idea of its position, or even whether it is more than a myth, the prospect is rather daunting, and few scientists would agree in the idea of setting a major telescope aside for a long period in order to take part in a chase which might easily prove fruitless in the end. ―Moore 91
He also suggests a possible means of discovering Planet Ten by using whatever probe makes the first flyby of Pluto:
After its encounter with Pluto, the probe will move outward―and if left to its own devices it will escape from the Solar System, as Pioneer 10 is in the process of doing and Pioneer 11 will do in the years following its pass of Saturn. But if a further burst of power could be applied, the probe could be put into a path which would keep it in orbit round the Sun. Now, suppose that it remains in radio contact, so that its position is known, and that it comes within the sphere of influence of Planet Ten. We could then use the movements of the vehicle to tell us where Planet Ten is; and as soon as we have even an approximate position, we should be able to identify the planet optically. ―Moore 91–92
Contact with New Horizons has been continuously maintained since its flyby of Pluto, and it has been used to explore the Kuiper Belt. In January 2019 it made a flyby of a small Kuiper Belt Object designated Arrokoth. But there are no plans to use New Horizons to hunt for Moore’s Planet Ten. More recent calculations have pushed the search zone much further out beyond the Kuiper Belt. It would take the probe over a century to reach the inner edge of this zone.
Asteroids and Comets
Before taking his leave of the subject of unmanned exploration of the Solar System, Moore makes a few predictions concerning some of the other bodies that orbit the Sun, but he does not pin himself down to specifics:
It is hard to say when the first deliberate asteroid probe will be launched. However, Pioneers and Mariners to the outer giants must pass through the main zone, and the chance of an encounter with, say, Ceres would be rather too tempting to be missed, particularly if it ranked as sheer bonus. Equally interesting are those asteroids which swing away from the principal swarm, and may come close to the Earth. Such is Icarus, only a mile or two in diameter, whose minimum distance from us is a mere 4,000,000 miles. At perihelion it penetrates to the region within the orbit of Mercury, and must become red-hot, though its aphelion distance is nearly twice that of the Earth. Icarus has a “year” only slightly longer than ours, and a path which is highly inclined. At its last near approach, in 1968, it was followed for some time by radar, and a moderate telescope would show it as a speck of light. I would hate to suggest a sojourn on Icarus, which, with its wild extremes of temperature, must be ranked as the Devil’s Island of the Solar System. On the other hand an automatic beacon there would be of great value, and I expect that one will be dispatched eventually, though it will have to be designed with particular care and probably belongs to the period well after 1992. ―Moore 94
As it happened, the first flyby of an asteroid took place in 1991, when the Galileo spacecraft passed close to 951 Gaspra on its way to Jupiter.
Moore also envisages the possibility of sending probes to rendezvous with comets:
Obviously one can never land a probe on a comet―there is nothing solid enough on which to land―but it would be worth sending a vehicle through the gaseous tail, or even through the main comet itself. The chances of survival in the latter case would be rather low, because a collision with a particle of any real size would be disastrous, but the experiment would nevertheless be interesting, and tentative plans have been drawn up for contacting comets within the next decade ...
Halley’s Comet, due once more in 1986, is uncooperative inasmuch as it travels round the Sun in a retrograde or wrong-way direction, which causes problems at once. It may be necessary to send a probe out to a distance greater than that of the comet and then swing it inward, catching the comet up, so to speak, as it approaches the Sun. Once again we find that the procedure is not nearly so simple as might be thought. ―Moore 95
Nevertheless, these difficulties were overcome and in March 1986 three spacecraft―two Russian and one European―carried out successful flybys of Halley’s Comet:
All things told, Moore’s predictions for the unmanned exploration of the Solar System in the years 1975–1992 were quite respectable.
And that’s a good a place to stop before the next stage of our journey.
References
- Patrick Moore, The Next Fifty Years in Space, With Drawings by Andrew Farmer, William Luscombe Publisher Limited, The Mitchell Beazley Group, Artists House, London (1976)
Image Credits
- The Next Fifty Years in Space: William Luscombe publisher Limited, Andrew Farmer (illustrator), foyled-again (photographer), Fair Use
- Voyager: Artist’s Impression of the Voyager Spacecraft (Voyager 1 and Voyager 2 were twins), NASA, Public Domain
- New Horizons: Artist’s Impression of the New Horizons Spacecraft, NASA, Public Domain
- Patrick Moore: Anonymous Photograph, Fair Use
