The Grand Tour

[George Thomas Wilson]

The Grand Tour

 

The Dream, the Reality, the Legacy

 

Part 1: The Dream

 

This is a review of the article entitled “Voyage to the Planets” in the August 1970 of the National Geographic Magazine.  It was written by Kenneth F. Weaver, Assistant Editor, with Paintings by Ludek Pesek.  The article tells the tale of NASA’s vision for the exploration of the solar system.  It was written in the heyday of the Space Age, shortly after the first of the Apollo moon landings, and after NASA had sent multiple probes to fly-by and study Venus and Mars.  After an introductory section, covering the history of mankind’s interaction with the planets, the article discusses each planet and their first (or next) robotic visitor – Earth, a future NASA probe; Venus, a Mariner spacecraft; Mercury, the same Mariner; Mars, a pair of Mariner orbiters; the Asteroids, Pioneer F; Jupiter, Pioneer F; Saturn, Grand Tour 1; Uranus, Grand Tour 2; Neptune Grand Tour 2; Pluto, Grand Tour 1.

As shepherds of old watched the starry night sky, they took comfort in the consistency of the heavens, each heavenly lamp stayed fixed in its niche.  Well, not quite all.  Amid the thousands of naked-eye stars, several of the brightest disobeyed the usual pattern.  Unaccountably and mysteriously, they drifted from night to night across the winking field of lights in their own fashion, coming and going.  Sometimes they disappeared for weeks at a time.  To some ancient shepherds, these five mavericks were stray animals.  The Romans gave them the names of their gods: Mercury, Venus, Mars, Jupiter, and Saturn.  And from earliest recorded times, astrologers ascribed mystical qualities to these celestial bodies, as well as the sun and moon, teaching that they affected the destinies of nations and of kings.  The Babylonians embraced the notion of astrology, and that belief persisted through the centuries in Greece, in Rome, in the Moslem East, in medieval Europe, and in the Orient.  Astrologers advised princes and kings, casting horoscopes, and studying the charts for omens of good and ill.  With the Renaissance, men began again to study the nature of the universe, and astrology became discredited in the Western World.  When Galileo, in 1609, first trained the newly invented telescope on the heavens, scientific investigation became more exact and much more exciting.  No longer were the planets simple points of light; they were small disks.  Venus, brightest of all, showed phases like those of the moon.  And Earth was not the only planet to have a satellite: Jupiter had its own family of moons.  As for Saturn, it was in time revealed as the most beautiful object in the sky, with a gleaming girdle of rings about its equator.

Important discoveries soon changed man’s most fundamental concepts of the universe.  Copernicus proclaimed, in 1543, that the sun was the center of things, not Earth.  Kepler showed that orbits were elliptical not circular; Newton’s gravitational force allowed orbits to be calculated; and, by 1840, two additional planets were added, Uranus and Neptune.  But even through the telescope, the planets yielded their secrets grudgingly.  By World War I, these dark bodies, which shine only by reflected sunlight, seemed to lose their appeal.  In the ‘20’s and ‘30’s, most professional astronomers turned their attention to the distant stars.  All that has changed in the past dozen years.  Solar-system astronomy is again in ferment, and the ‘70’s promise to be the decade of planetary exploration.  Powerful new tools, such as radar and radio telescopes, infrared and other detectors, sounding rockets and spacecraft were beginning to produce an avalanche of information and several surprises.  Venus, for example, once thought to be Earth’s twin, is really an inferno.  Ridiculously, it rotated backwards.  Mercury, which was thought to keep the same face toward the sun, does indeed turn its face from the fire.  And Mars, the abode of a race of intelligent canal builders, has proved to have no canals and no evidence of liquid water, or anything to support life as we know it.  During the past decade, besides landing men on the moon, NASA has completed five successful flybys of the planets, two to Venus and three to Mars.  The Soviet Union had plunged three probes into the dense atmosphere of Venus.  No one knows how ambitious the Russian plans are, but the U. S. hopes to send unmanned spacecraft racing toward every single one of the planets.  These robot from Earth will take pictures, sniff atmospheres, gauge temperatures and pressures, measure radiation and magnetic fields, and - in case of Mars – look for evidence of life.  Let us look at this family of the sun, with its 9 planets, 32 moons, asteroids, and comets.

 

Earth

 

Suppose we visit each planet, seeing it as if we were aboard an unmanned NASA spacecraft that will approach it sometime within the next few years.  As a base for comparison, let us start with Earth.  How would our planet look to men and instruments from another planet, orbiting above and then landing on the surface to make observations?  The author could imagine that these extraterrestrial astronauts might make brief entries in their log something like this:

Blue-and-white planet – only one in this system.  Water covers 71%.  Low cloud masses, in swirling patterns, block much of the view.  Atmosphere: 78% nitrogen, 21% oxygen, and 1% argon, carbon dioxide, and other gases; water vapor variable; no appreciable hydrogen or helium.  Atmospheric pressure, 14.7 pounds per square inch.  Atmosphere and water vapor block part of the radiation from the sun, 93,000,000 miles away.  Planet acts like a magnet.  Many meteoroids reach the atmosphere; most burn up before striking surface.  Land surface chiefly silicates; heavily modified by water and wind.  Smooth in places; elsewhere rough and marked by steep uplifts.  Crust sputters and spews molten material.  Must be very hot underneath.  Temperature variations moderate.  Coldest near poles (frozen water); minimum -127 degrees F.  Hottest near equator; maximum 136 degrees F.  Life abundant; many forms; heavily dependent on liquid water and – in most cases – oxygen.  Vegetation shows seasonal changes because planet is tilted, with one hemisphere, then the other, toward the sun during a 365-day orbit.  Rotation on axis, 24 hours.  One large moon; with Earth essentially a double planet.

 

Venus

 

In the autumn of 1973, if NASA’s plans hold, an Atlas-Centaur rocket will launch a Mariner spacecraft on a voyage to Venus and Mercury, the two planets lying between Earth and the sun.  It will be the first U. S. attempt to fly past Mercury and the first gravity-assisted mission to a planet: That is, the spacecraft will be aimed so that as it passes Venus the gravitational field of that planet will help swing it, somewhat like a ball on a string, and send it with proper speed and direction toward Mercury.  This mission, like all those planned for the planets in the ‘70’s, will be unmanned.  But imagine that you are aboard as the spaceship approaches its first goal, Venus.  The date is between February 3 and 6, 1974, and your spacecraft has been on it way for more than three months.  [All flight information in this article is based on tentative plans now being refined by Caltech’s Jet Propulsion Laboratory (JPL) and (for the Pioneer missions to Jupiter) NASA’s Ames Research Center.  Figures may vary on the actual missions.]

Time: One hour before closest approach.  We are coming in on Venus’s dark side.  Only a sliver of the lighted side of the planet is clearly visible.  Behind us, 28 million miles distant, our home planet has shrunk to a brilliant “star” – the brightest in the heavens except for the sun.  Our messages, traveling at the speed of light, take two and a half minutes to reach the 210-foot radio telescope at Goldstone, California.  The sun, only 67 million miles away, has grow a third larger than it appears from Earth.  Now twice as much solar heat and light beat down upon our spacecraft.  As we curve around the planet, the growing crescent rapidly enlarges.  At closest approach – about 3,100 miles away – a half Venus nearly fills our field of vision, shining brilliantly with a slight yellowish color.  Then the entire dazzling spectacle sweeps into view as we swing on around and head to Mercury; Venus is more than three times as bright as Earth if seen from the same distance.  During the flyby, our cameras are taking pictures of the scene and our instruments are recording temperature and other information about the planet’s environment.  All we can see, however, is an expanse of dense clouds.  That is all any man has ever seen of Venus.  And it may be all any man will ever see, for what lurks below that veil is an awesome world of unbearable heat and pressures and of terrifying distortions.

Back on Earth, scientists have just begun to crack the many mysteries of Venus.  By piecing together evidence from earth-based radar, from three Soviet probes, and from Mariner 2 and 5, they are beginning to find out what makes Venus such a grim place.  [See: “Mariner Scans a Lifeless Venus,” May 1963, National Geographic.]  Key to the matter is a remarkable atmosphere, now thought to be 95% carbon dioxide, that exerted a pressure a hundred times the pressure of Earth’s atmosphere.  Layers of clouds above Venus reach the astonishing altitude of 35 miles.  The thick atmosphere traps the sun’s energy and helps build up the most furnacelike heat yet found on any planet.  It is much like the glass of a greenhouse in holding in the energy by trapping the longer wavelengths.  At the equator, the temperatures are observed to be as high as 1,000 degrees F.  Radar has detected a low mountain range, but in general the surface of Venus is thought to be quite gentle in slope.  The thick atmosphere has the capacity to bend light sharply, just as a prism does.  At the top layer of the clouds, temperatures have been measured at about -35 degrees F., and somewhere between the frigid cloud tops and the searing surface must be a “comfortable” temperature.  One of the many mysteries about Venus is where its water has gone – if, indeed, it ever had any.  The amount of water vapor in the upper atmosphere is extremely low – no more than 1/1,000 the amount of Earth’s.  The rotation period of Venus is 243 days, turning clockwise, backward to the typical motion of the planets.  Because of the combination of this slow backward rotation and the 225 days it takes to orbit the sun, Venus “sees” the sun come up in the west every 117 days.  The makeup of the clouds is still being debated.  Some insist that they find evidence of water droplets and ice crystals.  Other substances suggested are compounds of mercury and a form of iron chloride which might explain the yellowish color.

 

Mercury

 

Catapulted by Venus’s gravitational field, our spacecraft bends it flight path by some 40 degrees and races toward Mercury, the solar system’s innermost and smallest planet.  On March 30, 1974, we reach its second goal.  Mercury has only about a third the diameter of Earth.  We approach it so fast and it looms up so swiftly that was almost feel vertigo.  Now our cameras and instruments race to record information.  After years of preparation and 5½ months in flight, we have only two hours to gather all the close-up information on Mercury we will get in this decade.  Earth lies 93 million miles behind, still a very bright point of light.  The scientists there, at the Jet Propulsion Laboratory in Pasadena, California, wait tensely for the information from our instruments and tape recorders; the signals take more than eight minutes to reach Earth.  The sun, now only 43 million miles away, appears more than twice as large as when seen from Earth; the solar radiation bombarding us is five times as intense as that striking Earth’s atmosphere.  If our spacecraft were truly designed for manned spaceflight, it would require more radiation shielding and temperature control.  The surface of Mercury filling our view is a rare sight, never clearly seen from Earth.  Now we can see it with perfect clarity; no atmospheric effects block the vista.  We are only about 600 miles from the surface, and our eye can distinguish objects as small as 1,700 feet across.  Everywhere we see evidence that this rocky cinder has been cratered by comets and asteroids, and it is not hard to imagine that it was once scorched by tremendous heat.

Early in solar-system history, the sun blazed for a short time, maybe ten thousand years, with a luminosity as much as thirty times as great than that of today.  Mercury was probably twice as measure then as it is now, but the sun evaporated away half its substance.  The lighter, more volatile elements escaped, leaving a heavy planet that is probably about 30% silicates, or rock, and 70% metals.  It is 5½ times as dense as water.  Even today Mercury bathes constantly in ferocious heat.  When the planet is aphelion, the farthermost point from the sun in its eccentric orbit, the flow of solar energy is five times as great as that reaching the vicinity of Earth.  When Mercury comes into perihelion, its closest approach, the searing radiation is ten times as great.  Temperatures reach 650 degrees F. on the equator, though they probably drop during the long night to -300 degrees F.  And there is apparently no atmosphere.  The was Mercury reflects and polarizes light is similar to that of the airless moon.  With low gravity (a third of Earth’s) and high temperatures, most gases would escape over the eons.  The pictures telemetered back to Earth in the Mercury mission will arouse unprecedented interest among scientists and laymen alike.  Astronomers have never seen Mercury really well.  The planet stays so close to the sun in its relatively tiny orbit.  It can sometimes be seen briefly as an evening star just after sunset, or a morning star just before dawn.  The widely held theory was that Mercury always kept the same face toward the sun, that its rotation, like that of the moon’s, was synchronous.  That period was 88 earth days.  But, in 1965, the 1,000-foot radio telescope at Arecibo, Puerto Rico, monitored the planet’s rotation at only about 59 days.  It was noticed that 59 was almost exactly two-thirds of 88.  It meant that Mercury spins three time for every two orbits.  So, Mercury’s year is 88 days, and its sidereal day (as seen from the stars) is 58.65 days.  But its solar day – from one midnight to the next – was exactly twice as long as its year, 176 days.  Conditions are not favorable for life, and no one seriously suggests it.

 

Mars

 

The first of the planets outside of Earth’s orbit is Mars, the red planet, whose color suggested blood and once chilled the hearts of men.  Mars, the planet of war, whose symbol represents a shield and spear, and whose two tiny moons, Phobos (Fear) and Deimos (Terror), were named for the war god’s attendants.  Edgar Rice Burroughs’ Mars was the home of Dejah Thoris, Princess of Helium.

Time: November 14, 1971.  Just 193 days ago our one-ton Mariner spacecraft left Cape Kennedy, propelled by an Atlas-Centaur rocket.  It is unmanned, but let us imagine we are aboard.  Now we are at our closest approach to Mars, only 1,000 miles above the ruddy surface.  The colors are burnt ocher in the bright areas and a grayer red in the dark, with none of the greens and blues observers “see” in their telescopes.  The bright greens and blues are very largely an optical illusion.  Our cameras are greedily recording the scene below.  This time will be no simple flyby, such as the previous three Mariner Mars missions.  Our spacecraft has gone into an orbit that will swing out to a distance of 10,500 miles and bring us back to a 1,000-mile altitude just 12 hours from now.  For the next 90 days we will orbit in this fashion, photographic strip after strip, mapping 70% of the entire planet.  We scan the dark and light areas, searching for familiar outlines.  Nix Olympica (Snow of Olympus), lies far to the northwest.  Below us and to the west is Solis Lacus (Lake of the Sun), and just ahead are the dark region Aurorae Sinus (Bay of Dawn) and the bright regions Candor and Xanthe.  What romantic names these are, given at a time when every educated man was steeped in mythology.  Craters dominate the landscape.  We can see no mountain chains, no bodies of water, no canals.  The atmosphere seems clear almost to the horizon, where a narrow rim of bluish haze, with an occasional bright patch, gives way to the blackness of space.  And out in the blackness, 130,000,000 miles away, shines a diminished sun, two-thirds its remembered size.  Only half as much solar energy is falling on Mars as on Earth.  Back on Earth, 400 seconds away as our telemetry signals travel, scientists at the Jet Propulsion Laboratory are processing Mariner’s TV pictures and information from our instruments about temperatures, atmospheric constituents, and possibly some materials from the surface.  Ten days from now another Mariner exactly like ours will arrive, go into a somewhat different orbit, and for 90 days flash back to Earth findings about seasonal changes.  Between these two Mariner 1971 missions, we hope to clear up some of the enigmas of Mars.

Before the Mariner 4 mission in 1964-65, when all we knew of Mars had been painfully gleaned through telescopes, many people believed that it was the only planet whose surface could be clearly seen.  Its solar day lasted only about 40 minutes longer than ours.  The tilt of its axis with respect to the orbit was only about 2 degrees greater than Earth’s, which gave the two planets much the same seasonal variations.  Surface temperatures, at least at midday on the equator were comparable to those on Earth on a spring day.  We could clearly see polar caps like Earth’s, presumably made of water ice, that waxed and waned with the seasons.  The spring “wave of darkening” suggested vegetation responding to the advancing moisture from the polar regions.  And of course there were the “canals.”  Since 1877, straight lines were observed on Mars.  They wee called canali (channels) in Italian, but mistranslated as canals, which suggested they were dug by intelligent beings.  But the three Mariner spacecrafts have demolished most of those notions.  From close-up pictures sent back to Earth, showing 20% of the Martian surface, and from infrared and ultraviolet studies, we now see a Mars that is quite different from Earth.  The Martian Atmosphere, chiefly carbon dioxide, measures less than 1% the density and pressure of Earth’s.  The planet enjoys little protection against the sun’s radiation, especially ultraviolet, that would kill any unprotected Earth organism.  At midday, the temperature might reach 80 degrees F., but at night it could drop to -150 degrees.  Mars appears much drier than Earth’s most arid deserts.  The polar caps in all probability are dry ice – frozen carbon dioxide – with small amounts of water ice.  Everything in the Mariner pictures indicates very gentle slopes on Mars.  There are no mountain ranges, no great faults, no extensive volcanic fields, in fact no evidence o volcanic activity.  The Arecibo radar has found a difference of about eight miles between the lowest and highest points on the planet.

Three radically different kinds of terrain show up in Mariner pictures: cratered, “chaotic,” and featureless.  As on the moon, craters appear prominently, even in the thin south polar cap.  But in depth and slope, they vary distinctly from those of the lunar surface, suggesting differences in the geological processes that modify them.  Many of the Martian craters appear as though their rims had been sandpapered off, and they show very flat bottoms.  A second kind of terrain, essentially free of craters, appears in the region between Aurorae Sinus and Margaritifer Sinus.  Its irregular, jumbled topography of short ridges and furrows, covering in one area as much as half a million square miles – the size of Alaska – has been given the name chaotic terrain.”  The bright circular “desert” of Hellas represents the third kind of topography of Mars.  It is called featureless because in a large smooth basin, some 1,200 miles across, hardly any craters can be seen.  Nothing on the moon looks like this, but it does somewhat resemble the great plains of Earth.  By contrast, the dark highland region to the west, Hellespontus, is heavily cratered.  Possibly Hellas is floored by some unusually light, porous material.  The Martian wind, perhaps reaching 100-mile-an-hour velocities, could move this material, and craters would quickly be filled in and lost.

 

Asteroids

 

Time:  Midsummer 1972.  The Pioneer F spacecraft, which left Earth 140 days ago, has long since crossed the orbit of Mars.  It is 125,000.000 miles from Earth, heading for Jupiter, largest of the planets.  To us, as imaginary passengers, it appears as a mere point of light; its banded disk will not be clearly visible for many weeks.  Now we are running the gauntlet of the asteroid belt.  The moment of truth has arrived.  Will we survive the passage through the minor planets and millions of smaller objects that swirl in this celestial grinding machine between Mars and Jupiter?  It will take some 200 days to cross the 150,000,000-mile-wide belt.  A particle even the size of a pea moving at 12 miles a second could completely disable our spacecraft.  What are the odds?  Fortunately very low, according to the experts planning the Pioneer flights.  We scan the blackness, hoping to see one of the minor planets, but in vain.  Calculations are that while traversing the belt we might be within viewing distance of no more than one object as large as 450 feet in diameter.  Perhaps 20 bodies as large as 130 feet across will come close enough to be detected, but only if we are looking in the right direction at the right time.

On New Year’s Day in 1801 an Italian, Giuseppi Piazzi, discovered a starlike body beyond the orbit of Mars, where a “missing planet” was supposed to be located.  Named Ceres, it proved to be the first and largest of a group of objects called asteroids that circle the sun in a wide belt between the orbits of Mars and Jupiter.  Ceres, an airless, lifeless ball 480 miles in diameter, is large enough to be termed a miniature planet, and so are a few others, such as Pallas, Juno, and Vesta.  But the great majority are irregular chunks no more than a mile across, and countless numbers ranging down to the size of a dust grain.  Perhaps 100,000 could be detected with the 200-inch telescope on Palomar Mountain, California.  Only one, Vesta, is ever visible to the naked eye.  If all were swept up together, they would be less than a thousandth the mass of Earth.  Nearly two thousand asteroids have been observed enough to be given numbers and names.  Many bear mythological names; others honor astronomers, flowers, cities, and women.  An easy assumption would be that the asteroids are the debris of a planet that exploded.  But the reverse may well be true.  The asteroids are probably part of the original record of the nebula, or dust cloud, from which we believe the sun and planets condensed some five billion years ago.  While most of these small bodies stay close to the asteroid belt, a few have eccentric orbits that invade the inner solar system, and some fly so close to our own planet that they might be called “Earth-grazers.”  Just a year ago this month, astronomers took aim at an unusual Earth-grazer, Geographos, which came within 5.6 million miles.  Discovered in 1951 during the National Geographic Society-Palomar Observatory Sky Survey, this planetoid was named in honor of the Society.  Huge craters on the moon testify to what happens when the orbit of one of these flying mountains brings them into collision with other celestial bodies.  Earth’s atmosphere burns up the small meteoroids. Rocks 100 feet across, like the one that blasted Arizona’s Meteor Crater, hit once every 50,000 years in North America.

 

Jupiter

 

Time: Between December 7, 1973, and March 17, 1974.  Safely through the asteroid belt, our buglike Pioneer F spacecraft approaches Jupiter, giant of the solar system.  Earth, now a bluish point, lies more than 500 million miles behind.  The sun, almost as far, shows a disk about a fifth its normal size.  Solar cells for energy would be of marginal use to us now: We receive only one twenty-seventh the solar radiation Earth receives.  So, on two long booms stretching beyond our big antenna dish, we have generators using radioactive materials to produce power to operate our electronic equipment.  Messages to Earth, carrying pictures and information on magnetic fields, ionized particles, temperature, and planetary chemistry, now take 47 minutes to travel that great distance one way.  There seems to be no end to the enormous bulk below us.  Although we are 80,000 miles away, this gargantuan hydrogen ball is so huge – enough to swallow up 1,300 Earths – that we cannot see an entire hemisphere.  Only when we are farther out could we view it as a sphere and see how it is flattened at the poles because of its high rotation rate.  Irregular bands of alternating yellow and bluish or brownish gray cover the surface – and yet perhaps one should not say surface, for all we see are clouds.  No man knows at what depth, perhaps thousands of miles down, Jupiter’s enormous pressures have turned its hydrogen into a metallic solid.  Two striking phenomena catch the eye: The Great Red Spot, one of the most curious objects of the entire solar system, stans out like a blemish on the southern hemisphere.  Its elliptical area, seeming to “float” among the clouds, is larger than Earth’s surface.  The most likely theory yet proposed suggests that a kind of eddy in the atmosphere caused by a depression or a high spot far below.  A crisp black circle marks the shadow of Io, one of Jupiter’s 12 moons.  Inexplicably, this shadow has been known to emit more radiant heat than the cloud layer around it.  Io itself has been easily visible to us as we approached, and so have the other three satellites Galileo first saw: Ganymede, larger than Mercury, Calisto, larger than Earth’s one moon, and Europa, a fifth the size of Earth.  Eight additional – and smaller – satellites give Jupiter a larger family than the sun’s.

We know all too little about Jupiter; it is a series of perplexing questions and riddles.  Yet what scientists do know and what they surmise make it in many ways the most exciting, the most provocative, body in the solar system.  We regard this vast ball, so differently from any of the rocky terrestrial planets, as a “deep-freeze sample” of the original cloud of dust and gas from which the solar system condensed.  Cold outer temperatures and high gravitational force (two and a third times that of Earth) have most likely prevented the primordial gases from escaping.  Jupiter may be considered almost a star.  If it were only a little more massive, gravitational contraction would release so much energy that it would turn into a nuclear furnace, like the sun, or any other star, and become incandescent.  Though Jupiter is merely a “near-star,” the giant planet gives off substantially more energy than it receives from the sun.  Gravitational contraction has been suggested as a cause.  Scientists suspect that temperatures rise steadily from -200 degrees F. at the cloud tops to as much as 20,000 degrees F. at the core.  And through some mechanism not clearly understood, the Jovian planet emits random bursts of intense radio energy at long wavelengths.  It is the most powerful radio object in the sky except the sun.  Apparently, these emissions are tied to the planets powerful magnetic field and radiation belts, something no other planet except Earth is known to have.  To add to puzzle, the radiocasts are affected by the position of Io, second closest of Jupiter’s moons.  The atmosphere consists largely of hydrogen and helium, which explains why the planet’s density is only a fourth that of Earth’s.  Pioneer F – and Pioneer G, scheduled to go to Jupiter 13 months later – will seek to establish the all-important proportions of hydrogen and helium.  These two gases, the lightest and simplest of all chemical elements, make up 99% of the universe.  Methane and ammonia, simple compounds when hydrogen joins with carbon and nitrogen respectively, have been detected.  What lies deeper can only be conjectured; water has often been suggested.

Floating high in the Jovian atmosphere are enormous bands of pastel-colored clouds, thought to be composed of frozen and liquid ammonia compounds.  They rotate at whirlwind speed; although Jupiter has eleven times the diameter of Earth, its rotation rate is more than twice as fast – one turn in less than ten hours.  No wonder the planet bulges at the equator.  Jupiter’s atmosphere – containing hydrogen, ammonia, methane, and water is very like that of early Earth, when life arose.  In Jupiter’s atmosphere today lightning and ultraviolet radiation may be producing amino acids, the building blocks of life.  Late in the 1970’s, if the present planning of NASA and JPL is carried out, two unmanned missions called the “Grand Tours” will be sent forth on journeys to the outer reaches of the solar system.  They will take advantage of an opportunity arising only once every 175 years, when the outer planets line up relatively close together along an arc, like pearls on a string.  The two 1,300-pound robot craft will aim first at Jupiter, whose enormous gravitational effect will carom them off at greatly increased velocity to the next plant, where the process will be repeated.  These might appropriately be called the “By Jove” missions.  One voyage, beginning in 1977, is planned to visit Jupiter, Saturn, and Pluto, making the trip in 8½ years instead of 40 it would take to go to Pluto if this game of celestial billiards were not played.  The other Grand Tour, starting in 1979, aims for Jupiter, Uranus, and Neptune.

 

Saturn

 

Time: September 12, 1980.  Since leaving Earth it has taken three years, even with Jupiter’s powerful kick, for us to reach Saturn – sixth Planet of our solar system.  The sun lies nearly 900 million miles behind us, twice as far as from Jupiter.  Its warmth and light are only a hundredth of what we are accustomed to; Saturn travels in eternal twilight.  A message to Earth – at the speed of light – now requires nearly an hour and a half.  We are diving under Saturn, staying well clear of its hazardous rings and ten moons.  Our path will then turn up to throw us out of the ecliptic (the plane in which the Earth orbits the sun) and toward Pluto, whose orbit is tilted 17 degrees to the ecliptic.  Some 281,000 miles away, Saturn – the most extravagant sight in the family of the sun – glows with a dull yellowish hue.  But the brilliant white of the rings suggests the glitter of countless diamonds.

The author has looked at this remarkable planet through one of the University of Arizona telescopes.  The half tilt of the rings presented a favorable view, and seeing that winter night was exceptional.  Even the faint bands on the planet itself were clearly discernable.  If he felt any disappointment, it was only that this exquisite spectacle, almost a billion miles away, presented the static quality of a carving in ice.  Somehow, he expected the rings in my eyepiece to whirl like a spinning top.  Controversy surrounds the rings.  Most specialist, however, would agree that they represent particles that never did accrete into a satellite (or just possibly a satellite that swung too close to Saturn and broke up in its gravitational grip); and that they consist of chunks of water ice, or ice-coated bits of rock, each in its own orbit.  Collisions among particles whose orbits crossed because they were elliptical or tilted have gradually forced all the particles into circular orbits in approximately the same flat plane thinner than a sheet of paper in proportion to its width.  Estimates of the rings’ thickness range from around a foot to a half a mile.  Every 15 years, the rings are seen edge on from Earth.  The matter of the rings’ thickness may be settled at the end of the year when they are seen edge on again.  Two bright rings are clearly visible, separated by the Cassini Division.  A third, the Crepe Ring, a dusky band, lies closer in.  And inside these, reaching almost to Saturn, is a fourth ring, extremely faint, observed last year.  Second planet in size, Saturn is another gas giant with a composition much like Jupiter’s.  If you could get it into a tub of water, it would float, for the density of Saturn is only 7/10 that of water.

 

Uranus

 

Time: July 28, 1985, six years since leaving Earth and four years from Jupiter.  We are moving into the bitter cold and unrelieved darkness of the outer solar system, crossing the orbits of far-ranging comets.  Uranus lies only 15,500 miles below, a pale greenish orb with faint markings.  Five moons make up its retinue.  Earth glimmers nearly two billion miles away, a distance requiring 2 hours and 45 minutes to span.

From here on out, we know so little about the planets that we can hardly ask questions.  Even with the best telescopes, cameras rarely if ever record true surface features on Uranus.  Its diameter and rotation period are imperfectly known, and its surface markings and atmosphere are still speculative.  Quite unlike its neighbors, Uranus lies on its side, so that at intervals in its orbit its poles point almost directly toward the sun.  If you watch from one of the poles, you would see the sun for 42 years, and then live in darkness for another 42.

 

Neptune

 

Time: November 28, 1988.  We see Neptune dead ahead, a bluish-green sphere whose features have never been photographed from Earth.  Farthest out of the giant planets, this twin of Uranus now moves nearly three billion miles from the sun.  So intense is the cold that our spacecraft would drop to 370 degrees F., except for the heat from radio-isotope thermal generators.  Radio time to Earth: four hours, six minutes.  The data our instruments are now recording, and the pictures being sent to Earth, will add immeasurably to our understanding of Neptune and its moons, for our knowledge is meager indeed.  Both Uranus and Neptune have densities greater than Jupiter and Saturn.  This fact suggests that the two outer planets are not as rich in hydrogen and helium, but must contain a higher proportion of water and ammonia ices, as do comets.

 

Pluto

 

In the 1840’s, scientists concluded that the gravitational tug of an unknown planet was forcing Uranus to wander from its predicted orbit.  The location where the missing planet should be was calculated, and Neptune was found within an hour’s search.  But Neptune’s pull seemingly did not fully account for the observed wanderings of Uranus.  Scientists insisted that still another planet would be found, and predicted its path around the sun.  In 1929, Clyde W. Tombaugh, at Lowell Observatory in Flagstaff, Arizona, undertook to look for “Planet X.”  Tombaugh’s technique was to take two photographs of a section of the sky a few nights apart, then compare the images.  Under a viewing device known as a blink comparator, he examined first one plate and then the other, in rapid succession, studying one small area at a time.  If any object had moved against the star background in that interval of several night, it would seem to jump.  Asteroids and spurious images complicated this tedious work.  But on February 18, 1930, after seven months’ painstaking inspection of some six million star-images, Tombaugh found his quarry – a yellowish body with a magnitude of about 15, only 1/4000 as bright as the faintest star you can see with the naked eye.  It was named Pluto, and the first two letters of the name became its symbol.  The outermost planet is so remote that it is exceedingly difficult to measure accurately.  Thus, we know almost nothing about it, except that it orbits the sun in 247 earth years, rotates in 6.4 days and appears to be no larger than Mars.  It is thought to be solid, not gaseous.  Its eccentric orbit brings it inside Neptune’s orbit near perihelion.  For this and other reasons, some astronomers think Pluto is actually an escaped satellite of Neptune.  Following his discovery, Tombaugh continued searching for other planets for 14 more years to no avail.  Ironically, the calculations of Uranus’s deviations have since been shown to be in error.  So, the discovery of the nineth planet was a happy accident.

On March 9, 1986, we encounter Pluto on our Grand Tour.  At that time the little planet, probably a snow-covered rock, is thirty times as far from the sun as Earth, and the solar energy falling on each square mile is a thousandth of that for Earth.  Our long voyage to the planets is ended.  The Grand Tour spacecraft moves on to an endless wandering beyond the solar system and into the mazes of the Milky Way.  We have traveled farther than any man before us, and have seen such wonders as no eyes have ever beheld.  With Immanuel Kant, the 18^th-century philosopher, we may say: “I have… Ventured on a dangerous journey, and I already behold the foothills of new lands.  Those who have the courage to continue the search will set foot upon them…”  THE END

 

Part 2: The Reality

 

The Flight of Mariner 10

 

Mariner 10, the first space probe designed to explore Mercury by way of a Venus gravity assist, was launched on November 3, 1973, on an Atlas-Centaur rocket.  The view from the imaginary alien spacecraft in the National Geographic article was recreated during the departure from the earth-moon system by Mariner 10.  Those images were used to calibrate the cameras.

The mission followed the narrative in the National Geographic very closely.  The spacecraft was the third American spacecraft to fly past Venus, but the first to carry a camera.  Since Venus was totally cloud covered, other instruments flew on Mariner 2 in 1962, and on Mariner 5 in 1967.  Mariner 10 reached Venus of February 5, 1974, flying by the planet at 3,584 miles over the night side.  Its first pictures of Venus were of a crescent planet.  As it swung around Venus, more and more of the planet came into view.  By using special filters with range into the ultraviolet, Mariner 10 took pictures that showed details in the cloud cover.  The photos show that the atmosphere rotates much faster than the planet, in about five days, with the fastest winds around the equator gradually slowing toward the poles.  This fact let to the observed chevron pattern in the Venusian atmosphere.  Besides the instruments to detect infrared, ultraviolet, magnetic, and plasma data, the spacecraft performed an occultation, where the spacecraft disappears behind the planet.  The probe sent a steady radio signal that passed through Venus’s atmosphere, allowing density measurements.  The gravity assist was successful.  Mariner 10’s heliocentric velocity dropped from 82,785 mph to 72,215 mph, allowing the spacecraft to fall in toward Mercury.

The spacecraft arrived at Mercury on March 29, 1974 and flew by the planet at 437 miles distance over the night side.  Approach pictures were taken starting hours before closest approach and departure pictures continues for hours after.  The probe was in a 176-day orbit, twice that of Mercury’s.  This meant that both planet and probe would return to the same place on September 21, 1974, which they did.  This time, the probe flew 29,869 miles over the south polar region of the planet.  After losing roll control in October 1974, a third and final encounter, the closest to Mercury, took place on March 16, 1975, at a range of 203 miles, passing almost directly over the north pole.  On March 24, 1975, the maneuvering gas was exhausted.  Commands were sent to turn off the radio transmitter and the mission ended.  The irony of Mariner 10’s orbit was that the same face was sunlit on each encounter, much like observing the moon from Earth.  Because of this fact, only about 45% of Mercury’s surface was mapped, from the over 2,800 photographs taken of the planet.  The occultation of Mercury showed that it has a tenuous atmosphere of helium; the magnetometer showed the planet has a strong magnetic field and a large, iron-rich core; and the infrared readings showed Mercury’s nighttime temperature drops to -297 degrees F., and its daytime temperature reaches 369 degrees F.

 

The Flights of Mariners 8 and 9

 

Mariner 8, the probe that the National Geographic article described, launched on May 9, 1971 on an Atlas-Centaur rocket.  The after separating from the Atlas first stage, the Centaur upper stage lost control and started tumbling.  It crashed in the Atlantic Ocean north of Puerto Rico.  [As of this article, Mariner 8 was the last launch failure of an American planetary probe.]  The Mariner ’71 mission intended to have two identical spacecrafts put into different orbits around Mars for different purposes.  Mariner 8 was supposed to do a general global mapping mission, while Mariner 9, its twin, was meant for more targeted mapping of future landing sites.  With Mariner 8 in the drink, the launch of Mariner 9, originally scheduled for May 19^th, was delayed, and a new, hybrid mission was devised to achieve as many of the original mission’s goals as possible.  Finally, on May 28, 1971, Mariner 9 took off on another Atlas-Centaur.  This time, both stages worked and Mariner 9 set sail for Mars.  When Mariner 9’s retrorockets fired, on November 14, 1971, Mariner 9 became the first spacecraft to orbit another planet.  Unfortunately, a global dust storm was underway, and the goal of mapping 70% of Mars’s surface looked in doubt.  Fortunately, the rugged little spacecraft lasted longer that its planned 3-month mission.  The storm lasted to mid-January, and Mariner 9 began its photography of the surface.  It took 7,329 images and mapped 85% of the surface.  The images revealed river beds, craters, and massive volcanoes.  Mariner 9 discovered a massive canyon system that was named Valles Marineris in its honor.  After depleting it supply of attitude control gas, the spacecraft was turned off on October 27, 1972.

 

The flights of Pioneers 10 and 11

 

Approved in February, 1969, Pioneer 10 (originally designated Pioneer F) was launched March 3, 1972 on an Atlas-Centaur-Star 3-stage rocket.  The probe was sent on a direct, 21-month flight to Jupiter, leaving Earth at 9 miles a second.  Even at that speed, it took 21 months to reach Jupiter.  It took 63 days to cross Mars’s orbit.  Between July 15, 1972 and February 15, 1973, it became the first spacecraft to traverse the asteroid belt.  After crossing the asteroid belt, Pioneer 10’s trajectory was tweaked to use Jupiter to slingshot it out of the solar system.  On November 6, 1973, at 16 million miles distance, “photography” of Jupiter was begun.  Unlike Mariner spacecrafts which are 3-axis stabilized, Pioneers are spin stabilized.  Pioneer 10’s photometer swept the planet, a line at a time, to build up a picture, either through a red filter or a blue one.  Combining red and blue filter images with a synthetic green filtered one created with data from the two images, a color image was produced.  By December 2, the image quality exceeded the best images from Earth.  A total of more than 500 images were transmitted.  The spacecraft fly over the equator of Jupiter at 82,178 miles above the cloud tops.  The radiation belt captured by the immense magnetic field was measured at 200,000 rads (electrons) and 56,000 rads (protons).  A full body dose of 500 rads is fatal to humans.  At closest approach, the probe’s speed reached 82,000 mph.  The trajectory took the spacecraft behind Jupiter for a radio occultation, and behind the moon, Io.  That maneuver showed that Io had an ionosphere extending 430 miles from the moon.  It also showed the Io orbited Jupiter in a doughnut-shaped cloud of hydrogen.  Cresent images of Jupiter were returned as Pioneer 10 moved away from the planet.  As Pioneer 10 drifted into the unknown, the last usable data was sent on April 27, 2002, and the last weak signal was detected January 23, 2003.  The last (unsuccessful) attempt to detect the probe was on March 4, 2006, ending the mission.

On April 5, 1973, thirteen months after its sister ship’s launch, Pioneer 11 (nee Pioneer G) lifted off on an Atlas-Centaur-Star rocket on a 20-month trip to Jupiter.  At the time of the launch, the Apollo moon program had been shut down, and NASA was going through a reorganization.  The Grand Tour mission was cancelled in December 1971, and money redirected to the Space Shuttle, and JPL was scrambling to come up with a cheaper alternative.  In May 1974, after Pioneer 10 had achieved all of its objectives, mission controllers at Ames retargeted Pioneer 11 to fly past Jupiter on a north-south trajectory, enabling a Saturn flyby in 1979.  On December 2, 1974, the probe passed Jupiter at 26,612 miles above the cloud tops.  The probe obtained detailed images of the Great Red Spot and of the immense polar regions.  Jupiter’s gravity altered the probe’s trajectory towards Saturn and to gain velocity.  The arc to Saturn took the probe 30 degrees above the plane of the solar system, an unexplored region, thus serving as a pathfinder for an upcoming mission – the dual Solar-Polar probes, later downsized to one probe and renamed, Ulysses.  By the time Pioneer 11 passed Saturn on September 1, 1979, at 13,000 miles distance from Saturn’s cloud tops, many events had occurred – the Grand Tour replacement had been approved, Mariners 11 and 12, cleared to fly to Jupiter and Saturn only; the probes were renamed Voyagers 1 and 2, and launched in 1977; and they both flew past Jupiter earlier in 1979.  The flyby distance of 13,000 was another pathfinder event – this was just outside the edge of the known ring system and exactly where Voyager 2 needed to pass if it was to continue to Uranus on an extended mission.  The probe discovered several small moons of Saturn, flying within 2,500 miles of one of them.  It also discovered another ring.  The magnetosphere and magnetic field of Saturn were charted, and performed occultations of Saturn and Titan, Saturn’s planet-sized moon, and the only moon with a sizable atmosphere.  By February 25, 1990, Pioneer 11 could no longer power its instruments, so it was shut down, ending the mission.

 

The Flights of Voyagers 1 and 2

 

As mentioned above, the Grand Tour was cancelled in December 1971.  In 1972, a scaled-down mission, Mariner Jupiter-Saturn, was proposed.  In the original Grand Tour plan, there was to be one launch in 1977 and another in 1979.  The new plan had both probes launched during the 1977 window.  On March 4, 1977, the probes, Mariners 11 and 12, were renamed Voyagers 1 and 2.  On August 20, 1977, the first probe was launched on a Titan IIIE rocket.  Since this spacecraft was on a longer, slower arc to Jupiter, it was named Voyager 2.  Voyager 1 was launched about two weeks later, on September 5, on another Titan IIIE.  Voyager 1 made flybys of Jupiter, Saturn, and Saturn’s moon, Titan.  NASA had a choice of either doing a Pluto or Titan flyby.  They opted for Titan since Pioneer 11 had shown that its atmosphere was thicker than Earth’s.  Voyager 1 began observing Jupiter on January 6, 1979.  During the Jupiter campaign, it studied and took high resolution photos of Jupiter and its major moons.  The probe came within 217,000 miles of the planet’s center on March 5, 1979, and flew by its moon Io three hours later at 13,000 miles distance.  Voyager 1 discovered active volcanism across the entire surface of Io.  After passing Jupiter, the probe photographed the never-before-seen planetary ring around Jupiter.  Voyager 1 finished photographing the Jovian system in April 1979.  Voyager 2 began its Jupiter observations on April 25, 1979.  It entered the Jovian system on July 8, and on July 9, 1979, the probe flew by Jupiter at 448,518 miles from planet center.  The probe continued observations of Jupiter until August 5, 1979, and the Jupiter campaign came to an end.  Between Voyager 1 and Voyager 2, almost seven months of continuous observations were made.

In the original plan for the Grand Tour, Jupiter was to be used to slingshot the probes to the four outer planets, two each.  Probe 1 was to go on to Saturn and Pluto, while Probe 2 was to continue flying to Uranus and Neptune.  Unfortunately (or not), Pluto was dropped from the agenda for a close look at Titan.  On the upside, the planned single flyby of Saturn had become three.  Together with Pioneer 11, the Voyager probes made flybys of the ringed planet in 1979, 1980, and 1981.  Pioneer’s encounter has already been discussed so let us start with Voyager 1.  Voyager 1 began long range observations of Saturn on August 22, 1980.  It entered the Saturnian system on November 12, 1980, with a flyby of Titan at 4,034 miles distance and a flyby of the planet at 114,542 miles from the center of mass.  Voyager discovered auroras on Saturn, examined the rings in detail, and photographed the major moons while discovering several minor ones.  The probe found Titan’s thick atmosphere completely opaque, with a thick hazy of organic smog.  An occultation was performed and the moon’s mass was measured.  Because observations were considered vital, the trajectory of Voyager 1 needed to pass below the south pole and out of the plane of the ecliptic.  The observations of Saturn ended on November 14, 1980.  With Voyager 1 successful in its exploration of Titan, Voyager 2 was cleared to extend its mission to include Uranus, and possibly Neptune.  The probe started observations of Saturn on June 5, 1981.  It entered the Saturnian system on August 22, 1981 and flew by Saturn at 100,062 miles from the planet’s center.  The spacecraft again studied the planet, its rings, and its moons, but this time with no close flybys.  The probe continued observations of Saturn until September 25, 1981 when Voyager 2’s Saturn campaign ended.

The original Grand Tour had Probe 2 leaving Earth in November 1979 and reaching Uranus on July 28, 1985.  Voyager 2 launch two years and three months earlier but, because of the detour to Saturn, it did not reach Uranus until January 24, 1986, six months later than imagined.  Observations of Uranus began on November 4, 1985.  Upon entering the Uranian system, the probe made a flyby of the moon, Miranda, at 18,024 miles.  After taking long-range images of Uranus’s other four known moons, it flew by Uranus at 66,500 miles from the planet’s center.  Most of the southern hemisphere was in sunlight at that time.  Voyager 2 discovered 11 previously unknown moons.  The planet’s day was measured at 17 hours 14 minutes.  Its magnetic field is misaligned with its rotation axis.  Uranus’s cloud features were hidden by a layer of haze, but false-color and contrast-enhanced images showed bands of concentric clouds around the south pole.  In addition to the haze, an ultraviolet “dayglow” was prevalent in the atmosphere.  Detailed images from the flyby of Miranda showed huge canyons made from geological faults.  One suggestion was that Miranda was shattered and reaggregated.  Before Voyager 2 arrived, earth-based observations had discovered a ring system around Uranus.  Voyager 2 discovered two additional rings in that system.  Voyager 2’s Uranus campaign ended on February 25, 1986 when observations were terminated.

Voyager 2 flew by Neptune on August 25, 1989, twelve years, and five days after it was launched.  The Grand Tour envisioned in the pages of National Geographic had the Neptune encounter occurring on November 28, 1988, nine months before the actual event.  Long range observations of Neptune began on June 5, 1989.  By the time Voyager 2 was approaching Neptune, partial rings had been discovered around the planet by earth-based telescopes.  Voyager 2 proved that these ring arcs were in fact complete, albeit lumpy, rings.  The probe flew by Neptune at only 3,076 miles above the cloud tops.  Four hours later, it made a flyby of the large moon, Triton at 24,736 miles distance.  Since the plane of Triton’s orbit is tilted to the plane of the ecliptic, Voyager 2 was directed to fly over the north pole of Neptune.  Around 60 lightning strikes were detected in the planet’s atmosphere during the flyby.  Voyager 2 discovered six new moons of Neptune, as well as additional rings in the planets ring system.  On Neptune itself, Voyager 2 discovered the “Great Dark Spot,” which was believed to be a region of clear gas, forming a window in the planet’s high-altitude methane cloud deck.  On October 2, 1989 observations of Neptune ceased and the “Grand Tour” was completed, almost.  On February 14, 1990, Voyager 1 took a “family portrait” of the solar system, just like the same imaginary alien spacecraft from the National Geographic article would see as it approached the system.  Besides the sun, six planets – Venus, Earth, Jupiter, Saturn, Uranus, and Neptune – posed for the portrait.  Mars proved to dim, and Mercury was washed out by the sun.  The image of Earth is the Famous “Pale Blue Dot.”  Both Voyagers are still in operation in interstellar space at the time of this writing, but that is another story.

 

Part 3: The Legacy

 

Earth

 

The view from the imaginary alien spacecraft in the National Geographic article was finally recreated starting with the departure from the earth-moon system by Mariner 10 in 1973.  [See: Flight of Mariner 10 above.]  Those images were used to calibrate the cameras.  Starting in 1990, with the Jupiter-bound spacecraft, Galileo, Earth was the target of flybys to provide gravity assists, boosting, or lowering the spacecraft’s velocity, pushing it farther out of, or dropping it farther into, the solar system.  These flybys not only allowed testing of cameras and instruments, but also to perform ground-truth studies, such as looking for bio-markers, or indications of life.  A list of NASA Earth flybys is as follows:

• Galileo (to Earth) 1990
• Galileo (to Jupiter) 1992
• NEAR (to Eros) 1998
• Cassini (to Jupiter) 1999
• Stardust (to 81P/Wild) 2001
• MESSENGER (to Venus) 2005
• Stardust (capsule drop-off) 2006
• EPOXI (to Earth) 2007
• EPOXI (to 103P/Hartley) 2008
• Stardust (to 9P/Tempel) 2009
• EPOXI (no target) 2010
• Juno (to Jupiter) 2013
• OSIRIS-REx (to Bennu) 2017
• Lucy (to Jupiter Trojans) 2022
• OSIRIS-APEX (to Apophis) 2023

Venus

 

List of further NASA Venus exploration:

• Pioneer Venus 1 (orbiter) 1978
• Pioneer Venus 2 (multiprobe) 1978 (4 entry probes, 1 landed)
• Galileo (flyby) 1990
• Magellan (orbiter) 1990 (radar mapper)
• Cassini (flyby) 1998
• Cassini (flyby) 1999
• MESSENGER (flyby) 2006
• MESSANGER (flyby) 2007
• Parker Solar Probe (7 flybys) 2018-24

 

Mercury

 

List of further NASA Mercury exploration:

• MESSENGER (flyby) 2008
• MESSENGER (flyby) 2008
• MESSENGER (flyby) 2009
• MESSENGER (orbiter) 2011

 

Mars

 

List of further NASA Mars exploration:

• Viking 1 (orbiter/lander) 1976
• Viking 2 (orbiter/lander) 1976
• Mars Observer (orbiter) 1993 [failed]
• Mars Surveyor (orbiter) 1997
• Mars Pathfinder (lander) 1997
• Sojourner (rover) 1997
• Mars Climate Orbiter (orbiter) 1999 [failed]
• Mars Polar Lander (lander) 1999 [failed]
• Deep Space 2 (2 penetrators) 1999 [failed]
• Mars Odyssey (orbiter) 2001
• Spirit (rover) 2004
• Opportunity (rover) 2004
• Mars Reconnaissance Orbiter (orbiter) 2006
• Phoenix (lander) 2008
• Dawn (flyby) 2009
• Curiosity (rover) 2012
• MAVEN (orbiter) 2014
• InSight (lander) 2018
• MarCO A & B (2 flybys) 2018
• Perseverance (rover) 2021
• Ingenuity (helicopter) 2021

 

Ceres and the Asteroids Belt

 

In the National Geographic article, the imaginary asteroid flyby was of Geographos, an object named after our society.  Unfortunately, the only flyby of that body (Clementine) failed before reaching that body.  Galileo flew by the main belt asteroid, Gaspra, on October 29, 1991 beginning a series of investigations of main belt, near-earth, and soon the Trojan, asteroids.  Ceres, the largest object in the asteroid belt, was first designated a planet when it was discovered in 1801, but later it was demoted to an asteroid once more objects in the belt were discovered.  In 2006, Ceres was promoted to the new designation, Dwarf Planet.  Being the only dwarf planet in the asteroid belt, Ceres deserved special attention.  On March 6, 2015, the ion-drive powered spacecraft, Dawn, settled into orbit around Ceres, making the small world the first dwarf planet to be explored.  Dawn had already explored the massive asteroid, Vesta, from orbit in 2011.  Since I had no other place to put them, I am listing NASA’s comet missions here as well.  A List of NASA Asteroid exploration:

• Galileo (flyby) 1991 Gaspra
• Galileo (flyby) 1993 Ida
• Clementine (flyby) 1994 Geographos [failed]
• NEAR (flyby) 1997 Mathilde
• NEAR (flyby) 1999 Eros
• Deep Space 1 (flyby) 1999 Braille
• Cassini (flyby) 2000 Masursky
• NEAR (orbiter) 2000 Eros
• NEAR (lander) 2001 Eros
• Stardust (flyby) 2002 Annefrank
• New Horizons (flyby) 2006 APL
• Dawn (orbiter) 2011 Vesta
• Dawn (orbiter) 2015 Ceres [Minor Planet: see below]
• OSIRIS-REx (orbiter/sampler) 2018 Bennu
• DART (flyby/impactor) 2022 Didymos
• Near-Earth Asteroid Scout (flyby) 2022 GE [failed]
• Lucy (flyby) 2023 Dinkinesh

And a List of NASA Comet exploration:

• ICE (flyby) 1985 Giacobini-Zinner
• ICE (flyby) 1986 Halley
• Deep Space 1 (flyby) 2001 Borrelly
• Contour (flyby) 2003 Encke [failed]
• Stardust (flyby/sampler) 2004 Wild
• Deep Impact (flyby/impactor) 2005 Temple
• EPOXI (flyby) 2010 Hartley
• NExT (flyby) 2011 Temple

 

Jupiter

 

A List of further NASA Jupiter exploration:

• Ulysses (flyby) 1992
• Galileo (orbiter/probe) 1995
• Cassini (flyby) 2000
• Ulysses (flyby) 2004
• New Horizons (flyby) 2007
• Juno (orbiter) 2016

 

Saturn

 

A List of further NASA Saturn exploration:

• Cassini (orbiter) 2004

 

Uranus

 

No further NASA Uranus exploration to date.

 

Neptune

 

No further NASA Neptune exploration to date.

 

Pluto and the Kuiper Belt

 

Since the August 1970 article, the outer edge of the solar system has been radically redefined.  Discovered in 1978, by the U. S. Naval Observatory, Charon, a moon of Pluto, showed that Pluto was much smaller than thought, with a diameter of 1,477 miles.  Charon is large in comparison to Pluto; its diameter (753 miles) is half that of the planet, and its mass is one-eighth.  Since the moon is so large compared to the planet, and the fact that the center of gravity between them is outside of Pluto, they both orbit that point at 12,178 miles apart, making them a true double-planet, albeit a dwarf one.  In 1990, minor planet Albion was discovered, the first Kuiper belt object since Pluto (1930) and Charon (1978).  At that time, the floodgates opened, and the number of Kuiper belt objects has increased to thousands.  In March 2003, a large object, Haumea, was found in the belt.  That was followed by the discoveries of Eris in January 2005, and Makemake in March 2005.  Eris, with a diameter of 1,445 miles, is slightly smaller than Pluto.  It has one moon, and thanks to that, its mass can be measured.  Eris is 27% more massive than Pluto, meaning it is rockier.  Haumea is the third largest Kuiper belt object, and has two moons.  It has an elongated shape, 1,443 miles long by 1,059 miles wide by 638 miles high.  Makemake has a diameter 60% that of Pluto, and has one moon.  New Horizons was launched on January 19, 2006.  At that time, Pluto was still a planet.  During its time as such, it had “shrunk” from Earth-size in 1930, to Mars-size in 1970, to smaller than the Earth’s moon in 1978.  Later in 2006, the International Astronomical Union (IAU) demoted Pluto from planet to a new designation, dwarf planet.  At the same time the IAU promoted Ceres from asteroid to dwarf planet and assigned Eris, Haumea, and Makemake that designation as well.  New Horizons was launched to a planet that was not a planet when it arrived; instead, it became the second probe to explore a dwarf planet.

As New Horizon approached Pluto, an intensive search for any additional moons of Pluto was undertaken with Earth-based telescopes – four were found.  This was done for collision avoidance purposes.  New Horizons began long-range observations of Pluto and its moons on January 4, 2015.  The closest approach to Pluto was on July 14, 2015, at a range of 7,750 miles from the center of Pluto.  The probe came within 17,900 miles of Charon.  Occultations of Pluto and of Charon were performed.  Both bodies were photographed, and their chemical compositions were mapped.  Photographs were also taken of the four, small moons of Pluto.  A list of NASA Pluto and Kuiper Belt exploration:

• New Horizons (flyby) 2015 Pluto [Minor Planet: see below]
• New Horizons (flyby) 2019 Arrokoth

 

Tom Wilson