One Day to Pluto: Looking Back at Humanity’s First-Time Successful Encounters With the Sun’s Planets

Tomorrow's historic flyby of the dwarf planet Pluto and its binary companion, Charon, will mark the completion of humanity's first-time exploration of each of the Solar System's nine traditional planets. Photo Credit: NASA
Tomorrow’s historic flyby of the dwarf planet Pluto and its binary companion, Charon, will mark the completion of humanity’s first-time exploration of each of the Solar System’s nine traditional planets. Photo Credit: NASA

Tomorrow, the United States will become the first nation to have conducted a close-range reconnaissance of all nine “traditional” planets in the Solar System, as NASA’s New Horizons spacecraft sweeps silently past the dwarf world Pluto, its binary companion Charon, and a system of four tiny moons—Nix, Hydra, Kerberos, and Styx—before continuing toward a possible rendezvous with at least one Kuiper Belt Object (KBO), as early as 2018-2019. It is a matter of great coincidence that New Horizons’ visit to Pluto falls on the very day of the 50th anniversary of NASA’s Mariner 4 mission, which completed the first successful flyby of Mars, back on 14 July 1965. Over the past five decades, a series of U.S. spacecraft have hurtled past Mercury (Mariner 10), Venus (Mariner 2), Mars (Mariner 4), Jupiter (Pioneer 10), Saturn (Pioneer 11), and Uranus and Neptune (Voyager 2), offering humanity its first detailed insight into the nature of these strange worlds. Tomorrow’s visitation of Pluto by New Horizons will bring this first-time inspection of each of the classical planets full-circle.

It would be neat and straightforward, of course, if this exploration had occurred in “planet-order,” but it was always apparent that Venus and Mars—as the closest and most easily reachable of the planets—appeared first on NASA’s radar. In February 1961, the Soviet Union’s Venera 1 spacecraft was launched to directly impact Venus, but contact was lost several days into the mission, and it was estimated to have passed within 62,000 miles (100,000 km) of the planet, sometime in mid-May of that same year. In July 1962, NASA attempted to launch Mariner 1 on a Venus flyby mission, but the spacecraft was lost less than five minutes into the flight, following a guidance system failure in its Atlas-Agena booster. Success was finally achieved when Mariner 2 was launched on 27 August 1962.

Mariner 2 launches atop an Atlas-Agena booster in August 1962, bound for the first successful encounter of a human-made machine with another planet. Photo Credit: NASA
Mariner 2 launches atop an Atlas-Agena booster in August 1962, bound for the first successful encounter of a human-made machine with another planet. Photo Credit: NASA

It had already suffered a substantial weight cut of two-thirds, due to the cancellation of a promised Centaur upper stage and the use of a less powerful Agena booster to deliver the spacecraft away from Earth and onto a trans-Venus trajectory. “To make the extremely demanding schedule—reminiscent of the crash program to build Explorer 1 after the launch of Sputnik in late 1957—Mariner would have to borrow designs and parts from the Ranger lunar probes, then in production,” NASA later reflected. “In fact, the mission would have to be designed in a week.”

Laden with instrumentation, Mariner 2’s primary goal was to measure temperature distributions across the planet’s cloud-bedecked surface and acquire basic data of its atmospheric constituents, using microwave and infrared radiometers. These pair were utilized primarily in the vicinity of Venus, whilst a battery of other instruments—a fluxgate magnetometer, a cosmic ray detector, a particle detector, a solar plasma spectrometer, and a magnetometer—were employed throughout the spacecraft’s interplanetary cruise, with one key finding being the definitive detection of the “solar wind.” During its journey to Venus, Mariner 2 completed the first successful interplanetary midcourse correction maneuver, but suffered a temporary loss of attitude control in September and a short circuit in one of its solar panels in October. The panel permanently failed in November, but by this stage the spacecraft was close enough to the Sun for its sole remaining panel to pick up the load and continue to supply adequate electrical power. Unfortunately, the midcourse correction maneuver added 47 mph (75 km/h) to Mariner 2’s velocity, instead of the intended 45 mph (72 km/h). This apparently minor increase was more than enough to effectively double the Venus flyby altitude, with the spacecraft now expected to pass not within 9,000 miles (15,000 km), but within a far larger 20,900 miles (33,600 km). However, even this wider flyby distance was judged acceptable for Mariner 2 to gather good scientific data.

Passing within 21,607 miles (34,773 km) of Venus on 14 December 1962, Mariner 2 transmitted data “at a far-from-blistering 8 1/3 bits per second,” yet provided a stunning perspective of the planet famously labeled as Earth’s “twisted sister.” Microwave radiometer measurements yielded temperatures of at least 425 degrees Celsius (797 degrees Fahrenheit), with no major differences between the night side, equator, or day side of the planet. These results hinted either that the surface was extremely hot or that the atmosphere was optically thick in nature. In fact, continuous cloud cover was determined up to an altitude of at least 37 miles (60 km). No indication of an appreciable magnetic field was found, nor was there a detectable magnetosphere or radiation belts, whilst the precise value of the “Astronomical Unit” (AU)—the median distance between Earth and the Sun—was determined, and an understanding that it revolves very slowly in a retrograde motion was gained. Several weeks later, in January 1963, its mission triumphantly concluded, Mariner 2 fell silent, and remains in solar orbit to this day.

“It’s a jubilant moment for JPL and the country,” noted the Jet Propulsion Laboratory (JPL) of Pasadena, Calif., reflecting upon Mariner 2. “After five years of playing catch-up to the Soviet Union in space exploration, the United States has achieved its first bona fide ‘first’—the first successful flyby of another planet. The mission delivers not only news about Venus itself, but discoveries about the realm of space between the planets. It will open a new era, decades of inspiring missions managed by the laboratory that take the world to all of the planets, from Mercury to Neptune, revealing sights in many cases unimagined.”

The first grainy image of Mars, as seen from Mariner 4 on 14 July 1965. Photo Credit: NASA
The first grainy image of Mars, as seen from Mariner 4 on 14 July 1965. Photo Credit: NASA

Mariner 2’s transmissions were monitored by the Deep Space Instrumentation Facility (DSIF), which consisted of a trio of tracking stations at 120-degree intervals around the globe, at Goldstone, Calif., near Woomera in Australia and at Hartebeesthoek, near Johannesburg in South Africa. The following year, 1963, the DSIF was renamed the Deep Space Network (DSN), and the station in South Africa was later moved to the mountainous environs of Robledo, near Madrid, Spain. Since then, this worldwide network of deep-space tracking assets has supported numerous missions to the inner and outer Solar System, including New Horizons itself.

Less than three years after the first flyby of Venus, NASA achieved a similar feat at Mars. Although several Soviet attempts had been made to reach the Red Planet, all had failed either during the launch phase or whilst in transit. Mariner 4 rocketed aloft on 28 November 1964, although its eight-month voyage to Mars proved far from untroubled. The spacecraft suffered problems with its star tracker during the first few weeks, losing lock on the target star Canopus no less than six times, which prompted the first attempt at a midcourse correction maneuver to be abandoned, whilst Mariner 4’s gyros were in spin-up. However, the maneuver was later accomplished successfully, and on 14-15 July 1965 the spacecraft passed within 6,118 miles (9,846 km) of Mars’ ochre-hued cloud-tops.

Jupiter, with the Great Red Spot visible at the limb on the far right side, as seen by Pioneer 10. Photo Credit: NASA
Jupiter, with the Great Red Spot visible at the limb on the far right side, as seen by Pioneer 10. Photo Credit: NASA

Its complement of scientific instruments—which included magnetometers, radiation, dust and cosmic ray detectors, and, significantly, a television camera—enabled it to reveal the Red Planet in more detail than had previously been possible. In stark contrast to what more recent missions would reveal, Mariner 4 showed a heavily cratered world, with no trace of a magnetic field or radiation belts and an atmospheric pressure at the surface of between 4.1 and 7.0 millibars. Twenty-one full images were acquired, together with a partial 22nd image, making Mariner 4 the first spacecraft in history to successfully return images of another planet from deep space. Those images appeared to dissipate the popular view that Mars either harbored life or had done so in its distant past. Although they covered barely 1 percent of the surface, the images revealed high-level haze, the effects of heavy micrometeoroid bombardment, and craters as wide as 75 miles (120 km). Nevertheless, the possibility of life remained at the forefront of scientific attention and was a key focus of subsequent missions to the planet. As for Mariner 4 itself, its attitude control system was exhausted by the fall of 1967 and communications were terminated just before Christmas, leaving it derelict in heliocentric orbit.

Beyond Mars lay the giant gaseous planets—Jupiter, Saturn, Uranus, and Neptune—which exerted their own influence upon humanity’s scientific fascination. It was recognized that a rare alignment of these four mysterious worlds in the late 1970s would permit a “Grand Tour” to explore each of them, with a relatively minimum expenditure of propellant and time. In order to prepare for such a possibility, in 1964 NASA opted to develop a pair of spacecraft to enter the outer Solar System for the first time. Initially dubbed the “Galactic Jupiter Probes,” they were later named Pioneer F and G (and eventually “10” and “11”). Pioneer 10 rose from Earth on 3 March 1972 and passed the orbit of the Moon in less than 11 hours, becoming the fastest human-made object at that time.

Eleven months later, it traveled through the asteroid belt—becoming the first spacecraft to do so and also the first mission to detect interplanetary atoms of helium—before hurtling within 82,177 miles (132,252 km) of Jupiter on 4 December 1973, passing deep into its hostile radiation environment. So intense was this radiation environment that one investigator used up 99 percent of the range of his instrument to obtain data, which survived the encounter with just one percent to spare, whilst two of Pioneer 10’s cosmic ray and energetic electron detectors became saturated less than 24 hours before Closest Approach. “We can say that we sent Pioneer 10 off to tweak a dragon’s tail and it did that and more,” said Robert Kraemer, then-head of the Office of Planetary Programs at NASA Headquarters in Washington, D.C. “It gave it a really good yank and … it managed to survive.”

Pioneer 11 became the first robotic visitor to Saturn, passing the ringed world in September 1979. Photo Credit: NASA
Pioneer 11 became the first robotic visitor to Saturn, passing the ringed world in September 1979. Photo Credit: NASA

In total, around 500 images were acquired as Pioneer 10 passed along the planet’s magnetic equator, experiencing a number of false commands, but successfully resolving a patchwork of bright and dark areas on the moons Ganymede and Europa. Passing behind Io, the spacecraft determined the existence and extent of not only a thin atmosphere, but also an unexpectedly high concentration of electrons near its surface, indicative of the presence of an ionosphere. It also made the surprising discovery that Io orbited within a vast hydrogen cloud, extending 500,000 miles (805,000 km). However, the flyby profile meant that none of the four “Galilean moons” were approached closely, as Pioneer 10 came within 870,000 miles (1.4 million km) of Callisto and 200,000 miles (321,000 km) of Europa. As for Jupiter itself, Pioneer 10 successfully imaged the Great Red Spot, revealed the “inverted” nature of the magnetic field and generated an infrared map of the planet, confirming that it radiated more heat than it received from sunlight.

Three years later, the spacecraft crossed the orbit of Saturn, then Uranus in 1979, and finally Pluto and Neptune by mid-1983, becoming our first emissary to journey beyond the last of the major planets. By this stage, its near-twin, Pioneer 11, had long since completed humanity’s first reconnaissance of Saturn, the most distant of the planets known from ancient times. Launched on 6 April 1973, it survived a passage through the asteroid belt a year later, passed Jupiter in November 1974 and commenced the long trek outward to Saturn, which it reached on 1 September 1979, hurtling just 13,000 miles (21,000 km) over the giant planet’s cloud-tops. In doing so, it was directed to pass through the planet’s ring-plane at the same position that the soon-to-arrive Voyager 1 and 2 spacecraft would use, in order to evaluate the potential for damage from ring-material impacts.

Ironically, Pioneer 11 imaged and narrowly missed colliding with a tiny, newly discovered moon, identified variously as either Epimetheus or Janus, which it passed within 2,500 miles (4,000 km). At the same time, the spacecraft detected a new ring—the very tenuous “F” ring—which has since demonstrated changeability on timescales of a matter of hours, making it possibly the most active ring of any planet in the Solar System. It is now understood that the material of the F ring is “shepherded” by the tiny moon Prometheus. During its encounter, Pioneer 11 also revealed a pair of new moons and examined Saturn’s magnetosphere, magnetic field and interior for the first time, as well as compiling the first in-situ thermal profile of the large moon Titan.

The crater-pocked surface of Mercury, as viewed by Mariner 10. Photo Credit: NASA
The crater-pocked surface of Mercury, as viewed by Mariner 10. Photo Credit: NASA

By this stage, Mercury—the heavily cratered innermost member of the Sun’s retinue of planets—had also been visited by a spacecraft. Due to the closeness of the planet to its host, the Mariner 10 spacecraft required additional thermal blankets, a sunshade, specially “tiltable” solar arrays and additional shielding to withstand radiation levels 4.5 times more extreme than when it left Earth. On 3 November 1973, Mariner 10 was launched to conduct what would effectively be a double flyby mission, becoming the first human-made machine to make use of a planetary “slingshot.” It employed the gravity of Venus in February 1974 to bend its trajectory sufficiently to accomplish no fewer than three rendezvous with Mercury over a year-long period between March 1974 and March 1975. Its first encounter came on 29 March 1974, during which it passed within 437 miles (703 km) of Mercury.

Due to the geometry of its orbit, the same face of the planet was presented to Mariner 10 on each occasion, which enabled the spacecraft to map no more than 45 percent of the surface, although it acquired around 3,500 “useful” images. Mercury’s surface immediately compared favorably with that of our Moon—at least in color and texture—and Mariner 10 revealed a tenuous, helium-rich atmosphere, a large iron core and traces of a magnetic field. Surface features included a mixture of highland and lowland plains, with areas of heavy cratering and relatively smooth terrain, suggestive of a major resurfacing event in Mercury’s distant past. The spacecraft also successfully pegged nighttime temperatures at -183 degrees Celsius (-297 degrees Fahrenheit) and extreme daytime temperatures as high as 187 degrees Celsius (369 degrees Fahrenheit). Passing the planet for the third and final time on 16 March 1975, Mariner 10 became our last robotic visitor to Mercury for more than three decades.

Beyond Saturn, in the farthest reaches of the known Solar System, the giant planets Uranus and Neptune remained unexplored by a spacecraft until a human generation ago. In fact, both were discovered in relatively recent times: Uranus by the astronomer William Herschel in March 1781 and Neptune, by telescopic and mathematic means, in September 1846. Both planets received their first visitor from Earth in the form of the same spacecraft—Voyager 2—which swept just 50,600 miles (81,500 km) past Uranus on 24 January 1986, only a handful of days before the Challenger accident, and within 3,000 miles (4,800 km) of Neptune on 24 August 1989.

Launched on 20 August 1977, Voyager 2 was originally tasked to rendezvous with Jupiter and Saturn, conducting a joint close-range reconnaissance of both planets, in conjunction with its twin, Voyager 1. However, it was recognized that by carefully planning the trajectory of Voyager 2 after its departure from Saturn, it might be possible to achieve an encounter with Uranus and Neptune. A year before the spacecraft rose from Earth, NASA provisionally approved an extended mission, conditional upon the successful completion of the Jupiter/Saturn encounter, and one particular instrument—the Infrared Imaging Spectrometer (IRIS)—was specially modified to enable it to overcome the predicted poor sensitivity at Uranus’ extreme distance from the Sun. In October 1981, after leaving Saturn behind, the spacecraft began a five-year Voyager Uranus Interstellar Mission (VUIM).

Viewed from 600,000 miles (965,000 km) beyond the planet on 25 January 1986, Uranus presents itself as a mere sliver of a crescent to Voyager 2's cameras. Photo Credit: NASA
Viewed from 600,000 miles (965,000 km) beyond the planet on 25 January 1986, Uranus presents itself as a mere sliver of a crescent to Voyager 2’s cameras. Photo Credit: NASA

The actual encounter with the strange aquamarine planet in the otherwise dreadful month of January 1986 was truly spectacular and the spacecraft’s trajectory team achieved impressive delivery results: Voyager 2 was just 62 miles (100 km) off-target, equivalent to sinking a golf-putt from a distance of 2,250 miles (3,630 km). The images revealed a relatively bland, quiescent world, with little visible atmospheric activity, prompting several members of the imaging team to wryly dub themselves “The Imagining Team.” Voyager 2 acquired stunning imagery of Uranus’ five large moons: Ariel, Umbriel, Titania, Oberon, and, specifically, tiny Miranda, the latter of which showed a surface tortured by aeons of relentless micrometeoroid bombardment, with canyons extending 12 miles (20 km) deep and networks of oval, racetrack-like features snaking their way across its terrain.

Forty-three months later, in August 1989, Voyager 2 encountered Neptune, which marked the final, first-time reconnaissance with a new planet in the 20th century. As described in a series of AmericaSpace articles, published last year to commemorate the 25th anniversary, the spacecraft revealed a distinctly more active world than Uranus, with clouds and major atmospheric features discovered several months before Closest Approach. These included the high-pressure, fast-rotating vortex of the Earth-sized Great Dark Spot, an attendant “bright companion” cloud, a smaller dark spot and a chevron-shaped “scooter.” Neptune also produced the fastest known winds in the Solar System, gusting at up to 1,240 mph (2,000 km/h), even more fierce than Saturn. The planet’s system of moons came under particular scrutiny from Voyager 2, not least the enigmatic Triton, which—like Pluto—has been theorized to have originated as a Kuiper Belt Object (KBO). Triton’s thin atmosphere, curious, cantaloupe-like terrain and the intriguing possibility that it harbored a form of cryogenic volcanism.

Each of these intrepid first-time visitors to our planetary neighbors—Mariner 2, Mariner 4, Pioneer 10, Pioneer 11, Mariner 10, and Voyager 2—remain in space, although only the latter continues to return scientific data, at an estimated distance of 10 billion miles (16.1 billion km) from Earth, with the expectation that it may remain partially functional for up to another decade, until as late as 2025. Following receipt of a very faint signal on 22 January 2003, Pioneer 10 operations ended in February, with power reserves predicted to be so low that further transmissions were likely not possible. “Originally designed for a 21-month mission, Pioneer 10 lasted more than 30 years,” said Project Manager Dr. Larry Lasher. “It was a workhorse that far exceeded its warranty and I guess you could say we got our money’s worth.” By this time, contact had also been lost with Pioneer 11, with which NASA had ceased communications in September 1995, due to insufficient power reserves. “This is the little spacecraft that could,” said then-NASA Administrator Dan Goldin, “a venerable explorer that has taught us a great deal about the Solar System and, in the end, about our innate drive to learn.”

In spite of its immense distance from the Sun, Neptune proved a surprise in terms of the dynamism of its atmosphere. Image Credit: NASA
In spite of its immense distance from the Sun, Neptune proved a surprise in terms of the dynamism of its atmosphere. Image Credit: NASA

As for our species first explorers to the inner, “rocky” planets, Mariner 10’s nitrogen supply became depleted on 24 March 1975, only eight days after its third successful encounter with Mercury, and its instruments were subsequently shut down. Unless it has sustained a catastrophic micrometeoroid impact, it likely remains in a looping heliocentric orbit. Likewise Mariner 2, our first successful visitor to Venus, returned its last transmission to Earth in early January 1963, and also remains in solar orbit. Following its July 1965 encounter with Mars, Mariner 4 suffered from imprecise antenna configuration and contact was lost until September 1967, when communications were briefly re-established and cosmic-dust measurements revealed that the spacecraft’s attitude had been changed by an apparent micrometeoroid shower. By early December, in any case, its attitude control propellant was exhausted and on the 21st contact with Mariner 4 was terminated, leaving it derelict in heliocentric orbit.

Tomorrow, on the 50th anniversary of our first successful close encounter with Mars, we will conclude our first-time exploration of each of the classical nine planets in the Solar System. It will truly be an epochal moment, when New Horizons hurtles silently past Pluto, making this outermost-known bastion of the Sun’s empire known to the inhabitants of another world, far away. And when the encounter is all over—and scientists can begin the process of assembling a comprehensive understanding of this dwarf planet from reams of telemetered data—New Horizons will become the fifth spacecraft to depart the Solar System forever and, carrying some of the ashes of Pluto’s discoverer, Clyde Tombaugh, will also become the first to transport human remains into interstellar space. Like its predecessors, it will add its own contribution to the proud heritage of Mariner 2, Mariner 4, Mariner 10, Pioneer 10, Pioneer 11, and Voyager, 2, by advancing the frontiers of our knowledge. For the first time in history, on 14 July 2015, all nine of the classical planets of the Solar System will move from unknown to known.



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  1. Ben,
    Great article… I am disappointed that NASA is not putting out more actual photos on an ongoing basis of the approach..Or is there someplace else I should be looking on a NASA website

    • Two reasons you aren’t getting instant gratification from New Horizons: First, the transmission rate is extremely low compared to just about any other probe save Voyager. Second, New Horizons has to physically turn around when it transmits, because all of its instruments and antennae are mounted to the body of the probe. Put another way, it can’t do all the important observations it was launched to do and transmit pretty pictures on an ongoing basis at the same time.

      Making as many observations as possible during the historic flyby are much more important than rapidly delivering pictures to the public. Wait a few days.

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