AnankeUsing a telescope at Mount Wilson Observatory, astronomer Seth Nicholson discovers Ananke, a tiny moon of Jupiter orbiting the huge planet at an average distance of 21 million miles and at a high inclination relative to Jupiter’s equator. Ananke is most likely a captured asteroid or the remnant of a captured asteroid, and other small Jovian moons in the same orbit may be other pieces of the captured (and shredded) body. Ananke is the first Jovian moon discovered in nearly two decades, and it will be over two more decades before another is found.

Gravitational assist

Mike MinovitchRecent Jet Propulsion Laboratory hire Michael Minovitch submits the first of a series of papers and technical memorandums on the possibility of using carefully-calculated gravitational assist maneuvers to speed transit time between celestial bodies while requiring minimal engine/fuel use. Where most previous scientific thought concentrated on using engine burns (and a lot of fuel) to cancel the effects of a planet’s gravity, Minovitch demonstrated that gravity could be a big help with a carefully calculated trajectory. Though nearly every planetary mission since then has capitalized on Minovitch’s research, it was initially rejected by JPL. Minovitch’s calculations are later revisited by Caltech grad student Gary Flandro, who flags down a particular combination of Minovitch’s pre-computed trajectories for a “grand tour” of the outer solar system, a mission which will eventually be known – in a somewhat scaled-down, less grand form – as Voyager.

The Grand Tour

VoyagerThe Grand Tour Outer Planets mission is proposed to NASA by the Jet Propulsion Laboratory. Using a combination of the gravitational assist trajectories computed by JPL’s Michael Minovitch in 1961, Caltech/JPL grad student Gary Flandro has identified a favorable alignment of the outer planets which would allow for a single spacecraft to reach Jupiter, Saturn, Uranus, Neptune and Pluto within two decades. Vehicles taking advantage of this planetary alignment must lift off at very precise times during 1977 and 1978, and the alignment will not occur again for nearly 200 years. An ambitious plan is laid out for multiple flyby vehicles with atmospheric probes for every gas planet and landers for specific moons of interest, launched by Saturn V rockets. Budget realities scale this plan back: flybys will be carried out by two cheaper Mariner vehicles (later renamed Voyager), while the atmospheric and satellite probes – eventually to be renamed Galileo and Cassini – wait even longer to reach their destinations, and the Pluto flyby is scrapped until the 21st century New Horizons probe lifts off. JPL also recommends an inner solar system tryout of the gravitational assist maneuvers required, resulting in the Mariner 10 mission to Venus and Mercury.

Pioneer 10 launched

Pioneer 10NASA launches Pioneer 10, the first spacecraft sent to study the huge planet Jupiter at close range. Its Atlas-Centaur booster gives it a good head start, propelling it to over 32,000 miles per hour en route to Jupiter, the fastest man-made object in history at this point. Pioneer 10 is also the first man-made vehicle to traverse the asteroid belt, with instruments detecting fewer large particles than anticipated. It will reach Jupiter in late 1973.

Mariner Jupiter/Saturn ’77

VoyagerHaving spent six years wrangling with various mission profiles for a “Grand Tour” of the outer solar system, made possible by a favorable planetary alignment occurring only once every 175 years, NASA finally authorizes a very stripped-down version of its original ambitious Grand Tour plans. The Mariner Jupiter/Saturn ’77 mission will consist of two twin unmanned spacecraft to be launched in 1977, each on a course to explore Jupiter, and then to use Jupiter’s gravity to deflect them to Saturn. These spacecraft will be renamed Voyager 1 and Voyager 2 just a few months before lifting off.

Pioneer 11 launched

Pioneer 11The unmanned space probe Pioneer 11 is launched on a course that will be one of the first real tests of the theory of gravity assist. Reaching Jupiter in 1975, it will use the giant planet’s gravity to throw it across the solar system to rendezvous with Saturn, the first human-made vehicle to visit that planet. The experience gained with Pioneer 11’s groundbreaking trajectory through the solar system will prove instrumental in the upcoming Mariner Jupter/Saturn ’77 mission, which is later be renamed Voyager.

Pioneer 10: Jupiter’s first visitor from Earth

JupiterNASA’s Pioneer 10 space probe, launched in 1972 and boosted to unprecedented speeds by a three-stage rocket, makes its closest pass to the planet Jupiter, the first human-made vehicle to do so. Zooming past Jupiter’s equator at a distance of less than 100,000 miles, Pioneer 10’s electronics are nearly fried by the giant planet’s intense radiation. Pioneer 10 presses on toward the edge of the solar system, continuing to report back through the 1990s.


LedaAstronomer Charles Kowal discovers Leda, a tiny, previously undiscovered moon of Jupiter, using Mount Palomar Observatory’s telescope. With a radius of less than seven miles and an inclined orbit, Leda is the first Jovian moon discovered in over two decades, and is among the last to be discovered using ground-based telescopes in the 20th century.

Pioneer 11 visits Jupiter

Pioneer 11 at JupiterNASA’s Pioneer 11 space probe passes close to Jupiter, barely 27,000 miles above the giant planet’s cloudtops, again encountering radiation capable of frying spacecraft electronics. Pioneer 11 captures the first images of Jupiter’s polar cloud structure and pulls off a daring gravity assist maneuver: the planet’s gravity flings Pioneer 11 up and over the north polar region and across the solar system for a 1979 rendezvous with Saturn, the first spacecraft to visit that planet.


ThemistoAstronomers catch fleeting glimpses of a new natural satellite of Jupiter, Themisto, though the initial estimates of its orbit are “off” enough that Themisto becomes “lost” and isn’t observed again until 2000. With a diameter of roughly five miles, Themisto marks the dividing line between the larger inner moons of Jupiter and the widely-scattered menagerie of asteroid-like outer moons orbiting the planet. Astronomers Elizabeth Roemer and Charles Kowal (who discovered another new Jovian moon in 1974) share credit for discovering the moon. Themisto is the last Jovian satellite to be discovered by ground-based telescope in the 20th century.

Now, Voyager

VoyagerNASA Administrator James Fletcher announces that the ambitious twin Mariner Jupiter/Saturn ’77 space probes, due to be launched later in the year, have been christened with new names: Voyager 1 and Voyager 2. The name change has been initiated by recently-promoted Voyager program manager John Casani, who thinks the spacecraft need a name that’s less of a mouthful (the name “Discoverer” was also considered). For the first time, NASA openly admits that one of the vehicles – Voyager 2 – may continue on to Uranus and Neptune should its Saturn flyby go well in 1981, depending on the spacecraft’s health.

Jupiter probe, Space Telescope approved

Hubble Space TelescopeCongress approves the largest NASA budget in ten years, including authorization and funding for two major unmanned spacecraft: a Space Telescope to be deployed into Earth orbit via Space Shuttle, and a yet-to-be-named Jupiter orbiter and atmospheric probe, originally proposed in the late 1960s as part of the outer planets Grand Tour mission plan. The Jupiter probe, which must be ready to launch in 1982 to take advantage of a planetary configuration providing the shortest distance between Earth and Jupiter, is the subject of a fierce budget fight in Congress. (This spacecraft will go on to be named Galileo.)

Voyager 2 launches

Voyager 2NASA launches Voyager 2 (weeks ahead of Voyager 1), giving the unmanned space probe the best shot of taking advantage of a favorable planetary alignment known as the “Grand Tour”. Using a series of carefully calculated gravity assists, Voyager has the potential to visit all four of the major outer gas planets – Jupiter, Saturn, Uranus and Neptune – in under 15 years without having to expend fuel to make the trip. If Voyager 2 survives long enough to visit Uranus or Neptune, it will become the first man-made spacecraft to visit either planet.

Voyager 1 leaves Earth

VoyagerThe unmanned robotic Voyager 1 space probe lifts off on a voyage to Jupiter, Saturn and beyond, taking advantage of a once-in-175-years alignment of the planets in the outer solar system. Originally designated Mariner 11, one of many planned space probes in the now greatly scaled-back Mariner Jupiter/Saturn ’77 program, Voyager 1 is also the first spacecraft to take a picture of the Earth and its moon from beyond the moon’s orbit, and will become the first human-made object to leave Earth’s solar system.

Voyaging blindly onward?

Voyager 1 (CG)NASA’s Voyager 1 space probe, en route to its first destination, develops a potentially mission-jeopardizing problem: the scan platform, which contains and aims many of Voyager’s scientific instruments, jams and becomes stuck in place. As Voyager 1 has yet to even reach Jupiter, this threatens to make it an expensive failure. Transmitting commands from Earth to give the scan platform a gentle three-axis workout, engineers at NASA manage to free the stuck instruments, salvaging Voyager 1’s mission to the outer planets and their moons. (A similar fault develops in Voyager 2 during its 1981 encounter with Saturn.)

Voyaging silently onward?

Voyager 2 (CG)NASA’s Voyager 2 space probe, leaving the inner solar system en route to a grand tour of the outer planets, suddenly stops transmitting to Earth, failing to acknowledge commands sent by its ground controllers. Any chance of the probe conducting its studies of Jupiter and Saturn, let alone Uranus or Neptune, is in serious jeopardy. Discovering a problem with Voyager 2’s ability to compensate for the Doppler shift in signals coming from Earth, NASA engineers devise a workaround to compensate for this problem from the ground, saving the mission.

Voyager 2’s mission, should it choose to accept it…

Voyager 2Following a communications blackout scare in April 1978, JPL uploads an autonomous command sequence to the Voyager 2 unmanned space probe, which would allow the spacecraft to carry out a self-guided mission to Jupiter and Saturn, the results of which would automatically be transmitted to Earth even if Voyager 2 can receive no further instructions from Earth. Due to the command storage limitations of Voyager 2’s onboard computer, this automatic backup mission plan makes no allowances for pictures of Jupiter, saving that capability for Saturn instead. In the event that Voyager 2 can no longer hear commands from Earth, the extended mission to Uranus and Neptune would be forfeited in favor of “minimum science return” from Jupiter and Saturn.

Voyagers survive the asteroid belt

JupiterVoyager 1 emerges unharmed from what is considered the outer limit of the asteroid belt between Mars and Jupiter, having entered this 223,000,000-mile-wide zone of space in December 1977. Voyager 2 is expected to emerge similarly unscathed in late October 1978. NASA’s Pioneer 10 and 11 spacecraft had already demonstrated, in the early 1970s, that passage through the asteroid belt without mission-jeopardizing damage is possible. Both spacecraft are already imaging Jupiter from a distance of less than 180,000,000 miles, now meeting or exceeding the resolution of the best photos of Jupiter taken from Earth-based telescopes.

Voyager 1: Jupiter ahead!

JupiterFrom a distance of 36 million miles, NASA/JPL’s unmanned spacecraft Voyager 1 can already see the planet Jupiter in far greater detail than the cameras aboard Pioneers 10 and 11. Over the next month, Voyager 1 records images as it closes in on its first planetary target, spotting roiling storm clouds and fluid cloud bands with unprecedented clarity; JPL assembles the images into a “movie.” Despite the size of Jupiter at the end of the sequence, Voyager 1 is still over a month away from its closest pass to the giant planet.

Voyager 1 and the rings of Jupiter

Jupiter's ringsAs it nears its closest approach to the planet Jupiter, NASA’s Voyager 1 space probe detects the first likely signs of a ring system around Jupiter’s equatorial region. Barely visible until Voyager 1 is behind the planet and can see them through scattered sunlight, the rings are only about 20 miles thick, but are over 150,000 miles in diameter. The lead time between Voyager 1’s visit and Voyager 2’s later flyby allows ground controllers to plan a better observation campaign for Voyager 1’s sister ship, and the rings are observed in more detail by the later Galileo and New Horizons missions.

Voyager 1 at Jupiter

JupiterThe unmanned NASA/JPL space probe Voyager 1 makes its closest approach to the giant planet Jupiter, a little over 200,000 miles away. While Voyager’s higher-resolution cameras trump any of the Pioneer images of Jupiter, the real revelation proves to be Jupiter’s four largest moons, revealing a smooth-but-cracked icy surface on Europa, craters on Ganymede and Callisto, and the colorful mountains of Io, whose biggest secret goes undiscovered until a few days after Voyager 1’s closest flyby.

The Volcanoes of Io

IoJPL navigation engineer Linda Morabito, double-checking raw Voyager 1 images to ensure that the unmanned space probe is properly aligned for its encounter with Saturn in 1980, discovers the first evidence of active volcanoes on another body in Earth’s solar system: a plume of sulfur erupting over 150 feet above the surface of Jupiter’s moon Io. Scientists rush to check Voyager 1’s other Io images, and find that Voyager’s cameras caught more than half a dozen eruptions in the act.


AdrasteaTiny Adrastea, a small, asteroid-like moon of Jupiter, is discovered in photos returned by Voyager 2 during its flyby of the planet. Adrastea orbits along the outer edge of Jupiter’s ring system, and is likely to be the body from which material for that ring is ejected. Its close orbit carries it around the planet at a speed faster than Jupiter’s rotation, one of the few bodies in the solar system locked into such a fast orbit.


AtlantisSpace Shuttle Atlantis lifts off on a mission lasting nearly five days, whose primary goal is to lift the interplanetary probe Galileo into orbit. Originally intended for launch in late 1982, Galileo is bound for Jupiter by way of a long, looping trajectory that sends it to Venus and back to Earth multiple times, picking up speed via gravitational assist with each visit. Galileo won’t actually reach Jupiter itself until December 1995. Aboard Atlantis for this flight are Commander Donald Williams, Pilot Michael McCulley, and mission specialists Franklin Chang-Diaz, Shannon Lucid, and Ellen S. Baker.

Galileo’s antenna problem

GalileoLaunched in 1989 via Space Shuttle, the unmanned Galileo probe reveals a significant technical problem during its first flyby of Earth: the umbrella-like high-gain antenna, allowing it to send its observations of Jupiter and its moons back to Earth at high speed, is stuck in a partly-open, partly-closed configuration that prevents its use. Ground engineers at JPL have to devise data compression schemes, and a tight record/playback schedule, that will allow Galileo to take all of its planned observations and send them back to Earth at a lower bit rate than planned. It is theorized that the long delays in Galileo’s launch – the probe was ready for launch in 1982 but had to wait through delays in the early shuttle program and was then kept in storage in the aftermath of the Challenger disaster – allowed the antenna’s lubricant to dry up. Galileo won’t reach its target planet, Jupiter, until 1995.

Comet Shoemaker-Levy 9

Shoemaker-Levy 9Astronomers on Earth discover a comet like none seen before: having flown close to Jupiter in July 1992, the comet has broken into multiple pieces in a configuration that its discoverers call “a string of pearls.” Calculations of the orbit of the newly detected comet, named Shoemaker-Levy 9 after the team that discovered it, reveal something stunning: the orbit of the fragments will bring them back to Jupiter in just over a year, at which point they are expected to collide with the planet rather than pass it by or go into orbit. Not only does this give Earth-based astronomers time to coordinate observations, but NASA has an ace in the hole: the entire event will be witnessed by the unmanned Galileo probe as it makes its final approach to the giant planet.

Jupiter eats a comet

Shoemaker-Levy 9Its collision with the solar system’s largest planet predicted over a year in advance, the fragments of Comet Shoemaker-Levy 9 begin impacting Jupiter’s atmosphere in an astronomical event lasting six days. With Earth-based telescopes watching, as well as cameras and instruments on the Hubble Space Telescope, Galileo and even Voyager 2, huge explosions are witnessed as the cometary chunks slam into Jupiter’s southern hemisphere at over 200,000 miles per hour, leaving dark “scars” larger than the diameter of Earth visible on the planet’s atmosphere and releasing more heat than the surface of the sun. Galileo is still over a year away from arriving at Jupiter.

Galileo takes the plunge

GalileoAfter five months of independent flight, the Galileo atmospheric probe slams into the atmosphere of giant planet Jupiter at a speed over 100,000mph, burrowing over a hundred miles into the huge planet’s dense atmosphere before the heat of entry and the atmospheric pressure crush the probe. Deploying a parachute to slow its descent, the probe survives for nearly an hour, its sensors finding a surprisingly dry atmosphere. As it plummets toward the center of Jupiter, the Galileo probe registers 450mph winds, but never finds any hints of anything resembling a solid surface. The sum total of the probe’s sensor readings – the entirety of our data gathered directly within the atmosphere of Jupiter – tops out at 460 kilobytes of data.

Mission accomplished! Now keep going.

GalileoBeating the odds imposed upon it by the unforgiving environment around the planet Jupiter and its major moons, and engineering challenges such as a high-gain antenna that never unfurled properly after its 1989 launch, NASA’s Galileo space probe completes its two-year mission. Since the spacecraft is still intact and reasonably healthy, NASA gains a two-year extension, which it calls the Galileo Europa Mission, focusing on the two innermost large moons, icy Europa and volcanic Io. Trajectories are planned to fly Galileo even closer to these moons than ever before, though the harsh radiation zone around Jupiter itself could fry Galileo’s main computer and end the mission at any time.

Galileo and the oceans of Callisto

CallistoThe unmanned NASA/JPL space probe Galileo makes a remarkable find at Callisto, the outermost of Jupiter’s four large “Galilean” moons: evidence that a saltwater ocean may lie beneath the moon’s pockmarked surface. Even more unusually, it may be the catalyst for Callisto’s magnetic field (a rarity for a satellite – not even all of the solar system’s planets have magnetic fields). Galileo’s instruments raise the possibility that the subsurface ocean may be conducting electricity and helping to generate that field (which current scientific models say Callisto should be too small to generate on its own). Scientists do not, however, believe that Callisto is a strong candidate to support life, unlike Europa.

Galileo and the atmosphere of Callisto

CallistoNASA reveals recent findings from the unmanned Galileo orbiter touring Jupiter and its moons, including the discovery of a tenuous atmosphere surrounding the outermost large moon of Jupiter, Callisto. The thin layer of carbon dioxide surrounding Callisto completes the set: Galileo has detected an atmosphere of one kind or another around each of Jupiter’s four Galilean moons, some of which are large enough to be planets. As for how the atmosphere is sustained in the extremely hostile environment around Jupiter, scientists speculate that the CO2 could be emitted from beneath Callisto’s own crust.

Galileo and the strange chemistry of Europa

EuropaNASA scientists announce a startling find on one of Jupiter’s moons: a chemical which must be manufactured on Earth occurs naturally on the surface of Europa. The Galileo orbiter detects naturally-occurring hydrogren peroxide on the surface of the icy moon (a surface which most planetary scientists now believe covers an ocean of water or “slush” just beneath the crust), probably caused by the constant bombardment of radiation-charged particles from Jupiter itself. The hydrogen peroxide on Europa is also found to be very short-lived, quickly breaking down into gaseous oxygen and hydrogen.

Galileo and the battery acid of Europa

EuropaA long-standing mystery of one of the moons of Jupiter is solved, opening many more questions. Data gathered by the Galileo unmanned orbiter reveals that a previously unidentifiable substance on Europa’s surface is sulfuric acid, a compound used as battery acid on Earth. Scientists quickly split into two camps on the origins of the acid: it may be welling up from inside Europa, or it may be volcanic material ejected from Io and then deposited on Europa as the moons’ orbits occasionally bring them into conjunction. Though the presence of sulfuric acid initially dashes hopes of finding life on Europa, the possibility is not completely ruled out by this finding.

Galileo vs. the Volcano

IoNASA/JPL’s unmanned Galileo space probe buzzes Jupiter’s volcanically active moon Io, zipping directly over one of its most active volcanic regions at an altitude of only 380 miles, Galileo’s closest Io flyby to date. But this close encounter, and the fate of Galileo itself, had been in doubt just hours before the flyby: entering Jupiter’s most intense radiation environment, Galileo had entered a failsafe mode in which the spacecraft shuts down until further commands are received from Earth. Ground controllers, called in to work emergency hours on a Sunday, revive Galileo a mere two hours before its close pass by Io.

Mission accomplished again! Now do more.

GalileoCompleting its extended two-year tour of Jupiter and its intriguing moons Io and Europa, NASA’s Galileo robotic probe is given another extension, this time called the Galileo Millennium Mission. Highlights are expected to include observing Jupiter in tandem with the Cassini unmanned probe, which will swing by Jupiter in 2000 to gain a gravity assist en route to its own final target, Saturn. Galileo will also resume its exploration of the major Jovian moon Ganymede, the solar system’s largest satellite.

Cassini swings by Jupiter

JupiterNASA’s Cassini space probe successfully completes a slingshot maneuver past Jupiter, a necessary milestone on the unmanned robot’s long journey to Saturn. While in the neighborhood, Cassini conducts tandem studies of Jupiter with the still-operational Galileo probe, which – despite numerous major malfunctions – has still survived three years longer in Jovian space than it was expected to. As large as Jupiter is, Cassini will still be lending its electronic eyes and ears to studies of the largest planet well into January. Cassini will reach Saturn’s neighborhood in 2004.

Hello, Himalia!

HimaliaNASA releases a photo taken by the Cassini space probe as it passed by Jupiter and its complex system of satellites in December 2000, showing the first-ever view of the tiny moon Himalia, taken from a distance of 2.7 million miles. Non-spherical in shape, but estimated to be roughly 100 miles across its widest face, Himalia is believed to be an asteroid permanently captured into an inclined orbit of Jupiter. It was discovered in 1904 from Earth-based telescopes. The New Horizons space probe will also attempt to image Himalia in 2007. The sixth largest satellite of Jupiter, Himalia is the first of the planet’s outer satellites beyond the orbit of Callisto to be photographed by a passing spacecraft.

Galileo’s close look at Io’s volcanoes

Mary WhitehouseNASA/JPL’s Galileo unmanned space probe makes the closest flybys yet of Io, the volcanically hyperactive moon of Jupiter which made headlines when Voyager scientists spotted an eruption in 1980. The new images give the best view yet of Io’s inhospitable surface, including a volcanic crater, 47 miles across, named after the Brazilian god of thunder, Tupan Patera – an eye-searingly colorful region even in natural light.

Galileo takes the Amalthea plunge

AmaltheaNearing the end of its fuel supply, NASA’s Galileo probe passes one of Jupiter’s innermost moons, tiny, asteroid-like Amalthea, at a distance of less than 100 miles, coming closer to Jupiter and its belts of intense radiation than ever before. This final flyby of a Jovian moon is a punishing one for Galileo: the failure of Galileo’s main antenna dish after its 1989 launch has forced the robotic explorer to store its findings on tape for later playback to Earth at a low bit rate, but in this case roughly half of Galileo’s measurements of the highly charged environment near Jupiter are lost to radiation-induced failure of the tape recording system. The intense radiation also causes enough faults in Galileo’s main computer that the probe puts itself in a failsafe mode and “sleeps” until further commands are received from Earth hours later.

Galileo lives!

GalileoNASA and JPL ground controllers coax the radiation-damaged tape recorder built into the Galileo probe into working again, finally allowing several weeks worth of scientific data to be replayed to Earth. The damage was suffered during Galileo’s closest pass by Jupiter to study the tiny moon Amalthea the previous month. With layoffs looming for many of Galileo’s ground control staff in early 2003, this is Galileo’s last chance to relay its scientific findings back to Earth. It is on a trajectory that will end its mission by plunging into Jupiter in 2003.