[12.0] Missions To Jupiter (1)

v2.0.0 / chapter 12 of 20 / 01 jul 16 / greg goebel

* Jupiter is both the largest of the solar system's "gas giant" planets, and also the nearest of the four to Earth. Its size, proximity, and system of moons has made it the best-studied of the outer planets. This chapter outlines the history of Jupiter exploration into the beginning of the 21st century.

Jupiter & moons montage

[12.2] PIONEER 10 & 11
[12.3] VOYAGER 1 & 2


* The ancients knew Jupiter only as a bright point of light that moved through the heavens, shining not quite as brightly as the planet Venus but more brightly than Sirius, the brightest star. In 1610, Galileo turned his simple telescope on Jupiter, and found much to his amazement that it was not only a disk with faint banded markings, but was flanked by four pinpoints of light, all in a row. The pinpoints moved from day to day, and after plotting their motions he realized they were orbiting the planet.

One of Galileo's scientific rivals, Simon Marius, who claimed to have discovered the moons first, named them "Io", "Europa", "Ganymede", and "Callisto", with Io being the moon nearest to Jupiter and Callisto being the most distant. His claim didn't stick, and though his names were retained, the four bodies became known as the "Galilean moons" of Jupiter. They proved to be substantial worlds in themselves, and they would actually be faintly visible to the naked eye, if the brightness of Jupiter didn't flood them out.

The discovery of the Galilean moons had a particular significance in that it helped promote the then-controversial concept that the Earth was not the center of the Universe, that there were other systems of worlds that did not rotate around the Earth. In fact, there was great sensation and some outrage over the discovery, with critics charging that the moons were a fabrication or a hallucination. Such objections quickly faded away; anyone with a reasonable telescope could see the moons, and soon the evidence for them was undeniable by any reasonable standard.

The discovery led to another important scientific breakthrough. Galileo had attempted to determine the velocity of the speed of light by using lanterns to trade signals with an assistant at a distant location, but of course light travels much too fast (300,000 kilometers per second) to be measured with such a crude procedure. However, observations of the Galilean moons quickly determined their movements, allowing eclipses of the moons by the planet to be predicted. In 1675, the Dutch astronomer Ole Romer performed observations of these eclipses that showed them to be off the predicted time. Romer was able to see that the variation in time corresponded to the difference in the distance of Jupiter from Earth, and concluded that the change was due to the speed of light. He was able to calculate the speed of light from his observations to within about 75% of the actual value, an impressive accomplishment given the tools available to him.

* As telescopes improved, astronomers gradually began to uncover more details about Jupiter. The planet was patterned with a series of parallel bands that proved to be brightly colored and undergoing continuous subtle changes, as if they were some sort of cloud pattern. The light bands eventually became known as "zones" while the dark bands became known as "belts".

Nobody could identify specific landmarks under the clouds, but in 1664 the British scholar Robert Hooke observed a great spot on the southern hemisphere of the planet. In some sources, discovery of this spot is credited to the Jean-Dominique Cassini in 1665. Although this spot was not consistently observed during the 18th century, it was recorded on sketches in 1830 and has been a fixed characteristic ever since that time.

Eventually, astronomers nailed down the gross characteristics of Jupiter. The giant planet orbits the Sun at an average distance of 780 million kilometers, or about five astronomical units. It completes one orbit around the Sun every 11.9 Earth years, and has a day 9.9 hours long. Jupiter has a mass about 318 times that of Earth, making it not only the most massive planet in the Solar System, but more massive than everything else in the Solar System, excluding the Sun, combined. Its average diameter is 11.2 times that of Earth, giving it a volume 1,300 times greater than that of our planet. The ratio of the planet's volume to mass gives it an average density only a quarter of that of the Earth.

The force of gravity at the planet's highest levels is 2.5 times that of Earth. Although for a long time astronomers believed Jupiter might have a solid surface under a layer of clouds, the fact that the planet is noticeably bulged at the equator, with an equatorial diameter of 143,000 kilometers and polar diameter of 133,700 kilometers, and its low density argued that it was nothing more than a huge ball of gas. It appears to be about 90% hydrogen and 10% helium by volume -- the ratio changing to 75% and 25% if measured by mass -- with a relatively small rocky-iron core. The atmosphere grows more dense with depth but has no definite solid surface. Spectroscopic analysis also showed quantities of hydrogen-rich compounds, such as ammonia, water, and methane, in the upper reaches of the atmosphere.

Incidentally, if Jupiter were more massive, it would not have an appreciably greater diameter, since gravitational pressures would compress the core of the planet. If Jupiter were about 80 times more massive, that central compression would result in temperatures and pressures high enough to initial fusion reactions, creating a small red star.

In the mid-1950s, the American astronomer Kenneth Lynn Franklin observed that Jupiter emits strong radio noise, implying the planet has a strong magnetic field, ten times more intense than that of the Earth. The magnetic poles are inclined about ten degrees to the Jupiter's axis of rotation. The radio noise results from the interaction of solar wind particles with the intense magnetic fields near the planet's poles, which also creates "auroras" like the Earth's "northern lights". The intense magnetic fields were clearly created by compressed hydrogen, which has metallic properties and so creates a conductive inner shell to the planet. The magnetic fields likely trapped high-energy particles, giving the planet "radiation belts" much more intense than those of Earth.

* Astronomers were also able to determine gross details of the four Galilean moons:

All four of the Galilean moons are tidally locked to Jupiter: they rotate so they keep one face to the planet at all times. Interestingly, Io, Europa, and Ganymede are locked in "resonant" orbits, with Io making two orbits for every orbit of Europa, and Europa in turn making two orbits for every orbit of Ganymede. Astronomers could also calculate the mass and densities of these four worlds, but beyond that, they didn't know much about their compositions or surface features.

* A fifth moon of Jupiter was discovered by the American astronomer Edward Emerson Barnard on 9 September 1892. He discovered it by "eyeball" observation with the 91-centimeter refractor at Lick Observatory. The moon was very tiny and this was a remarkable feat of observation; in fact it was the last moon to be discovered without the use of photography or electronic imaging. Barnard had the right to name the moon, but he couldn't come up with one, and so it was named "Amalthea" after his death by the French astronomer Camille Flammarion. Over the next century, a number of other small moons of Jupiter were discovered:

Note that these moons were not actually named until 1975, being originally designated simply by the numeric order of their discovery. All of these moons are only a few tens of kilometers across. Amalthea is inside the orbit of Io, while the other eight are outside the orbit of Callisto and have "irregular" orbits, clearly elliptical and with high orbital inclinations relative to the plane of Jupiter's equator.

The outer eight irregular moons are dark objects, and seemed to be arranged into two groups of four moons each. The first group includes, from nearest to Jupiter to farthest, Leda, Himalia, Lysithia, and Elara. They all have orbital inclinations of roughly 30 degrees to the Jovian equator, and "prograde" orbits, in the same direction as Jupiter's rotation and the orbits of the inner "regular" moons. The orbits are close, separated by less than one to a few Jupiter radii.

The second group is outside the first, and includes -- from nearest to farthest -- Ananke, Carme, Pasiphae, and Sinope. All have reverse or "retrograde" orbits with inclinations of roughly 150 degrees. The grouping is somewhat looser than with the other four outer irregular moons; the orbits of Pasiphae and Sinope are very close, well less than a Jupiter radius, but there is a substantial separation between those two moons and Carme, over ten Jupiter radii, and well as a similar separation between Carme and Ananke.

The irregular orbits of the outer moons suggested to astronomers that they had been captured by Jupiter after the planet's formation. The organization of their orbits into groups also suggested that they were fragments of earlier, larger objects. The only puzzle was how Jupiter could have captured them; an object falling around Jupiter from deep space would, by simple energy conservation, fly back out to deep space again. The general assumption was that Jupiter still had a tenuous "nebula" around it in its early days that helped capture the outer irregular moons, dragging them into the "Hill sphere" -- the orbital radius outside of which the moons would be pulled away by the Sun. Interaction with the nebula possibly caused fragmentation of the moons as well.


[12.2] PIONEER 10 & 11

* Although NASA JPL had considered sending a probe to Jupiter as far back as the late 1950s, giving a target date of 1963 for the launch, that schedule proved ridiculously optimistic. JPL underestimated the difficulty of building a deep-space probe, and at time the US space program's top objective was manned space flight anyway. Actual launch of Jupiter probes took over a decade longer, and at least at first, JPL wasn't the one to do it. Although JPL had the charter for planetary exploration, NASA headquarters liked to promote a degree of competition between NASA centers, and in the 1960s and early authorized the NASA Ames center in the San Francisco Bay area to conduct deep-space missions in the form of a series of five "Pioneer" probes.

The name "Pioneer" was borrowed from a spotty series of Moon and space-studies probes launched by the US Air Force and US Army in the late 1950s, discussed to an extent previously. The Ames Pioneer probes, which were intended to perform "space physics studies", were more successful, with four Pioneers successfully launched from 1965 through 1968, though the fifth spacecraft was lost before reaching orbit.

In 1967, the space science community associated with NASA began to lobby for a Jupiter mission, with a scientific advisory board making a formal recommendation for the launch of two Jupiter probes as early as 1972. NASA headquarters like the idea, and in February 1969 instructed NASA Ames to build and launch two identical Jupiter probes.

The first Jupiter launch "window" would be open in late February through early March 1972. The first of the pair of Ames-built spacecraft, "Pioneer F", was scheduled for launch in that slot, while the second, "Pioneer G", would be launched in the next window, 13 months later. One or both of the spacecraft would use a "gravitational slingshot" technique, stealing momentum from Jupiter in a close flyby to provide a course and velocity kick, to proceed on to Saturn after its flyby of Jupiter. The launch deadline was only a short time away, and so the project was managed on a fast-track basis, with TRW Corporation selected as the spacecraft contractor. Ames also solicited proposals for experiments from the space science community, and selected eleven experiments for each of the two spacecraft to perform.

The science experiments were intended to study the solar wind, cosmic rays, the radiation belts around the giant planets, the scattering of solar ultraviolet radiation from the planets, and their thermal emission. The probes were also to be used for other research not directly associated with any payload element. For example, their flight paths were to be monitored to see if they were perturbed by the gravitational effect of some unknown and distant tenth planet.

The two spacecraft each weighed 258 kilograms, including 30 kilograms for the science payload and 27 kilograms for propellant. The heart of each spacecraft was a hexagonal bus that held the science packages, computers, guidance and communications systems, and propellant. A rigid high-gain dish antenna 2.9 meters in diameter sat on top of the bus. Three booms extended from the bus, one carrying a magnetometer and the other two each carrying a SNAP-19 RTG, providing a sum of 155 watts of power at launch and 140 watts at Jupiter.

Pioneer 10/11

The trajectories of both spacecraft would ultimately take them out of the solar system completely, and so each was fitted with an identical plaque that was optimistically intended to provide clues about the origin of the spacecraft in case any galactic civilization picked them up. The plaques were made of gold-anodized aluminum, and displayed a man and a woman; spectral lines of neutral atomic hydrogen; a diagram of the solar system; a drawing of the Pioneer probe itself; and a smaller diagram of the Pioneer pointing to the third planet from the Sun.

* Pioneer F was launched from Cape Canaveral by an Atlas-Centaur booster on 2 March 1972, and was successfully placed onto its interplanetary trajectory, to be renamed "Pioneer 10". A little over a year later, on 5 April 1973, Pioneer G was also launched successfully, to be renamed "Pioneer 11".

Pioneer 10 crossed the orbit of Mars on 25 May 1972, and entered the asteroid belt on 15 July; it was the first spacecraft to do so. Concerns over the possibility of a collision with a piece of sky junk in the asteroid belt had led to the inclusion of a meteor detector, mounted on the back of the high gain antenna, which was of course pointed back at Earth and roughly opposite to the spacecraft's direction of motion. The detector consisted of 234 cells filled with a mixture of argon and nitrogen gas. If a cell were punctured, it would leak gas at a rate proportional to the size of the hole and the change in gas density would be reflected by an electric current flowing through the cell. On 15 February 1973, seven months after entering the asteroid belt, Pioneer 10 passed out unharmed, worries over collisions with asteroid belt objects having proven exaggerated.

Pioneer 10 performed observations of the deep space environment all through its flight, taking measurements of solar wind particles, and using its imaging polarimeter camera to measure the "zodiacal light", or the faint glow of interplanetary dust particles on the plane of the ecliptic, where the orbits of most of the planets lie. The imaging polarimeter was almost ruined during a scan of the zodiacal light. The instrument would burn out if it was pointed at the Sun. The spacecraft was performing a scan while it was in line of sight of the NASA Deep Space Network station in South Africa, when a fishing trawler cut the transatlantic cable to the Azores that was relaying information from Ames to control the probe. The command to stop the scan didn't reach the spacecraft. Mission controllers managed to patch together a link through the White House communications system to rescue the probe with only minutes to spare.

* Pioneer 10 began its observations of Jupiter on 3 November 1973. Observations continued for two months. On 29 November, the probe passed inside the orbits of all seven of Jupiter's outermost moons and into the planet's "bow shock", where the strong Jovian magnetic field meets the solar wind. On 3 December, Pioneer 10 made its closest approach to Jupiter, to then sail continue on a path out of the solar system.

Pioneer 11 followed almost exactly a year later, performing its closest approach to Jupiter on 2 December 1974. It passed over the planet's north pole, giving a view never seen from Earth, and then went on a slingshot to Saturn, making its closest approach to the ringed planet on 1 September 1979. Pioneer 10 crossed Neptune's orbit and left the solar system on 13 June 1988.

Pioneer 10 and 11 have been somewhat forgotten, since they were relatively simple probes and did not return as much data as their successors. However, they were the first successful outer-planet probes and so still occupy a significant place of honor in the history of spaceflight. Those tuned to space exploration found the first images ever taken up close of Jupiter and Saturn a breathtaking experience -- a true shock of the new.


[12.3] VOYAGER 1 & 2

* While NASA Ames had been working on the Pioneer probes, JPL had been characteristically considering a much more ambitious project.

By the mid-1960s, JPL engineers had not only discovered the gravitational slingshot technique, but had also determined that in the late 1970s there would be a rare alignment of the planets, occurring only once every 175 years, that would allow a single probe to perform consecutive flybys of Jupiter, Saturn, Uranus, and Neptune. The slingshot trajectories would not only allow a single probe to visit all four planets, but it would greatly reduce the time it took to perform the visits. The scheme was known as the "Grand Tour".

JPL officials lobbied for a Grand Tour mission, but money was tight, and they were only able to obtain approval for two probes for consecutive flybys of Jupiter and Saturn, originally designated "Mariner 11" and "Mariner 12". The two probes evolved into "Voyager 1" and "Voyager 2", inheriting, as mentioned previously, the name of a canceled Mars lander program that was eventually resurrected as Viking. The Voyager program was formally initiated on 1 July 1972. In response to an "announcement of opportunity" for experiments to be performed by the probes, JPL received 77 proposals. 28 of the proposals were accepted. Edward Stone, a Caltech physicist, was named project scientist.

* The Voyagers were derived from the earlier JPL Mariner planetary probes, but they were bigger and more sophisticated. They weighed about 815 kilograms each. They were built around a ten-sided spacecraft bus, with a 3.7 meter high-gain dish antenna on top, plus three RTGs extending from a boom to one side, a pair of magnetometers on another boom, an instrument scan platform on a third boom, and two long whip antennas at right angles to each other.

Voyager 2

The Voyagers were "three-axis stabilized", not spin-stabilized like Pioneer 10 & 11. The probes were controlled by an onboard redundant computer system that could handle flight operations semi-autonomously, reducing the need for interaction with mission control, and could also be reprogrammed. Each Voyager spacecraft carried 11 experiments, including:

As with many other space probes, radio science experiments were also planned in which measurement of the spacecraft's radio signals were used to obtain planetary data. Instead of the plaque carried by Pioneer 10 & 11, each Voyager carried a gold-plated copper analog phonograph record, which contained greetings in 60 languages, 38 sounds, 90 minutes of music samples, and 115 images. The items were selected by a committee chaired by populist astronomer Carl Sagan as representative of human culture on Earth. Each record came with an aluminum jacket, cartridge, needle, and graphical instructions.

* Voyager 2 was launched by a Titan IIIE / Centaur booster from Cape Canaveral on the morning of 20 August 1977. Voyager 1 was launched 16 days later. Despite the fact that Voyager 1 was launched second, it was scheduled to reach Jupiter four months ahead of Voyager 2, since it used a shorter trajectory.

The first year of the dual mission was marked by glitches, bugs, and breakdowns. The most significant was the failure of Voyager 2's main radio receiver in April 1978. The mission planners ended up taking extraordinary measures to stay in touch with the probe. JPL engineers had to stay up nights to write software for the probe's onboard computer system to kill or work around bugs. However, both probes survived in good working condition to fly by Jupiter. Voyager 1's formal flyby phase began in January 1979. The probe crossed the orbit of Sinope on 10 February 1979, and made its closest approach, 349,050 kilometers, on 5 March. Voyager 2 followed that summer, with closest approach on 9 July 1979. Its mission was modified in light of the data returned by Voyager 1 to provide improved coverage.

The two probes returned spectacular data. Voyager 1 sent back 18,000 images, while Voyager 2 sent back 13,000. The probes provided the first detailed images of the Galilean moons, discovered three new moons inside the orbit of Io that would presently be named "Metis", "Adrastea", and "Thebe". The probes also discovered a very faint planetary ring system.

The two Voyagers went on to Saturn. JPL was able to obtain funding to send Voyager 2 on to Uranus and Neptune as well, and the Grand Tour happened after all.



* While JPL was working on getting the Voyagers aloft, the lab was also considering what to do next. In 1976, Dr. James van Allen's Science Working Group had recommended development of a "Jupiter Orbiter-Probe (JOP)". As its name implied, JOP was to be a two-part spacecraft, with a probe component that would fall into the Jovian atmosphere, while the orbiter surveyed the planet and its moons in detail. By the spring of 1977 the idea had congealed into a request to Congress for funding. JOP ended up competing for funding against the "Large Space Telescope (LST)", which would eventually emerge as the Hubble Space Telescope, and the request for JOP was initially rejected. After intense lobbying, it was reconsidered and accepted. That marked a beginning of what would become a massive struggle to get the probe off the ground.

By October 1977, JOP had taken form as a project with a cost of $410 million USD, launch in January 1982, arrival at Jupiter in 1984, and mission termination in 1986. Unfortunately, at the time NASA's first priority was the space shuttle program, and as troubles with shuttle development piled up, funds for other programs suffered. JOP's initial funding was only $21 million USD. This was still enough to keep the project going, with the probe given the name "Galileo" in 1978, and funding was increased.

Galileo seemed to be on track, but the shuttle had other effects on the Galileo project that would prove more far-reaching than anyone could imagine. In order to justify the shuttle program, NASA was insisting that it fly all US payloads, and that included Galileo. Program officials and project scientists protested loudly: Galileo could be launched on a Titan IIIC booster, which was flying and available, instead of waiting on the shuttle, which was moving along fitfully at the time. NASA headquarters insisted that Galileo fly on the shuttle. Nonetheless, John R. Casani, the JPL Galileo project manager, warned his people to do nothing that would prevent the spacecraft from fitting into a Titan payload shroud.

* As Galileo emerged, it was a large and advanced spacecraft, weighing 2,550 kilograms, including the entry probe, and with a length of 5.5 meters. It carried 19 instruments, including 12 on the orbiter and 7 on the atmospheric entry probe. The probe was spin-stabilized, with most on the spacecraft systems on a "spun" section, and most of the instruments on a smaller "de-spun" section that rotated in the opposite direction to the spacecraft rotation to keep them stationary.

The spun section was built around an eight-sided bus with spacecraft electronics, part of the command and data system, and a retrorocket system developed by Messerschmitt-Boelkow-Blohm of Germany. Three booms extended from the bus, including one for a magnetometer to measure the magnetic field of Jupiter and its moons, and two for RTGs. The spun section also carried instruments to detect low-energy and high-energy charged particles, and dust detectors. The high-gain antenna was mounted on top of the bus. It was an umbrella-like structure, obtained from the NASA Tracking & Data Relay Satellite (TDRS), that was folded compactly for launch and would unwrap once in space.

The despun section carried:

The atmospheric probe weighed 346 kilograms and consisted of a spherical instrument package, encased in a conical heat shield with a diameter of 86 centimeters. The probe was powered by a lithium-sulfur battery. It carried instruments to analyze the composition and structure of Jupiter's atmosphere as it descended, as well as detect lightning flashes. After entry into the Jovian atmosphere, it was to discard its heat shield and deploy a 2.5 meter wide parachute to slow its descent.

Galileo probe

* The designers were worried about the rotating joint between the spun and despun sections of the probe, which was an obvious source of possible problems, and were also concerned about the lack of redundancy on the spacecraft. The biggest worry was still the shuttle program, which continued to suffer delays. A closely related problem was that Galileo would have to be launched by the IUS upper stage, carried by the shuttle. IUS didn't have enough kick to send Galileo directly to Jupiter, and so NASA considered using a gravitational assist from a Mars flyby to allow IUS to do the job.

All the various delays in trying to get the shuttle and the upper stage into operation caught up with Galileo. In January 1980, NASA headquarters decided to postpone the launch from 1982 to early 1984, with arrival at Jupiter in 1986. To get around the IUS problem, Galileo was to be split into separate orbiter and atmospheric probes, both of which would be launched by the shuttle. This sent the design team back to the drawing board, and the delays and duplication of spacecraft increased the cost of the mission to $650 million USD.

That wasn't the end of the zigs and zags:

After that, things settled down for a few years. With launch date set for 21 May 1986, JPL moved quickly on implementing the probe, and on 19 December 1985 Galileo left Pasadena on a custom-built tractor-trailer rig that took it to the Kennedy Space Center in Florida for launch preparations. On 24 January 1986, the JPL Galileo team threw a party to celebrate what seemed to be the light at the end of the tunnel.

On 28 January 1986, space shuttle Challenger blew up after liftoff, killing all seven crew.

* Of course the shuttle fleet had to be grounded, leading to another launch delay. Since Galileo wasn't clearly going anywhere for a while, in early 1987 it was trucked back to JPL. There it was disassembled so that its components could be tested to see if they were still functional after such a long time in storage, with the bad components replaced. The two expensive RTGs could not be replaced, and once initially fueled their radioactive isotopes faded down their half-life curve with absolute predictability, whether the probe was flying or not. Mission planners had to factor in a lower power budget. How much lower depended on when the probe could actually be launched.

Not only was the shuttle fleet grounded, but the shuttle-Centaur upper stage had been killed for good out of safety concerns, and Galileo would have to be launched on IUS. The problem was that IUS as it finally came to exist was not as powerful as originally intended, and even the Earth-flyby Delta-VEGA trajectory wouldn't get Galileo to Jupiter.

However, JPL had designed software to calculate the gravity assist trajectories that would allow Galileo to tour the moons of Jupiter, and Robert Diehl, head of the mission design team, was able to use the software to calculate a new interplanetary trajectory for Galileo. The new flight plan would use one Venus and two Earth flybys for gravity assist, and so was referred to as "Venus-Earth-Earth Gravity Assist (VEEGA)". The VEEGA trajectory was something of a lucky accident for Galileo, since the proper alignment of planets for it only occurred once every few decades. The flight plan envisioned launch of Galileo in October or November 1989, with a Venus flyby in February 1990, followed by the first Earth flyby in December 1990, and the second Earth flyby two years later.

Now the IUS could still send Galileo to Jupiter, if by a roundabout path. One bit of good news about the VEEGA trajectory was that it would send Galileo through the asteroid belt twice, once before each Earth flyby, allowing the probe to perform the first detailed observations of asteroids. The bad news was that the probe would now take six years to reach Jupiter.

Another piece of bad news was that Galileo would cruise closer to the Sun than the designers had accounted for, and the solar heat would almost certainly ruin the high-gain antenna. The solution was to leave the antenna furled, put a sunshade on its cap, and keep the probe pointed straight at the Sun until the spacecraft reached more distant space, where the antenna could be deployed. An auxiliary low-gain antenna had to be fitted to the base of the spacecraft to permit communications in the interim. Along with a few small other changes, the long flight cruise to Jupiter and the modifications to protect the antenna raised mission cost to $1.36 billion USD.

Galileo faced one final obstacle before it could go into space. Anti-nuclear activists worried that a launch accident, or a guidance error during the two Earth flybys, would cause the plutonium in the spacecraft's RTGs to be released into the Earth environment. They began public protests. NASA was by no means unprepared for such an attack, and in fact the agency had put a massive amount of effort into ensuring that the RTGs were as safe as possible, as well as into preparing answers to almost every possible objection. Mainstream environmental organizations regarded the matter as of such marginal significance that they did not join in to the quarrel. The activist group, which included the prominent "monkey-wrencher" Jeremy Rifkin, took NASA to court, but NASA's won the case easily.

Galileo was finally launched by the space shuttle on 18 October 1989. JPL threw a party in relief. Given the past history of the project, however, it would have been surprising if anyone at the party thought their troubles were over for good.

* The mission did seem to go well at the outset. The spacecraft's VEEGA trajectory took it past Venus on 19 February 1990, with Galileo performing useful scientific observations during the flyby. At the end of the year, it performed its first flyby of Earth, and then looped back around the Sun for its second pass. Then the project ran up against one more one more potentially deadly obstacle. In April 1991, mission controllers sent a command to the probe to tell it to unfurl its high gain antenna. It didn't. Over the next months and years ground controllers tried to get the antenna unstuck by various tricks, such as temperature-cycling it, pointing it to the Sun and then into darkness; or hammering the deployment motors.

It stayed stuck. Analysis concluded that the lubricant used to hold the antenna's retaining pins had become "frozen" while Galileo was in its long storage, waiting for launch. While there was some hope the antenna might come unstuck when the atmospheric probe was launched, that could not be counted on.

All that was left was to salvage the mission as best as possible. At the distance of Jupiter, under normal circumstances the low-gain antenna would only be able to send data at a rate of about 10 bits per second. Earth-based receivers were enhanced to provide better sensitivity, giving the receiving channel a bandwidth of 100 bits per second. Galileo's computers were reprogrammed to provide improved data compression. Mission planners still felt the probe would be able to achieve its main objectives. They held their breath and prayed that nothing else would go wrong.



* Galileo was able to proceed with its mission, and performed significant observations while en route to Jupiter. On 29 October 1991, the spacecraft the first close-up pictures of an asteroid when it flew by asteroid Gaspra.

On 8 December 1992, Galileo cruised by Earth on the final leg of the VEEGA trajectory, taking images of the Earth and Moon. On 28 August 1993, the probe took highly detailed images of the asteroid Ida, and also spotted a tiny moon of Ida, which was named Dactyl. In late July 1994, while still 18 months away from Jupiter, Galileo was able to photograph Jupiter's far side when more than 20 fragments of the Comet Shoemaker-Levy plunged into the planet's atmosphere over a six-day period.

The comet had been discovered in March 1993 by Eugene and Carolyn Shoemaker, and David Levy. Analysis of its orbit showed that it had been "captured" into a long orbit around Jupiter by the giant planet, possibly sometime between 1920 and 1930. The comet had broken up following a relatively close fly-by of Jupiter in July 1992, and the fragments were going to collide with the planet when they fell back down the arc of their orbit.

The impacts were tremendous, one momentarily increasing the brightness of the planet by 15%. Incidentally, amateur astronomers have observed other, smaller impact events on Jupiter since that time -- the big planet tends to collect them -- with one in 2009, two in 2010, one in 2012, and one in 2016. The aftermath of the 2009 impact was observed by the NASA Hubble Space Telescope.

A year after the comet impact, on 13 July 1994, the atmospheric probe was released to allow it to follow its Jupiter-impact trajectory. In October 1995, the mission was faced with another critical problem when the spacecraft's data recorders became stuck in the "rewind" position for 15 hours, wearing out a section of tape. Engineers were able to unstick the recorder and reprogrammed it to avoid such a fault in the future, as well as the bad section of tape.

The spacecraft entered the Jovian system in late 1995, six years after launch, with the atmospheric probe entering Jupiter's atmosphere on 7 December 1995. The probe analyzed the chemical composition of the atmosphere, measured the size of cloud droplets, detected lightning bolts, and gauged the flow of heat through the atmosphere as it descended. A few hours later, the Galileo orbiter performed its orbital insertion engine burn. After years of troubles, the probe was finally in orbit around Jupiter.

* Galileo provided much more data in far more detail than the Voyager probes. The best resolution of the Voyager images was 500 meters per pixel, while the best resolution of Galileo images was 10 meters per pixel. Galileo's performance around Jupiter did much to make up for the many frustrations and hardships endured getting the probe there. The probe survived over twice as long as designed, even though it had been exposed to more than three times as much radiation as it had been built to tolerate. It returned more than 14,000 images.

Galileo performed its last flyby, of the little moon Amalthea, on 5 November 2002. By that time the spacecraft was almost out of fuel, and the mission was finally ended on 21 September 2003, when it was sent into Jupiter's atmosphere to perform one final set of observations, after over 30 orbits through the planet's moon system. Though its development had been a cautionary tale for anyone considering such an ambitious project, Galileo turned out to be an outstanding success in the end.

* As a very minor footnote ... just before Galileo was sent into Jupiter's atmosphere, a story made the rounds along the fringe that the radioactive material in the probe's RTGs might detonate as a fission bomb, initiate fusion reactions in the Jovian atmosphere, and make the planet blow up. Of course, as we all know now, this is exactly what happened, and the radiation from the explosion wiped out human life on Earth, returning the planet to the cockroaches.