Exploration of the outer solar system
author Paul Boșcu, June 2017

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Beyond Mars lie the far planets: Jupiter, Saturn, Uranus, and Neptune. Even though they are a great distance from the Sun, they are not even close to the edge of the solar system. Beyond Neptune is a large icy area called the Kuiper Belt that extends outward seven billion miles. Within it there are untold numbers of celestial bodies orbiting the Sun. One of these Kuiper Belt Objects is Pluto, formerly a full-fledged planet, but now considered a dwarf planet. In ancient times people could see only two of the far planets in the nighttime sky: Jupiter and Saturn.

Jupiter was named for the mythical Roman god of light and sky. He was the supreme god also known as Jove or dies pater (shining father). His counterpart in Greek mythology was named Zeus. Saturn was named after the god of agriculture, who was also Jupiter’s father. His Greek counterpart was called Kronos.

Following the invention of the telescope, Uranus, Neptune, and Pluto were discovered. Uranus was named for the father of the god Saturn. Neptune was the god of the sea and Jupiter’s brother in Roman mythology. Pluto was named after the Greek god of the underworld.

When the space age began, humans sent robotic spacecraft to investigate the far planets. They returned images of strange and marvelous worlds composed of gas and slush instead of rock. Many new moons were revealed. Some of these moons are covered with ice and have atmospheres. There could be liquid water beneath that ice teeming with life. This possibility is particularly appealing to space scientists and to all people who wonder if life extends beyond Earth.

Dwarf planets are a new category of celestial bodies. The designation was created in 2006 by IAU resolution. Even though Pluto is the best-known dwarf planet, it was not the first one discovered. This distinction goes to Ceres, a small world named after a Roman goddess.

Ceres was discovered in 1801 by the Italian astronomer Giuseppe Piazzi. He found it in the massive asteroid belt lying between Mars and Jupiter.

Clyde Tombaugh is credited with discovering the dwarf planet Pluto. Tombaugh made the discovery on February 18, 1930, while working at the Lowell Observatory in Flagstaff, Arizona. This famous observatory was founded in the 1890s by Percival Lowell. Lowell’s widow wanted to name the planet after her late husband. This was not allowed, because it would have broken the tradition of using names from Greek and Roman mythology. The name Pluto was finally selected from many suggestions made by the public.

Another dwarf planet is Eris. It was discovered in July 2005 by astronomers at the California Institute of Technology in Pasadena, California. Like Pluto, Eris is a trans-Neptunian object, meaning it lies beyond Neptune. Eris is named after a Greek goddess.

Americans weren’t content with just exploring the Moon and the inner solar system. They longed to expand their pioneering spirit to the vast reaches of the outer solar system. Thus, the Pioneer Project was born. Pioneer 10 and 11 stand out from the crowd as the first spacecraft to make the trip to Saturn and Jupiter. These planets are referred to as outer planets or outer solar system planets because they lie outside the asteroid belt; the other outer planets are Uranus and Neptune.

During the early 1970s the United States began a series of interplanetary missions designed to explore the far planets. The first of these missions was aptly named Pioneer. Pioneer spacecraft were the first to investigate Jupiter and Saturn. The missions were managed by NASA’s Ames Research Center in Moffett Field, California, for the agency’s Office of Space Science.

In 1965, scientist Gary Flandro was a student at the California Institute of Technology. While doing a summer project on the topic of space travel, he realized that the giant planets Jupiter, Saturn, Uranus, and Neptune were all coming around to the same side of the Solar System. This meant that a spaceship could use the gravity of each planet to pull it along, speeding it to the next planet. It would take less than half as much time to visit all four planets as anyone had previously calculated. Flandro called this the “Grand Tour,” and his summer project became part of NASA’s plan to explore the outer planets in the 1970s.

Pioneer 10 was launched atop an Atlas-Centaur rocket from Cape Canaveral Air Station in Florida. It was the first mission ever sent to the outer solar system. Ultimately, it became the first human-made object to leave the solar system for interstellar space. Pioneer 10 was the first spacecraft to travel through the asteroid belt between Mars and Jupiter. Scientists had feared that this would be a dangerous area of space. They learned that the asteroids in the belt are spread far apart and do not pose a significant hazard to spacecraft flying through.

Pioneer 10 returned valuable images and other data from Jupiter. Pioneer 10 also found that Jupiter has a very strong magnetic field that produces high levels of radiation in its immediate vicinity. After its Jupiter flyby, Pioneer 10’s trajectory took it out of the solar system. Pioneer 10 sent back the first close-up pictures of Jupiter’s cloud systems, with its storms so big that they would cover the entire Pacific Ocean.

One of Pioneer 10’s primary objectives was to determine how close it could reach without damage from Jupiter’s radiation. The total dose was a combination of the intensity and duration of exposure. A close approach increased the intensity of the radiation, but it also accelerated the craft’s speed so that the exposure duration was less. The scientists opted for a distance of about one and a half Jupiter’s diameter.

The twin to Pioneer 10, Pioneer 11 launched on April 5, 1973. As it made its Jupiter flyby, the spacecraft came within 34,000 kilometers of the clouds surrounding Jupiter and was able to photograph the planet and Callisto, one of its primary moons. After its successful Jupiter flyby, Pioneer 11 used Jupiter’s gravity to change its trajectory and send itself on to Saturn. The spacecraft’s Saturn flyby was equally successful, sending home images of Saturn, as well as its rings and moons.

Both Pioneer 10 and Pioneer 11 stayed in contact with Earth for more than two decades. Pioneer 10 was still making contact with Earth as recently as 2003, when it was 12 billion kilometers from Earth. The spacecraft’s power has likely declined now to the point where sending signals home is no longer possible. Pioneer 11’s mission came to completion in 1995, because no further communications have been received since then. The likely reason for this communications shutdown is that the spacecraft’s onboard power generator has expired.

The fact that these spacecraft remained in contact as long as they did is a testament to superior design and mission planning. The two spacecraft sent back valuable data about particles and fields in the far reaches of the solar system. In addition, the simple tracking of their orbits has yielded interesting information about chaos theory and orbital interactions in complex systems. And last but certainly not least, Pioneer 10 was used as a test target for flight engineers learning how to talk to faraway spacecraft.

Over the years the instruments aboard the spacecraft began to fail or were turned off by NASA to conserve power. In 1997 NASA ceased routine tracking of the spacecraft due to budget reasons. The spacecraft was the most distant human-made object in space until February 1998, when it was passed by an even faster spacecraft called Voyager 1.

As of March 2008, Pioneer 10 was more than eight billion miles from Earth and was heading toward the star Aldebaran (the eye in the constellation Taurus), which is eighty-two light-years away. It will take the spacecraft over two million years to reach the star.

Even though they’ve lost touch with Earth, Pioneer 10 and 11 are still capable of communicating. As the first man-made objects to be sent out of the solar system, both spacecraft bear a plaque designed by leading scientists as a message for any future intelligent beings that might encounter them. The plaque includes a drawing of a naked man and woman next to a schematic of the spacecraft, for scale. It also includes a diagram of a hydrogen atom, markers to indicate the position of our Sun, and a map of the solar system with a special marker to point out Earth.

Where might these plaques come in handy? Well, both spacecraft are on trajectories that will eventually take them to the stars: Pioneer 10 is heading toward the star Aldebaran, in the constellation Taurus, and will take about 2 million years to get there. Pioneer 11 is heading toward the constellation Aquila, a journey that’ll take about 4 million years to complete.

Science writer Eric Burgess suggested the plaque to Carl Sagan in 1971. Sagan recruited his wife Linda Salzman to do the artwork and Frank Drake, his colleague at Cornell, to design an appropriate plaque. NASA agreed to include it on Pioneer 10 and 11.

The Pioneer spacecraft were built with special power systems based on radioisotope thermoelectric generators (RTGs). RTGs generate electricity from the heat released during the natural radioactive decay of a plutonium pellet.

Even though sending plutonium into space is controversial, NASA has used this power source on all of its missions to the far planets. The planets are too far from the Sun to make solar power a feasible and reliable choice for these spacecraft.

Voyager 1 and Voyager 2 are the main players in the most successful and longest-lasting missions of solar-system exploration. The two spacecraft, launched in 1977, were conceived of as a way to take advantage of a rare planetary alignment that would allow them to visit and study Jupiter and Saturn, as well as Uranus and Neptune if all went well. The particular orbital locations of these four planets provided a prime opportunity for the spacecraft to conserve both fuel and power by using the gravitational attraction of one planet to propel them toward their next goal, a technique known as gravity-assisted trajectory.

The identically designed twin Voyager spacecraft were stabilized and controlled using gyroscopes. Data was managed through a digital tape recorder; it was sent and received via the spacecraft’s 12-foot (3.7- meter) antenna. The wide- and narrow-angled cameras that made up the imaging subsystem were controlled via computer.

The twin spacecraft mission took advantage of a rare orbital positioning of Jupiter, Saturn, Uranus, and Neptune that permitted a multi planet tour with relatively low fuel requirements and flight time. The alignment allowed each spacecraft, following a particular trajectory, to use its fall into a planet’s gravitational fi eld to increase its velocity and alter its direction enough to fling it to its next destination.

The various instruments on board were designed to detect and measure the solar wind and other charged particles, cosmic radiation, magnetic field intensities, and plasma waves.

Voyager 1 arrived at Jupiter first to fulfill the initial part of its mission: observing Jupiter and its moons. The spacecraft took images and gathered other data during its month-long trip through Jupiter’s system. One of its major discoveries was the existence of active volcanoes erupting on Jupiter’s moon Io. As Voyager 1 used Jupiter’s gravity to swing onward toward Saturn, it was able to take close observations of Saturn’s moon Titan, as well as Saturn’s rings. Its closest approach to Saturn occurred in November 1980.

Within a few days of approaching Jupiter, Voyager 1 took some of the most surprising pictures ever seen. They showed that Jupiter’s moon Io was not a dead and dormant orb like our Moon. Instead, Io is covered with recent lava, and huge, new volcanoes are constantly erupting.

The mysterious, large moon Titan was a prime target of the Voyager 1 mission, and the spacecraft trajectory was carefully designed to allow for a close flyby. Unfortunately, Titan’s thick, hazy atmosphere completely blocked any views of the surface from orbit, and the much-anticipated pictures of Titan showed nothing more than a fuzzy orange ball.

Despite the Titan disappointment, the data Voyager 1 collected over the years has contributed immensely to astronomers’ understanding of interplanetary space. With its mission completed, Voyager 1 began making its way out of the solar system; it currently can lay claim to the title of “farthest man-made object from the Earth.”

Voyager 2 has become known as one of the greatest and most-prolific exploratory space missions in history because it made significant discoveries everywhere it went. It encountered and documented new rings of Jupiter. It also showed the Great Red Spot, a storm vortex located on the South equatorial belt of Jupiter. Voyager 1 and various Earth-based astronomers had viewed the huge storm system before but never with such detail and accuracy. It took close-up pictures of Jupiter’s moon Europa, revealing a network of cracks and ridges and a very young surface. It made measurements of Saturn’s temperature.

After Voyager 2 completed exploring Saturn at close range, it proved to have sufficient fuel and functionality left to continue on what scientists had hoped would be the next leg of its journey: a visit to Uranus. the mission navigators at NASA redirected Voyager 2 so it could continue its grand tour of the solar system past Uranus and Neptune.

Both Voyager 1 and Voyager 2 carry golden photograph records that can be played. Handy instructions are included on the spacecraft itself for how to build a device to playback the record. The gold-plated copper records, filled with a wide variety of sounds and images intended to capture the range of human experience, are meant to serve as a time capsule of sorts.

Scientists, led by the famed astronomer Carl Sagan, carefully debated the impression that each and every sound and image might give a future extraterrestrial civilization. Here’s what they decided to include on the records: Audio: Voices in numerous languages, natural sounds, and music from many cultures, Visual: Photographs of people, buildings, animals, and other natural and artificial scenes. The cover of the record includes reference points to help determine the location of the solar system in which Earth resides.

The disks carry printed messages from former President Jimmy Carter and Kurt Waldheim , former secretary general of the United Nations.

The Voyager spacecraft were built and designed to last for about five years. Yet 40 years later, they’re still sending data and information back to Earth (albeit not a lot these days), thereby contributing to the history of space research. In 2013 Voyager 1 left our solar system to enter interstellar space. As of 2017 Voyager 2 is at the outer reaches of our solar system. It’ll be at least 40,000 years before either spacecraft makes a close encounter with another star system. How long can the Voyager spacecraft continue producing data? Scientists estimate that the two spacecraft are good for at least another ten years, or until the probes finally run out of power.

In 1998, Voyager 1 passed Pioneer 10’s distance to become the man-made object that’s most distant from the Sun. The meager information the twin spacecraft now send back to Earth focuses on interstellar studies. Both spacecraft investigated the very edge of the Sun’s influence and the interaction of that region with the space between the stars.

Like the plucky little Pioneers, both Voyagers are traveling so fast that the Sun’s gravity can never stop them. Their power supplies should be fine until at least the year 2020, so scientists still have many years of data to look forward to from deep space. Eventually, the spacecraft will no longer have enough power to send a signal back to Earth, similar to what occurred with the Pioneer spacecraft. In the vacuum of space, however, they’ll continue silently on their journey until something, or someone, gets in their way.

The Voyager spacecraft proved to be so hardy after completing their planetary journeys that they were sent on a new mission called the Voyager Interstellar Mission (VIM). The purpose of the VIM is to use the instruments on the spacecraft to explore the outermost edge of the heliosphere. This is the region of space dominated by energy effects from the Sun.

Perhaps the most famous scientist in North America in the 1970s and 1980s was Carl Edward Sagan. As professor at Cornell University, he studied the atmospheres and the geology of planets. According to Sagan, robot spacecraft were the best way to explore the Solar System. Sagan also convinced other scientists that it was worth studying the idea that life might exist on another planet, even if the possibility were remote.

The books he wrote, such as The Cosmic Connection and Pale Blue Dot, were very popular. He also hosted a television series called Cosmos, and was well known for saying “billions and billions.” The movie Contact was based on a novel that Sagan wrote. He inspired many young people to learn about science and study astronomy.

Sagan took leave from Cornell in 1978 and moved with his wife, Ann Druyan to Los Angeles to work on Cosmos. The 13-part PBS series debuted in 1980. He explored the universe in his “spaceship of the mind,” walking on a cosmic calendar and sliding down a black hole. Controversy followed Carl Sagan. Jealous scientists blocked his admission to the National Academy of Sciences in 1992. But 300 people expressed support for his work at his 60th birthday in 1994. Students said that they had majored in science and decided to study planets because of Sagan. Sagan contracted pneumonia and died on December 20, 1996.

Through television, books, and speeches, Carl Sagan became the most publicly recognized scientist of the 1970s. “The popularization of science...the communication not just of the findings but of the methods of science seems to me as natural as breathing,” he said at a conference in Seattle in 1994. “After all, when you’re in love, you want to tell the world.”

Born to Jewish parents in Brooklyn, New York in1934, Carl fell in love with science at an early age. School bored him even after he skipped several grades. Astounding (later Analog) science-fiction magazine and Edgar Rice Burroughs’s novels nurtured his creative imagination.

Sagan worked at Indiana University under Nobel Prize-winning biologist Hermann Muller. Hermann Muller shared Sagan’s love of science fiction and the belief that there was life on other planets. Sagan got his bachelor’s degree in physics in 1955 and his masters’ in 1956. In his 1960 Ph.D. thesis, he proposed that water vapor contributed to Venus’s greenhouse effect. Mariner 2 flyby data in 1962 supported his conclusions, though other gases later proved to cause the greenhouse.

Sagan received a fellowship to the University of California at Berkeley in 1960. This led to an appointment with the Smithsonian Astrophysical Observatory and an assistant astronomy professorship at Harvard in 1962. Before moving to Massachusetts, he studied exobiology at Stanford under Nobel winner Joshua Lederberg. During this time, Sagan worked on a secret government program to detonate an atomic bomb on the Moon.

Sagan was an outstanding teacher at Harvard. But his research was considered too speculative and superficial by traditional scientists. So, in 1968, he was denied tenure. Sagan then took a job with Cornell. Sagan thought that microbes were plentiful in the solar system. He advocated NASA’s quarantine of astronauts to prevent contamination. Carl Sagan became a household name in 1973 after the first of many appearances on the Tonight Show.

Travel times to the outer solar system are long and conditions are harsh, meaning that next-generation orbiter missions, following the flybys of the Pioneer and Voyager missions, needed to be large. Two such flagship missions, Galileo and Cassini, visited Jupiter and Saturn, respectively, with impressive results.

NASA’s Galileo spacecraft arrived in orbit around Jupiter in 1995. Galileo’s entry probe descended through the clouds of Jupiter to take measurements, while its orbiter made multiple flybys of Jupiter and its moons over the next eight years.

NASA’s Cassini spacecraft reached Saturn orbit in 2004, and the accompanying Huygens probe (built by the European Space Agency, or ESA) landed on the surface of Titan in early 2005.

Exploration of the outer solar system began in earnest with the United States’ Galileo mission in the late 1980s. Named after the Renaissance astronomer Galileo Galilei, who discovered Jupiter’s four large moons, the Galileo mission to Jupiter featured a robotic spacecraft designed to study the Jovian system (Jupiter and its moons) in detail. The Galileo spacecraft was launched into orbit by Space Shuttle Atlantis on October 18, 1989 . It was the first spacecraft to orbit a planet in the outer solar system.

Galileo consisted of an orbiter and an atmospheric probe. One section contained instruments and equipment; another section was devoted to a powerful high-gain antenna for transmitting data. Instruments onboard Galileo included many devices geared toward studying surface geology, composition, radiation, particles, magnetic fields, and other aspects of Jupiter and its moons.

Although hampered in data return by its crippled high-gain antenna, the Galileo spacecraft was otherwise a triumph of engineering in that it returned images and other measurements of Jupiter for around eight years. This duration surpassed all estimates of the amount of radiation damage the spacecraft could endure and still operate.

The spacecraft swung by Venus once and Earth twice as part of gravity assist maneuvers. These are maneuvers in which a spacecraft flies in close enough to a planet to get a boost from the orbital momentum of a planet traveling around the Sun.

Along the way, Galileo took spectacular pictures of the Earth and its moon together; it also performed the first close-up flybys of two asteroids, Gaspra and Ida. Galileo even had prime seating when Comet Shoemaker-Levy 9 collided with Jupiter in July 1994 and returned images back to Earth before Jupiter rotated enough to be seen by Earth-based telescopes. The spacecraft finally arrived at Jupiter in 1995.

The high-gain antenna had been folded and tucked behind a sunshade for most of the journey to Jupiter. When the time came to open the antenna, it only opened part way because of friction in the antenna ribs. No amount of maneuvering by scientists on Earth could open the antenna completely, so the mission had to rely on the less-powerful, slower-transmitting low-gain antenna instead. Although the Galileo mission still performed useful science, its total data return was severely limited by the loss of the high-gain antenna.

The probe component of Galileo, designed to study Jupiter’s atmosphere, was jettisoned from the main Galileo spacecraft several months before the latter reached Jupiter. When the probe reached Jupiter’s atmosphere it slowed down as it flew through, collecting data along the way via its onboard instruments. These instruments included mass spectrometers, cloud-detection equipment, sensors for measuring temperature and pressure, and particle detectors for studying Jupiter’s radiation belts.

The Galileo probe sent data back to the orbiter (for later relay to Earth) for nearly an hour before it stopped communicating. As expected, the probe most likely melted and, eventually, disintegrated as it moved deeper into Jupiter’s atmosphere. The probe found higher-than-expected concentrations of certain chemical elements called noble gases (argon, krypton, and xenon, among others). The presence of these elements, which don’t react with most substances, suggests that Jupiter may have formed in a colder part of the solar system than originally thought.

The Galileo mission answered a number of questions about Jupiter’s satellites(moons). Its four large satellites — Io, Europa, Ganymede, and Callisto — are particularly interesting because of their varied appearances.

The Voyager missions of the late 1970s discovered volcanoes on Io, but Galileo one-upped them by confirming that Io is the most volcanically active body in the solar system. (In fact, it’s about 100 times more volcanically active than Earth is today!) Io’s tidal heating results in material being ejected in volcanic plumes and lava flows, which cover the moon’s surface. (Tidal heating is the way that the interior of a planet or satellite is heated due to the squishing and pulling that takes place due to varying gravitational forces.)

Another major discovery by Galileo was evidence for a liquid water ocean under the icy surface of Europa. Images from the Galileo mission showed a cracked surface with iceberg-like features. Results from Galileo’s magnetometer gave the most decisive evidence for an ocean on Europa, with the discovery of an induced magnetic field consistent with a large subsurface volume of salty water, similar to terrestrial seawater. Although not yet definitively confirmed, Galileo results very strongly suggest that Europa’s subsurface contains an ocean with more water than all of Earth’s oceans combined. And where there’s water, there’s the possibility for life.

Galileo also took images of Ganymede, the only moon in the solar system with its own magnetic field, and Callisto. Callisto is an old cratered body, whereas Ganymede has undergone some cracking and faulting in the past due to the slight degree of tidal heating that it receives. Interestingly, magnetic field measurements suggest that both Ganymede and Callisto could also have subsurface liquid oceans, though these bodies of water would be much farther beneath the surface than the ocean on Europa.

Io, Europa, and Ganymede are in an orbital resonance with Jupiter. An orbital resonance is the state that occurs when orbiting planets, satellites, or other celestial bodies exert a specific gravitational force upon the other bodies in the circuit. So for every time Ganymede goes around Jupiter once, Europa goes around the planet twice, and Io goes around it four times.

Rather than run the risk of an out-of-control spacecraft crashing into Europa’s surface in the future (and possibly contaminating a subsurface biosphere), NASA mission planners made the difficult choice to send the still-functioning Galileo deep into Jupiter’s atmosphere, where it could burn up without causing any potential damage to Europa. In September 2003, Galileo sent its final signals back to Earth before entering Jupiter’s atmosphere.

Data from Galileo was used to study the composition of Jupiter’s atmosphere, including clouds made from ammonia ice, and measure wind speeds around the Great Red Spot (a giant storm in Jupiter’s atmosphere) and other features. Images also revealed the fine structure of Jupiter’s faint ring system. Galileo, as an orbiter, was never sterilized after being built on Earth. In addition to the possible microbes that could still be living inside the spacecraft, another danger came from the nuclear power source that fueled it.

At the end of Galileo’s mission, NASA mission planners faced a painful choice about the fate of the aging spacecraft, which was degrading. Such spacecraft are normally left to have their orbits decay naturally, but mission designers faced an additional complication when operating in the Jovian system: The discovery of a likely subsurface ocean on Europa meant that the prospects for life there were high enough that planetary protection constraints came into play.

The Galileo orbiter was originally designed for a two-year mission. It ended up lasting for fourteen years. The Galileo mission was hugely successful. The spacecraft traveled more than 2.8 billion miles during its long journey.

After two years of study, Galileo completed its primary mission in 1997, but the length of its overall mission was extended multiple times to add additional flybys of the satellites and studies of Jupiter’s atmosphere and rings. Galileo spent a total of eight years in orbit around Jupiter and managed to complete almost all of its mission objectives despite its nonfunctional high gain antenna.

Cassini-Huygens, a flagship-style NASA mission to the Saturn system, was nearly 20 years in the making. The final product, Cassini-Huygens, was a joint venture between NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI). The mission consisted of two basic parts: The Cassini orbiter, named after a famous Italian-French astronomer, was built by NASA; the Huygens space probe, named after a famous Dutch astronomer, was built by ESA. ASI built the high-gain antenna for Cassini, as well as the radar system used to see through Titan’s clouds. The mission focused on Saturn and its large moon Titan.

On October 15, 1997, the spacecraft was launched atop a Titan IV rocket from the Kennedy Space Center. Over the next three years it received two gravity assists from Venus and one each from Earth and Jupiter. Cassini arrived at Saturn in July 2004, becoming the first spacecraft ever to orbit the planet.

The Cassini orbiter was charged with studying Saturn’s rings, clouds, and other atmospheric conditions; investigating its satellites; and performing a wide-ranging investigation of Titan’s atmosphere and surface. The Huygens probe was responsible for returning surface and atmospheric data during its trip through Titan’s atmosphere and for about an hour and a half after landing.

Upon arriving at Saturn in 2004, Cassini flew through the plane of Saturn’s rings and had its only close approach to the dark outer moon Phoebe. Cassini has made almost 100 orbits of Saturn as of this writing, with each orbit targeted with a close flyby of one or more satellites. The spacecraft’s discoveries are many and varied. For example, Cassini took detailed images of Phoebe, including some that led scientists to believe that water ice exists in the moon’s subsurface layer.

Cassini images also showed a huge ridge encircling the middle of the moon Iapetus like a Belt and discovered four new small moons, including one, Daphnis, that orbits inside Saturn’s rings. Imagery from a range of 1 million kilometres showed a chain of ‘white dots’ on Iapetus, seemingly mountain peaks rising to heights of 10 to 20 km, contiguous with a dark linear feature.

When Giovanni Domenico Cassini discovered Iapetus in 1671 he inferred from the fact that it was apparent only on one side of its orbit that for some reason the leading hemisphere was dark. When the Voyagers passed through the system they confirmed this, but the dark terrain (which was named Cassini Regio) appeared featureless. Two days after entering orbit of Saturn, the Cassini spacecraft turned its cameras to Iapetus and revealed there to be basins on the dark terrain– evidently the relics of a heavy bombardment that occurred early in the moon’s history.

Tiny Enceladus, which is the sixth-largest of Saturn’s moons, was officially discovered in 1789. Cassini flew past Enceladus and spotted startling signs of activity for such a small moon: huge geysers of material were observed being ejected from Enceladus’s surface. The geyser sites are located at the moon’s South Pole, and the plumes contain primarily water ice (in addition to other materials). The geysers are associated with areas of extremely high temperatures, consistent with pockets of warm, liquid water under the surface that could act as reservoirs.

The discovery of geysers on Enceladus means that this small satellite joins only Jupiter’s moon Io and Neptune’s moon Triton as extraterrestrial sites of ongoing geologic activity. Some material ejected from the geysers falls back onto the surface of Enceladus, and the rest of it goes into orbit. Enceladus is covered with fresh, bright frost from the deposited geyser material, explaining the very bright surface first seen by Voyager.

The Voyagers had shown Enceladus to have a sharp line of demarcation between an area that was heavily cratered and a seemingly smooth plain which indicated that the small icy moon had been extensively resurfaced. This impression was reinforced by the early views from Cassini.

It appears there is liquid water at shallow depth beneath the south pole, and the ‘tiger stripes’, which run in parallel, 40 km apart, for about 140 km, are fractures through which it is able to reach the surface. It has been suggested that this activity is driven by the heat due to the tidal stress resulting from the eccentric orbit. If the ice contained ammonia, this antifreeze would lower the melting point of water ice by 100°C and reduce its density sufficiently for solid-state convection to occur.

Instruments onboard Cassini were specifically designed to see through the moon Titan’s thick, hazy atmosphere in order to provide scientists with a clear view of the surface below. Data and images gathered by Cassini have shown Titan to possess a surprisingly familiar surface. Although Titan’s surface temperatures are cold enough that exotic substances such as ethane and methane are liquid, these materials flow over the surface and form lakes and valleys just like liquid water does on Earth.

Much of Titan’s surface currently appears dry, but lakes of methane have been found near the moon’s North Pole. These are the first lakes found beyond Earth, with implications for hydrocarbons and organic chemistry creating possibilities for life on Titan.

The best views of the surface were provided by the spacecraft’s microwave radar, but this was effective only when close to the moon and supplied long narrow swaths of the surface, each of which had an area of approximately 1 per cent of the globe.

To complement the orbital observations of Titan from the Cassini orbiter, the Huygens space probe, which hitched a ride to Saturn aboard Cassini, was designed to take detailed measurements of Titan’s atmosphere and surface from the moon itself. Because the probe was too far from Earth to accept data from or send data to NASA directly, its findings were relayed through Cassini. As the probe slowly parachuted to Titan’s surface, it took images and analyzed Titan’s winds and the chemical composition of its atmosphere.

As hoped, the probe came down over a boundary between the light and dark areas. The imagery appeared to show a network of dark drainage channels on a bright area leading down to a shoreline, with several offshore islands on the dark area. The wind carried the probe out over the dark area for touchdown.

The ground level view showed a solid surface littered with ‘rocks’. Since the camera was very close to the ground, the sense of perspective was deceptive: the rocks were not the boulders they appeared; they were actually only a few centimetres in size, the nearest being no more than 1 metre away, and they were assuredly not silicate rock but lumps of water ice.

Images sent back from Huygens during its descent showed what appeared to be drainage channels: Dark swatches crossed larger, light swatches as they led into very large (sealike) dark swatches. These channels are much smaller than those that can be seen from the orbiting Cassini spacecraft, supporting the theory that liquid has traveled over much of Titan’s surface at various times. In all, Huygens sent about 700 images to Cassini for relay back to Earth. Unfortunately, a software glitch resulted in the loss of about half of those images.

Huygens was primarily designed to study Titan’s atmosphere, but NASA engineers wanted to be prepared in case the probe survived its landing. Of course, they didn’t know whether Huygens was headed for dry land or a global ocean, so they made sure it could survive a landing on either a wet or dry surface — if it had landed in liquid, it would even have floated.

Huygens’s battery was intended to last for three hours at most, and much of that time was occupied with the probe’s descent to the surface, so there was little time available for sending back data. The probe managed to transmit data from the surface for 90 minutes.

New Horizons launched on Jan. 19, 2006; it swung past Jupiter for a gravity boost and scientific studies in February 2007, and conducted a six-month-long reconnaissance flyby study of Pluto and its moons in summer 2015, culminating with Pluto closest approach on July 14, 2015. As part of an extended mission, pending NASA approval, the spacecraft is expected to head farther into the Kuiper Belt to examine another of the ancient, icy mini-worlds in that vast region, at least a billion miles beyond Neptune’s orbit.

Generally, New Horizons seeks to understand where Pluto and its moons “fit in” with the other objects in the solar system, such as the inner rocky planets (Earth, Mars, Venus and Mercury) and the outer gas giants (Jupiter, Saturn, Uranus and Neptune). Pluto and its largest moon, Charon, belong to a third category known as "ice dwarfs." They have solid surfaces but, unlike the terrestrial planets, a significant portion of their mass is icy material.

Using Hubble Space Telescope images, New Horizons team members have discovered four previously unknown moons of Pluto: Nix, Hydra, Styx and Kerberos. A close-up look at these worlds from a spacecraft promises to tell an incredible story about the origins and outskirts of our solar system. New Horizons is exploring – for the first time – how ice dwarf planets like Pluto and Kuiper Belt bodies have evolved over time.

New Horizons launched in 2006, on an Atlas V rocket from Cape Canaveral Air Force Station in Florida. A power outage and high winds delayed two previous launch attempts, but New Horizons made it safely into space on the third try. The spacecraft's first destination was Jupiter, in February and March 2007. New Horizons passed by less than 1.4 million miles (2.4 million km) of the solar system's largest planet, making it the first spacecraft to swing by since the Galileo probe finished its mission at Jupiter in 2003.

New Horizons was so busy gathering data in its July 2015 encounter that, as planned, the spacecraft didn't communicate with Earth during its closest approach to Pluto and Charon. Controllers celebrated when New Horizons phoned home and they knew that data was on the way. Early pictures from New Horizons showed a surprisingly young surface, with a mountain range on Pluto as high as 11,000 feet (3,500 meters). Believed to be about 100 million years old at most, this range likely pointed to recent geological activity on the surface, but how was a mystery.

Pluto's distance — about 3 billion miles (5 billion kilometers) from Earth — presented power challenges for New Horizon's designers, since the sun's rays are too weak to generate power. There are also long communications delays for those staying in touch with the 1,054-lb. (478 kilograms) spacecraft; at Pluto, it took 4.5 hours for a one-way message to get there from Earth.

Evidence of ionized gas tens of thousands of miles beyond Pluto found around the same time pointed to the planet's atmosphere being lost to space after the solar wind crashes into it. Later in July, team members presented evidence of a haze above Pluto's surface — another surprise. At the time, the models they had suggested the haze is created when sunlight breaks up methane in Pluto's atmosphere.

Over the Northern Hemisphere summer of 2014, investigators used the Hubble Space Telescope to see if there were any Kuiper Belt objects within reach of New Horizons after it concluded its Pluto mission. Scientists identified three candidates, with each of them at least 1 billion miles (1.6 billion km) beyond the dwarf planet. With NASA approval in 2016, New Horizons’ mission was extended to take a closer look at one of these worlds, a Kuiper Belt object dubbed 2014 MU 69. It will reach this object on Jan. 1, 2019.

Meanwhile, even after the mission ends, a group of scientists, artists, engineers and more are proposing placing a sort of message from Earth on the free hard drive space on the New Horizons spacecraft. "When New Horizons gets past Pluto, [and] has done all its data and is going on the slow boat to the heliopause [the boundary between the solar system and interstellar space], then it might be possible to just reprogram about 100 megabytes of its memory and upload a new sights and sounds of Earth that are not created by a small group of scientists but, in fact, are globally crowdsourced," said Jill Tarter, who is the co-founder of the SETI (Search for Extraterrestrial Intelligence) Institute, in 2013.

Juno's principal goal is to understand the origin and evolution of Jupiter. Underneath its dense cloud cover, Jupiter safeguards secrets to the fundamental processes and conditions that governed our solar system during its formation. As our primary example of a giant planet, Jupiter can also provide critical knowledge for understanding the planetary systems being discovered around other stars. With its suite of science instruments, Juno will investigate the existence of a solid planetary core, map Jupiter's intense magnetic field, measure the amount of water and ammonia in the deep atmosphere, and observe the planet's auroras.

Juno will let us take a giant step forward in our understanding of how giant planets form and the role these titans played in putting together the rest of the solar system. Juno is one of three New Frontiers probes that NASA is currently operating or building. The others are New Horizons, which flew by Pluto in 2015, and OSIRIS-REx, which is expected to fly to asteroid 101955 Bennu in 2020 to collect a sample and return it to Earth.

Juno was selected in 2005 and was originally expected to launch in June 2009, but was delayed until August 2011 due to NASA budgetary restrictions. Juno launched from Cape Canaveral Air Force Station in 2011. While eight other spacecraft have flown in Jupiter's neighborhood in decades past, part of what makes Juno stand apart is its ability to generate solar power from Jupiter's neighborhood. In February 2016, the Juno spacecraft did a maneuver to put in on course for the gas giant for a July 4, 2016, arrival.

Several spacecraft have flown by Jupiter en route to other locations in the solar system (such as Pioneer 10 and 11, Voyager 1 and 2, and New Horizons). To date, however, only one mission stayed for the long term: Galileo. After being launched from space shuttle Atlantis in October 1989, Galileo arrived at Jupiter in 1995 and spent eight years studying the planet and its moons. Juno aims to go further. It will focus solely on Jupiter, and hopes to answer some of the questions scientists have about the planet.

Even during the brief flybys, they have been able to glimpse interesting information about Jupiter and its moons. For example, New Horizons caught a large outburst on the volcanic moon Io. To date, however, only one mission stayed for the long term: Galileo. Galileo's discoveries include finding potential salt-water oceans under the crusts of Europa, Callisto and Ganymede. It also sent a descent probe into Jupiter's atmosphere. Much of the mission's value also came from spending nearly a decade in Jupiter's system, allowing scientists the rare chance to do up-close, lengthy observations of the largest planet in the solar system.

Juno’s mission goals, as stated by NASA are: how much water does Jupiter have in its atmosphere? This is important to figure out if our formation theories of the solar system are correct, or if they need some work. What is Jupiter's atmosphere like? Specifically, what are the properties at every layer such as gas composition, temperature and cloud motions? Figuring out the weather on Jupiter will help us learn more about gas giant weather generally. What are the magnetic and gravity fields of Jupiter? This will give scientists some hints of what the interior structure of Jupiter looks like.