On July 4, 2016, the Juno probe is scheduled to reach Jupiter, and begin a campaign to get closer to the Jovian giant than any other spacecraft in history.
Including Juno, there have been nine space probes that have studied Jupiter up close. The first was Pioneer 10, which sailed past the striped giant in 1973. Here's a rundown of those nine missions, and what they have accomplished.
Most of the probes that studied Jupiter had multiple mission objectives — Pioneer 10 also studied the asteroid belt, and then flew past Jupiter to study more distant regions of the solar system. But Jupiter is the primary focus of the Juno mission. The probe will come closer to the top of Jupiter's colorful clouds than any probe before it, and after about 20 months of observation, Juno will end its mission by crashing into the gas giant. [Complete Coverage of the Juno Mission to Jupiter]
One of the things Pioneer 10 studied was the intense radiation that surrounds Jupiter. The massive planet has "the harshest radiation environment in the solar system," according to NASA. A strong magnetic field draws the radiation into a ring the shape of a doughnut around Jupiter, which poses a problem for visiting probes: Pioneer 10 fulfilled all of its Jupiter mission objectives except one, which failed because the radiation "triggered false commands" in the probe's electronics, according to NASA. In order to avoid the radiation, Juno will make more than 30 orbits that take it into the narrow space between Jupiter and the radiation doughnut. Launched in March 1972, Pioneer 10 lived up to its name by becoming the first space probe to ever cruise past the Jovian system. The spacecraft came to within 81,000 miles (130,365 kilometers) of the tops of the clouds that cover Jupiter.
Before its pass by Jupiter, Pioneer 10 studied the asteroid belt, and after the flyby it continued on a journey to exit the solar system. Its last message to Earth was received on Jan. 23, 2003. It carries a golden plaque that includes a map of Earth's location.
Launched in April 1973, the Pioneer 11 spacecraft followed its sister satellite, Pioneer 10, through the asteroid belt and past Jupiter. It then became the first spacecraft to fly past Saturn, and discovered a previously unseen member of the planet's ring system, known as the "F ring."
Pioneer 11 came to within 26,569 miles (42,760 km) of Jupiter's cloud tops — three times closer than Pioneer 10. It also captured stunning images of Jupiter's Great Red Spot, and gathered information about the radiation belt around the planet, and it also carries a golden plaque with a map of Earth's location.
NASA lost contact with Pioneer 11 in 1995.
Voyager 1 & Voyager 2
Voyager 1 left Earth in September 1977, and made its close approach of Jupiter in March 1979. It captured more than 18,000 images of the planet.
Voyager 1's sibling spacecraft, Voyager 2, launched earlier — on Aug. 20, 1977 — but reached Jupiter slightly later, in July 1979. Voyager 2's observations of the Jovian system revealed, among other things, the presence of active volcanos on Jupiter's moon Io. On Jupiter's moon Europa, Voyager 1 had imaged blurry streaks, which Voyager 2 was better able to resolve — they turned out to be massive cracks in the otherwise smooth, icy crust. The cracks indicate the presence of a subsurface ocean.
The Voyager 1 and 2 probes, like the sibling Pioneer probes before them, were built to study multiple objects and regions in the solar system, with the ultimate goal of leaving our planetary neighborhood. Unlike the Pioneer probes, scientists are still receiving data back from both Voyager probes, and Voyager 2 is NASA's longest-running mission. In recent years, the probes have revealed new information about the environment at the edge of the solar system and the interstellar space beyond.
Following the successful flybys of Jupiter by the Pioneer and Voyager probes, NASA launched a mission to orbit the giant planet. Named after Galileo Galilei, who first spied Jupiter's four largest moons through a telescope, the Galileo probe was launched in October 1989, and arrived at Jupiter in December 1995.
In addition to orbiting the planet, Galileo dropped a probe onto the gaseous planet to measure things such as temperature, wind speed and atmospheric pressure. Orbiting around the planet, Galileo studied the planet's magnetosphere, and its amazing thunderstorms. Galileo's primary mission ended in 1997, but an extended mission allowed the probe to continue operations into 2003. During that time, the probe continued to study Jupiter, as well as the moons Io and Europa.
Galileo's mission ended with a dramatic plunge into Jupiter — the same fate planned for Juno. Probes that venture into space typically still carry microbes from Earth, despite scientists' best efforts to sterilize them. Because it's possible that Jupiter's moons harbor life, scientists don't want to risk contaminating those bodies with microbes from Earth. That's why probes are often destroyed (by crashing into an extremely hostile environment, such as Jupiter's) or left to orbit a body for thousands of years.
The Ulysses mission was another multipurpose operation that included, among other things, a study of Jupiter's atmosphere, as well as its magnetic field and radiation belt.
A joint effort between NASA and the European Space Agency (ESA), the Ulysses probe's primary objective was to orbit the sun and study it from pole to pole, and to take measurements of the heliosphere — the bubble that wraps around the solar system, consisting of charged particles (the solar wind) and the sun's magnetic field.
Ulysses launched in 1990, and began its life in orbit around the Earth. It was then kicked outward into the solar system, where it swung around Jupiter in February 1992. With the assist from Jupiter, Ulysses entered into a new orbit that allowed the probe to observe the southern and northern poles of the sun.
The Cassini-Huygens mission is a cooperative venture by NASA, ESA and the Italian Space Agency to explore the Saturn system. The probe has sent back a trove of magnetic images of Saturn and its rings, as well as the planet's many moons.
The spacecraft was launched in October 1997, and flew past Jupiter to get a gravity boost for its journey to Saturn. The probe made a close approach of Jupiter in December 2000, and took images of Jupiter over a total of six months, generating on the order of 26,000 images of the gas giant. Researchers have learned a great deal about Jupiter from those views.
The Huygens portion of the Cassini-Huygens probe dropped onto Saturn's moon Titan in 2005 to study the moon's atmosphere and composition. The Cassini orbiter continues to study Saturn, but its days are numbered. Starting late this year, the probe will begin making a series of daring dives between Saturn and its innermost ring, where Cassini can take up-close images of the planet and potentially sample material from the rings. In September 2017, the probe will intentionally dive into Saturn.
NASA's New Horizons mission made headlines in July 2015 when it made the first-ever close approach of the Pluto system. But New Horizons also flew past Jupiter, where it got a gravity assist to help speed up its 10-year journey to Pluto.
During its flyby of Jupiter in February 2007, New Horizons took dramatic photos of the storms that rage across the planet, as well as portraits of many of Jupiter's moons.
New Horizons is now heading into the Kuiper Belt, the region beyond the orbit of Pluto, populated by small, icy, rocky bodies. The New Horizons team is awaiting approval to fly past another object in the Kuiper Belt.
The Juno probe launched in August 2011 on a five-year journey to Jupiter. The mission's main objectives are to learn about Jupiter's precise mass and internal composition, as well as the water content of its atmosphere. Because massive planets typically form early in the life of a solar system, these variables can provide information about the history of the planetary neighborhood where they live.
In order to get close to Jupiter, Juno will enter into an orbit that takes it in between Jupiter's cloud tops and the doughnut of radiation that surrounds it. Juno will loop through this interior region more than 30 times in 20 months.
To enter into this orbit, Juno will first have to slow down as it reaches Jupiter. That maneuver is set to take place July 4, after which scientists can breathe a sigh of relief, and wait for the data to roll in.
Editor's Note: This article previously stated that Juno will orbit through the Jupiter system for about 20 weeks. The probe will actually stay for about 20 months.