An astronomer once said that the solar system consists of the Sun, Jupiter and “a bunch of junk.” Of course, this quip should not be taken too seriously, because each planet is unique in its own way, and their degrees of interest have nothing to do with their size. However, it points to a simple and undeniable fact: Jupiter is truly huge. It is the solar system’s largest planet, with a mass 2.5 times greater than the mass of all the other seven planets combined. Jupiter can thus be rightly called the king of the solar system.

In this article, we will analyze how humanity has explored Jupiter and discuss plans to study it in the near future.

Medici Stars and the Speed ​​of Light

Jupiter is one of the brightest objects in the night sky. This meant that it was well known to ancient astronomers, who described in detail the 12-year cycle of its movement. It is believed that the Chinese zodiac appeared precisely because of this periodicity. In some chronicles there are also references to a small star accompanying Jupiter. It is quite possible that a historical observer with particularly acute vision managed to discern Jupiter’s largest moon, Ganymede.

size comparison of Ganymede, Ceres, Eris, Pluto, and Moon.
Comparison of the sizes of Ganymede, the dwarf planets Ceres, Eris and Pluto, and our Moon. Image from NASA

The matter of who was the first to look at Jupiter through a telescope is debatable. At one time, the contest was between Galileo Galilei and Simon Marius. It is now believed that Galileo was the first, while Mari made his observations literally the next day after the Italian astronomer. In any case, it was Galileo who told the whole world that the planet had four companions. The Italian astronomer proposed calling them Medici stars to honor his financial patron, the Tuscan Duke Medici. But ironically,they were ultimately given the names proposed by Simon Marius: Io, Europa, Ganymede and Callisto.

Jupiter Moons
Photo of Jupter’s moons, from left to right: Io, Europa, Ganymede and Callisto. Photo:

The advent of more powerful telescopes gradually allowed astronomers to learn more about Jupiter. It turned out that the planet was covered with cloud belts that rotate around it at different speeds. In 1665, Giovanni Cassini identified a distinctive red formation on Jupiter. Many scientists believe this was the first sighting of the Great Red Spot, a giant atmospheric vortex that has existed for at least several centuries.

Jupiter's Great Red Spot
Jupiter’s Great Red Spot, photographed by the Hubble Telescope in 2014. Photo by NASA

Observations of Jupiter’s moons led to another significant discovery. In 1671, Danish astronomer Ole Roemer discovered that the observed positions of the moons did not coincide with the parameters he had calculated, and that the magnitude of the deviation depended on the distance from the Earth. Based on these observations, Roemer concluded that the speed of light is finite and then estimated it at 220,000 km/s. Even though this figure is about a quarter lower off the true value (300,000 km/s), it was still a huge scientific breakthrough for its time.

Over time, astronomers were able to determine the mass and size of Jupiter, discovering it to be the largest planet in the solar system. Now we know that Jupiter has a diameter of 140,000 km, eleven times larger than that of our planet. As for the mass, Jupiter’s is a whopping 318 times larger than Earth’s. For comparison, the mass of Saturn (the second largest planet in the solar system) is only 95 times larger than Earth’s.

sizes of the planets of the solar system
A visual comparison of the sizes of the planets of the solar system. The largest are, of course, Jupiter and Saturn. Slightly smaller are Uranus and Neptune (blue and light blue), followed by Earth and Venus, and the smallest are Mars and Mercury. Image by

As scientific knowledge accumulated, astronomers gradually realized that Jupiter was radically different from rocky planets like Earth or Mars. The planet is a gas giant, composed of more than 90% hydrogen and helium. Jupiter does not have a solid surface. As it sinks into its depths, the pressure increases so much that the gasses first begin to behave like a liquid, and then like a solid.

Due to Jupiter’s enormous size, there is a fairly popular myth that it is a “failed star”, which was just a little short of the mass necessary for thermonuclear reactions to start inside it. However, this is not the case. To become a failed star (brown dwarf), Jupiter would have to have at least 12 times its current mass. This is too big a gap for this contingency to have been a serious possibility.

One of the most important discoveries related to Jupiter was made in 1955, when it was revealed to be a very active source of radio emission, second only to the Sun in this respect. This is due to the previously- mentioned features of the planet’s internal structure. Due to the extremely high pressure in Jupiter’s depths, hydrogen acquires the properties of a metal. The powerful electric currents arising in it generate a very powerful magnetic field, which gives off radio pulses.

Flights of space sappers

Although astronomers were able to determine the main characteristics of Jupiter by the beginning of the space age, they of course wanted to know much more. They were especially interested in how the planet was formed, what its internal structure was, and what exactly goes on in its atmosphere. Of particular interest was Jupiter’s complement of moons. At that time, information about them was extremely scarce, but at least it was already known that the largest moons were comparable in size to Mercury.

NASA began developing its first Jupiter mission in the second half of the 1960s. The designers faced a number of serious challenges, the key of which were ensuring a reliable power supply and stable communications. Another major issue was safety. Firstly, at that time it was unknown how much of a danger the asteroid belt posed to spacecraft. Secondly, scientists had rather vague ideas about the power of Jupiter’s radiation belts.

As a result, NASA decided to first send a couple of relatively simple probes to Jupiter that would play the role of space “sappers” and assess the safety of the route for subsequent more complex and expensive missions. They were given the names Pioneer 10 and Pioneer 11.

Pioneer 11 near Jupiter
Pioneer 11 flying near Jupiter. Image by NASA

Pioneer 10 was launched in March 1972. In July of that year, it became the first spacecraft in history to enter the Main Asteroid Belt. The instruments installed on board the probe did not record a noticeable increase in the number of particles compared to the environs of the Earth. In February 1973, Pioneer 10 successfully passed through the asteroid belt, proving that it can be safely navigated.

Pioneer 10 made a flyby of Jupiter in December 1973. The device collected data on the composition of the gas giant’s atmosphere, determined its mass, and measured its magnetic field strength. It turned out that the heat flow emanating from Jupiter is 2.5 times higher than the energy the planet receives from the Sun. This is believed to be due to its gravitational compression.

One of the most sensational discoveries came from the planet’s radiation belts, whose power exceeded all scientists’ expectations. Pioneer 10 was exposed to radiation doses ten thousand times greater than the power of Earth’s radiation belts. Due to the huge level of radiation, the probe’s equipment began to generate false commands, communication was interrupted several times, and some images were lost. However, Pioneer 10 nevertheless managed to survive.

A year later, Pioneer 11 visited the vicinity of Jupiter, confirming its predecessor’s main conclusions. Later, based on the data the probe collected, NASA engineers built a safe flight path to Jupiter for a pair of more complex and expensive devices, which were designated Voyager.

Voyager’s Great Voyage

The Voyager project was based on a discovery made in the 1960s. Astronomers realized at that point that in the late 1970s, all four giant planets of the solar system would line up in a giant arc, which happens only once every 175 years. This provided a unique opportunity to explore them all in one go, and Jupiter was the first point on this great journey.

The Voyager mission began in 1977. Two years later, the pair of probes reached the solar system’s largest planet. Discoveries were not long in coming, as the probe revealed that Jupiter has rings. Unlike the famous rings of Saturn, they are very dim and mostly made of dust.

The devices observed in detail the dynamics of Jupiter’s atmospheric processes. For the first time, detailed images of the Great Red Spot were obtained. At the time of the probes’ visit, its diameter was 21 thousand km, which is almost twice the diameter of the Earth.

The probes also discovered three previously unknown satellites of Jupiter and took detailed photographs of four moons discovered by Galileo. It turned out that Ganymede is the largest moon in the solar system (and is even larger than Mercury) and has its own magnetic field, while Europa has a smooth icy surface covered with cracks. The images taken by the probes became the starting point for the theory that the moon may have an ocean hiding under its surface.

But the main sensation was Io, where the probes spotted active volcanoes. This discovery may not seem so significant now, but we must take into account the historical context. By the 1970s, many astronomers had developed a picture of the solar system as a static place with no geologically active worlds other than Earth. Io showed that this view was completely wrong. Io’s surface has hundreds of volcanoes that are continuously erupting, constantly changing its landscape. The length of their lava flows can reach hundreds of kilometers. The moon also has entire lakes filled with molten sulfur.

Surface of Io
Surface of Io. Photo by NASA

It is not surprising that the discovery of Io’s volcanoes is now widely considered to be the most important of all the discoveries made by the Voyager probes during their epic journey.

Problematic Galileo

Pioneer and Voyager studied Jupiter from a flyby path. Despite the importance of their missions, they physically could not linger in the vicinity of the gas giant for even one minute extra. To continue revealing the secrets of the king of the solar system, scientists needed to place a device into a permanent orbit of Jupiter.

This task was assigned to the Galileo mission. It consisted of an orbiter of the same name, as well as an atmospheric probe, which was planned to be dropped on Jupiter. Initially, NASA was going to launch Galileo at the end of 1986, but then the Challenger shuttle disaster occurred. As a result, Galileo’s launch had to be delayed for four years. This setback did not have the best effect on the mission’s future fate.

The problem was that NASA initially planned to launch Galileo using the new Centaur G upper stage rocket, which was capable of ensuring a direct flight of the spacecraft to Jupiter without gravity maneuvers. But after the death of Challenger, NASA decided that it was too risky to take this upper stage on board the shuttle. As a result, engineers had to use a less powerful upper stage, which was no longer capable of directly sending Galileo to Jupiter. They thus had to develop a new flight route that included performing a previously unplanned gravity assist maneuver at Venus for a boost.

This created an unexpected problem. Since the main antenna of the device was not designed to withstand high temperatures, NASA decided not to deploy it until the probe was at a safe distance from the Sun. Until this point, Galileo used a low-power auxiliary antenna. Thus, when the device finally began to deploy its main antenna, it jammed and could not open. As a result, throughout his subsequent mission he had to communicate exclusively through an auxiliary antenna. Because of its low power, Galileo transmitted much less data than if its main antenna had remained operational.

Be that as it may, Galileo reached Jupiter in 1995. During its flight, a sensational event occurred: the gas giant collided with fragments of Comet Shoemaker-Levy 9. Unfortunately, Galileo was not able to directly observe the bombardment of Jupiter, since the debris fell on the opposite side of the planet. However, it nevertheless recorded powerful atmospheric disturbances and subsequently photographed the comet’s impact sites.

Galileo with Jupiter and Io
Galileo with Jupiter and Io in the background. Image by NASA

After Galileo entered orbit around Jupiter, the probe separated from it, and entered the planet’s atmosphere in December 1995. It eventually managed to dive to a depth of about 150 km before being crushed by the enormous pressure. As often happens in science, the data transmitted by the probe largely did not coincide with expectations based on then-existing models of Jupiter’s atmosphere. For example, the earthly messenger failed to detect the expected layer of water vapor. Jupiter’s clouds turned out to be thinner, and thunderstorm activity was less than expected. However, the planet’s wind speeds significantly exceeded expectations, as the probe managed to record gusts reaching 550 km/h.

As for Galileo itself, the spacecraft studied Jupiter until 2003. Due to problems with its antenna, the device transmitted significantly less data than scientists would have liked. Nevertheless, Galieo managed to make a number of important discoveries. Among them were the discovery of ammonia clouds on Jupiter, the identification of the planet’s global structure and dynamics of its magnetosphere, and the discovery of how the planet’s rings were formed.

In addition, Galileo confirmed the existence of Ganymede’s magnetosphere and provided evidence that a giant ocean of liquid water lies beneath Europa’s surface. Subsequent analysis of his data indicated that similar oceans may also be hidden in the depths of Ganymede and Callisto.

The Galileo mission was completed in 2003. To eliminate the possibility of a future collision between the spacecraft and the planet’s moons (and thus the contamination of their surfaces by terrestrial organisms), the device was deliberately aimed at Jupiter and burned up in its atmosphere.

It’s worth noting that since Jupiter is the most massive planet in the solar system, NASA has used its gravity several times as free propulsion to boost missions heading to the far reaches of the solar system. In the 1990s and 2000s, the Cassini and New Horizons missions took advantage of this opportunity, while the Ulysses probe used Jupiter’s gravity to change the inclination of its orbit and move out of the ecliptic plane. This allowed it to conduct the first-ever studies of the polar regions of the Sun.

Juno and future Jupiter missions

Despite all the technical problems, the Galileo mission overall was quite successful. However, some of the secrets of the king of the solar system remained unsolved. One of them was related to the planet’s internal structure. The fact is that scientists had two main theories of how Jupiter was formed. According to the first, Jupiter was initially a solid object, which then accumulated matter from its protoplanetary disk, increasing its mass. According to the other theory, the planet was formed as a result of the direct collapse of a protoplanetary disk. In the first version, Jupiter must have a compact, rocky core, while in the second, the core may not exist at all.

To find out which of these two scenarios is true, NASA launched the Juno mission in 2011. In addition to studying Jupiter’s internal structure of the planet, the mission was also entrusted with the task of studying the planet’s gravitational and magnetic fields.

Jupiter, photo by Juno
Jupiter in unusual colors, photo taken by Juno’s camera. Photo by NASA / Juno photo gallery

Juno reached Jupiter in 2016. There again were some problems. Due to a malfunction of the main engine, the device was unable to reach its planned orbit. However, this did not stop Juno from carrying out its planned measurements. Once again, the results surprised scientists. It turned out that Jupiter has a core, and that it is much larger and contains more heavy elements than expected. One theory holds that a catastrophic collision that occurred at the dawn of the solar system is responsible. Computer simulations show that the newborn Jupiter could have collided with an object whose mass was equal to about 10 Earths. Such an impact would have led to the complete destruction of the gas giant’s original core and its mixing with the matter of the dead protoplanet. This scenario could explain why Jupiter’s core is so large.

Juno also photographed Jupiter’s poles for the first time in history. Moreover, the spacecraft was able to carry out an orbit different from what it was designed for, thereby opening up additional possibilities. After NASA extended Juno’s life several times, it appears that the spacecraft will be able in the future to make a series of close flybys of several of Jupiter’s large moons. In 2021, Juno made a visit to Ganymede, which was followed by a close flyby of Europa in 2022. This year, it’s Io’s turn. Juno is still in service, and is expected to continue operating until at least 2025.

At the beginning of the next decade, two missions will arrive to Jupiter at once: the European JUICE and American Europa Clipper. Their main target will not be the planet itself, but its icy moons, whose surfaces conceal giant oceans are hidden. Scientists currently believe that they are one of the most promising places to search for life in the solar system.

Photo of Europe, whichJuno made in September 2022. Photo by NASA

JUICE was launched this year and is expected to reach Jupiter in the summer of 2030. The flight plan calls for it to make a series of close flybys of Europa, Ganymede, and Callisto. At the end of its mission, JUICE will enter a permanent orbit around Ganymede and land on its surface.

Europa Clipper will launch next year and will also reach Jupiter in 2030. Its main target will be the moon Europa. Europa Clipper will be tasked with a series of close flybys of the moon. If the device manages to find some signs of habitability in its ocean, this will certainly speed up the implementation of the Europa Lander project. As part of this, NASA plans to land a device on Europa that will study its surface and, possibly, try to reach the ocean.

China recently announced that it also has plans to study the icy moons of Jupiter. In particular, the People’s Republic is going to launch its Tianwen-4 apparatus, which will enter orbit around Callisto. The launch of this mission is currently scheduled for the end of this decade.

China’s interest in Callisto is understandable. Although this icy moon is now not as well-known as Europa or Ganymede, it is considered the most promising place for creating a potential base. Thus, the Chinese mission could be the first sign on the path to the future colonization of Jupiter’s satellites.