On the 7th day of January in the present year, 1610, at the first hour of the night, when I was viewing the constellations of the heavens through a spyglass, the planet Jupiter presented itself to my view ..

Galileo would record the above statement in his groundbreaking text Siderius Nuncius. It was the first scientific publication based on telescopic observations. It included descriptions of the moon’s surface with its craters and mountains. It also talked about the vast number of stars invisible to the naked eye.

But it was at the end of the text in which he reported his most important observation. One that would turn heads. It was his discovery of what he called, Medicean stars.

On the night of 7th January 1610, Galileo saw three little stars arranged in a straight line around Jupiter. Small but very bright. He assumed them to be fixed stars. Intrigued, he returned the next night to observe them and found that the three stars had changed position. Just as things got interesting, on the 9th his observations would be interrupted by clouds.

He would continue the next two nights. On the 11th of January he reached the conclusion that these stars were revolving around Jupiter. And also that there were not three stars, but four of them.

Galileo’s description of Jupiter and the stars he spotted.

Galileo had just spotted Jupiter’s moons. Four of the seventy nine we know exist as of today. Io, Europa, Ganymede and Callisto. The first set of bodies known to orbit a planet that wasn’t ours. And that particular observation would play a part in changing how we view our universe. Though it would take other pieces of the puzzle to get rid of the geocentric model, Jupiter and its moons would play a pivotal role in that transition.

Jupiter is the fourth brightest object in the sky with an apparent magnitude m of -2.94(The more negative, brighter it is. The moon’s m is -12.74). So it has been visible to humankind ever since we began looking up. Cultures all around the world had a name for the planet. Chinese astronomer Gan de had written Suixing Jing (Treatise on Jupiter) as early as 4th century BC. Two thousand years later, the fascination with the gas giant hasn’t ended.

In 1598, King Phillip III of Spain offered a reward of 6000 ducats plus life pension for the discoverer of longitude. Not the first, nor the last reward to be offered, it was aimed at solving the longitude problem. One of the greatest dilemmas of its time.

Sailors around the world were unable to calculate their longitude on their journeys which led to them getting lost and stranded. And in some extreme cases, it led to their deaths. The Scilly naval disaster killed over a thousand people. While gauging latitude was relatively easy, longitude was much more complex.

Galileo attempted to solve the problem by using Jupiter and its moons as a clock to measure longitude. Using the skies to navigate and tell time wasn’t a new concept. The sky looks different from different places on earth. This is what celestial navigation makes use of.

Through the years of observations Galileo had recorded the time the moons take to orbit Jupiter. He created tables of data which predicted the time when each of the moons would disappear and reappear. By using the timing of these eclipses it was possible to determine longitude. The time at which the eclipse is observed from two places is recorded. The difference in time between these two locations translates to longitude, east or west depending on if you are ahead or behind.

Galileo’s solution would be ignored because it was difficult to view Jupiter on a moving ship and there were other potential problems like clouds. However, the method of using the eclipses of Jupiter’s moons worked well on land. With telescopes mounted to the ground, viewing and recording the predicable eclipses became popular due to its effectiveness for mapping longitude.

Other astronomers published their tables of such data as well. As measurements got more accurate, maps would be redrawn, changing the geography of the world. And the pursuit of accuracy would lead us to an insight into the nature of light.

Image credit — Chuttersnap on Unsplash.

Jean Dominique Cassini, Italian born turned French astronomer would publish his tables of data on Jupiter and its moons in 1668. The most accurate set at the time. He had already made a few important discoveries and this only added to his reputation. He was brought to work at the royal observatory in Paris where he would eventually become the director.

Once he was there, he got to work on improving the available data on the Jovian system. He collaborated with scientists around the world collecting data to make better measurements. Cassini would make observations of the eclipses from Paris while his envoys reported from Denmark. Armed with this observational data they could calculate longitude accurately. The new boundaries were dictated by the celestial objects and the maps were redone based on the new lines. France was shown to be smaller than previously thought. King Louis XIV could only jokingly complain that he was losing more land to astronomers than his enemies!

There remained one curious fact about the observations though. One of Jupiter’s moons, Io appeared to be irregular in its orbit. Sometimes it would appear ahead of schedule and sometimes it would appear later than predicted. In 1676, Danish astronomer Ole Romer, working with Cassini at the time, would independently figure out that it was due to the finite speed of light that Io’s orbit seemed irregular.

At that point in time, it was inconclusive whether the speed of light was infinite or not. In any case, it was extremely fast. So for those who believed light had a speed limit, it would be quite difficult to come up with an experiment to prove it. Only at distances large enough can we see its true nature.

When it is at its closest to us, Jupiter is 588 million kilometers away and at its farthest, 968 million kilometers. Due to light’s finite speed, the additional distance it has to cover results in us viewing Io’s orbit as irregular.

Cassini had already noted the discrepancy when he published his tables earlier and also rightly guessed that light was taking time to reach us but he would not take a stand on it as he felt there were other forces at play. He and Romer would hold opposing views on the matter and the finite speed of light would not be accepted until other discoveries were made. But their names will forever be attached to that history. Along with Jupiter and its moons.

A lot of astronomy had been done from the time of Gan De to Romer. Observing our giant planet had given us clues about how the planets were moving and how light behaved. But that was only the beginning.

In the next couple of centuries, our technology would grow parallel to our understanding of the universe. As the space age began, Galileo and Cassini would be immortalized as spacecrafts that would go and visit Jupiter. And there was a lot to learn from it.

Rick Guidice’s artistic impression of Pioneer 10 — Wikimedia

From a distance of around 12.2 billion kilometers, the Pioneer 10 spacecraft sent its final signal to Earth on 23rd January 2003. Somewhere beyond Pluto, the probe is floating away in the emptiness of space. Or it probably has gotten destroyed by space debris, which would mean the plaque on it intended for any intelligent life that came across it would have gotten destroyed too. But whether the spacecraft is intact or not, its legacy most certainly is.

Fittingly named, the Pioneer 10 and 11 launched in 1972 and 1973 respectively would pave the way for future space exploration missions to the outer solar system. The first man made object to visit Jupiter, it demonstrated that spacecrafts could safely travel across the asteroid belt that lies between Mars and Jupiter. It sent back the first close up images of the Jovian system.

The Pioneer spacecrafts recorded data on Jupiter’s giant magnetic field and the intense radiation belt around it. Through infrared measurements, it found that Jupiter gives off more heat than it receives from the sun.

It was almost set up like a scouting mission. The data collected from the Pioneers made it possible for future space missions to build more robust hardware and electronics that would be able to withstand the intense radiation it would face on such voyages.

While we lost contact with both the Pioneer spacecrafts. The Voyager 1 and 2 are still in contact with Earth forty years after their launch in 1977. Both have now entered interstellar space.

The Voyagers would make a set of surprising discoveries. The spacecrafts discovered new moons, detected lightning on Jupiter, and found active volcanoes on Io. The first time we had observed such phenomena on a body other than the Earth. It was a great surprise to astronomers. And it was also the mission that discovered Jupiter’s rings. Not as impressive as Saturn’s rings but its existence was unexpected.

These four spacecrafts only performed flybys of Jupiter. And it still sent back a wealth of new information on its atmosphere, its magnetic field, and its satellites. Time had now come for an orbiter probe. A mission dedicated to the giant planet itself.

Enter, the Galileo spacecraft. Launched in 1989, it would take six years to reach Jupiter and spend another eight in its system before being deliberately sent into the planet’s atmosphere to crash and burn.

The Galileo spacecraft would make an important discovery even before it arrived at its destination. Dactyl, a tiny moon orbiting the asteroid Ida. The first evidence of an asteroid moon. Neatly paralleling the real Galileo discovering the first planet moons nearly 500 years earlier. After that discovery, it would witness one more significant event before reaching Jupiter.

For five days, starting from the 16th of July 1994, fragments of comet Shoemaker-Levy 9 racing at a speed of 60 kilometers per second would smash into Jupiter’s atmosphere. Keeping up with the tradition of firsts, it was the first time we had observed a major impact on a planet by a comet or an asteroid.

The Galileo spacecraft, around 240 million kilometers away at the time would directly observe the collisions and resulting fireball explosions. The comet impacts would leave a dark spot on the planet’s clouded surface, visible for months. Such an impact on Earth would have been an extinction level threat.

A year after the Shoemaker-Levy impacts, Galileo arrived at Jupiter on December 8th, 1995. For the next eight years it would orbit the planet thirty four times. And in that time, a bunch of new discoveries were made and a few questions from previous missions were answered.

One of the highlights of the mission was the atmospheric probe carried by the spacecraft which was released into Jupiter’s atmosphere. It collected data for fifty seven minutes before it stopped working.

It found the temperatures of the upper atmosphere to be much higher than expected. The winds of Jupiter were blowing at close to 700 kilometers per hour. Both these findings suggested an internal source of energy which pushed heat upwards and drove the winds. This is unlike Earth where winds are powered by solar heating. It studied the chemical make-up of Jupiter. Measuring the amounts of hydrogen, helium and other common elements.

The source of the rings discovered by the Voyager mission was found to be dust ejected as interplanetary meteoroids smash into the small inner moons.

Part of the mission was to study the moons. Until then, not too much was known about them but the Galileo mission would change all that. The extent of Io’s volcanic activity was observed and it turned out to be hundred times that of the Earth. Ganymede was found to have a magnetic field, the first known moon to possess one. Evidence suggested that Europa’s icy crust may have an ocean of liquid water under it. And liquid water meant the possibility of life.

And that is why at the end of its mission, to avoid the risk of contaminating the moon with any earthly bacteria, the Galileo spacecraft was sent into Jupiter deliberately to vaporize. The first man made satellite on another planet had completed its mission. And once again, our understanding had been updated.

So what do we know about Jupiter today after all these years of observing it?

The banded appearance of light and dark clouds on Jupiter's upper atmosphere is immediately visible to us. The lighter colored areas are called zones and the darker colored areas are called belts. While it looks calm on the surface, it is anything but.

These winds move at great speeds in opposite directions. The interaction between these conflicting patterns of winds causes turbulence and gives rise to storms and cyclones. Water vapor clouds give rise to thunderstorms and lightning. The most famous storm on Jupiter is the great red spot, a massive storm that could fit two to three earths inside it, which we first observed in the 17th century. Recent observations have suggested that the great red spot is shrinking.

Jupiter is a gas giant. It has no solid surface. Made up of almost entirely hydrogen and helium, the most abundant gases in the cloud which birthed the sun and the planets. In trace amounts, the planet also contains ammonia, methane, water vapor, and other gases.

Being a giant, it naturally has the biggest gravitational influence after the sun. The most widely accepted models of how the solar system formed include scenarios where Jupiter’s gravity played a role. Its gravity is also the reason why it gets impacted by comets and asteroids a lot more than any planet in our system.

Though it is classified as a gas giant, much of the hydrogen in Jupiter exists in a liquid state. A little below the upper atmosphere, temperatures and pressures get higher and we have a large layer of liquid molecular hydrogen. Even further below it gets so extreme that electrons get stripped away from the hydrogen molecules and start to exist freely. In this layer of Jupiter’s interior is where liquid metallic hydrogen exists. It starts displaying properties of a metal. Normally hydrogen doesn’t conduct electricity very well but in this state of liquid metallic form, it is believed to be the cause of Jupiter’s magnetic field.

Whether a core exists at the center of the planet is still unclear. Even if it did, as models of planetary formation suggest, we don’t know how it would behave at such intense temperature and pressure.

Juno, the current spacecraft orbiting Jupiter is trying to answer these questions. Especially the ones concerning the planet’s interior. It entered orbit on July 5th, 2016 and is expected to stay there until 2021. The images of Jupiter used in this article were taken by Juno. Apart from these beautiful images, a lot of data has been collected out of which new studies are being published. One study suggesting a planet colliding into Jupiter early on causing its core to break apart came out while I was writing this. Only time will tell what else we will learn. But just like the trend with the gas giant has always been, we can expect surprises.

Jupiter is without a doubt one of the most fascinating planets in our solar system. Every part of the planet’s system is a story on its own. From its internal structure to its moons, there is so much to know. We sent a probe into it and barely scratched the surface. So we keep at it.

The best part about studying Jupiter is that it makes a wide range of astronomical concepts accessible. And maybe that is why it has had such a long storied history with humanity.

So go on, take a look at the sky tonight and join the tradition of observing a giant. It’s quite easy to spot. It is the fourth brightest object in the sky after all. You won’t even need a spyglass.

I like science, history, and writing.

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