The exploration of Io, Jupiter's innermost Galilean and third-largest moon, began with its discovery in 1610 and continues today with Earth-based observations and visits by spacecraft to the Jupiter system. Italian astronomer Galileo Galilei was the first to record an observation of Io on January 8, 1610, though Simon Marius may have also observed Io at around the same time. During the 17th century, observations of Io and the other Galilean satellites helped with the measurement of longitude by map makers and surveyors, with validation of Kepler's Third Law of planetary motion, and with measurement of the speed of light. Based on ephemerides produced by astronomer Giovanni Cassini and others, Pierre-Simon Laplace created a mathematical theory to explain the resonant orbits of three of Jupiter's moons, Io, Europa, and Ganymede. This resonance was later found to have a profound effect on the geologies of these moons. Improved telescope technology in the late 19th and 20th centuries allowed astronomers to resolve large-scale surface features on Io as well as to estimate its diameter and mass.
The advent of uncrewed spaceflight in the 1950s and 1960s provided an opportunity to observe Io up-close. In the 1960s the moon's effect on Jupiter's magnetic field was discovered. The flybys of the two Pioneer probes, Pioneer 10 and 11 in 1973 and 1974, provided the first accurate measurement of Io's mass and size. Data from the Pioneers also revealed an intense belt of radiation near Io and suggested the presence of an atmosphere. In 1979, the two Voyager spacecraft flew through the Jupiter system. Voyager 1, during its encounter in March 1979, observed active volcanism on Io for the first time and mapped its surface in great detail, particularly the side that faces Jupiter. The Voyagers observed the Io plasma torus and Io's sulfur dioxide () atmosphere for the first time. NASA launched the Galileo spacecraft in 1989, which entered Jupiter's orbit in December 1995. Galileo allowed detailed study of both the planet and its satellites, including six flybys of Io between late 1999 and early 2002 that provided high-resolution images and spectra of Io's surface, confirming the presence of high-temperature silicate volcanism on Io. Distant observations by Galileo allowed planetary scientists to study changes on the surface that resulted from the moon's active volcanism.
In 2016, Juno arrived at Jupiter, and while the mission was designed to study Jupiter's atmosphere and interior, it has performed several distant observations of Io using its visible-light telescope, JunoCAM, and its near-infrared spectrometer and imager, JIRAM.
NASA and the European Space Agency (ESA) have made plans to return to the Jupiter system in the 2020s. ESA launched the Jupiter Icy Moon Explorer (Juice) to explore Ganymede, Europa, and Callisto in 2023, while NASA launched the Europa Clipper in 2024. Both will arrive in the Jupiter system in the late 2020s and early 2030s and should be able to acquire distant observations of Io. The proposed NASA Discovery mission Io Volcano Observer, currently going through a competitive process to be selected, would explore Io as its primary mission. In the meantime, Io continues to be observed by the Hubble Space Telescope as well as by Earth-based astronomers using improved telescopes such as Keck and the European Southern Observatory.
Discovery: 1610
The first recorded observation of Io was made by Tuscan astronomer Galileo Galilei on January 7, 1610 using a 20x-power, refracting telescope at the University of Padua in the Republic of Venice. The discovery was made possible by the invention of the telescope in the Netherlands a little more than a year earlier and by Galileo's innovations to improve the magnification of the new instrument. During his observation of Jupiter on the evening of January 7, Galileo spotted two stars to the east of Jupiter and another one to the west. Jupiter and these three stars appeared to be in a line parallel to the ecliptic. The star furthest to the east from Jupiter turned out to be Callisto while the star to the west of Jupiter was Ganymede. The third star, the closest one to the east of Jupiter, was a combination of the light from Io and Europa as Galileo's telescope, while having a high magnification for a telescope from his time, was too low-powered to separate the two moons into distinct points of light. Galileo observed Jupiter the next evening, January 8, 1610, this time seeing three stars to the west of Jupiter, suggesting that Jupiter had moved to the west of the three stars. During this observation, the three stars in a line to the west of Jupiter were (from east to west): Io, Europa, and Ganymede. This was the first time that Io and Europa were observed and recorded as distinct points of light so this date, January 8, 1610 is used as the discovery date for the two moons by the International Astronomical Union. Galileo continued to observe the Jupiter system for the next month and a half. On January 13, Galileo observed all four of what would later be known as the Galilean moons of Jupiter for the first time in a single observation, though he had observed all four at various times in the preceding days. On January 15, he observed the motions of three of these satellites, including Io, and came to the conclusions that these objects were not background stars, but were in fact, "three stars in the heavens moving about Jupiter, as Venus and Mercury round the Sun." These were the first moons of a planet other than the Earth to be discovered.
The discoveries of Io and the other Galilean satellites of Jupiter were published in Galileo's Sidereus Nuncius in March 1610. While the Jovian moons he discovered would later be known as the Galilean satellites, after himself, he proposed the name Medicea Sidera (Medicean Stars) after his new patrons, the de'Medici family of his native Florence. Initially, he proposed the name Cosmica Sidera (Cosmic Stars), after the head of the family, Cosimo II de' Medici, however both Cosimo and Galileo decided on the change to honor the family as a whole. However, Galileo did not name each of the four moons individually beyond a numerical system in which Io was referred to as Jupiter I. By December 1610, thanks to the publication of Sidereus Nuncius, the news of Galileo's discovery had spread throughout Europe. With high-powered telescopes like Galileo's becoming more available, other astronomers, such as Thomas Harriot in England, Nicolas-Claude Fabri de Peiresc and Joseph Gaultier de la Vallette in France, Johannes Kepler in Bavaria, and Christopher Clavius in Rome, were able to observe Io and the other Medicean Stars during fall and winter of 1610–1611.
In his book Mundus Iovialis ("The World of Jupiter"), published in 1614, Simon Marius, the court astronomer to the Margraves of Brandenburg-Ansbach, claimed to have discovered Io and the other moons of Jupiter in 1609, one week before Galileo's discovery. According to Marius, he began observing the Jupiter system in late November 1609. He continued to observe the moons of Jupiter through December 1609, but did not record his observations until December 29, 1609 when he came to the conclusion "that these stars moved round Jupiter, just as the five solar planets, Mercury, Venus, Mars, Jupiter, and Saturn revolve round the Sun." However, Marius' observations were dated based on the Julian calendar, which was 10 days behind the Gregorian calendar used by Galileo. So Marius' first recorded observation from December 29, 1609 equates to Galileo's second observation of the Jupiter system on January 8, 1610. Galileo doubted this claim and dismissed the work of Marius as plagiarism. Given that Galileo published his work before Marius and that his first recorded observation came one day before Marius', Galileo is credited with the discovery. Despite this, it is one of Marius' naming schemes for the moons of Jupiter that is regularly used today. Based on a suggestion from Johannes Kepler in October 1613, he proposed that each moon was given its own name based on the lovers of the Greek mythological Zeus or his Roman equivalent, Jupiter. He named the innermost large moon of Jupiter after the Greek mythological figure Io.
Io as a tool: 1610–1809
For the next two and a half centuries, because of the satellite's small size and distance, Io remained a featureless, 5th-magnitude point of light in astronomers' telescopes. So, the determination of its orbital period, along with those of the other Galilean satellites, was an early focus for astronomers. By June 1611, Galileo himself had determined that Io's orbital period was 42.5 hours long, only 2.5 minutes longer than the modern estimate. Simon Marius' estimate was only one minute longer in the data published in Mundus Iovalis. The orbital periods generated for Io and the other Jovian satellites provided an additional validation for Kepler's Third Law of planetary motion.
From these estimates of the orbital periods of Io and the other Galilean moons, astronomers hoped to generate ephemeris tables predicting the positions of each moon with respect to Jupiter, as well as when each moon would transit the face of Jupiter or be eclipsed by it. One benefit of such predictions, particularly those of satellite eclipses by Jupiter since they were subject to less observer error, would be determining an observer's longitude on Earth with respect to the prime meridian. By observing an eclipse of a Jovian satellite, an observer could determine the current time at the prime meridian by looking up the eclipse in an ephemeris table. Io was particularly useful for this purpose since its shorter orbital period and closer distance to Jupiter made eclipses more frequent and less affected by Jupiter's axial tilt. Knowing the time at the prime meridian and the local time, the observer's longitude could then be calculated. Galileo attempted to produce a table predicting the positions of the Jovian satellites and eclipse timings after he negotiated first with Spain and then with The Netherlands to create a system for measuring longitude at sea using eclipse timings. However, he was never able to generate accurate predictions far enough ahead in time to be useful so he never published his tables. This left the tables published by Simon Marius in Mundus Iovialis and Giovanni Battista Hodierna in 1654 as the most accurate ephemeris tables available, even though they too were unable to predict the moons' positions with sufficient accuracy.
Giovanni Cassini published a much more accurate ephemeris table in 1668 using his observations from the previous 16 years. Using this table, Cassini generated a more accurate map of France by observing eclipses of the Jovian satellites at various locations across the country. This showed that previous maps had depicted some shorelines as extending farther than they really did, which caused the apparent area of France to shrink, and led King Louis XIV to comment that "he was losing more territory to his astronomers than to his enemies." Eclipse timings of the Jovian moons would continue to be used to determine longitude for another hundred years for tasks such as surveying the Mason–Dixon line and geodesy measurements. Efforts were made to utilize this method for marine navigation, but it proved to be impossible to make the necessary observations with sufficient accuracy from the moving deck of a ship; it would not be until the invention of the marine chronometer in the mid-18th century that determining longitude at sea became practical.