On Monday, NASA is set to hold a press conference about some "surprising activity" on Jupiter’s icy moon of Europa. There’s no indication as to what that activity might be, other than it involves the Hubble Space Telescope, but it’s garnering more than one 2010 reference around the internet. Europa has always been a fascinating world that has challenged our view of the nature of the Solar System.
Europa was discovered by accident. Galileo Galilei turned his telescope to observe Jupiter on January 7th, 1610, when he made a startling discovery: the planet was accompanied by four small objects, and over the next several nights, he determined that these objects orbited the planet. He wasn’t alone in this observation: a German astronomer named Simon Marius spotted the same points of light on the night of the 6th and gave each object their present names: Io, Europa, Ganymede, and Callisto.
The discovery of these moons was a major advance in the understanding of how the Solar System worked. Galileo realized that Jupiter was the center of an orbital system, which contradicted the teachings of the day. David A. Weintraub wrote about Galilelo in his book Is Pluto a Planet: A Historical Journey through the Solar System, "The motion of the Medicean stars around Jupiter demonstrat[ed] that Aristotle was wrong about the Earth being the only center of motion in the universe."
The discovery placed Galileo in opposition to the Catholic Church, which deemed his research and findings heresy. He was admonished in 1616, and later interrogated and forced to recant. In 1634, he was placed under house arrest for the remainder of his life.
But the existence of these new worlds was impossible to suppress, and other astronomers began their own observations. In 1892, American astronomer Edward Emerson Barnard discovered Amalthea, a fifth satellite orbiting Jupiter, but it would be decades before astronomers got their next good look at the Jovian system.
While much of the world’s attention toward space programs was directed to the Apollo program in the 1960s, NASA scientists set their sights on other destinations in the Solar System. Scientists determined that by using the gravity of other planets, they could send probes further out into the Solar System more efficiently than if they used a direct shot.
In 1966, NASA engineers Gary Flandro and Roger Bourke proposed the ‘Grand Tour’ mission and in 1970 received $13 million in funding from Congress. But time was running out. Calculations showed that there was a narrowing window of opportunity to send a probe out to the furthest reaches of the Solar System.
NASA had begun to test out the gravitational boost theory with their Pioneer program between 1965 and 1968, sending several probes to the inner Solar System to study the Sun. It decided to take this same approach to the outer planets, and proposed a new set of missions that would fly by Jupiter, designated Pioneer 10 and 11.
On March 2nd, 1972, Pioneer 10 launched from Cape Canaveral in Florida, and encountered Jupiter for the first time in November 1973. Pioneer 11 followed in 1973, and captured close-up images of Jupiter and four of its moons, including Europa before it continued on to Saturn.
Pioneer 10 was equipped with a variety of instruments to study Jupiter’s atmosphere, radiation, and magnetic field. While studying the gas giant was the probe’s primary focus, it captured the first grainy photographs of the Europa as it flew by in December at a distance of 321,000 km. The images revealed some color variation on the moon’s surface, but not much else. The probe captured images of some of the other Jovian moons as it flew by the planet, and continued to transmit until 2003.
The mission was the first to the outer planets, and notably, the first to take a close look at Europa, and the two Pioneer missions helped to lay the groundwork for a more ambitious mission: Voyager.
After Pioneer 10 launched, NASA received approval for another Mariner mission, Mariner Jupiter Saturn, renamed Voyager in 1977.
Voyager was more sophisticated than its predecessors, and was designed to gather and return a wide range of amount of information about the system’s outer planets. Unlike Pioneer, Voyager would carry computers onboard to help pilot the spacecraft, and run its instruments.
Voyager 2 lifted off on August 20th, 1977, followed by Voyager 1 16 days later on September 5th.
Voyager 1 overtook its sibling in December 1977 and spotted Jupiter on January 6th, 1979. It captured numerous images and readings from the system as it approached, and took the first detailed images of Europa as it flew past two months later on March 5th. It revealed huge cracks across the surface of the moon, which scientists assumed were the results of plate tectonic activity and ice on its surface.
However, it was Voyager 2’s observations that caused astronomers to sit up and take notice of the moon. As it passed by Europa on April 25th, it took high-resolution photographs, revealing something completely unexpected: it was incredibly smooth. There were no mountains, as one might expect with considerable tectonic activity, and it had few signs of impact from craters.
Taking a close look at the cracks on the surface of the moon, scientists found that they looked similar to the fracturing of sea ice. The surface characteristics led them to believe that Europa wasn’t covered by a typical rocky surface, but one that was entirely ice, potentially hiding an ocean of liquid water.
This was something new: up to this point, liquid water was only known to exist on one place: Earth. Given water’s integral relation to life, it fueled further speculation that Europa could hold the building blocks for extraterrestrial life, which would fundamentally alter how we see our place in the universe.
As scientists studied the pictures, science fiction authors jumped on the change to include the moon, most famously Arthur C. Clarke, who used it as a setting in his sequel to 2001: A Space Odyssey, which included the following alien admonishment:
ALL THESE WORLDS
ARE YOURS EXCEPT
USE THEM TOGETHER
USE THEM IN PEACE
Following the success of the Voyager mission, NASA began planning another, one that would orbit Jupiter and collect even more information about the planet and its moons. Originally titled Jupiter Orbiter Probe, it was renamed Galileo and was scheduled for launch in 1984.
Unlike other probes, Galileo would be launched from orbit, carried up by one of NASA’s new space shuttles. Program delays and the destruction of the Challenger forced NASA to push back its launch, and it wasn’t until 1989 that it finally went into space aboard Space Shuttle Atlantis during the STS-34 mission. Galileo took its time to reach its destination: it used Venus and a pair of Earth flybys to head out to the Asteroid belt and Jupiter, and spotted the gas giant in time to see Shoemaker-Levy 9 crash into it. On December 8th, 1995, it arrived in orbit, and set about studying the planet and its satellites.
For two years, Galileo focused extensively on Jupiter, but once its primary mission concluded in 1997, it was extended to cover its moons, especially io and Europa. It began to take high-resolution pictures of the surface of the icy moon, providing new details about the nature of the crust.
Over the coming years, Galileo returned a considerable amount of information about Europa: it confirmed the composition of its thin atmosphere and helped to provide compelling evidence of a subsurface liquid ocean covering the planet, aided in part by tidal flexing as the moon orbits Jupiter. NASA ended Galileo’s mission in 2003 after Jupiter’s intense radiation degraded the spacecraft, and ultimately crashed the probe into it.
The space agency has kept its eyes on the moon in the intervening years, with the Hubble Space Telescope potentially spotting plumes coming off the surface, which appeared similar to those of Saturn’s moon Enceladus, which also harbors a liquid ocean.
The likely presence of a subsurface ocean makes Europa a prime target for future missions. NASA had planned a followup to Galileo, The Europa Orbiter, but ultimately canceled it in 2002 while a second Jupiter Orbiter, JIMO, was canceled in 2005.
Most recently, NASA’s Juno spacecraft arrived in orbit around Jupiter earlier this summer, but its focus will be limited to the gas giant. (It did spot Europa as it arrived, and provided a neat time lapse of the four moons orbiting Jupiter) It will be deorbited in 2018 so as to not accidentally crash into Europa or any of the other moons and contaminate them.
Planning for a dedicated mission to study Europa is underway: the Europa Clipper went into development in 2015, which would send a probe and lander to the moon, to be launched in the 2020s. The mission would provide our best look yet at Europa, and possibly confirm the presence of an ocean under its surface. However, the mission is subject to NASA’s funding, and as of this spring, NASA faces a major cut: $49 million dollars for 2017, down from $175 million in the 2016 budget.
At almost every opportunity, scientists learn more about our Solar System each time Europa is examined up close, from the nature of how the planets move to the potential for liquid water elsewhere in our Solar System. Hopefully, NASA’s announcement tomorrow will follow in that tradition.
Updated to correct Gary Flandro and Roger Bourke’s titles to engineers.