Many centuries ago, Galileo peered through his telescope and saw four dim objects orbiting Jupiter. At the time, he assumed that those objects were fixed stars, and he named them the Cosmica Sidera (‘Cosimo's stars’, after his tutee Cosimo de’ Midici, later Grand Duke of Tuscany).

Today, we know that these bodies - Io, Europa, Ganymede, and Callisto - are actually moons orbiting Jupiter, collectively referred to as the Galilean Moons. Actually, we know that Jupiter has at least 67 moons (!), but the Galilean moons are particularly interesting. Ganymede is the most massive of all moons in the Solar System; Callisto has the oldest and most heavily cratered surface in the Solar System; Io is the most volcanically-active object in the Solar System; and then there is Europa…

Famed for its role in 2001: A Space Odyssey, Europa is slightly smaller than Earth’s Moon, has a water-ice crust, and a thin atmosphere composed primarily of oxygen. If you’re thinking of somewhere that might host extraterrestrial life, that’s a promising start. Moreover, it is believed that an ocean of salty, liquid water could exist beneath the surface (heating due to geological activity could keep the water from turning to ice, and provide energy sources for life). And…NASA has recently detected the presence of compounds associated with organic material, as well as water vapour plumes twenty times higher than Mt. Everest, on Europa. In short, Europa potentially has all the ingredients for life - water, warmth, chemicals from its core - as we know it on Earth.

No wonder, then, that scientists have been pushing to send a probe to Europa, to search for extraterrestrial life. And plans to visit the icy world have just received a boost: the White House has recently released a budget that includes millions of dollars earmarked for the research and development of a NASA mission to Europa. A potential launch date is set in the mid-2020s, with a flight time to Jupiter expected to be anything between 2 and 7 years. The primary objective of the mission, if it goes ahead, will be to investigate whether the icy moon has conditions that could harbour life.

Also in the mid-2020s, the European Space Agency (ESA) should have a spacecraft of its own - the Jupiter Icy Moon Explorer (JUICE) - visiting the Jovian moon system, focusing in particular on Ganymede, Callisto, and Europa, all three of which are thought to have significant bodies of liquid water below their surfaces. JUICE’s broad objective will also be to characterise the moons as potential hosts for extraterrestrial life.

While there are thought to be billions and billions of habitable, Earth-like planets in the Milky Way Galaxy, even the closest star to Earth is - at more than four light years away - an enormous distance from us. How exciting to think, then, that extraterrestrial life might have developed much closer to home!

PS: the history of the discovery of the Galilean moons is interesting in its own right. Simon Marius, a German astronomer, discovered them concurrently with but independently of Galileo, and the names Marius suggested are the names still in use today. Perhaps more remarkably, there is evidence to suggest that Gan De, a Chinese astronomer, may have discovered the moon Ganymede more than two thousand years before Galileo!