The long-awaited fly-by of Pluto by the New Horizons spacecraft, on 14 July 2015, was an event 85 years in the making, following Pluto’s discovery by Clyde Tombaugh in 1930. Since then, Pluto has gone from planet to dwarf planet, but despite protestations from the New Horizons team, its reclassification never really changed the mission or the importance of what it would find at Pluto.
Liquid water existing on the surface of Mars, in the here and now, has been the holy grail of Martian exploration for some time, but in October 2015 NASA announced that the orbiting Mars Reconnaissance Orbiter (MRO) had turned water-diviner to find compelling evidence that water is bursting out onto the red surface and trickling down the slopes of crater walls and hillsides.
While 2014 was the year the Rosetta spacecraft celebrated making it into orbit around a comet, 2015 was the year it got down to some serious hard work. Its comet, with the tongue-twisting name 67P/Churyumov–Gerasimenko, made its closest approach (186 million kilometres) to the Sun, a period known as perihelion when the comet would be expected to be at its most active. Rosetta was there to witness this.
At number 10 in our top stories of the year: one of the most perplexing problems of modern cosmology and astrophysics was solved. For decades scientists had puzzled over where all the lithium in stars had disappeared to. Fortunately for the Big Bang model, Italian scientists at the International School for Advanced Studies in Trieste discovered that it was the stars that were to blame.