This article was written by Alan Duffy from Swinburne University of Technology and was originally published by The Conversation.
This was a golden year for planetary exploration thanks to all of the NASA and European Space Agency missions that were planned and implemented decades ago. Not since Apollo and the epic space race of the Cold War has space featured so heavily in the public eye. So here are the top 10 space events that I got most excited about in 2015.
1. Pluto Flyby by New Horizons
For me the biggest scientific result of the year, if not the decade, was the revelation by New Horizons that the frozen distant world of Pluto was as active and varied as any we had yet explored.
Organic material staining the surface orange, kilometres high water-ice mountains plunging into freshly resurfaced nitrogen-ice sheets and a collapsing atmosphere all made this dwarf planet astoundingly exciting and well worth the almost decade long journey to reach.
While this world is definitely not a planet (having a system of moons almost as massive in total as it is) the distant frozen dwarf planet is truly King amongst the new class of Plutoids that represent a distinct phase of planet formation.
2. Water (but no atmosphere) on Mars
The announcement that Mars has flowing water on its barren surface was of huge importance as (at least on Earth), where there's flowing water, there's life. It meant we need to reassess the conditions under which water can exist and hence the possibilities for life.
The red planet made the list because of the incredible discovery by NASA's MAVEN spacecraft that the Sun was responsible for stripping the Martian atmosphere away, turning a water-rich world nearly four billion years ago into the inhospitable desert it is today.
The atmosphere isn't locked away in rocks underneath the surface, à la Total Recall, but instead has been lost to space meaning future colonists may well have to bring more of their supplies than thought.
3. Philae called home (and more Rosetta discoveries)
The idea that there's a spacecraft sitting on a comet still astounds me.
That #WakeUpPhile came true (briefly) in 2015 is a great reason to feature Rosetta again this year and explore the scientific discoveries from the comet, such as how the iconic tail forms from comet ice as well as the discovery of organic material. These compounds are the precursors to several different amino acids as found in organisms on Earth, meaning objects like Comet 67P could have brought the ingredients for life to the planet's earliest days.
One of the big surprises is that the water in this comet has three times more 'heavy' water (where hydrogen is replaced with its heavier isotope deuterium) than in the oceans of Earth. Wherever the water came from that made life possible on Earth it wasn't from comets like this.
4. Alien worlds discovered by Kepler
NASA's Kepler spacecraft continued to search for exoplanets around stars in our Milky Way, even after the failure of critical gyroscope stabilisers, bringing the total to 1,030 confirmed worlds (with thousands more candidates to be followed up).
One of the most exciting discoveries was of a rocky world similar in size to Earth orbiting a Sun-like star, Earth's 'cousin. Although this is far from saying it's Earth-like. We don't know if Kepler-452b is habitable yet as we can't measure its atmosphere.
However, a rocky world orbiting in the 'goldilocks zone' suitable for liquid water to exist was a huge step forward in our ultimate search for Earth 2.0 and finding life beyond Earth.
5. Breakthrough Listen to hunt for ET
An incredible US$100 million initiative funded by Russian billionaire Yuri Milner to use two of the world's largest radio telescopes including Australia's Parkes (aka The Dish) to search the closet million stars and 100 nearest galaxies for alien signals. Breakthrough Listen will also use the optical Lick observatory in the chance that aliens have upgraded from radio/TV signals to laser-based communications.
In the decade-long search astronomers will upgrade the telescopes to the benefit of astronomy worldwide, and learn incredible things about the stars of our Milky Way. Most excitingly of all we may be able to answer one of the most important questions of all time: are we alone?
6. The Martian
Rarely does a film convey the science so well that it could be considered an educational resource, but The Martian managed it.
Exceptionally detailed consideration of the physics of orbital dynamics, life support systems, astro-biology and some poignant moments of the cost of space exploration to astronauts as much as their families left behind.
The initial sandstorm was more plot device than physical reality, as while global sandstorms occur, the air pressure is so low on Mars (and we now know why thanks to discovery 2 above) that a few air molecules at hurricane speed would only feel like a light breeze in the denser air of Earth.
7. Super blood moon (lunar perigee eclipse)
One of the most observed events of the celestial year, the lunar eclipse was also, in some circles, reported as being the harbinger of the apocalypse.
This was because during a lunar eclipse the Moon turns 'blood' red as sunlight travelling through our atmosphere scatters onto the Moon. Only longer, red, wavelengths of light make it through the atmosphere which is why the sun low on the horizon appears red. The result is that the 'blood' is being illuminated by all the sunrises and sunsets of Earth.
This eclipse occurred when the Moon was at its closest point to Earth (known as a perigee or 'super' moon), making this a beautiful sight for most, and a terrifying one for some. If you missed it this year we won't see a similar super lunar eclipse until 2033.
8. Dawn exploration of Ceres
NASA's Dawn mission to Ceres, a dwarf planet and largest body in the asteroid belt, was overshadowed by the flashier, high speed flyby of Pluto by the New Horizons mission.
Yet as Dawn drifted towards Ceres on the faintest of thrust from its ion engine it spotted a huge surprise. Ceres had bizarre bright regions shining out from an asphalt-dark world. These were so unexpected that NASA even created an online poll so everyone could make a guess.
9. Launch of LISA Pathfinder
The race to test Einstein's final prediction of gravitational waves heated up this year, and 2016 will only be more exciting.
The European Space Agency successfully launched LISA Pathfinder in December meaning that, technically, it won't begin its mission (orbiting between the Earth and Sun) until February. It still makes the list as this is a critical first step and technology demonstrator of the ultimate space-based gravitational wave detector, LISA, or as it is now called, the New Gravitational wave Observatory.
The spacecraft formerly known as LISA will consist of three satellites in space precisely measuring their respective distances with lasers. These can then measure the change in their separation as a ripple in spacetime itself, caused by the titanic collisions of distant blackholes, passes by. An audacious goal which is why checking the technology is even possible first with LISA Pathfinder is a smart move.
10. Growing space lettuce
Aboard the International Space Station astronauts took their first bite of space-grown lettuce.
While the jokes write themselves - e.g. the astronauts were growing cos(mos) lettuce but it tasted like rocket - the demonstration that we can grow our own food in space will be critical for our possible migration from Earth to the rest of the solar system.
In the future we may look back on this moment as the biggest reason why 2015 had a bumper crop of astronomical events. Sorry.
Alan Duffy, Research Fellow, Swinburne University of Technology
This article was originally published by The Conversation. Read the original article.