The past 18 months has witnessed a frenzy of activity in space exploration. From the confirmation of gravitational waves, to the discovery of water on the Red Planet, it is a momentous time for astronomy and astrophysics. We've even revisited the planetary status of Pluto, everyone's favourite space underdog.

Excitement around space discoveries probably hasn't been higher since the golden era when humans first set foot on the moon almost 50 years ago.

For astrophysicist Professor Rachel Webster, easily the biggest recent discovery is the confirmation of the existence of gravitational waves, a discovery that she says changes the way we see the universe.

Here then are our top five events and discoveries, and a preview of what to expect next.

Gravitational Waves are discovered

Albert Einstein predicted it, but it took astrophysicists more than 50 years of trial and error to prove him right – gravitational waves, which are 'ripples' in the fabric of space-time that are caused by a significant and dramatic event, such as two black holes colliding – exist.

Professor Rachel Webster, from the School of Physics, University of Melbourne, says "it's a pretty big deal".

"Einstein was very keen on ideas of symmetry so he was taking his understanding of light and applying it to gravity," she says.

The discovery will allow scientists to delve deeper into time and peer inside some of the biggest objects in the universe, such as the earliest moments after the Big Bang.

Instead of relying solely on light to observe the universe – which is an imperfect observational tool because it is blocked by solid objects, and is absorbed and scattered by dust and gas between the source and the telescope – we can now use gravitational waves. These travel unimpeded through space and provide a totally new way to observe universe-defining events like black holes colliding and super-energetic neutron stars.

For those involved in discoveries like this, it can be the culmination of a lifetime's work. Professor Andrew Melatos, who was part of the discovery team and is a node-leader of the Australian Research Council Centre of Excellence for Gravitational Wave Discovery, has had a career-defining year.

"The discovery is its own reward, any of us will tell you that," says the University of Melbourne astrophysicist.

"It is a history-making moment. Just as we still remember the discoveries of Newton (gravity) and Kepler (laws of planetary motion), people will still be looking back and talking about this discovery in 500 years time."

Hubble Telescope turns 25

The iconic Hubble humbly celebrated a quarter of a century of scientific discoveries in 2015.

The telescope floats about 547 kilometres above the earth, and has provided ground-breaking observations of planets, galaxies, and other parts of the universe that were previously restricted to our imaginations.

The many breakthroughs made thanks to Hubble's ageing telescope include the discovery that there is a massive black hole at the heart of almost every galaxy, and the discovery that the universe is expanding at a faster rate than previously thought.

Astrophysicist Dr Michele Trenti, from the School of Physics, University of Melbourne, has spent his career analysing images from the Hubble. He says many people have a "real connection" with Hubble because of the "astonishingly beautiful pictures" it takes.

"Hubble had its own story – it was a faulty instrument at first, and then it got fixed – and the details of nearby galaxies are beautiful," he says.

Dr Trenti has recently used Hubble to discover the oldest galaxy so far discovered, at 13.4 billion years old.

NASA considers Hubble "the people's telescope" as its discoveries over the years have captured the public's attention and inspired millions of people. While the Hubble has outlived everyone's expectations, it will be joined by the James Webb Telescope in 2018, which will be able to delve even deeper into the universe, and see objects up to 100 times fainter than Hubble can see.

"The James Webb Space Telescope will be much more powerful than Hubble for infrared observations, but it will not be able to observe at ultraviolet and optical wavelengths," Dr Trenti says.

"Therefore, Hubble will continue to play a key role for astronomical observations from space, and it is likely that the mission will be extended."

Water on Mars

In 2015, NASA scientists presented evidence of flowing water on the Red Planet. Under certain circumstances, when the planet is above minus 10 degrees Celsius, water is believed to trickle down canyons and rock walls, leaving stains in its wake.

Despite the excitement surrounding this, and the possibility of life on our nearest planetary neighbour, Professor Webster says that water does not necessarily mean that life has ever existed on Mars, or that we will one day find living organisms in regions with periodic flowing water.

"Martian life forms may not actually choose those locations as the most stable, hospitable sites to colonise. Instead they may live underground," she wrote at the time of the discovery.

NASA is now focused on designing experiments that can further explore the possibility of life on Mars. But it might be another decade or more before a carefully constructed experiment can explore these regions and directly detect actual life forms.

The question of whether the Red Planet had water has been one that science has been trying to answer for years. The theory is that where there is water, there is life, or at least the possibility to create a habitable environment for humans one day.

As early as the 1970s, images of Mars were beamed back to Earth showing what appeared to be ancient, dried-up lakes.

It is not known where the water is coming from but the fact that water exists means that there is a possibility that life could be supported in some way.

Finding out the source of this water is the next challenge for scientists.

Juno lands on Jupiter

In early July, NASA announced that a spacecraft the size of a tennis court, that had spent the past five years hurtling towards the biggest planet in our solar system, had sent back a photo to earth.

That photo is a haunting image that includes Jupiter's Great Red Spot and three Jovian moons. The $1.1 billion spacecraft, Juno, is on a mission to help scientists discover whether the gas giant has a core of heavy elements and to better understand Jupiter's magnetic and gravitational fields, composition and interior structure. It will also look for clues about the planet's formation and the source of its furious winds, which can reach speeds of 618 km/h.

Astronomer Dr Tanya Hill, an honorary researcher in the School of Physics, University of Melbourne and Senior Curator at Museum Victoria, says Juno was the first time since 1999 that a mission had entered the planet's orbit. "What I love about the Juno mission is that it is going to reveal so much of what's beneath the clouds," Dr Hill says. "Does Jupiter have a core? Is there water? There's lots that we have to learn."

To even get to Jupiter is a huge achievement. The plane's mass is 300 times that of earth, it has an enormous gravitational pull and has a metal-frying radiation belt that is the most powerful in the solar system. Juno will circumnavigate the planet 37 times, at a distance of 4700 km, which is far away enough to avoid being destroyed by radiation.

Juno travelled 2.25 billion kilometres in the five years since it was launched, and will operate for about 20 months before performing a death-dive into Jupiter's atmosphere.

Pluto is a planet … again?

In 2006, our solar system lost a planet after the smallest and most distant member, Pluto, was downgraded to a dwarf planet.

But in March, there was discussion that Pluto's status as a planet could be returned. Images from New Horizon, the space probe that is studying Pluto, appear to show clouds in Pluto's atmosphere, which, for some scientists, is an indicator of planet status.

But Professor Webster, who is a member of the International Astronomical Union that made the original decision to declassify Pluto in 2006, says nothing has changed. And she adds that the existence of clouds has nothing to do with what defines a planet.

"Planets are defined through the International Astronomical Union, and there has been no meeting about this since 2006. So there may be a movement to reopen the discussion, but it hasn't been redefined," she says.

There is indeed a push for the decision to be reconsidered. But Professor Webster believes this is for historical, rather than scientific reasons.

"People have grown up with Pluto as a planet: they get attached. I take a rather different view, and that is that science evolves," she says. "Sometimes, we think, 'this is the way the world is', and then we learn more and say, 'actually, it's slightly different'. We modify things and that's what we did."

The Future

The discoveries of the past two years are paving the way for even more breakthroughs. Professor Webster says many of these discoveries, especially gravitational waves, will change the way we understand the universe.

"I don't think we fully understand where it will take us, but it will take us somewhere interesting," she says. "For the next generation of students coming through it's not a prediction anymore, it exists, and it's a new way of thinking about the universe."

And here is a taste of some of the more intriguing projects that are in the pipeline:

• The OSIRIS-REx probe was successfully launched in early September and is on its way to land on an asteroid that regularly orbits past the Earth. Scientists believe that the carbon-rich asteroid, named Bennu, could help answer questions about how the Earth was "seeded" with the origins of life.
• NASA continues to work on the Orion spacecraft, which could carry people to Mars and beyond within the next dozen years.
• University of Melbourne students are working on a cube satellite that will be launched into space in 2018 via the Melbourne Space Program.
• In 2017, China will launch a mission to the Moon, and aim to return lunar samples to the Earth.
• Adelaide will host the International Space Congress in 2017.

Banner Image: the Hubble Telescope/Wikipedia