Astronomers and scientists, not to mention science fiction writers, have often considered whether Mars can be colonised by human beings. The nearest planet to Earth, Mars has a thin atmosphere, desolate landscape and a higher radiation exposure yet still the allure of having a colony persists.
The renowned science journal, Nature, has devised an interactive game to see how terraforming Mars might work. The game is short, easy to do and consists of interactive multi-choice questions. Access to the game can be found at this hyperlink: nature.com -terriform mars game
Aliens might exist. But there are three reasons why they’re not visiting us
Steven Spielberg’s new film, Disclosure Day, explores the idea of extraterrestrial life.
UniversalCarol Oliver, UNSW Sydney
The United States government’s recent release of hundreds of previously classified Unidentified Anomalous Phenomena (UAPs) cases spanning the 1940s to the present, along with the new Steven Spielberg movie, Disclosure Day, about extraterrestrial life, has fuelled the idea that aliens are visiting Earth.
In fact, polls in Australia, the US and elsewhere indicate around a third of the public believes aliens are here.
However, while what we know about the universe suggests aliens may exist, there are three compelling reasons why they probably aren’t visiting us.
Space is big – very big
To begin with, space is vast – beyond our imagination.
Proxima Centauri, the nearest star to our Sun, is about 40 trillion kilometres away, 268,000 times farther than the Sun is from Earth. That’s 4.3 light years as astronomers measure it. A light year is the distance light travels in one year at 300,000km per second.
We can only travel across space at a fraction of the speed of light with current technology. Even our fastest spacecraft, the Parker Solar Probe, travels at a top speed of roughly 191 kilometres per second – 0.064% the speed of light.
At that speed, it would take about 6,650 years to reach Proxima Centauri, and that’s just in our local stellar neighbourhood. So interstellar travel within human lifespans would require much higher velocities.
Let’s assume we did have the means to travel close to the speed of light. That introduces the first problem with travelling at that velocity. Albert Einstein demonstrated that time is relative; the rate of time flow is not the same everywhere in the universe. The faster a spaceship travels from Earth, the slower time will pass for its passengers. This is called time dilation.
For example, when NASA astronaut Scott Kelly arrived back on Earth from a year on the International Space Station, he was milliseconds younger than his identical twin because time moves more slowly for objects in motion, and the International Space Station travels at roughly 28,150 kilometres per hour.
This difference was negligible for the Kelly twins. But for any aliens cartwheeling through our skies, it would be significantly more because of the journey to Earth and back from a distant star system at a necessarily higher speed. They would go home to a planet much older than the one they left – perhaps by a century or more. They would be time exiles.
A photograph from the Apollo 17 mission in December 1972.NASA
Unimaginably high energy requirements
Then there’s the unimaginably high energy requirement for interstellar travel.
The mass of the spaceship increases with velocity, so an increasing amount of energy is required to accelerate it.
At the speed of light, the ship becomes infinitely massive, requiring an infinite amount of energy. This is clearly impossible.
Another significant issue is that space is a vacuum – but not completely. There are just enough particles to worry about. They can potentially cause fatal radiation for passengers and the instruments of a high velocity spacecraft, or destroy it. Sparsely spread hydrogen atoms turn into intense radiation at near light speed, and the heat that is generated would ablate and eventually destroy the hull.
Faster-than-light travel, according to physicist Miguel Alcubierre, is possible, but it comes with its own set of issues and a currently impossible energy requirement.
That raises the question of why spend all this energy to travel to Earth? Anything we have, an advanced civilisation (as they would have to be to get here) would be able to make on their planet.
A unique biosphere
Yet another issue is our biosphere, unique to Earth as far as scientists know.
Life and the planet co-evolved. Complex life would not exist on Earth if cyanobacteria, a type of single-celled microbe, had not pumped oxygen into our mostly nitrogen atmosphere 2.4 billion years ago.
It’s therefore not toxic for us, but oxygen is reactive and could be highly corrosive for aliens. And while they could wear protective suits like humans do when going to inhospitable environments, reports of visiting aliens do not include any descriptions of spacesuits.
So, are aliens out there?
If aliens are not here, are they out there?
It’s an interesting question, scientifically and philosophically. Scientists do not have enough information yet, but they are working on the question.
About 6,200 exoplanets have been found in more than 4,700 solar systems, though none are like Earth or our Solar System.
Most stars could have at least one planet, and there are more than 100 billion stars in our galaxy alone. The number of planets is therefore astronomical, and some may be habitable.
Closer to home, there are worlds with potential for microbial life either past or present – Mars, Europa (a moon of Jupiter), and Enceladus and Titan (moons of Saturn). If we discover life began twice in our Solar System, that will increase the odds of life elsewhere.
Since 1960, we’ve had the capability to look for intelligence elsewhere, piggybacking on normal radio astronomy. The biggest search for alien life projects are carried out by the SETI Institute in California and the Breakthrough Listen project based at Oxford University in the United Kingdom.
Nothing has been found across all the searches made. Finding intelligence in our time frame – about a hundred years – in the 13.8-billion-year history of the universe is challenging.
However, as a 1959 Nature paper noted, while it’s difficult to estimate the chance of success, if we don’t search, the chance drops to zero.
Gaia has been the most successful ESA space mission ever, so why did they turn Gaia off? What did Gaia achieve? And perhaps most importantly, why was it my favourite space telescope?
Running on empty
Gaia was retired for a simple reason: after more than 11 years in space, it ran out of the cold gas propellant it needed to keep scanning the sky.
The telescope did its last observation on 15 January 2025. The ESA team then performed testing for a few weeks, before telling Gaia to leave its home at a point in space called L2 and start orbiting the Sun away from Earth.
L2 is one of five “Lagrangian points” around Earth and the Sun where gravitational conditions make for a nice, stable orbit. L2 is located 1.5 million kilometres from Earth on the “dark side”, opposite the Sun.
L2 is a highly prized location because it’s a stable spot to orbit, it’s close enough to Earth for easy communication, and spacecraft can use the Sun behind them for solar power while looking away from the Sun out into space.
It’s also too far away from Earth to send anyone on a repair mission, so once your spacecraft gets there it’s on its own.
Gaia used its thrusters for the last time to push itself away from L2, and is now drifting around the Sun in a “retirement orbit” where it won’t get in anybody’s way.
To do this, it measured the precise positions and motions of 1.46 billion objects in space. Gaia also measured brightnesses and variability and those data were used to provide temperatures, gravitational parameters, stellar types and more for millions of stars. One of the key pieces of information Gaia provided was the distance to millions of stars.
A cosmic measuring tape
I’m a radio astronomer, which means I use radio telescopes here on Earth to explore the Universe. Radio light is the longest wavelength of light, invisible to human eyes, and I use it to investigate magnetic stars.
But even though I’m a radio astronomer and Gaia was an optical telescope, looking at the same wavelengths of light our eyes can see, I use Gaia data almost every single day.
I used it today to find out how far away, how bright, and how fast a star was. Before Gaia, I would probably never have known how far away that star was.
This is essential for figuring out how bright the stars I study really are, which helps me understand the physics of what’s happening in and around them.
A huge success
Gaia has contributed to thousands of articles in astronomy journals. Papers released by the Gaia collaboration have been cited well over 20,000 times in total.
Gaia has produced too many science results to share here. To take just one example, Gaia improved our understanding of the structure of our own galaxy by showing that it has multiple spiral arms that are less sharply defined than we previously thought.
Not really the end for Gaia
It’s difficult to express how revolutionary Gaia has been for astronomy, but we can let the numbers speak for themselves. Around five astronomy journal articles are published every day that use Gaia data, making Gaia the most successful ESA mission ever. And that won’t come to a complete stop when Gaia retires.
The Gaia collaboration has published three data releases so far. This is where the collaboration performs the processing and checks on the data, adds some important analysis and releases all of that in one big hit.
And luckily, there are two more big data releases with even more information to come. The fourth data release is expected in mid to late 2026. The fifth and final data release, containing all of the Gaia data from the whole mission, will come out sometime in the 2030s.
This article is my own small tribute to a telescope that changed astronomy as we know it. So I will end by saying a huge thank you to everyone who has ever worked on this amazing space mission, whether it was engineering and operations, turning the data into the amazing resource it is, or any of the other many jobs that make a mission successful. And thank you to those who continue to work on the data as we speak.
Finally, thank you to my favourite space telescope. Goodbye, Gaia, I’ll miss you.
In 2017, NASA discovered and later confirmed the first interstellar object to enter our Solar System.
It wasn’t aliens. But artist impressions of the object (called ‘Oumuamua, the Hawaiian word for “scout”) do resemble an alien spaceship out of a sci-fi novel. This strange depiction is because astronomers don’t quite know how to classify the interstellar visitor.
Its speed and path around the Sun don’t match a typical asteroid, but it also has no bright tail or nucleus (icy core) we normally associate with comets. However, 'Oumuamua has erratic motions that are consistent with gas escaping from its surface. This “dark comet” has had astronomers scratching their heads ever since.
Flash forward to today, and more of these mysterious objects have been discovered, with another ten announced just last week. While their nature and origins remain elusive, astronomers recently confirmed dark comets fall into two main categories: smaller objects that reside in our inner Solar System, and larger objects (100 metres or more) that remain beyond the orbit of Jupiter.
In fact, 3200 Phaethon – the parent body of the famous Geminid meteor shower – may be one of these objects.
How do dark comets differ from normal comets?
Comets, often described as the Solar System’s “dirty snowballs”, are icy bodies made of rock, dust and ices. These relics of the early Solar System are critical to unlocking key mysteries around our planet’s formation, the origins of Earth’s water, and even the ingredients for life.
Astronomers are able to study comets as they make their close approach to our Sun. Their brilliant tails form as sunlight vaporises their icy surfaces. But not all comets put on such a dazzling display.
The newly discovered dark comets challenge our typical understanding of these celestial wanderers.
Image of comet C/2023 A3 Tsuchinshan-ATLAS taken by astronauts aboard the International Space Station.NASA
Dark comets are more elusive than their bright siblings. They lack the glowing tails and instead resemble asteroids, appearing as a faint point of light against the vast darkness of space.
However, their orbits set them apart. Like bright comets, dark comets follow elongated, elliptical paths that bring them close to the Sun before sweeping back out to the farthest reaches of the Solar System.
They go beyond Pluto, some even making it out to the Oort Cloud, a vast bubble of tiny objects at the fringe of our Solar System. Their speed and paths are what allow astronomers to determine their origins.
A comparison of dark comets and bright comets set against the Milky Way. On the left, a small, rocky, dark comet represents their typical size of one metre to a few hundred metres wide. On the right is a larger, icy, bright comet with a glowing tail, whose size ranges from 750 metres to 20 kilometres wide. The stark difference in size explains why dark comets lack the bright, visible tails of their larger, more iconic counterparts.Composition: Dr Kirsten Banks; Background image: R. Wesson/ESO; Dark comet: Nicole Smith/University of Michigan, made with Midjourney; Bright comet: Linda Davison
But what makes these comets so dark? There are three main reasons: size, spin and composition or age.
Dark comets are often small, just a few metres to a few hundred metres wide. This leaves less surface area for material to escape and form into the beautiful tails we see on typical comets. They often spin quite rapidly and disperse escaping gas and dust in all directions, making them less visible.
Lastly, their composition and age may result in weaker or no gas loss, as the materials that go into the tails of bright comets are depleted over time.
These hidden travellers may be just as important for astronomical studies, and they may even be related to their bright counterparts. Now, the challenge is to find more dark comets.
How can we find dark comets?
How do we even find these mysterious dark comets in the first place? As they get closer to the Sun, we don’t see spectacular tails of debris.
Instead, we rely on the light they reflect from our Sun.
Several astronomical images are combined to capture the faint, fast moving object ‘Oumuamua in the centre. The white streaks are stars.ESO/K. Meech et al.
These little guys might be stealthy for our eyes, but they are often no match for our large telescopes around the world. The discovery of ten new dark comets revealed last week was all thanks to one amazing instrument, the Dark Energy Camera (DECam) on a large telescope in Chile.
This camera can’t “see” dark energy directly, but it was designed to take massive photos of our universe – for us to see distant stars, galaxies and even hidden Solar System objects.
In their recent study, astronomers pieced together that some of these nightly images contained likely dark comets.
A) The Dark Energy Camera (DECam), mounted on the Victor M. Blanco four-metre telescope at the Cerro Tololo Inter-American Observatory in the Chilean Andes (Credit: Dark Energy Survey). B) Two examples of newly discovered dark comets within the DECam data, from Seligman et al. (2024).
The good news is, we are starting to focus more attention on these objects and on how to find them.
In even better news, in 2025, we’ll have a brand new mega camera in Chile ready to find them. This will be the Vera C. Rubin Observatory, with the largest digital camera ever built.
It will allow us to take more images of our night sky more quickly, and see objects that are even fainter. It’s likely that in the next ten years we could double or even triple the amount of known dark comets, and start to understand their interesting origin stories.
There could be more 'Oumuamua-like objects out there, just waiting for us to find them.
Any night now, a “new star” or nova will appear in the night sky. While it won’t set the sky ablaze, it’s a special opportunity to see a rare event that’s usually difficult to predict in advance.
The star in question is T Coronae Borealis (T CrB, pronounced “T Cor Bor”). It lies in the constellation of the northern crown, prominent in the Northern Hemisphere but also visible in the northern sky from Australia and Aotearoa New Zealand over the next few months.
Most of the time T CrB, which is 3,000 light years away, is much too faint to be seen. But once every 80 years or so, it brightly erupts.
A brand new star suddenly seems to appear, although not for long. Just a few nights later it will have rapidly faded, disappearing back into the darkness.
A burst of life
During the prime of their lives, stars are powered by nuclear fusion reactions deep inside their cores. Most commonly, hydrogen is turned into helium creating enough energy to keep the star stable and shining for billions of years.
But T CrB is well past its prime and is now a stellar remnant known as a white dwarf. Its internal nuclear fire has been quenched, allowing gravity to dramatically compress the dead star.
T CrB also has a stellar companion – a red giant that has puffed up as it enters old age. The white dwarf mops up the swollen red giant’s gas, and this forms what’s known as an accretion disc around the dead star.
The matter keeps piling up on a star that’s already compressed to its limit, forcing a continual rise in pressure and temperature. Conditions become so extreme, they mimic what once would’ve been found inside the star’s core. Its surface ignites in a runaway thermonuclear reaction.
When this happens, the energy released makes T CrB shine 1,500 times brighter than usual. Here on Earth, it briefly appears in the night sky. With this dramatic reset, the star has then expelled the gas and the cycle can begin all over again.
Animation of a nova erupting as thermonuclear reactions ignite on the smaller white dwarf star. Credit: NASA/Conceptual Image Lab/Goddard Space Flight Center.
How do we know it’s due?
T CrB is the brightest of a rare class of recurrent novae that repeat within a hundred years – a time scale that allows astronomers to detect their recurrent nature.
Only ten recurrent novae are currently known, although more novae may be recurrent – just on much greater timescales that aren’t as easily tracked.
The earliest known date of T CrB erupting is from the year 1217, based on observations recorded in a medieval monastic chronicle. It’s remarkable that astronomers can now predict its eruptions so precisely as long as the nova follows its usual pattern.
The star’s two most recent eruptions – in 1866 and 1946 – showed the exact same features. About ten years prior to the eruption, T CrB’s brightness increased a little (known as a high state) followed by a short fading or dip about a year out from the explosion.
T CrB entered its high state in 2015 and the pre-eruption dip was spotted in March 2023, setting astronomers on alert. What causes these phenomena are just some of the current mysteries surrounding T CrB.
How can I see it?
Start stargazing now! It’s a good idea to get used to seeing Corona Borealis as it is now, so that you get the full impact of the “new” star.
Corona Borealis currently reaches its best observing position (known as a meridian transit) around 8:30pm to 9pm local time across Australia and Aotearoa. The farther north you are located, the higher the constellation will be in the sky.
The nova is expected to be a reasonable brightness (magnitude 2.5): about as bright as Imai (Delta Crucis), the fourth brightest star in the Southern Cross. So it will be easy to see even from a city location, if you know where to look.
We won’t have much time
We won’t have long once it goes off. The maximum brightness will only last a few hours; within a week T CrB will have faded and you’ll need binoculars to see it.
It almost certainly will be an amateur astronomer that alerts the professional community to the moment when T CrB outbursts.
These dedicated and knowledgeable people routinely monitor stars from their backyards on the chance of “what if” and therefore fill an important gap in night sky observations.
The American Association of Variable Star Observing (AAVSO) has a log of over 270,000 submitted observations on T CrB alone. Amateur astronomers are collaborating here and around the world to continually monitor T CrB for the first signs of eruption.
Hopefully the nova will erupt as expected sometime before October, because after that Corona Borealis leaves our evening sky in the Southern Hemisphere.
After 13 years in orbit around Saturn and 20 years in space, the Cassini Mission has come to an end. Launched on 15 October 1997 as a cooperative project between NASA, the European Space Agency (ESA) and the Italian Space Agency, the four year prime mission carried the Huygens probe to Saturn with its tour being extended twice. Amongst many discoveries two key ones were on Saturn's Moons, the global ocean and indication of hydrothermal activity within Enceladus, and liquid methane seas on Titan.
As Cassini's rocket fuel used for adjusting its course was almost exhausted rendering control of the course of the space craft impossible, the decision was taken by NASA to dispose of the orbiter in Saturn's atmosphere. This ensures that Cassini cannot collide nor contaminate Saturn's moons and affect any future studies of habitability and potential life on those moons. The orbiter will plunge into Saturn's atmosphere on September 15, 2017.
Mars composite image - (c) Jet Propulsion Laboratory
Mars, the planet which has conjured many a science fiction movie and not a few novels. Seemingly close, the nearest planetary neighbour to Earth is nonetheless very distant to this planet and travel over such a large distance poses both technical challenges and extreme health risks for humans attempting the voyage.
In terms of orbit around the Sun, Mars is always 48.3 million or more kilometres away from the Earth which equates to around 140 times further than the moon. In terms of transit time, this distance would mean around 210 days to travel from the Earth to Mars depending on the best available launch window and capability in velocity based on the existing propulsion systems or those in the design stages. Once at Mars, a mission team would need to wait around 496 or so days before being able to commence a return flight to Earth.
What sort of space vehicle would be able to both propel the pay load safely to Mars and be able to sustain a flight crew during transit to the planet ? It would need to have sufficient speed to cover the distance with a size and scale to include the necessary life support functions including air and water to sustain the crew. Existing fuel burning engines would not suffice due to the amount of fuel which would need to be carried and a solar powered vehicle would be slow moving indeed. Mounting a nuclear engine on a manned space vehicle carries its own multi-level high risks.
The technology to reach Mars is not insurmountable however the physical health ramifications for astronauts may yet pose an almost impassable barrier. Existing data from Moon missions, Skylab and the International Space Station have demonstrated the punishing effect of long term exposure to weightlessness on the human body - bones waste away at a rate of one percent of bone mass per month; fluid can collect behind eyeballs and cause blurred vision; radiation from solar flares and cosmic rays (which are high energy particles travelling at close to light speed) pose a direct threat to DNA and human brain cells. These are only the known factors as already identified with potentially many more.
So Mars travel remains a dream at the moment. NASA and private companies such as SpaceX have the intention to reach the red planet but for the now the only visitors will remain remote mobile robotic devices scanning the landscape for, as yet, undetected discoveries.
Strata at the base of Mount Sharp, Mars: (c) NASA Mars curiosity rover
Despite the realisation that there are literally billions of stars, exoplanets, planets and other celestial bodies in the Universe, the perplexing reality is that proof of life on other planets has been impossible to obtain despite decades of effort. Programs such as SETI and observational work through telescopes (Hubble, Spitzer, Kepler and in the future TESS and James Webb) has done little to establish the existence of life on another planet. Chopra and Lineweaver, in a controversial paper, have proposed a Gaian bottleneck theory to explain the low or non-existence of life while making the telling observation thatarchaeological excavations have not unearthed alien spaceships and optical and radio searches for extraterresttrial intelligence have not been successful.
The
Gaia hypothesis, (theory or principle), contends that organisms interact with their inorganic
surroundings on Earth to form a self-regulating complex system that plays a critical role in maintaining the conditions for life on the planet. Within this paradigm are key components such as the way in which the biosphere and the evolution of life forms affect the stability of global climate, ocean
salinity, oxygen in the atmosphere and other environmental variables that
affect the habitability of the planet.
According to Chopra and Lineweaver, a Gaian bottleneck exists whereby if life emerges on a planet, it only rarely evolves quickly enough to provide activities which regulate greenhouse gases and albedo, thereby maintaining surface temperatures compatible with liquid water and habitability. This bottleneck theory therefore suggests that first, extinction is the cosmic default for most life on the surfaces of wet rocky planets and second, rocky planets need to be inhabited to remain habitable. Almost a Catch 22 situation. The emergence of life's ability to modify its environment and regulate initially abiotic feedback mechanisms is termed 'Gaian regulation'. As far as Chopra and Linewear are concerned, without rapid evolution of Gaian regulation, early extinction would be the most common outcome for planetary life. As continuing efforts are made to search the universe to locate and identify life, the Gaian bottleneck model suggests 'that the vast majority of fossils in the Universe will be from extinct microbial life.'
Three scenarious are shown in Fig 1 above - A) Emergence bottleneck, life rarely emerges even on rocky wet planets; B) No bottleneck, life emerges with high probability and lasts for billions of years; C) Gaian bottleneck, life emerges but goes extinct within a billion years.
So, the chances of finding any form of life, under this model is close to zero and what evidence may be found will have long been dead.
Chopra and Lineweaver's paper can be accessed here:
Within some astronomy circles there is a view that searching for extraterrestrial life in the universe is a good and worthy goal, both from a scientific point of view and the resources needed to carry out such research. Hollywood films such as 'ET', 'Close Encounters of the Third Kind' and 'The Day the Earth Stood Still' paint a picture of higher intelligence being understanding and interaction with people on this planet being one of positivism or curiousity. But on what basis would an assumption along these lines bear any resemblance to reality? The recent release of a muddled mess of a film called 'Skyline' presents a different view of aliens. In this film they are portrayed as overtly hostile and their arrival on earth is aggressive with humans seen as resources to be harvested. In 'Skyline', the human race is not successful at beating off the invaders and succumbs to the more powerful alien force. It should not presumed that any life form outside of this planet would subscribe to the same values, beliefs or behaviour often recognisable to homo sapiens. The Universe in fact is a highly hostile and volatile environment and should any intelligence be found which has the capacity to reach this planet, it is unlikely to have benign behaviour or adherence to ethical and moral values.
The recent pronuncement that Gliese 581g, a planet orbiting the red dwarf star Gliese 581 may be suitable for life should not come as any surprise. Located approximately 20.5 light years away from Earth in the constellation of Libra, the planet is located in the middle of the "Goldilocks" zone, or what is defined as a habitable part of space near its parent star. The existence of liquid water is considered a strong possibility and this condition is generally considered a precursor for life. The planet was discovered by the Lick-Carnegie Exoplanet Survey following a period of over ten years of observations. Gliese 581g is believed to be the first Goldilocks planet ever found being the most earth-like planet with the potential for harboring life. The search for life in the universe continues both through optical astronomy and radio astronomy (the SETI program). The question remains, if life is found, what would the human race do?
