JS Communications Skill

Seán Hogan

Black holes are a common concept in Astrophysics, and are one of the celestial bodies (an object in space) that nearly all people on Earth are familiar with. A large planet that sucks in all the surroundings in an inescapable vortex. Well, that is where the science-fiction description of black holes overpowers the physically real one. In this blog post, I am going to describe black holes as they are, to the best of the scientific community’s knowledge. As well as tackle some common misconceptions about black holes. So hopefully by the end of this, you will be able to help your friends from getting sucked into an inescapable vortex of misinformation.


So… what IS a black hole?

A black hole is a celestial body where its escape velocity is greater than the speed of light. Which might seem like I have just used another bit of physics jargon to explain black holes, but I promise, this is pretty easy to understand.

Escape velocity is defined by the formula below, where G in Newton’s Gravitational Constant, a value which is the same for every planet, M is the mass of the planet and r the radius of the planet. Escape velocity is the minimum speed an object must constantly travel at to escape the influence of gravity of an object. You can see that the mass of the object trying to escape the gravitational pull of a planet or celestial body is not used in the formula, only properties of the planet itself.

Hee Hoo

The Formula for Escape Velocity

So in order for the escape velocity to greater than the speed of life, the ratio between the mass of a celestial body and its radius needs to me 6.74 x 1026kg for every metre of radius. Numbers at this scale can be hard to comprehend, so hopefully to give context; Earth has this ratio at 9.37 x 1017 and our Sun has this ratio at 2.86 x 1021. Don’t leave the fact that 21 is only 5 less than 26 make you think those two values are close, the Sun would need 100,000 times the amount of mass. To put it into better perspective, in order for Earth to be a black hole, it would need to be a sphere of radius 8mm.


Could a black hole appear right by the Earth and suck us all in and destroy life as we know it?

No. That will not happen. If humanity is wiped out from something from the stars, it will not be a random black hoe appearing. In order for something of such an incredible density to be created, random chance is not enough. Most black holes form with the death of stars. As a star dies, its own gravitational force has the star collapse in on itself, called a supernova. So a star’s death is needed to form a black hole, the circle of life. The only star of notable proximity to us is, well the Sun. Our Sun is not approaching the end of its lifetime and when it does, it will not have enough mass to turn into a black hole.

An Artist’s Depiction of a Supernova

Okay, but they do suck you in, right?

Well, yes and no? All celestial bodies have some mass, meaning they have some gravity, which means they “suck” you in (in physics we would generally say “attracts you”). This idea that black holes are dragging everything towards them to absorb and destroy them is plain incorrect. In order to get to the point where you get taken by the black hole, you would be at a distance that means you would be brought down to the surface of any planet of equivalent mass. To give perspective, at the centre of the Milky Way there exists a supermassive (meaning at least 100,000 times the mass of the sun) black hole that the rest of the galaxy orbits. Or if you prefer, if I was to replace every planet, every moon and the Sun in our solar system with black holes of similar masses, Earth’s trajectory would remain exactly the same. So be very careful when travelling near a black hole, as crossing the event horizon, the point where light can no longer escape might not even be noticed by you, looking out of a black hole looks completely normal. But then again if you decide to play with fire and go anywhere near a black hole, it is kind of your own fault.


Fair enough, what happens in a Black hole?

Well, that is where the knowledge of astrophysicists ends unfortunately. Black holes are incredibly complex objects. Since no light can escape after it passes the Schwarzschild radius (the point where even light cannot escape), no information about what is inside can be transferred back, it is physically impossible for us to know what happens in black hole. Some physicists use the term “hairless” to describe that very little information we can gleam from a black hole. The main way we can even figure out they are there are from their gravitational effect on other celestial bodies. However, black holes are theorised to slowly lose mass via a very complicated theory called Hawking Radiation. So given enough time, we should see what is left after a black hole, but for black holes of a size large enough they could be observed by humans, that timeframe is too long. I am talking about a number with over 60 zeros, for the smaller black holes. That is even larger than physicists believe the universe has existed for, by nearly by a factor of a number with 50 zeroes! Hopefully some time far, far, far in the future, some black hole will fully evaporate, and humanity (or what comes after us) can see the traces of those dark voids.


I saw that film Interstellar, doesn’t time go slower near a black hole?

That is a really, REALLY difficult question. But that’s good! Asking difficult questions are exactly what fields like astrophysics are all about. Regardless, I doubt I will be able to give you a satisfactory answer but it is to do with the general relativity model of time, or spacetime rather. Trying my absolute best here to explain what I know briefly and clearly, in most of our models regarding time, space and time are intrinsically linked. Objects will larger masses are able to “warp” space around them, like placing a heavy object on a trampoline (this is also how gravity is somewhat explained). Since space gets warped severely by this massive objects, so will time. That is brushing over a lot of  details so I highly recommend going out and doing your own bit of reading and research on the topic.


That was a lot, is there any fun quick facts I could tell friends at parties about Black holes?

Okay, since you were so good to read the entire way through my blog. I will tell you what people theorise happens if you fall in a black hole. It is called Spaghettification. Simply put, if you were to go in feet first, like a slide, you would be stretched out thin like a piece of spaghetti. Your feet would be experiencing a stronger gravity force than you head, so will be pulled faster and will stretch out your body. Cool but a little body-horror-esque if you try and work out the details of the process.

Spaghettification Pictured. It probably would feel amazing for a brief period

Thank you very much for reading, I hope you go further and do further research on space. There truly is no limit to the cosmos, and our Earth is only one tiny part of it.


References for Further Reading

[1] H. R. Smith, “What Is a Black Hole?” Accessible from https://www.nasa.gov/audience/forstudents/k-4/stories/nasa-knows/what-is-a-black-hole-k4.html Aug 2018

[2] L. Lerner, “Black holes, explained” Accessible from https://news.uchicago.edu/explainer/black-holes-explained#:~:text=Black%20holes%20are%20regions%20in,not%20even%20light%20can%20escape. Oct 2022


As the Earth’s population and surface temperature continue to rise exponentially, the questions on many scientists’ minds are: Where to next? Could humanity survive long-term living in deep space? Is there life on Mars? – credits to Bowie for that one

Pancosmorio Theory

With the Pancosmorio theory of human sustainability, we get the “all world limit” which attempts to tackle all these questions.  In order for humans to sustain life in space they need a self-restoring, Earth-like, natural ecosystem.  Life on Earth evolved because of the unique conditions on our planet, that are not replicated elsewhere in our Solar System.  To sustain life on some other planet, this planet needs to be like Earth in order to allow for humanity’s physical and social needs to be fulfilled.

The first factor to survival is gravity.  Humanity has evolved such that our bodies utilize energy most efficiently under Earth’s gravity.  Gravity induces a gradient in the fluid pressure within the body which sustains its functions and without this pressure human beings cannot survive.  There is no other place in our Solar System with the same gravity as Earth, this inevitable gravity imbalance would be detrimental to humanity – not ideal.

The next contributing factor is oxygen.  Earth generates the oxygen necessary for humanity’s survival.  Again, there is no other planet in our solar system that produces oxygen to the extent that Earth does. This is as a result of the myriad of different plant species that grow on Earth which would need to be replicated elsewhere for human survival.  If this system at any point failed to provide oxygen to the human settlement it would mean instant doom.

Another factor is energy.  The energy required to sustain this type of settlement would be enormous.  The further a planet is from the sun the less solar energy the planet receives.  Mars receives less than half the amount of sunlight than we receive here on Earth.  It would be similar to running an electric car on a phone battery.

Well, what about the gravity and oxygen levels on Mars, I hear you asking?  Gravity on Mars is approximately 38% of the surface gravity on Earth.  The oxygen level on Mars is only 0.13% compared to the 21% we experience in Earth’s atmosphere.

Sounds pretty impossible, right?  Does it turn out that Bowie knew nothing about space travel?

Fig 2: Depiction of Life on the Red Planet. Reference: NASA

Life on Mars

Well Elon Musk is on Bowie’s side.  The SpaceX founder maintains that the future of humanity is at stake, and his great plan to save us all from impending doom is to pack us into plus-sized rockets, sardine style, and shoot us all to Mars, as he claims, in a Battlestar Galactica type effort.  His goal is to have transported one million people to Mars by 2050.  He plans to build 100 starships every year over a 10-year period, with each starship leaving for Mars in the key 30-day window that opens every 26 months.  This interval is to take advantage of the period when Earth and Mars are closest in their solar orbits.

Just in case the prospect of being one of the first rocketed to Mars by Musk is sounding appealing to you, here are the simple requirements.  The fee for any brave pioneer is $100,000, the conditions are dangerous and cramped, furthermore Musk remarks, “you might not make it back” – still sound so appealing?

If it does, time to get saving, because according to Musk’s plan the first starship is set to take off in 2028, landing on Mars 6 months later in 2029.  However, there is a slight problem with this goal, his fully integrated starship is yet to reach space.  The main component of the plan, the starship, doesn’t fully exist yet.

Now don’t despair, life on Mars is still possible, Bowie’s dream can still be fulfilled.  NASA is planning on slowly, cautiously sending humans to Mars.  Budding explorers and scientists are due to take their first tentative steps onto the Red Planet in the late 2030s or early 2040s.  NASA is also preparing the adventurers for living on Mars.  In June of this year, four volunteers will participate in a year-long mission living in a habitat that will simulate life on Mars.  During the mission, the crew members will carry out all the tasks that will be necessary for humanity to survive on Mars.  So, I’m not claiming life on Mars is impossible, just that Musk’s timeline needs some work!

If you’re still interested in being one of humanity’s first to set foot on our next-door cosmic neighbour, perhaps going with NASA is your best bet!  Just one request before I sign off, say hi to Matt Damon when you get there.


[1] Irons, Lee G, Irons, Morgan A, (2023), “Pancosmorio (world limit) theory of the sustainability of human migration and settlement in space”, https://doi.org/10.3389/fspas.2023.1081340

[2] NASA, “A step towards Mars”, https://www.nasa.gov/chapea/about

[3] SpaceX, “Mars and Beyond”, https://www.spacex.com/human-spaceflight/mars/

The Big Crunch

The Big Crunch is one suggested theory for how the Universe may end. The Friedmann equations, which physicist Alexander Friedmann derived in 1922, are what gave rise to the concept of the big crunch. Under the presumption that the Universe is homogeneous and isotropic, these equations relate to the Universe’s expansion or contraction. The force of gravitational attraction and the outward momentum created of the big bang were thought to be the two principal forces that determined whether the Universe would expand or contract. The Universe would then  rather simply constrict if gravity were to overpower expansion. This happens when the Universe’s actual density exceeds the critical density determined by the Friedmann equations. For comparison, our cosmos has a critical density of roughly 5.9 protons per cubic meter and an actual density of roughly 1 proton every 4 cubic meters. Exceeding the critical density would result in the collapse of the cosmos or the creation of a super-dense black hole.

The Big Bounce

Then, you might be curious as to what occurs after collapse. Many scientists believe it would just “bounce” back, also known as ‘the Big Bounce’. When the Universe collapses it will become the size of a Planck’s length, or roughly 10-35 m. The Universe’s Volume is very close to 0 at this scale, Therefore as the Universe still has Mass it’s density is almost infinite. In that situation, the processes of the Big Bang such as inflation would all repeat again recreating the Universe. This is currently theorised to be due to Loop Quantum gravity. The Universe could repeat itself endlessly as a result of this process, restarting time and time again. Possibly, our own Universe is not the first in this cycle simply just another repeat in a long line of repeats. It is also speculated that every time the Universe begins again, it will be the same Universe i.e., the timeline we exist in today will be the same timeline in the new Universe and all the events of our timeline will once again occur in the new Universe.

Dark Energy

The Big Crunch as a thoery was acceptable up until two international teams of scientists discovered dark energy in 1998. The real driving force behind expansion. In 2004, more in depth measurements of dark energy were possible thanks to the Chandra X-ray observatory. They discovered that although dark energy has a fixed value, the amount of dark energy is increasing as the Universe gets bigger. This measurement suggests that the Universe is always expanding. There is little room for a Big Crunch in an expanding cosmos. However, we still don’t fully understand dark energy. Fortunately for us, the Nancy Grace Roman space telescope is set to launch in May 2027. The Nancy Grace Roman telescope’s objective is to learn more about the origins of dark energy and its function in the early cosmos. While the likelihood of the Big Crunch is currently minimal, there is still a possibility that with greater knowledge of the universe, collapse could still be possible. This is something that the Nancy Grace Roman will hopefully clarify.



Dr. Edward  J. Wollack(2015) What is the Ultimate fate of the Universe? -Nasa https://map.gsfc.nasa.gov/universe/uni_fate.html

William Harris(2021) How the Big Crunch Theory Works – How stuff works https://science.howstuffworks.com/dictionary/astronomy-terms/big-crunch.htm

Ashley Balzer(2020) Nasa’s WFIRST will help uncover the Universe’s fate -Nasa https://www.nasa.gov/feature/goddard/2019/nasa-s-wfirst-will-help-uncover-universe-s-fate

Anna Heiney(2004) Chandra Discovery Sheds Light on Dark Energy – Nasa https://www.nasa.gov/missions/science/f_dkenergy.html

Britt Grisworld(2014) What is the Universe Made Of?-Nasa


                                                               Cosmological Enigma-Dark Energy 



“It should be possible to explain the laws of physics to a barmaid.”

“When forced to summarize the general theories of relativity in one sentence: Time and space and gravitation have no separate existence from matter.”

-Albert Einstein

Read more

In 1990, we launched the Hubble telescope into orbit as the first sophisticated orbital observatory. This was an incredible achievement, but also allowed us to study things never before possible. The high resolution spectrograph allows us to observe and record ultra violet waves that could never make it through the earth’s atmosphere. This is hughely impactful to our observations and allows us to see the universe clearer than ever before.

Upon its launch, the telescope was malfunctioning and ineffective at making precise recordings, but through multiple missions and spacewalks, the telescope was fully functional and meeting its full potential. With fully functioning parts, we were able to make some remarkable discoveries. Through the observation of nearby cepheid variable stars, we were finally able to make an accurate calculation of the Hubble constant. While this had been estimated previously, we now had a reasonable calculation of the universe’s rate of expansion. Not only did we find values of important constants, we were able to get a clearer picture of the universe’s history as a whole. In the Hubble Deep Field, a photo including over 1,500 galaxies, we saw some of the “story” of the universe.

Hubble Ultra Deep Field | ESA/Hubble

Hubble Deep Field

This telescope was a huge success. It far outlived its expected lifespan and brought numerous incredible discoveries to mankind. So yes, the Hubble telescope was potentially the most important advancement in the study of the universe to date. Now the James Webb has taken over the mission and we can fully appreciate the impact the Hubble space telescope had on our understanding of the universe today.

Mercury, Venus, Mars, Jupiter and Saturn are visible in the night sky. They were known by the ancient Babylonians and their current names are derived from Roman gods. Uranus was first sighted in 1690 and recognised as a planet almost a century later, the first discovered since ancient times. Neptune was predicted mathematically, based on its gravitational effects, leading to its observation in the 1840s, along with a whole host of other solar system objects.


It wasn’t until 1930 that Pluto was discovered. Using a blink comparator to scan the night sky for small changes in position, it took 23-year old Clyde Tombaugh ten months to discover it, and made headlines around the world. Initial proposals were to name it after Percival Lowell, the universities founder, or his wife. It was an 11-year old English schoolgirl, Venetia Burnley, who proposed Pluto. She had been learning about the Romans and Greeks, and a classical name was deemed appropriate. Like the king of the underworld, Pluto sits alone in a cold, dark and distant realm. 


Pluto was thought to be the mysterious Planet X, responsible for perturbations in Neptunes orbit, and several times the mass of the Earth. However, this was clearly not the case, as 1950s observations by Gerard Kuipier showed that it had a much smaller radius than the Earth. Later it was found that Pluto was highly reflective, and if it was as big as previously thought, should be incredibly bright. Finally, the discovery of Pluto’s moon Charon in the ‘70s determined Pluto’s size once and for all, at a radius of 1200 km, and 0.2% the mass of the Earth; nowhere near as large as originally proposed. 


In the 1990s, lots of objects similar in size and location to Pluto became known. Eventually, its classification as a planet began to become controversial. The discovery of Eris, with a greater mass than Pluto, led to a desire to formally outline the definition of a planet. In 2006, the International Astronomical Union defined a planet as follows:


  1. It orbits the Sun.
  2. It has formed a spherical shape under its own gravity
  3. It has cleared its neighbourhood of bodies of comparable size, due to its own gravitational dominance. 


Unfortunately, Pluto fails to meet the third criterion, making up only a fraction of the total mass of all the objects in its orbit. Thus, it was stripped of its status as a planet. A new designation, “Dwarf Planet” was created, and Pluto, along with Eris and several other large non-planets. Between its confirmation as a planet, and the point at which it ceased to be one, Pluto had only only completed a fraction of its 250-year orbit around the sun. 


In 2015, the New Horizons spacecraft came within 12,500 km of Pluto, sending back stunning images of its surface. 

One of the biggest threats to the world’s telecommunications infrastructure is large emissions of radiation and magnetic energy from solar flares and coronal mass ejections (CME), also known as solar storms. As human civilization has become more and more dependent on the internet and technological infrastructure, the rare occurrence of severe space weather events has posed a much larger threat to industry and human civilization as a whole than ever before. Researcher Abdu Jyothi of the University of California, in her research paper, termed the impact of a solar superstorm event as the ‘Internet Apocalypse,’ where she examines the worst-case scenario of global internet outages from damaged electronic systems caused by rare solar superstorms.

The unique behaviour of the sun’s magnetic field gives rise to the ejection of radiation, particles, and matter from the surface of the sun, called space weather. The sun is made up of plasma, which is an extremely hot gas of ionised particles. The magnetic field of the sun is created in a system which is called the solar dynamo [2], where the motion of the electrically charged plasma in a magnetic field induces a current, which in turn generates more magnetic field [10]. Astrophysicists have deduced the shape of the magnetic fields at the surface of the sun by examining the motion of the plasma in corona loops in the sun’s atmosphere.

Image 1: Corona loops of plasma at the surface of the sun

Sunspots are regions of relatively lower temperatures on the surface of the sun where very strong magnetic fields prevent heat from within the sun from reaching the surface. In these regions, strong magnetic fields become entangled and reorganized. This causes a sudden explosion of energy in the form of a solar flare often accompanied by a coronal mass ejection, which is the ejection of electrically charged solar matter from the sunspot [3].

Some of the electromagnetic energy released by the flares, in the form of x-rays, and ejected particles can reach the earth. However, the earth has its own protective mechanisms against the regular occurrence of mild solar flares and CMEs. The upper layers of the earth’s atmosphere absorb the influx of x-rays. The earth is also surrounded by its own magnetic field, called the magnetosphere, which acts as a protective shield against the ejected solar matter from a CME that reaches the earth. Therefore, telecommunication infrastructure on the surface of the earth avoids the harmful effects. However, telecommunications satellites and GPS satellites further away from the earth’s surface are left more exposed and have been damaged or rendered inoperable due to solar flares [4]. Human health can also be compromised by direct exposure to harmful radiation and high energy particles emitted due to solar activity. On the surface of the earth, we are shielded from the harmful effects of space weather, however, astronauts in space must use special protective gear due to the extra exposure.

One of the positive side effects of space weather interacting with the earth is the spectacular display of the aurora borealis, more commonly known as the northern or southern lights, where charged particles become trapped in the earth’s magnetosphere and accelerate towards the earth’s poles. They collide with atoms and molecules in the earth’s atmosphere, releasing a burst of light and a colourful display in the night sky [5].

Image 2: The deflection of coronal mass ejections by the earth’s magnetic field

Image 3: The Aurora Borealis

More worryingly, there is the unlikely chance of a large-scale coronal mass ejection striking the earth in its direct path causing widespread damage to electrical infrastructure even on the surface of the earth. Such an event has been named a ‘solar superstorm.’ The enormous ejection of electrically charged solar matter causes shock waves in the magnetosphere and releases its energy toward the earth in a geomagnetic storm. As the earth’s magnetic field varies, electric currents are induced on the earth’s conducting surfaces by electromagnetic induction. These are called geomagnetically induced currents (GIC) [1]. This, in turn, induces electrical currents in the power grid and other grounded conductors, potentially destroying the electrical transformers and repeaters which keep the power grid running and damaging the vast network of long-distance cables which provide internet.

There has also been growing concern about the weakening of the earth’s magnetic field over the past few centuries, with some physicists believing it is because of the long overdue flip of the earth’s magnetic poles, something which occurs around every 200,000 years, but has not happened in over 750,000 years. This could potentially leave humans and telecommunication infrastructure on earth more exposed to more moderate and frequent space weather events.

The last large-scale geomagnetic storm, called the Carrington Event, was recorded in September 1859. Its main impact was on the mode of telecommunication at the time, the telegraph network, with reports of telegraph wires catching fire, electrical shocks, and messages sending even when it was disconnected from power. The CME was so strong that auroras could be seen from as far south as the Caribbean! In March 1989, magnetic disturbances caused by a strong solar storm wiped out the entire electrical grid in the Canadian province of Quebec [7]. Of course, since 1859, modern civilisation has become very dependent on electrical infrastructure to provide homes and businesses with power and internet for our constant connectivity demands, so a storm on the scale of the Carrington Event could have catastrophic implications for the world’s economy and society in general. A study by the National Academy of Sciences estimated that the damage caused by a Carrington-like event today could cost over $2 trillion and multiple years to repair [8]. By analysing the records of solar storms over the past 50 years, Peter Riley of Predictive Sciences inc. calculated that the probability of such an event happening in the next 10 years is 12%.

So what can be done to minimise the damage caused by a large-scale geomagnetic storm? As the sun is just coming out of a period of inactivity in its solar cycle, we have not experienced a significant solar storm to test the resilience of modern technological infrastructure against such events. However, nowadays we have a series of satellites which monitor solar activity, such as NASA’s Advanced Composition Explorer [9], which gives forewarning of a large incoming solar storm that would take at least 13 hours to reach earth. This gives power grid operators enough time to shut down their stations and minimise damage caused as it passes. Even with this precaution, an unprecedented Carrington-like event will likely cause widespread damage to the earth’s telecommunications and internet infrastructure, so better damage prevention and recovery plans will need to be put in place to ensure the maintenance of vital technological systems that the world’s population depends so heavily on.



[1] Sangeetha Abdu Jyothi. 2021. Solar Superstorms: Planning for an Internet Apocalypse. In ACM SIGCOMM 2021 Conference (SIGCOMM ’21), August 23–27, 2021, Virtual Event, USA. ACM, New York, NY, USA, 13 pages. https: //doi.org/10.1145/3452296.3472916

[2] NASA. 2022. Understanding the Magnetic Sun. [online] Available at: <https://www.nasa.gov/feature/goddard/2016/understanding-the-magnetic-sun>.

[3] Spaceplace.nasa.gov. 2022. Sunspots and Solar Flares | NASA Space Place – NASA Science for Kids. [online] Available at: <https://spaceplace.nasa.gov/solar-activity/en/>.

[4] Encyclopedia Britannica. 2022. How solar flares can affect the satellites and activity on the surface of the Earth. [online] Available at: <https://www.britannica.com/video/183276/overview-solar-flares>.

[5] NASA. 2022. Aurora: Illuminating the Sun-Earth Connection. [online] Available at: <https://www.nasa.gov/aurora>.

[6] Raeng.org.uk. 2022. [online] Available at: <https://www.raeng.org.uk/publications/reports/space-weather-full-report>.

[7] NASA. 2022. The Day the Sun Brought Darkness. [online] Available at: <https://www.nasa.gov/topics/earth/features/sun_darkness.html>.

[8] Science.nasa.gov. 2022. Near Miss: The Solar Superstorm of July 2012 | Science Mission Directorate. [online] Available at: <https://science.nasa.gov/science-news/science-at-nasa/2014/23jul_superstorm/>.

[9] O’Callaghan, J., 2022. New Studies Warn of Cataclysmic Solar Superstorms. [online] Scientific American. Available at: <https://www.scientificamerican.com/article/new-studies-warn-of-cataclysmic-solar-superstorms/>.

[10] Paul Bushby, Joanne MasonUnderstanding the Solar Dynamo. Astronomy & Geophysics, Volume 45, Issue 4, August 2004, Pages 4.7–4.13. https://doi.org/10.1046/j.1468-4004.2003.45407.x

[11] Youtube.com. 2020. Could Solar Storms Destroy Civilisation? Solar Flares & Coronal Mass Ejection [online] Available at: <https://www.youtube.com/watch?v=oHHSSJDJ4oo>.

Image Sources

Image 1: Vatican Observatory. 2022. Coronal Loops on the Sun – Vatican Observatory. [online] Available at: <https://www.vaticanobservatory.org/sacred-space-astronomy/coronal-loops-on-the-sun/>.

Image 2 & 3: GoOpti low-cost transfers. 2022. Aurora Borealis: where to see the Northern Lights in 2021?. [online] Available at: <https://www.goopti.com/en/about/goopti_blog/aurora-borealis-where-to-see-the-northern-lights-in-2020>.

Featured image: Cassini’s narrow angle camera captures three images of Epimetheus (smaller moon) passing between the spacecraft and Janus, on the 14 February 2010 [4].

There are two of Saturn’s many moons, named Janus and Epimetheus, that have an interesting relationship with each other. They are both small rocky worlds, only 196km and 135km across at their widest [1,2]. They both have nearly circular orbits at a distance of about 151,000km from the centre of Saturn. If you had observed where these moons were in the Saturn system last year in 2021, you would have found that Epimetheus was about 50km closer to Saturn than Janus, its slightly smaller orbit being inside that of Janus’. But if you were to look again next year, 2023, this would no longer be the case. Epimetheus will be further from Saturn than Janus and its orbit will be completely outside Janus’. This is because in 2022, Janus and Epimetheus will do something that they only do once every four years – swap orbits [5].

This interaction took place multiple times while NASA’s Cassini spacecraft was investigating the Saturn system before the mission ended in 2017. It first imaged the moons in 2005 two months before the moons switched places [3], and the featured image of this article is a close approach of the two moons that the spacecraft captured in 2010.

How does this switching of places between these moons work exactly? To explain it, it’s useful to think about some of the basic physics of orbits.

Suppose we have a rocket in a perfectly circular orbit around a planet (figure 1). If we point the rocket in the direction it’s moving in, and fire the engine for a short amount of time, it will of course speed up. As well as that, the extra energy given to the rocket from firing its engine increases the size of the orbit. The orbit will be stretched from a circle into an ellipse. The point in the orbit directly opposite the rocket, 180 degrees or half an orbit away, will move away from the planet. This point in the orbit, that is now the furthest point from the planet, is called the apoapsis. The point where the rocket fired it’s engine is now the closest point in the orbit to the planet, and is called the periapsis.

At the periapsis the rocket is now moving faster than it was in the circular orbit, since it sped up by firing the engine. But as it moves along the orbit, getting further away from the planet, it will slow down. It will be travelling slowest at apoapsis. When it passes this point and starts to get closer to the planet again, its speed will increase once more. This is all a result of Kepler’s second law of planetary motion. Without going into detail, it basically says that the further an orbiting object is from the object that it is orbiting, the more slowly it moves.

Figure 1: (a) a rocket in a perfectly circular orbit around a planet, arrows indicating the direction it orbits in (b) The rocket fires it’s engine for a short enough time that it doesn’t move too far in its orbit. (c) After its finished firing the engine, the rocket is in an elliptical orbit, moving fastest at periapsis and slowest at apoapsis. Diagrams by yours truly.

Now what about these moons of Saturn that swap orbits? Suppose they start out with the situation in figure 2, with the orbit of one moon being slightly outside the orbit of the inner moon (Since they switch, either moon can be Janus or Epimetheus). Now another of Kepler’s laws, the third one, essentially says that the larger an orbit is, the more time it takes the object to go around once. But this is not only because the orbiting object has more distance to travel, it’s also because it moves more slowly. If you google the average orbital velocities of the planets, you’ll see that they decrease as the planets get further from the Sun, mercury moving the fastest and Neptune the slowest.

Figure 2: Janus and Epimetheus, either being the inner or outer moon, have roughly a 50km difference in their orbits. The inner moon in red, moves slightly faster than the outer moon in blue, meaning as time passes it gets further and further ahead of the outer moon. Distances/sizes not to scale.

So the outer moon in the slightly larger orbit moves a little more slowly than the inner moon in the smaller orbit. This inner moon gradually moves further and further ahead of the outer, slower one. The inner moon will take 4 years to “lap” the outer moon, i.e. to go all the way around and start catching up on the outer moon from behind, as seen in figure 3 when the moons are at position 1.

They never actually get closer than about 15,000 km from each other[2]. But what happens is that the two attract each other gravitationally. The inner moon, being behind the outer moon, is pulled forward in its orbit by gravity. The outer moon, being in front of the inner one, experiences the same force pulling it backward.

Figure 3: What WOULD happen if the gravitational force between the two moons were like a short burst like from a rocket, force F in diagram. The Moons start out on the black orbits. The inner moon is accelerated so that it ends up on the red orbit, with new apoapsis above it’s old orbit (red dot, position 2). The outer moon is decelerated so that its new periapsis is below its old orbit(blue dot position 2). Distances/sizes once again not to scale.

Just like the rocket in figure 1, the inner moon has a force pulling it forward, in the direction it is orbiting, so its orbit is stretched such that the point directly opposite it moves outward, the red dot in position 2, figure 3. Similarly, the outer moon is pulled back by the inner moon, so a force is pushing it in the opposite direction to its motion, so it slows down. What happens in that situation is the opposite of figure 1 – the point directly opposite the moon in its orbit will move inward. That point, the blue one at position 2 in figure 3, will now be the closest point in the outer moon’s orbit to Saturn, or periapsis.

If these gravitational forces were just short lived boosts to the moons’ speeds, as if they had rocket engines on them, their orbits might look like what’s seen in figure 3. But this is not exactly the case. Technically gravitational forces extend to infinity, so there is no sharp start or stop point for the moons’ interaction. During their close encounter, they are continuously attracting each other. The outer moon is continuously being slowed by the inner moon behind it. Its orbit is continuously being decreased in size so that in the end, it is on a nearly circularly orbit closer to Saturn than the used-to-be inner moon. Similarly, the inner moon’s orbit is continuously being increased by the forward pull of the outer moon so that it ends up on a nearly circular orbit further from Saturn than the used-to-be outer moon.

So now the inner moon is the outer moon, and the outer moon is the inner moon! The new inner moon, being on a smaller orbit, moves more quickly and gets ahead, as predicted by Kepler’s third law. Now the moons can return to figure 2 where the whole process can start over again, only with the moons switched.

One other thing to note: Janus has about four times the mass of Epimetheus [3]. This means when the switching of orbits occurs, the change in Janus’ orbit radius will be less than the change in Epimetheus’ orbit radius. The gravitational forces attracting the moons during their close approach are equal. The same force on Janus, the heavier moon, will mean less acceleration – heavier objects require more force to accelerate them at the same rate. Less acceleration ultimately means less of an increase or decrease in the size of Janus’ orbit after the encounter.

Janus and Epimetheus are the only known example of this particular kind of “co orbital relationship” in our solar system [1] and an example of some of the strange things that are allowed by the laws of orbital mechanics.


[1] “Epimetheus”, NASA Science, solar system exploration, last updated December 19 2019


[2] “Janus”, NASA Science, solar system exploration, last updated December 19 2019


[3] Emily Lakdawalla, “The Orbital Dance of Epimetheus and Janus”, Planetary Society, February 7 2006


[4] “Cruising Past Janus”, NASA Science, solar system exploration, last updated October 4 2018


[5] “Janus Moon and its Dance around Saturn: The Co-Orbitals”, The Planets.org


At this stage Albert Einstein is a household name across the globe, his name being synonymous with the word ‘genius’. His theories and thought experiments have had an immense impact on our understanding of physics, and he seemed able to imagine ideas that no one else possibly could. This post tells the story of how, in 1929, Einstein retracted one of his theories – calling it the “biggest blunder” of his life.

Einstein had included in his equations of gravity what he called ‘the cosmoligical constant’, a constant represented by capital lambda, which allowed him to describe a static universe. This model of the universe complied with what was the generally accepted theory at the time in 1917, that the universe was indeed stationary.

Then, in 1929, Edwin Hubble (whom the Hubble telescope is named for) presented convincing evidence that the universe is in fact expanding. This caused Einstein to abandon his cosmological constant (i.e. presuming its value to be zero), believing it to be a mistake.

But that wasn’t the end of the story. Years went by and physicists repeatedly inserted, removed and reinserted lambda into the equations describing the universe, unable to decide whether or not it was necessary. Finally, in 1997/8, two teams of theorists, one led by Saul Perlmutter, published papers outlining the need for Einstein’s cosmological constant.

Through their analysis of the most distant supernovae ever observed – one of which was SN1997ap – and their redshifts, they had reached the conclusion that the distant supernovae were roughly fifteen percent farther away than where the prior models placed them. This could only mean they were accelerating away from us. The only known thing that ‘naturally’ accounts for this acceleration was Einstein’s lambda, and so it was reinserted into Einstein’s equations one last time. Einstein’s equations now perfectly matched the observed state of the universe.

So while Einstein’s initial use for the cosmological constant was incorrect, it proved vital to forming an accurate picture of our world. The great theorist had once again foreseen a factor no one else could – this time a good 70 years before anyone, including himself, was able to prove it.

With space travel becoming a more regular event with us travelling further and
for longer, it’s not insane to wonder whether we would be able to survive on
another planet. There is a lot of factors to consider when determining whether
a planet is habitable so for now, we will just be focusing on the composition.
For a planet to be habitable, it must be terrestrial. Terrestrial planets all have
the same basic structure, a central metallic core with a surrounding silicate
mantle. To sustain life, a planet requires a rapidly rotating magnetic field to
protect it from flares from nearby stars. This magnetic field is believed to be
generated by electric currents in the conductive metallic core of the planet.
The electric currents are created by convection currents due to heat escaping
from the core. These convection currents occur due to the temperature
difference between the solid inner core, the molten core and the cooler outer
crust of the planet. The silicate mantel of the planet is an important aspect of
its habitability. It lies between the molten core and the outer crust of the
planet, and it acts as an insulator for the molten core. This is important
because if the core is cold, no magnetic field is generated, and the planet will
no longer be protected from flares. For example, the Earth’s magnetic field
protects the Earth from harmful particles, such as solar radiation, from the
There are four elements needed for life to exist: carbon, hydrogen, oxygen and
nitrogen. These elements are also the most common chemically reactive
elements in the universe. For example, these four elements together comprise
over 96% of the Earth’s collective biomass. Oxygen is the only element that’s in
abundance in the planets crust. This is because many of the elements, such as
hydrogen and nitrogen, along with their simplest and most common
compounds, such as carbon dioxide, carbon monoxide, methane, ammonia,
and water, are gaseous at warm temperatures. They were trapped under the
crust when planets were formed and then later formed the planets’
atmosphere when they are released through volcanoes and other plate
tectonics. Plate tectonics, caused by the pressure as the elements expand as
gases, are crucial to habitability as they cool the deep interior as hot plumes
well up to the surface and cold plates drop down to the core-mantle boundary
by cycling material between the surface and interior. This cooling drives
convection in the metallic outer core, bringing us back to the above, in which
convection produces the geomagnetic field that shields the atmosphere and
protects the surface from the solar wind. One of the factors in determining
whether a planet, may be habitable or not is the existence of water. Water is a
key part in the habitability of a planet as it is composed of two (hydrogen and
oxygen) of the four elements most vital for life.
The habitability of a planet also depends greatly on the mass of the planet. A
planet with low mass is not suitable for habitation because low mass planets
have smaller diameters and thus higher surface-to-volume ratios than their
larger cousins. Such bodies tend to lose the energy left over from their
formation quickly and end up geologically dead, lacking the volcanoes,
earthquakes and tectonic activity which supply the surface with the four vital
elements needed to sustain life. For a planet to be habitable, its mass should
be roughly within one order of magnitude of the Earth’s mass.
The habitability of a planet also depends on whether they have not accreted
the gaseous outer layers of hydrogen and helium found on gas giants. A gas
giant does not have a surface and their gravity is huge. This means that they
are not viable for hosting life. A gas giant is a giant planet composed mainly of
hydrogen and helium. Gas giants are sometimes known as failed stars because
they contain the same basic elements as a star.
As we can see the habitability of a planet depends greatly on its composition
and that there is a lot of factors that prohibit life but our knowledge of the
universe is expanding so maybe one day we will find a planet that ticks all the
boxes for habitability