The Rare Earth hypothesis

Ever since the pioneering work of Frank Drake (1930-) in 1960, astronomers have been looking for radio signals from extraterrestrial civilisations and have failed to find anything. This could be because Earth-like planets containing complex life forms (such as ourselves) are rare in the Universe and only a series of highly improbable events led to the evolution of intelligent life on Earth. In 2000 geologist and palaeontologist Peter Ward and astronomer Donald Brownlee published a book in which they explained the term ‘Rare Earth Hypothesis’ which they had coined to describe this viewpoint.

However, they were not the first people to arrive at this conclusion. These ideas had been circulating for decades before the publication of their book. For example, the astronomers John Barrow and Frank Tipler discussed them in detail in their 1987 book ‘The Anthropic Cosmological Principle’.

A little background – the Drake Equation

Back in 1961, Frank Drake invented an equation to estimate the number of intelligent civilizations within our galaxy with whom we could potentially communicate, to which he gave the symbol N.  His equation, known as the Drake equation, consists of seven numbers multiplied together:

N=  R* x FP x NE x FL x FI x FC x L

in which

  • R* is the average number of stars formed per year in our galaxy. Current estimates are that this has a value of around 10.
  • FP is the fraction of the stars within our galaxy which have a planetary system with one or more planets, expressed on a scale of 0 to 1. A value of 1 means that all stars have planets. 0 would mean that no stars have planets. Current estimates are that FP is very close to 1.
  • NE is for the average number of bodies, either planets or moons of planets, with the right conditions to support life. For this to happen liquid water must exist somewhere on the planet.  A reasonable value which many astronomers would agree with is 0.4, meaning that out of every 10 stars which have planets, 4 have bodies which could support life.
  • FL is the fraction of bodies with the right conditions to support life, on which life actually evolves, expressed on a scale of 0 to 1.  A value of 1 means that on all  planets with the right conditions life will evolve. There is no consensus among astronomers about the value of FL.
  • FI is the fraction of bodies having life, on which life has evolved into intelligent civilisations, again expressed on a scale of 0 to 1. Again, there is no consensus among astronomers about what this value is.
  • FC is the fraction of bodies with intelligent life which develop a technology that releases signs of their existence into space. For example, on Earth TV and radio signals escape into space and could be picked up by a nearby alien intelligence with a sensitive enough receiver tuned to the right frequency. No one knows what the value of FC is.
  • L is the average lifetime of a civilisation in years.  Again, there is no consensus on this point.

As the values of many of the terms in the Drake Equation are not known to any degree of accuracy, it cannot be used to provide a meaningful estimate of N.  However, it is still very useful to illustrate the factors involved

Could we be alone? Could FI could be very very low?

Perhaps the term with the biggest degree of uncertainty in the Drake equation is FI, the fraction of bodies having life on which life has evolved into intelligent civilisations. I will outline the arguments for believing that this number is very low indeed.

The Earth is roughly 4.6 billion years old. Simple single-celled lifeforms emerged 300 million years later, a relatively short time after the Earth had cooled enough for liquid water to exist. These simple cells, called prokaryotes, cannot form complex organisms where different types of cells perform different functions. However, individual prokaryotes  can group together in colonies, forming a kind of slime.

Colonies of prokaryotes

All complex life on Earth is based upon cells called eukaryotes. These cells have a nucleus (containing the genetic material of the cell), structures called mitochondria, which regulate the cell’s energy and other specialised units known as organelles.

The first eukaryotes didn’t emerge until 2 billion years after prokaryotes, indicating that it was a much bigger step in evolution than the emergence of the first simple cells. It could well be the case that the large number of sub-steps needed in the evolution from prokaryotes to eukaryotes means that even where simple lifeforms have emerged,  in the vast majority of cases there has been no further evolution beyond this stage.

Even after the emergence of these complex eukaryotes, it would take over one billion years before multi-cellular life forms such as the first plants and animals appeared. In these organisms cells are specialised, so different types of cells perform different functions within the organism. Given the large amount of time taken to move from eukaryotes to complex organisms, it might well be the case that even if something akin to eukaryotes emerge on a planet, which have the potential to eventually evolve into multi-celled organisms, evolution proceeds no further. The average time taken to evolve from complex cells to multi-celled organisms given favourable conditions might be 2, 5 or even 10 billions years.

The role of mass extinction events

It has taken 600 million years from the appearance of the first animals to the emergence of Homo sapiens in Africa, around 200,000 years ago. Over this vast amount of time the vast majority of species have disappeared and have been replaced by other species which are better suited to the changing environment. However, the disappearance of species and emergence of new ones doesn’t occur at an even rate. Every 50-100 million years there have been catastrophic events which have caused periodic mass extinctions when a large number of species failed to survive. The most dramatic of these was at the end of the Permian period, around 230 million years ago, when 95% of land and 70% of sea species became extinct.

Fossil of a Trilobite – one of many species to disappear in the mass extinction at the end of the Permian Period

Perhaps the best known mass extinction occurred 65 million years ago when a massive comet or asteroid 10 km in diameter hit the Earth at a speed of up 50,000 km per hour. Its high speed coupled with its huge mass meant that it smashed into the Earth in the area now known as the Yucatan peninsula with an energy 6 billion times greater than the atomic bomb dropped on Hiroshima at the end of the Second World War.

KT extinction asteroid

The impact melted much of the local crust and blasted molten material outwards. Any object near to the impact site would have been instantly vapourised. Such was the energy of the impact that some of the Earth’s crust was thrown upwards with so much velocity that it went out into space. Over the next few hours molten rock, dust and ash rained down on an area millions of square kilometres in area. This hot material would have ignited fires, destroying plant and animal life within a large area.

Kt ejecta map

The impact 65 million years ago occurred in what is now the Yucatan peninsula in Mexico.  The outer ring shows the area which become covered in debris.

A fine cloud of dust from the impact circled the entire Earth. This blocked out sunlight, causing the Earth’s temperature to fall by about 15 degrees Celsius. The sudden drop in temperatures and lack of sunlight getting through meant that plant growth stopped for several months.  As a result, many species of herbivorous animals which fed on those plants became extinct and the carnivores which fed on the herbivores became extinct too. Those species which did survive had their numbers drastically reduced.  Nearly 75% per cent of the species of animals alive before the impact became extinct, including every species of dinosaur. After the disappearance of the large reptiles, mammals became the dominant land animals.

Although we as a species are now probably sufficiently advanced as to be able to survive a mass extinction event, even if only in small numbers, this would not have been the case if one had occurred earlier in our development. If one had happened 150,000 years ago, when Homo sapiens were few in number and less equipped to withstand a major famine – those hunter gatherers would have had nothing to hunt and nothing to gather for many months – we would have become extinct long before we could develop civilisation.

Indeed,  if mass extinctions were to occur every million years, rather than every 50 million years, it is difficult to see how any intelligent species could ever evolve. It would be wiped out before it became fully established. The reason why mass extinctions occur infrequently is due to the unusual layout of the Solar System, and I’ll talk about this next.

Our special Solar System

The Sun is a single star in an uncrowded region of space, in the outer regions of the Milky Way galaxy. The nearest star to the Sun is over 4 light years (40 trillion km) away, more than ten thousand times further than the outermost planet Neptune. If the Sun were in a more crowded region of the galaxy, a passing star’s gravity could easily disrupt the Earth into a different orbit, which might be closer to Sun, making it too hot to support life, or further away, making it too cold to support life.

The giant planet Jupiter is more than 300 times the mass of the Earth. As comets enter the inner Solar System from its outer reaches, Jupiter’s gravity slings most of these fast-moving ice balls out of the Solar System before they can get close to Earth.  Without Jupiter, comets like the one which hit the Earth 65 million years ago would collide with our planet much more frequently.

Image from NASA

Observations of planets detected around other stars have shown that arrangements of planets similar to our Solar System, with small inner rocky planets surrounded by massive giants in the outer reaches, are relatively rare.

Another factor which may be essential to the emergence of complex lifeforms is the presence of a large moon close to the planet. The Moon is 25% of the Earth’s diameter and is only 400 000 km away, a very short distance in astronomical terms.

Relative sizes of the Earth and the Moon

The Moon  is much larger in comparison to its parent planet than any other moon in the Solar System. This means that the Moon’s gravity stabilises the Earth’s tilt so that it doesn’t vary too much from its current value. Without the Moon there would be massive swings in the tilt between 0 degrees, when there would be no seasons, and 50 degrees, where there would be extreme seasons where much of the planet would be in darkness or full daylight for months at a time.

The Earth’s magnetic field

The Earth’s magnetic field is generated by convection currents stirred up by rotation in its liquid outer core, which is made out of iron. In fact for any planet to have a strong magnetic field, part of its interior must consist of a liquid which conducts electricity and it must be rotating rapidly enough to generate convection currents.

Earth Interior

Without a magnetic field the planet would not have a protective ozone layer shielding its surface from deadly ultraviolet radiation. It would be very difficult for advanced lifeforms to exist on the surface.  It is not known how many planets around other stars satisfy the criteria to have a magnetic field. However, the Earth is the only one of the inner planets in our Solar System (Mercury, Venus, Earth and Mars) to have a strong magnetic field.

Planetary size

In order for life to evolve, the planet must be large enough to retain a significant  atmosphere, without which liquid water cannot exist. Mars is roughly one tenth the mass of the Earth and its gravity is so low that its atmospheric pressure is only 0.6% of that of Earth – too low for liquid water to exist.

Man has been lucky to survive

In addition to periodic mass extinctions which wipe out most species, all populations come under pressure due to sudden changes in climate, such as ice ages, major volcanic eruptions, loss of habitat, disease and competition from other species for food and shelter. The first Homo sapiens were confined to a single region of Africa and were few in number, and thus vulnerable to becoming extinct. 70,000 years ago a super-volcano erupted in Indonesia causing the Sun to be partially blocked out and resulting in a fall in global temperatures. Only an estimated 15,000 humans are thought to have survived (Edwards 2010).


If we take all the factors I’ve talked about into account, for single-cell organisms to evolve into intelligent lifeforms capable of building civilisations, all of the following must happen.

  • Single cell creatures must have evolved in into complex cells, something akin to eukaryotes.
  • To reduce the frequency of mass extinctions, the planetary system must have a large giant planet in its outer reaches.
  • To ensure the planet remains at a habitable temperature, the parent star must be a single star in a less crowded region of the galaxy.
  • To ensure the planet remains at a habitable temperature over a long period of time, the parent star’s energy output must not vary too much.  This constraint rules out advanced life around the most common type of star in the galaxy, red dwarfs .
  • To reduce the frequency of dramatic changes in climate leading to mass extinctions, the planet must have a large satellite.
  • The planet must rotate rapidly enough to generate a magnetic field, as without this complex life would not be possible on its surface.
  • The planet must be large enough to hold an atmosphere.
  • Multi cellular organisms must have evolved.
  • The planet must have a significant fraction of its surface as dry land. The assumption is that although aquatic creature such as dolphins are undoubtedly intelligent, only land based creatures could build civilisations.
  • A land-based intelligent species has emerged during a survival of the fittest.
  • The intelligent species has managed to survive major environmental changes such as ice ages and has built a civilisation.

Putting all these together, followers of the Rare Earth hypothesis believe that the probability of all the above happening on a planet on which simple life has evolved could be as low as a billion to one.

If it were this low, then the expected number of intelligent communicating civilisations in a galaxy such as ours would be very low.  If the disputed figures within the Drake equation were guessed to be on the low side, N could be as low as 0.000016.  This would mean that not only are we the only intelligent communicating civilisation in our own Milky Way galaxy, but also that there are none in any galaxies within tens of millions of light years of us.

If this is the case then the Earth would not be just be an ordinary planet orbiting an ordinary star in an ordinary galaxy.  It would be a very special place indeed!

I hope you have enjoyed this post. To find out more about the Science Geek’s blog, click here or at the Science Geek Home link at the top of this page.


Edwards, L. (2010) Human were once an endangered species, Available at: (Accessed: 7 April 2018).



American manned spaceflight in 2018?

As readers of a previous post will know, since the retirement of the Space Shuttle in July 2011, America has been unable to put any astronauts into orbit around the Earth. Instead, it has been reliant on the Russian Soyuz spacecraft to ferry astronauts to and from the International Space Station (ISS). This situation may finally change in 2018; in the final two months of the year there are two missions tentatively planned to take astronauts to the ISS on American spacecraft. Interestingly, as a result of a change in space policy by the Obama administration eight years ago, both these missions will be in spacecraft designed and built by private companies, rather than NASA.

The Boeing CT-100 Starliner Space Capsule – image from NASA. In late 2018 this spacecraft may take astronauts to and from the ISS.

In a major speech in 2010, US President Obama announced a major shift in the function of NASA in American human space flight.

By buying the services of space transportation — rather than the vehicles themselves — we can continue to ensure rigorous safety standards are met. But we will also accelerate the pace of innovations as companies — from young startups to established leaders — compete to design and build and launch new means of carrying people and materials out of our atmosphere. ….

Some have said, for instance, that this plan gives up our leadership in space by failing to produce plans within NASA to reach low Earth orbit, …. But we will actually reach space faster and more often under this new plan, in ways that will help us improve our technological capacity and lower our costs, which are both essential for the long-term sustainability of space flight.’

(White House press release 2010)

Image from Wikimedia Commons

So, rather than build its own spacecraft to replace the Space Shuttle, NASA awarded grants to private companies to support research and development into human space flight. The program had a number of phases. In the first phase five companies were awarded grants to partially fund the research and development of the key technologies and capabilities that could ultimately be used in human space transportation systems. In the next phases, NASA awarded further grants to four companies to develop spacecraft that could send to astronauts to the ISS after the Space Shuttle’s retirement.

Image from NASA

After another selection process, in 2014 NASA made the final decision that the winners of the contracts for up to six crewed flights to transport astronauts to and from the ISS were as follows.

  • Boeing – They were given a contract worth up to $4.2 billion, to transport astronauts on their CT-100 Starliner spacecraft.
  • Space X –  This is a company set up by Elon Musk, the co-founder of Paypal. They were given a contract worth up to $2.6 billion to transport astronauts on their Dragon V2 – pictured below.

For more details see the reference below (NASA 2014).



 The Dragon V2 Spacecraft – Image from NASA 

When the final decision was made it was hoped that the winning companies would be able to launch manned missions to the ISS by 2017. However, perhaps unsurprisingly, there have been numerous delays in the development of both spacecraft and the launch dates have slipped.

According to the current launch schedule ( ) , the target dates for unmanned test flight of both spacecraft are August 2018, although an exact date hasn’t been specified. If there are no further delays and these test flights do take place in August and are successful, then in November 2018 the Boeing CT 100 spacecraft will be the first American spacecraft to carry astronauts into orbit since the retirement of the Space Shuttle. This will be followed by Dragon v2 the following month.

Opportunities for space tourism

The contract terms are that both companies will charge NASA around $60 million for each seat on a flight to the ISS. This is slightly cheaper than the amount it pays to the Russian space agency for a seat aboard Soyuz. The real boost is that, rather than the money going to the Russian space agency, it will go to American companies, boosting American high technology industries and creating American jobs.

Once they have fulfilled their contractual commitments to NASA, both companies are free to sell to additional spare capacity to space tourists willing to spend around $60 million dollars for a flight into orbit. This would be a very different type of space tourism than that offered by Virgin Galactic where customers will pay $250,000 for a three hour flight of which only two minutes are above an altitude of 100 km, which is defined as the boundary of space.

.Virgin Galactic rocket motor

Artist impression of Virgin Galactic’s SpaceShipTwo accelerating into Space – Image from Virgin Galactic

In February 2017 Elon Musk made the bold announcement that two individuals, who I must assume are extremely wealthy, had approached him and put down a ‘substantial deposit’ for a private spaceflight around the Moon in the Dragon v2 capsule.  At the time this was widely reported in the media e.g.

Musk refused to say who the individuals were or how much they had paid, but I would expect that the total cost of the spaceflights will be over $100 million dollars each.  To achieve enough speed to escape from the Earth’s gravity and reach the Moon the mission would use SpaceX’s new booster, the Falcon Heavy rocket, which was first launched in February 2018.

The Falcon Heavy launcher – image from Wikimedia Commons. This launcher could be used to launch a Dragon V2 spacecraft around the Moon

The spaceflight would be likely to follow a path known as free-return trajectory. I’ll talk about free-return trajectories in a later post, but essentially the idea is that it uses the Moon’s gravity to slingshot the spacecraft back to Earth, thus minimising the amount of fuel needed.

A typical free-return trajectory – image from Wikimedia Commons.

The original announcement said the spaceflight would be in 2018. However, according  to reports earlier this month, like this one, Elon Musk has said that the mission will be delayed because SpaceX will be focussing its effort on developing a new launcher with twice the thrust of Falcon Heavy. This is currently called the ‘Big Falcon Rocket’, but  I expect it it will be given a different name as the project progresses.

Therefore, I think that although this spaceflight will take place, it is unlikely to happen before 2020. However when it does occur I am sure that many people will follow it with great excitement. It will be the first time that humans have ventured outside the low Earth orbit since the last Apollo moon-flight in 1972.

Footnote – the Orion spacecraft

Even though NASA is now commissioning private companies to transport astronauts into low Earth orbit, it has not abandoned developing its own manned spacecraft altogether. It is currently developing the Orion spacecraft and a new launcher called the Space Launch System. Around 2023-5 the spacecraft is expected to take its first crew into orbit around the Earth, and it will have the capability take a crew of up to four beyond low Earth Orbit, perhaps on a mission around the Moon or to a nearby asteroid.

I hope you have enjoyed this post. To find out more about the Science Geek’s blog, click here or at the Science Geek Home link at the top of this page.


NASA (2014) NASA chooses American companies to transport U.S. astronauts to International Space Station, Available at: (Accessed: 7 February 2018).

The White House (2010) Remarks by the president on space exploration in the 21st century, Available at: 6 February 2018).


The early days of the space race

In my previous post I talked about two significant successes for the Soviet Union in 1957: the first artificial satellite in orbit in October and the first living creature, a dog named Laika, in orbit in November. In December of that year the Americans had a humiliating failure when the Vanguard spacecraft exploded in a massive fireball on the launch pad.

Vanguard TV-3 a few seconds after launch

To boost American prestige and to show the American public that the US wasn’t falling further behind the Soviets, it was important that America get a satellite into orbit as soon as possible. They achieved this when Explorer 1 went into orbit on 1 February 1958.

Explorer 1- Image from NASA

Explorer 1 had a payload of numerous science instruments designed under the direction of James Van Allen (1914-2006), a space scientist at the University of Iowa. It made the discovery that the Earth is surrounded by a belt of electrically charged particles trapped in its magnetic field.  The radiation in these belts is so intense that the readings from the Geiger counter on the spacecraft went off the scale when it passed through them. The belts are invisible to telescopes on Earth which is why they had not been detected previously. Today, in honour of Van Allen, they are known as the Van Allen radiation belts.

The Van Allen radiation belts

Explorer 1, like the Soviet Sputnik 1 and 2 spacecraft, had no solar cells to generate its own electricity. Electrical power came from a non-rechargeable battery. Three and a half months after launch, the battery ran out of charge so the spacecraft couldn’t transmit or receive signals and its instruments stopped working. However, Explorer 1 continued to orbit the Earth as a ‘dead’ satellite for much longer than Sputnik 1 or 2, for reasons which can be seen in the diagram below.

Only Sputnik 1 shown, Sputnik-2 had a similar orbit

As shown above, Explorer 1 was placed in a much higher orbit than Sputnik 1 and 2. At its closest approach it was 358 km above the surface of the Earth and its furthest 2,550 km. The higher a spacecraft’s orbit, the longer it remains in space because the traces of atmosphere which slow it down by friction are much less. Therefore, even though it couldn’t transmit any signals back to Earth, Explorer 1 remained in orbit until March 1970. This was over 12 years after its initial launch, whereas the Sputniks survived in space for around 4 months.

After the success of Explorer 1, the Americans successfully launched 4 other spacecraft into orbit in 1958. However, there were also 18 launch failures, meaning that nearly 80% of American launches in 1958 failed to get into orbit.

Luna 1, 2 and 3

Despite the American successes in 1958, the next big advances in space exploration were all made in 1959 by the Soviet Union. In January 1959 the Soviets launched Luna 1.  This was the first ever spacecraft to reach escape velocity, a speed high enough to enable it to escape from the Earth’s gravity altogether. It flew 6,000 km above the Moon’s surface and during its journey provided direct measurement of the solar wind, a stream of electrically charged particles coming from the Sun. Luna 1 didn’t have a camera, so was unable to send back any pictures of the Moon, but its instruments made the discovery that, unlike the Earth, the Moon has no magnetic field. Interestingly, Luna 1 was actually intended to hit the Moon’s surface but it missed its target due to a navigational error (Zak 2016) so, after passing the Moon, it went into orbit around the Sun, where it remains to this day.

Since 1959 Luna has been orbiting the Sun in an orbit which lies mainly between the Earth and Mars

Luna 2 was launched in September 1959 and this time succeeded in crash landing onto the Moon, becoming the first ever spacecraft to land on another celestial body. This again was a massive propaganda coup for the Soviets.  It even boosted the reputation of the Communist system as a whole, according to some writers of the day: only a successful and thriving country could achieve such great scientific feats.  Although their system couldn’t deliver the same level of material wealth for its citizens as free market capitalism, in 1959 the Soviets were ahead of America in space technology. It would not be until 1964 that America would successfully crash land a spacecraft on the Moon (see notes).

Even more exciting, however, was Luna 3. In October of the same year it became the first ever spacecraft to take pictures of the far side of the Moon, which had never before been seen from Earth and had remained an enigma throughout history.

Luna 3’s images  caused immense excitement around the world. These and subsequent pictures showed that the far side looks very different from the near side. It has a battered, heavily cratered appearance with a relatively small portion of its surface covered by the smooth dark areas (known as seas, or the Latin word maria).

Near and far sides of the Moon

The near and far sides of the Moon (image from NASA)

The greatest leap of all in the early days of space exploration came 18 months later. On 21 April 1961, a Vostok spacecraft containing cosmonaut Yuri Gagarin (1934-1968) launched successfully, performed a single orbit of the Earth, and safely landed 108 minutes later. This was a massive propaganda triumph for the Soviet Union and Gagarin, an air force pilot, became instantly famous throughout the world.

Yuri Gagarin – image from Wikimedia Commons

The American reaction to Gagarin’s flight was swift. A month later President John F Kennedy made the following address to the United States Congress:

“I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the Moon and returning him safely to the Earth. No single space project in this period will be more impressive to mankind, or more important in the long-range exploration of space; and none will be so difficult or expensive to accomplish.”


President J F Kennedy giving his address on 25 May 1961 – Image from NASA

This was an incredibly ambitious goal, given that in May 1961 America had not yet even placed a man in orbit. Nevertheless, Kennedy was advised that, given sufficient investment by the richest country in the world, a manned landing could be achieved before 1970. There was a good chance that the Soviets simply would not have the money to develop the new spacecraft and technologies required for this incredible leap forward.

So, the American government funded the largest commitment every undertaken by a nation in peacetime. At its peak the programme employed nearly half a million people and its total cost in today’s money was around $180 billion.

All this effort came to successful fruition on July 20 1969, when Neil Armstrong and Buzz Aldrin became the first human beings to land on the Moon. When Armstrong stepped out of the spacecraft he said the immortal words:

That’s one small step for man, one giant leap for mankind.

The Americans had won the space race.

Apollo 11 Astronaut Buzz Aldrin on the Moon – image from NASA



Actually, this is not strictly true.  Ranger 4 crashed on the far side of the Moon in April 1962, thus becoming the first American spacecraft to land on another celestial body. However, due a computer malfunction, it returned no scientific data. The first American probe to land successfully at its planned landing site and send data was Ranger 7 in July 1964.


Zak, A (2016) USSR launches the first artificial planet.  Available at: (Accessed: 30 September 2017).

4 October 1957 – the start of the space age

Exactly sixty years ago today, on 4 October 1957, the Soviet Union launched the first artificial satellite, Sputnik 1, into orbit around the Earth. This is considered to be the beginning of the space age. Before this date there were no man made satellites in space but on every single day since then there have been artificial satellites around the Earth. Today there are over 1000 active satellites in orbit (Union of Concerned Scientists 2017) and many times that number of defunct ones.

Image from NASA

Sputnik 1 is shown above. It consisted of a shiny metal sphere, 58.5 cm in diameter, made out of an aluminium alloy. To the sphere were attached four radio aerials. Unlike later satellites, Sputnik 1 carried no scientific instruments and wasn’t fitted with a TV camera to take pictures. It had no solar cells to generate electricity and was powered by three non-rechargeable batteries. Its only piece of equipment was a radio transmitter, which transmitted regular pulses on two different frequencies: 20 MHz and 40 MHz. Anybody with a short wave receiver tuned to either of these frequencies could pick up Sputnik’s signal as it passed overhead. Despite its small size, it was also possible to view the satellite just after sunset or just before sunrise through binoculars. It appeared as a faint rapidly moving point of light.

American Reaction

The year 1957 was during the cold war between the Eastern bloc – the Soviet Union and its allies – and the West. Before the launch of Sputnik, most people in America took for granted their country’s technological superiority. America had been the first country to develop the atomic bomb, led the way in computing and electronics and, in the years following the end of World War II, had been ahead of the Soviets in missile development. So most people naturally assumed that America would be the first country to place a satellite into orbit.

Front page of the New York Times from 5 October 1957- the day after the launch of Sputnik 1.

The launch of Sputnik 1 caught many in America by surprise and led a lot of people in America to fear that Soviet technology had not only caught up with but also overtaken that of America. Many were fearful of the potential military implications of the launch. If the Soviets had built a rocket which was powerful enough to launch a satellite into orbit, then the same rocket could be be used to attack America with nuclear weapons. For this reason, the period of time immediately after the Sputnik launch is often known as the “Sputnik Crisis”.

The Sputnik Crisis led to a big increase in US government funding for high technology industries such as missile defence. In February 1958, 4 months after the launch, president Dwight Eisenhower authorised the formation of the Advanced Research Projects Agency, later renamed the Defense Advanced Research Projects Agency (DARPA), within the Department of Defense  to develop emerging technologies such as missile defence for the U.S. military.  Note from Mrs Geek, the language consultant: this paragraph uses two spellings of the same word: the Science Geek is British so talks about ‘defence’, but he’s writing about an American agency with ‘defense’ in its title.  

Dwight Eisenhower – Image from Wikimedia commons

As well as the formation of DARPA, the Sputnik Crisis also led directly to the passing of the National Aeronautics and Space Act and the creation of the National Aeronautics and Space Administration (NASA), a civilian-led organisation concerned with America’s non-military efforts in space exploration.

The Soviet programme after Sputnik

As mentioned above,  Sputnik 1 had no power source such as solar cells to generate electricity, so the batteries on board could not be recharged. After 22 days the spacecraft ran out of power and ceased transmitting, although it still remained in orbit and could be observed from Earth.

As shown in the diagram above,  Sputnik was placed in an elliptical (oval-shaped) orbit.  When it was closest to Earth – its perigee – it was 215 km away, and when it was at its its furthest – its apogee) it was 939 km away.

Although 215 km is high enough above the Earth’s surface to be classed as space, at this altitude there are still significant traces of the Earth’s atmosphere. Every time Sputnik 1 dipped into the lower part of its orbit these thin traces of atmosphere slowed down the spacecraft, which was moving at around 28,000 km/h, by friction.  This process, which is known as orbital-decay, caused Sputnik 1 to lose energy causing it to move slightly closer to Earth on each orbit. By 4 Jan 1958 Sputnik 1 had dropped down to an altitude of 150 km. At this altitude the traces of atmosphere were thick enough so that friction slowed the spacecraft down to a speed at which it could no longer remain in orbit. Sputnik 1  then fell back to Earth, the frictional heating causing it to burn up in the process.

On 3 November 1957, one month after the launch of Sputnik, the Soviets launched another spacecraft, Sputnik 2. Sputnik 2 was much larger than its predecessor and had instruments to measure electrically charged particles, x-rays and ultraviolet emissions from the Sun. It also carried a passenger – a female dog called Laika, who became the first living creature to go into orbit.

Mockup of Laika in the Sputnik-2 capsule- image from NASA

As you can see from the diagram above, it was a tight squeeze to get Laika into the capsule and for the duration of the spaceflight she was barely able to move. Sadly for Laika, it was a one way ticket. Sputnik 2 was placed into a similar elliptical orbit as Sputnik 1 and, like Sputnik 1, it was destined to burn up on its return back to Earth five months later. In 1957 the technology for a spacecraft to return safely back to Earth from orbit did not yet exist. In addition, Sputnik 2 could only carry enough food, water and oxygen for Laika to survive for 7 days.

After the Sputnik-2 mission the official Soviet account was that Laika had survived a week in space and had been humanely euthanised by poisoning her seventh and final daily ration of food. This was the story which appeared in nearly all books on the early history of spaceflight written before 1990. After the fall of the Soviet Union in 1991, however, different accounts of the Sputnik 2 mission emerged and it was suggested that she had died much earlier in the mission from lack of oxygen, or when the cabin had overheated. This was confirmed in 2002, when Dimitri Malashenkov of the Institute for Biological Problems in Moscow stated that although she had survived the initial launch, she had died within a few hours from a combination of overheating and panic (Whitehouse 2012).  For political reasons, the Soviets had rushed the launch of Sputnik 2  and did not have the ability in November 1957 to keep a small capsule at an even temperature.

Despite surviving for a few hours in orbit, Laika’s place in the history of space exploration is assured. The information from Sputnik-2 proved that a living organism could tolerate a substantial time in weightlessness and paved the way for later human spaceflights in the 1960s. Today there are monuments to Laika in various places in Russia.

Laika surrounded by other pioneers of early Soviet space exploration


The launch of Sputnik 2 caused widespread fear amongst many in the US government, who felt that the US was falling further behind the Soviets. This fear was compounded when the Americans attempted to launch their first satellite into orbit Vanguard Test Vehicle 3 (TV3) on 6 December 1957. The launch  was shown live on television all over the United States, but it proved to to be a PR disaster when, two seconds after lift off, the engines failed, the rocket fell back to the launch pad and the fuel tanks ruptured which caused the rocket to explode in a massive fireball.

Vanguard TV-3 a few seconds after launch – to view a  short video of this launch click here

Despite the failure of Vanguard, the Americans successfully launched Explorer 1 into orbit in February 1958, marking the start of the space race between the two great world superpowers.  At the beginning of this post, I mention that today is the 60th anniversary of the space age, so next year we will be remembering the 60th anniversary of the beginning of this competition between the Soviets and the Americans to achieve particular goals in space exploration. The impetus of the space race led the to the Soviets putting the first man in space in April 1961, and the Americans putting the first man on the Moon, in July 1969. I shall talk about the early days of the space race in my next post.

I hope you have enjoyed this post. To find out more about the Science Geek’s blog, click here or at the Science Geek Home link at the top of this page.


Union of Concerned Scientists (2017) UCS Satellite Database, Available at: (Accessed: 22 August 2017).

Whitehouse, D (2002) First dog in space died within hours, Available at: (Accessed: 20 August 2017).


12-13 August 2017 – the Perseids

Anyone who is disappointed that they will be missing the total eclipse can console themselves with another astronomical event – providing that they live in the northern hemisphere, that the weather is favourable (no clouds, please) and that they can get away from populated areas with too much light pollution. This astronomical event is the Perseids, a meteor shower which appears at the same time every year, for reasons I’ll explain below.

Meteors, also known as shooting stars, are bright streaks of light lasting a few seconds. These are caused by small lumps of rock or metal called meteoroids hitting the Earth’s atmosphere at a very high speed (in the case of the Perseids around 200,000 km/h). As they pass through the atmosphere they get heated up by friction to a temperature of thousands of degrees and start to glow. This causes them to emit a streak of light as they pass through the Earth’s atmosphere. Most meteoroids get heated to such a high temperature that they vaporise and disappear from view.


Image from Wikimedia Commons

Most meteoroids are very small and are vaporised at an altitude of 50 km or higher.  If the meteoroid is large enough, bigger than 1 cm in diameter, some of it can survive the passage through the Earth’s atmosphere and the part that hits the ground is known as a meteorite.

What causes the Perseids?

A comet called Swift-Tuttle orbits the Sun every 133 years. As it orbits the Sun it leaves a cloud of debris in its wake, where material has crumbled away from the surface. Over a long period of time all this material has formed a wide oval-shaped ring in the same orbit as the comet.

Swift Tuttle Debris

Once a year, and at the same time each year, the Earth passes through this ring of debris. When this happens some of the particles hit the Earth’s atmosphere. This is what causes the Perseids meteor shower. As you can see from the diagram the ring is fairly wide and is composed of an huge number of smaller particles.  The Earth first crosses this ring in late July and takes until late August to get the other side. The thickest part of it is encountered around 12-13 August and this is the date on which the Perseid shower is at its most prolific.

When is the best time to observe the Perseids?

The best time of day to observe the Perseids, or in fact any meteor shower, is just before dawn. The diagram below shows the Earth passing through the debris cloud.

Meteoroids dawn

The diagram shows the Earth rotating on its axis. B indicates midday, when the Sun is highest in the sky, D midnight, A sunrise and C sunset.

In the hours after midnight (D to A), an observer is on the side of the Earth facing towards the Earth’s direction of travel, so there are far more meteoroids entering the Earth’s atmosphere. The time of day when the most meteoroids hit the earth is actually at dawn (A), but at that time the brightness of the early morning sky makes them difficult to see. The best time to see meteors is therefore just before it starts getting light in the morning.

If you have the chance to observe a meteor shower over a period of hours or even minutes, you will notice that if you follow the meteor trails backwards, all of them appear to originate from the same place. This is particularly noticeable if you take a long exposure photograph. The point in the sky from where the meteors appear to originate is called the radiant.


Image from Wikimedia Commons

The picture above shows a long exposure photograph of a meteor shower. The radiant is marked with a small circle. The Perseids’ radiant lies in the constellation Perseus, which is directly overhead around dawn at a latitude of 60 degrees North. My readers in the southern hemisphere won’t be able to see them, but you have your own meteor showers invisible to those of us up here!

The best way to observe the Perseids is to go a really dark place well away from light pollution and as I said previously the best time is in the early morning just before the sky starts getting light. In Manchester, where Mrs Geek and I live, the sunrise on 13 August is about 5:45 am and the best time to see the Perseids would be about three hours before this.  Mrs Geek always tries to persuade me to get up in the middle of the night and drive into the countryside but, alas, I am too lazy.  Another complicating factor is that the Moon rises at 11:15 pm on the night of 12 August and will be very bright, so the shower will be more difficult to view than in other years when we are at a different stage in the lunar cycle.

When astronomers measures the strength of a meteor shower, they use a term called the Zenithal Hourly Rate (ZHR). The ZHR is the maximum number of meteors an observer could see if the sky were completely dark, there were no cloud cover and the radiant were directly overhead.  The diagram below shows how the ZHR varied with date for the Perseids last year.

perseids zhr

(McCure 2016)

Fingers crossed for clear skies on the night of 12-13 August, but if we are unlucky, or if you are from down under, here’s a table showing some of the other showers, although none are on average as prolific as the Perseids:

Meteor Showers


McClure, B (2016) Perseid outburst expected in 2016, Available at: (Accessed: 25 July 2016)

Voyager 40th anniversary

Nearly 40 years ago, on 20 August 1977, the Voyager 2 space probe was launched from Cape Canaveral, Florida, on a mission to study the Solar System’s four outermost planets. It was followed 15 days later by the launch of an identical spacecraft, Voyager 1.

The Voyager spacecraft -Image from NASA

Although Voyager 1 was launched after Voyager 2, it followed a different path which meant that it actually visited its first target, the planet Jupiter, four months earlier.

Image from Wikimedia Commons

The diagram above shows the trajectory of the Voyager spacecraft. The first targets, the giant planets Jupiter and Saturn, had previously been visited by NASA’s Pioneer 10 and 11 spacecraft, but the Voyagers had better instruments and were able to take more accurate observations. Among the discoveries made by Voyager was that Io, one of Jupiter’s moons, has a number of active volcanoes. This made Io the first place other than the Earth where volcanoes had been seen to erupt.

Jupiter’s moon Io – Image from NASA

Voyager 1 passed close to Saturn’s giant moon Titan. Titan is 5,150 km in diameter, nearly twice as large as the Moon, and is in fact slightly larger than the planet Mercury. It is the only moon in the Solar System with a thick atmosphere, which surrounds it with a dense haze hundreds of kilometres deep, making it impossible to see any surface details. At Titan’s surface the atmospheric density is roughly 5 times greater than the Earth’s at sea level.  Voyager 1 discovered that it consists mainly of nitrogen (98.4%) and methane (1.4%) with trace amounts of other gases such as ethane.

Saturn’s moon Titan – Image from NASA

These discoveries led to speculation that, like Earth, Titan has complex weather patterns. Being so far from the Sun, Titan’s surface is very cold, around minus 180 degrees.  This temperature is near the boiling point of hydrocarbons such as ethane and methane, which on Earth are found in natural gas. Voyager 1’s discoveries suggested that on Titan it rains liquid hydrocarbons in a similar way to how it rains water on Earth and that there were lakes of liquid hydrocarbons on Titan’s surface. A later mission by the spacecraft Cassini to Saturn and its moons did indeed detect many lakes of liquid methane on Titan (NASA 2016).

After passing Saturn, Voyager 2 visited the two outermost planets Uranus and Neptune (shown below).

Neptune – image from NASA

Uranus and Neptune are of similar size and are both much smaller than the gas giants Jupiter and Saturn. They are mainly composed of frozen water, ammonia and methane and are sometimes called the “ice giants”  So far Voyager 2 is the only mission to have visited the ice giants and most of what we know about Uranus and Neptune came from this spacecraft. No further missions to Uranus or Neptune are planned. Although it is possible that a mission to the ice giants could be launched by NASA in the the 2030s (Clark 2015), this is very much at the proposal stage. I think that this won’t happen, given the high cost of space missions and other higher priority targets such as exploring Mars.

After completing their primary mission to study the outer planets, the Voyagers left the Solar System. They are still in contact with Earth and are now sending back data about interstellar space. For more details on this part of the Voyagers’ mission click here.

The golden record

Each Voyager contained a golden record, the cover of which is shown below.

Voyager Record Cover

Image from NASA

The purpose of the golden record was twofold. If either of the Voyager spacecraft were ever found by an alien intelligence in the far future then, as I’ll explain later, from the golden record cover they would be able to work out the location of the Sun within the galaxy and, by playing the record, get some information about the sounds and sights of Earth in the 1970s.

If you look at the bottom left hand corner of the golden record cover there is a small circle with 15 lines coming out of it. This is shown in more detail in the diagram below.

This diagram was also put on a plaque on the earlier Pioneer probes and was devised by Frank Drake (1930-), who has been heavily involved in the search for extraterrestrial intelligence (SETI). The centre of the diagram, from which the lines radiate, represents the Sun.  The right-hand end of the longest line (at 3 o’clock) represents the centre of the galaxy.  The end of each of the remaining lines represents an object called a pulsar and the length of the line between the Sun and the pulsar represents the distance to the pulsar.

Pulsars are rapidly rotating objects which emit radio waves in regular pulses a few seconds or fractions of a second apart. Each pulsar has its own distinct interval or period between pulses and the marks drawn on the lines depict the length of the pulsar period not in seconds but in multiples of a fundamental time unit that an alien might understand (see note below).  The alien civilisation could then have sufficient information to identify the fourteen pulsars and the distance of the Sun from each pulsar, and thus work out the location of our Solar system.

On the upper part of the golden record cover is a picture of the record itself plus instructions for playing it.

Voyager Record

Image from NASA

The content of the record was agreed by a committee chaired by Carl Sagan (1934-1996) itself, and includes:

  • greetings in 55 different languages,
  • natural sounds and some music from Earth
  • 116 pictures of a variety of objects such as the planets in the solar system, human anatomy, groups of children, important landmarks, interesting places, and man-made structures such as airports, large telescopes, the Golden Gate Bridge in San Francisco, and an American highway in the rush-hour.
  • a printed message from the then United States president Jimmy Carter, part of which is given below:

“…This is a present from a small different world, a token of our sounds, our science, our images, our music, our thoughts, and our feelings.  We are attempting to survive in our time so that we we may live into yours. We hope someday, having solved the problems we face, to join a community of galactic civilisations. This record represents our hope and our determination and our goodwill in a vast and awesome universe.”

Where are the space probes now?

As well as the two Voyagers, three other spacecraft have been launched on a trajectory to take them out of the Solar System. They are New Horizons, which passed the dwarf planet Pluto in 2015 and Pioneer 10 and 11, which were the first spacecraft launched to leave the solar system. As you can see from the table, the Pioneer spacecraft  are no longer in contact with Earth.  There are two possible reasons for this: either their power supply has run out, or the radio transmitter is no longer lined up with the Earth and is beaming out signals in a completely different direction. The Voyager spacecraft are still in contact and should have enough power to continue transmitting messages back to the Earth until the mid 2020s.


In the table, the distance of the probes is given both in kilometers and astronomical units (AU). 1 AU is the average distance from the Earth to the Sun and is equal to just under 150,000,000 km. The outermost planet Neptune is about 30 AU from the Sun and the nearest star about 250,000 AU away. The speed column shows how fast the spacecraft is moving away from the Sun in AU per year.

Will the Voyagers ever be found?

Even astronomers who think that alien life is widespread within our galaxy think that it is very unlikely, given the vastness of space, that the Voyager spacecraft will ever be found.  The spacecraft are small and their power sources will have long died, thus making them unable to transmit a signal which could be picked up. However if they were ever to be intercepted by an alien civilisation it is fascinating to think what they would make of them.

Note about the pulsar diagram

Clearly units of time such as hours, minutes and seconds, which have been invented by humans, would be meaningless to an alien. Atomic hydrogen is the most common element in the Universe and Drake used the transition of a hydrogen atom, shown on the bottom right hand corner of the record cover, to create a “fundamental time unit” which he hoped an extraterrestrial race would understand.  When a hydrogen atom flips from one state to another it emits radio waves at a frequency of 1,420,406,000 waves per second, a fact that any advanced civilisation would be well aware of. So each individual wave corresponds to a time interval of 0.000 000 000 704 seconds. All the pulsar periods are expressed in multiples of this fundamental time interval.


Clark, S (2015) Uranus, Neptune in NASA’s sights for new robotic mission, Available at: (Accessed: 15 June 2017).

NASA (2016) Cassini explores a methane sea on Titan, Available at: (Accessed: 15 June 2017).


Life in our galaxy?

With the recent discovery of three planets orbiting the red dwarf star Trappist-1 which have a similar size, mass and average surface temperature as the Earth, there has been considerable speculation as to whether one or more of these planets supports life.

What the surface of Trappist 1f, one of the planets orbiting Trappist 1, might look like – Image from NASA

Although there are challenges to complex lifetime forms  evolving on a planet around a red dwarf – which I discussed in a previous post – red dwarfs are the most common type of star in our galaxy. In this post I’ll discuss the likelihood that life has evolved in other places within our galaxy, including on planets around red dwarfs.

The Drake Equation

Frank Drake (1930-) is an American astronomer who is known as the ‘father of SETI’ – the Search for Extra Terrestrial Intelligence. Beginning in 1960, he was the first person to search for radio signals from aliens.

.  FrankDrake

Frank Drake- Image from Wikimedia Common 

In 1961 he invented an equation to estimate the number of intelligent civilizations within our galaxy with whom we could potentially communicate, to which he gave the symbol N.  This equation, which is known as the Drake equation, consists of seven numbers multiplied together:

N=  R*  x  FP  x  NE x FL x FI x FC x L

As I’ll explain below, some of these numbers are known to a reasonable accuracy, whereas others are not well known and astronomers differ widely their views of what the values should be.

  • R* is the average number of stars formed per year in our galaxy.  Current estimates are that this has a value of around 10.
  • FP is the fraction of the stars within our galaxy which have a planetary system with one or more planets, expressed on a scale of 0 to 1. A value of 1 means that all stars have planets. 0 would mean that no stars have planets. Planets are difficult to detect around other stars, because they are far too faint to be seen directly and have to be detected by other techniques. In 1961 Drake estimated that FP lay in the region of 0.2 to 0.5 (i.e between 20% and 50% of stars had planets). Current estimates are somewhat higher and that FP is very close to 1.
  • NE is for the average number of bodies, either planets or moons of planets, with the right conditions to support life. Current estimates for this vary considerably.  If most stars were like Trappist-1 then this value would be as high as 3. A reasonable value, which many astronomers would agree with, is 0.4, meaning that out of every 10 stars which have planets, 4 have bodies which could support life.

The Trappist-1 system – image from NASA

  • FL is the fraction of bodies, with the right conditions to support life, on which life actually evolves, expressed on a scale of 0 to 1.  A value of 1 means that on all  planets with the right conditions life will evolve. There is no consensus among astronomers about the value of FL. If, in the future, life is found in many other places in our solar system which have the right conditions  e.g. Mars, or in the warm underground oceans of Saturn’s moon Enceladus (see here for more information) then it would be reasonable to assume that, given the right conditions, in general life will evolve and FL is nearly 1 (see note 1).

Enceladus Ice Volcano

A geyser of warm water erupting from an underground ocean on Enceladus. Image from NASA

  • FI is the fraction of bodies having life, on which life has evolved into intelligent civilisations, expressed on a scale of 0 to 1. Again, there is no consensus among astronomers about what this value should be. Enthusiasts for extra terrestrial intelligence such as Drake believe that the value is close to 1, meaning that intelligent life will always evolve. Others, who believe that it was a highly improbable chain of events which led to the eventual evolution of man from single celled creatures, believe the value is very low.
  • FC is the fraction of bodies with intelligent life which develop a technology that releases signs of their existence into space. For example, on Earth TV and radio signals escape into space and could be picked up by a nearby alien intelligence with a sensitive enough receiver tuned to the right frequency. No one knows what the value of FC is, but current estimates are around 0.2.
  • L is the average lifetime of a civilisation in years. This could be very short if civilisations end up destroying themselves once they have discovered nuclear weapons – or it could be hundreds of millions of years.

The Optimists’ View.

As said previously, no one really knows what the values of most of the terms in the Drake equation are. If we go for values at the high end (FP= 1, NE=0.4, FL=1, FI=1, FC=0.2, L= 100 million) then we get the following:

N= 10 x 1 x 0.4 x 1 x 1 x 0.2 x 100,000,000

which works out as 80 million intelligent communicating civilisations in our galaxy!

One of the problems with such a large number is that we would expect a significant fraction of civilisations to be more advanced than us. Humans have only been civilised for a few thousand years and have already travelled into space.  If a civilisation had been around for more than 1 million years, for example, it is likely that they would have developed the ability to travel the vast distances to other planetary systems and would have already attempted to make contact with us. The fact that they haven’t may mean that civilisations much more advanced than us are rare.

It is also possible (although in my opinion extremely unlikely) that intelligent civilisations do exist and have been observing our planet for a long period of time. They deliberately do not contact us to avoid interfering with our development although they may decide to reveal themselves to us when we reach a certain level of development. This is known as the zoo hypothesis and has appeared in many science fiction stories.

What is clear is that for nearly 60 years, since the pioneering work of Drake in 1960, astronomers have been looking for radio signals from nearby civilisations over a wide range of radio frequencies and have failed to find anything.

Could we be alone ?

Other astronomers believe that some of the values in the Drake equation are very low. There are a large number of steps which occurred between the emergence of the first primitive single-celled life forms and the evolution of man. Each of the individual steps may have a very low probability. So FI the probability of life evolving into intelligent civilisations would be extremely small. For most of the Earth’s lifetime there were only single-celled organisms and, perhaps on most places where life emerges, it never gets beyond this point.

Another point is that mammals only become became the dominant life form after the extinction of the dinosaurs 65 millions years ago. Before that large small-brained reptiles were the dominant life form. Having greater intelligence does not always give an advantage over other traits such as size, speed and physical strength in the survival of the fittest.  There is therefore no guarantee that evolution will result in life forms with the intelligence necessary to develop civilisations.

In addition, dramatic events such as sudden changes in climate can cause any species to become extinct. Roughly 70,000 years ago, an enormous eruption occurred in what is now Sumatra, leaving behind Lake Toba. This triggered a major environmental change which caused the near extinction of the human race.  Humanity could have easily disappeared at this point. Although this has been recently disputed (BBC 2010).


Lake Toba, site of a supervolcano eruption 70,000 years ago – Image from Wikimedia Commons

For these reasons some scientists, such as the British theoretical physicist and popular science writer John Barrow, believe that FI could be around 0.000000001 or even lower. If it were this low, and we take the low end values for for the other parameters, then the expected number of intelligent communicating civilizations in the galaxy would be 0.000016. What this means that if we took 60,000 galaxies similar to our own Milky Way we would on average expect to find only one communicating civilisation. Ourselves!

If this is the case then the Earth would not be just be an ordinary planet orbiting an ordinary star in an ordinary galaxy.  It would be a very special place indeed, for it would be the only place for tens of millions of light years where intelligent life exists.


1 The Earth was formed about 4.6 billion year ago, and at first its temperature was thousands of degrees – far too hot for life to exist.  The first life forms appeared relatively early in the Earth’s history, when it was less than 1 billion years old when conditions became cool enough for life to exist.  This might seem to indicate that, if conditions are right, then life will evolve relatively quickly. Indicating that, perhaps, FL is close to 1.


BBC (2010) Toba super-volcano catastrophe idea ‘dismissed’, Available at: (Accessed: 15 Apr 2015).