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Skywatching

A near miss - in 1908?

The old, black-and-white pictures show men up to their knees in mud and water, making measurements with theodolites and other instruments, with their heads surrounded by a fog of mosquitoes.

These pictures were taken in 1921, in Siberia, when scientists were trying to have a closer look at what happened there in 1908.

On the morning of June 30, 1908, at Tunguska, Siberia there was a huge explosion. Trees were flattened for tens of kilometres, and glasses rattled on shelves in Paris. Due to political instabilities, the First World War and then the Revolution, it was not until 1921 that scientists made it to that remote location to investigate what happened.

The widely held theory was that something had come in from space at high speed, entered the atmosphere and exploded, causing the blast wave that flattened the trees. Something big enough to do that should have left fragments that reached the ground. In absolutely horrible conditions, these dedicated individuals were there to survey the site and find some of those bits.

Paradoxically, they found evidence of a huge explosion, but found no crater and no cosmic debris at all. That is how the situation remains. The most widely held theory at the moment is that the object that caused the blast was made of ice.

Then, most of it would have vaporized in the atmosphere and anything left would have melted long ago, providing more habitat for breeding mosquitoes. There is still no explanation that everyone is happy with, but some new research has come up with an idea that seems to fit the bill, an ominous one.

Some Russian scientists have been researching the event. They calculated what would have happened if a lump of ice came into the atmosphere at around 20 kilometres a second: a typical velocity for such objects.

Their conclusion was that unless it was coming straight down, it would have been vaporized long before it got low enough to cause an explosion that produced damage at ground level.

The few witness statements from the time of the event indicate an oblique path through the atmosphere. They therefore suggested something else, a lump of iron 200 metres across, which did not hit the ground at all; it simply shot through our atmosphere at high speed and went back into space.

Something that big could absorb all the heat produced by a few seconds in the Earth's atmosphere, and something that massive would not slow down much. Its path would be an almost straight line that happened to just miss the Earth, so it passed by through the lower atmosphere. 

A speed of 20 km/s is about 50 times the speed of sound — hypersonic. At such speeds, the air does not have time to get out of the way. It is trapped in front of blunt objects, compressed and heated to around 10,000 degrees. Huge shock waves would be produced, which, if the body passed close enough to the ground, could have done the observed damage.

If this object had hit the ground, it would have blown a crater three kilometres across. The environmental consequences would have been huge.

With no plate tectonics to erase them, the Moon is covered with craters, some many kilometres across, a record of impacts over billions of years. The Earth has been hit too, and on the oldest rocks that have not yet been recycled, such as those of the Canadian Shield we find old, large craters.

The jury might still be out on what actually took place over Tunguska in 1908.

However, over the Earth's 4.5 billion-year history, we have been hit many times, and it will happen again. This is why there are projects dedicated to searching for asteroids and other bodies with impact potential.

  • After dark, Saturn and brilliant Jupiter lie close together low in the south
  • Mars rising in the east.
  • Venus, even brighter, rises in the early hours. It is worth getting out the telescope. The Moon will reach First Quarter on the 23rd. 


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Close encounter with Mars

"No one would have believed, at the end of the 19th Century, that human affairs were being watched by ..... intellects vast, and cool and unsympathetic, regarding this Earth with envious eyes, and who slowly and surely drew their plans against us...."

This is how H.G. Wells began his book War of the Worlds. The intellects in question were those of the Martians, sitting on their drying, cooling world and thinking a move to somewhere wetter and warmer would be in order.

In Wells' story, the invasion happened when Earth and Mars were passing close to one another. These close encounters are the best times to observe the Red Planet, and we are having one of those encounters now.

These evenings we see Mars in our skies overnight. It is bright, red, and shining steadily, like a lamp, not twinkling.

Earth is the third planet out from the Sun, and Mars is the fourth. Since planets closer to their stars have to move faster in their orbits, and in addition have further to travel than planets further out, the Earth overtakes Mars on the inside track every 26 months.

Since at that moment Mars, Earth and the Sun are in line, with Mars on the opposite side of the Earth from the Sun, we say that Mars is in opposition. Since Mars' orbit is a bit elliptical, and Earth's is too, but less so, some oppositions are closer than others.

If Mars is at the point in its orbit closest to the Sun, and Earth at its most distant point, the two planets can be really close. The encounter in 2008 brought the two planets to around 57 million kilometres of one another, which is almost as close as it gets.

This time round the closest we will get, in mid-October, will be about 61.5 million kilometres. Considering that some oppositions might have the planets passing at a distance of over 100 million kilometres, our current opportunity to observe Mars is worth taking seriously.

We won't have another encounter as good as this until 2035.

Mars can be a tantalizing object to observe. At the moment even a small telescope will show Mars as a reddish-orange disc, a desert planet.

The main impediment to getting a really good view of the planet is turbulence in our atmosphere, making the planet look like a coin at the bottom of a stream. Fortunately, during this encounter, Mars gets quite high in the sky, which reduces the effect of the turbulence.

Those hazy, anti-cyclonic days, where the stars don't twinkle much are the best. However, the key is patience. Amidst the flashing and rippling, there are occasional moments of steadiness that allow us to see the planet in much more detail.

The challenges in getting really good views of the planet are the probable cause of one of the great misunderstandings about Mars, one that persisted until as recently as the 1960s. This was a consequence of staring too hard, for too long, under poor observing conditions.

In 1877, Italian astronomer Giovanni Schiaparelli reported seeing irregular lines on Mars: naturally occurring channels.  Of course, he used the Italian word for channels, canali.

English speaking astronomers mistranslated this word as canals, artificial waterways, made by Martians for water management on their drying planet. American astronomer Percival Lowell built an observatory at Flagstaff, Arizona, just to map the Martian canals. He worked hard, stared hard and worked long hours, and mapped the canals.

It was probably Lowell who launched our obsession with Martian invasions and other stories about Mars.

Of course, the great irony is that in the end, we invaded Mars.  We have sent more spacecraft there than to any other planet. One day there will be intelligent beings on Mars — probably us.

  • After dark, Saturn and brilliant Jupiter lie low in the south
  • Mars is rising in the east at the same time.
  • Venus, even brighter, rises in the early hours.
  • If you have a telescope, all these planets are worth a look.
  • The Moon will be New on the 16th. 


Pulse of the universe

Long ago, in a galaxy 3.6 billion light years away, something strange happened.

Whatever it was produced a short pulse of radio waves about a thousandth of a second long. The energy was comparable with the total energy produced by the Sun in a century.

To produce such a short pulse, the source cannot be larger than a thousandth of a light second in size, around 300 kilometres at most.

We can think of a few phenomena that can produce such a high-energy pulse. Our ideas include a black hole, colliding neutron stars, or the energy released by tangled magnetic fields in a magnetar, the highly magnetized core of an exploded giant star.

At that time, 3.6 billion years ago, the Earth was about 1.5 billion years old. Primitive life, such as stromatolites, inhabited the young world's oceans. The atmosphere was not yet breathable. However, algae in the oceans were busy photosynthesizing oxygen. 

About 100,000 years after it was produced, the pulse left its galaxy, radiating out in all directions into intergalactic space.

The material in space is more rarefied than any vacuum we can achieve in the laboratory; however, there are still a few atoms, free electrons and very weak magnetic fields.

These affect the radio waves, in near-empty space, the effect is slight, but over billions of light years it builds up into something significant.

When it was produced, the pulse spanned a wide range of radio frequencies; a radio tuned to it would have received a sharp click and radios at different frequencies would have received the signal at exactly the same time.

However, gradually, as it travelled through intergalactic space, the electrons present oscillated with the pulse and reradiated it. The result was a delay, which was smallest at the high frequencies and highest at low frequencies.

The click became a descending tone, where radios tuned to high frequencies would pick up the pulse before radios at lower frequencies would.

This process, known as dispersion, allows us to estimate how far the pulse has travelled. This is important in helping us identify the source and the energies involved in the process that produced it.

About 100,000 years ago, when Neanderthals were the dominant species of human and Cro Magnon man, our species, was still about 50,000 years in the future, the pulse entered our galaxy, the Milky Way.

About 3,000 years ago, when the Egyptian empire held sway in the Middle East, the pulse entered our neighbourhood, the zone containing the familiar stars in our night sky.

When Karl Jansky discovered cosmic radio waves and started the science of radio astronomy in 1932, the pulse was about 88 light years away.

Finally, when it arrived at the Earth, after travelling for 3.6 billion years, it so happened that the side of the Earth facing the incoming pulse had radio telescopes searching for such pulses.

The pulse signal was collected by the antennas, magnified by very sensitive amplifiers and digitized. Then, sophisticated software, searching the incoming signal for these dispersed pulses from far off, flagged the signal and sent a message to a computer terminal.

The observer saw it, turned to her companion and said: "I think we've detected another Fast Radio Burst.”

The advent of radio telescopes such as the CHIME radio telescope, operating at the Dominion Radio Astrophysical Observatory, is ideally suited for detecting these fast radio bursts, or FRBs, because it can see a large area of sky at the same time. It turns out that FRBs are common, with many occurring a day.

We would really like to know what sort of bizarre physics is making them.

  • After dark, Saturn and brilliant Jupiter lie low in the south
  • Mars rising in the east.
  • Venus, which is even brighter, rises in the early hours.
  • The Moon will reach Last Quarter on Oct 9.


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Life after stellar death

We hear, occasionally, of someone proposing what looks at first sight like a really improbable project.

The latest one is an experiment to detect signs of life on planets orbiting white dwarf stars. The instrument making such a project possible is the soon-to-be-launched James Webb Space Telescope. Optimistic is the word.

Long ago, astrophysicist Subrahmanyan Chandrasekhar calculated that in later life, if the core of a star exceeds 1.4 times the mass of the Sun, it would end its life in a huge explosion, a supernova.

On the other hand, smaller stars, like the Sun, would have a much more drawn out end; they would become white dwarf stars. This number is now known as Chandrasekhar's Limit. This means the Sun will end up as a white dwarf star.

Stars get their energy through nuclear fusion, where very hot, extremely compressed hydrogen becomes helium and other elements, liberating a lot of energy in the process. This is the origin of the heat and light that renders our planet inhabitable. 

For almost all stars, all the fuel they will ever have was acquired when they formed. This means that at some point, a star is going to start running short of fuel. Paradoxically, when the fuel shortage starts to bite, the star swells enormously into a red giant.

Its brightness increases by hundreds or even thousands of times. Such squandering of the last remaining fuel means this stage is not going to last long.

When the Sun reaches that point, Mercury and Venus will end up engulfed and Earth might be. In any case, it will be incinerated. The frozen planets and moons in the outer Solar System will become warmer and maybe suitable for life.

However, the inevitable end of the Sun's fuel supply comes soon. The outer parts of the Sun get sneezed off into space leaving the naked core a white-hot lump of nuclear waste products, mainly helium, about the size of the Earth, where a teaspoonful of this core material would weigh more than a tonne. It has become a white dwarf.

However, despite being white hot, the small size means the rate at which energy is radiated into space becomes fractions of a per cent of what the original star produced. 

In the Solar System, the outer planets would become more frozen than before, and the inner planets,
incinerated cinders, will now freeze. Intriguingly, although the white dwarf has no fuel, it is so miserly with its energy radiation that it will take billions or tens of billions of years to cool off.

This raises an interesting possibility. A planet orbiting a white dwarf star would not suffer from space weather - disruption of technical infrastructure due to their star's bad behaviour.

A white dwarf star also shines with a fairly steady brightness for billions of years, providing in some ways a better environment than our ancestors had on Earth. The Sun has brightened steadily by about 30% since life first appeared on Earth and our terrestrial ecosystems had to adapt to it.

However, the problem is the size of a white dwarf's Goldilocks Zone. This is the distance from a star where the heat would be just right for liquid water to exist on a planet's surface. Bright stars have quite deep Goldilocks Zones. Very dim stars have very narrow ones, which lie close to the star.

Our Solar System doesn’t have any planets that will be just right when the Sun becomes a white dwarf. Even Mercury is too far.

One would think that any planets close enough to become comfortable when their star becomes a white dwarf will have been fried by the star during its maturity and then incinerated and engulfed when the star became a red giant.

Prospects aren't good. However, there is only one way to know the sort of planets that orbit white dwarfs, and that is to look for them.

  • After dark, Saturn and brilliant Jupiter lie low in the south
  • Mars rises in the east.
  • Venus, which is even brighter, rises in the early hours.
  • The Moon will be Full on the Oct. 1.


More Skywatching articles

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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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