A near miss - in 1908?

Photo: https://rjmorwood.files.wordpress.com/

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. 

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.