Chapter 8: Interplanetary Bodies

Map showing the approximate location of the Tunguska event of 1908.

On the morning of June 30, 1908, a mysterious explosion occurred in the skies over Siberia. Herdsmen 500 kilometers (300 miles) away reported "deafening bangs" and a fiery cloud on the horizon. Closer to the explosion, it sounded like a crack of thunder, flung carpenters from a building, and knocked crockery off shelves. An eyewitness just 60 kilometers (40 miles) from the blast reported:

 

"The whole northern part of the sky appeared to be covered with fire... I felt great heat as if my shirt had caught fire [and] there was a...mighty crash... I was thrown onto the ground about seven meters from the porch... A hot wind, as from a cannon, blew past the huts from the north... Many panes in the windows were blown out, and the door of the barn was broken."

Probably the closest observers were a group of reindeer herders asleep in their tents about 30 kilometers (20 miles) from the site. They were blown into the air and knocked unconscious. One man was blown into a tree and later died. "Everything around was shrouded in smoke and fog from the burning fallen trees." The forest fires continued for weeks. What could have caused this dramatic and frightening event?

The Tunguska event, named after a nearby river, was caused by an object from space that struck the Earth's atmosphere and exploded. Seismic vibrations from the airburst were recorded 1000 kilometers (600 miles) away. Observers about 170 kilometers (110 miles) from the explosion saw the object in the cloudless sky as a brilliant, fiery ball, much larger than a full Moon in apparent size. It exploded before it could reach the ground, about 5-10 kilometers above the surface. The object detonated due to immense frictional heating as it sped through the atmosphere. An explosion this size would have completely obliterated the impactor. The force of the blast flattened trees over thousands of square kilometers.

Trees were knocked down and burned over hundreds of square km by the Tunguska meteoroid impact.

The mysterious object turned out to be a modest bit of interplanetary debris, probably rocky in composition, with a diameter of about 50 to 60 meters. Many such fragments circle the Sun — this one was merely the largest to hit the Earth in the last century or so. It was sheer luck that it hit one of the most remote parts of the Earth's surface, and so few people were hurt or killed. Had it hit a densely populated area, the devastation would have been enormous.

 

There has been a lot of controversy about this event, partially due to its uniqueness in modern history, its remoteness, and the resulting delay between the event and its investigation by scientists. Alternate explanations have been proposed, ranging from a UFO to a nuclear bomb to an antimatter explosion. However, the scientific evidence supports this straightforward scenario — the impact of an ordinary interplanetary object.

Recent studies reveal that explosions the size of the Tunguska event may happen every few centuries (and less often over populated land, which represents less than a seventh of the Earth's area). Much larger objects have hit the Earth in the past, but they are rare. For example, an iron asteroid fragment about 100 meters across hit what is now Arizona about 20,000 years ago, leaving a well-preserved crater a kilometer wide (now called Barringer Meteor Crater). A 10-kilometer asteroid hit the Earth 65 million years ago, ending the reign of the dinosaurs. Small impacts are much more common than these large ones. Brick-sized meteorites fall from the sky every year. Several houses, one person, and a car have been hit in recent decades. Tiny dust grains are even more common - you will see them on any clear night if you watch long enough. These dust grains incinerate in the atmosphere, and the resulting bright streaks of light are meteors, misleadingly called "shooting stars."

156 bolides were detected on a single (pointed) photographic plate of the all sky fish-eye photographic camera during the Leonid meteor shower in 1998 at Modra observatory. The exposure time was 4 hours.

Meteor Crater in Northern Arizona.

By sheer chance, the largest object to hit the Earth since Tunguska also fell in a fairly remote part of Russia. On February 13, 2013, an object streaked across the morning sky in Chelyabinsk in the southern Ural region. It was captured by cell phones and webcams on car dashboards. The object exploded at an altitude of 30 kilometers, causing a shock wave and a hail of debris. The kinetic energy as it approached the Earth was equal to 500 kilotons of TNT or 30 times the energy of the Hiroshima nuclear blast. The object was estimated to be about 20 meters in size before its breakup, about a third the size of the Tunguska impactor. Its explosion caused panic across the region and 1500 people were injured seriously enough to need medical attention, mostly for cuts from broken glass caused by the shock wave. People scrambled to collect fragments that had fallen to Earth, many still warm from their high-speed passage through the atmosphere. The episode was a reminder that mayhem from the sky can arrive at any time.

Interplanetary space contains many small bodies of different sizes. All of them move in elliptical orbits around the Sun as prescribed by Kepler's laws. Occasionally their orbits intersect those of planets, leading to a collision. If the bodies are large enough, they leave sizable craters that we can see on the surfaces of planets and moons throughout the Solar System. Scientists have been able to explain these "visitors in the sky," which had mystified people for thousands of years. The physical nature of comets, meteors, and asteroids gives us clues to the formation of the Solar System.