Forty kilometers north of Pretoria lies a ring of hills a kilometer in diameter and 100 meters high. These hills are the walls of an impact crater left by an asteroid which hit there some 200 000 years ago. The Tswaing crater is similar in size to the well-known Barringer meteor crater in Arizona. The crater walls at Tswaing were originally about twice as high as they are today.

The photograph shows you what it looks like today.

The crater is called Tswaing in seTswana, the local language of the area, or Soutpan in Afrikaans. Both names mean Salt Pan and this derives from the lake of salty water that fills the centre.

There is a museum adjacent to the crater. A path leads from the museum to the crater, along the rim, and down to the central lake. The crater is covered with indigenous trees and bushes, and attracts a variety of bird life.

HOW TO GET TO TSWAING

Tswaing is reached by taking the M35 (Soutpan) road from Pretoria going north past Ondersterpoort.From Johannesburg

· Take the N 1 (north) through Pretoria towards Polokwane.

· Drive past Zambesi/Cullinan off-ramp.

· Take N 4 (west)(Rustenburg) at next off-ramp (Bakwena Platinum Freeway).

· Drive for 19 km west.

· Small Toll fee at Doornpoort Plaza.

· Take M 80 Pretoria/Soshanguve off-ramp and go towards Soshanguve.

· Drive north along M 80 (Mabopane Freeway) until a point where this freeway. stops at three-way stop-sign intersection (distance about 18 km).

· Turn right towards M 35 Soutpan (old name for Tswaing) at this intersection.

· Turn left at T-junction with M 35 Soutpan or Tswaing Road.

· Drive on for about 15km north past Soshanguve’s informal settlements.

· Watch out for game fence (left) and Coca-Cola sign indicating Tswaing Crater 2,7 km.

· Turn left at brown tourist signs after 2,7 km into gravel road leading to main gate

The map below is not to scale but gives a rough idea of how the crater looks.

For more information on visiting the Tswaing crater site and museum, which is run by the National Flagship Institution, see the official Tswaing Meteorite Crater home page.
What Made the Crater?

Drilling in the crater has shown that the rocks forming it have been shocked by an explosion, and are not volcanic in origin. In the explosion that formed the crater the incoming object would have been largely vaporised - there is no large buried object in or under the crater. The absence of such a buried object at the Barringer crater in Arizona caused confusion as to its origins for many years, although in that case traces of the original iron asteroid have been found.
How Big was the Bang?

The explosion that occurs on impact is caused by the rapid release of the kinetic energy (energy due to its motion) of the object as it comes to a stop in about one hundredth of a second. The impact speed of an object falling from rest in space onto the Earth is 40 000 kilometres per hour. Incoming objects would not have been at rest and typically impact with a speed of around 60 000 kilometres per hour, ie around 16 kilometres per second.

The kinetic energy released when an object travelling at this speed is brought to rest in the impact is 100 million Joules per kilogram of mass. This is 25 times greater than the energy produced by the detonation of the same mass of high explosive such as TNT.

It has been calculated that the energy required to produce the Barringer crater was equivalent to the explosion of 15 million tonnes of TNT. As the Tswaing crater is slightly smaller than the Barringer crater, we shall assume that the energy of the explosion there was equivalent to 10 million tonnes of TNT. This happens to match the energy calculated to have been released in the Tunguska airburst event in Siberia in 1908.

The Tswaing impact energy release also matches the power of the first US hydrogen (fusion) bomb, codenamed "Mike", exploded at Eniwetok on 31 October 1952. It is one third the power of the largest Soviet test bomb, which had a yield of 58 Megatons, and was dropped on Novaya Zemla on 31 October 1961. However, 10 Megatons is 500 times the power of the 20 kiloton atomic (fission) bomb that devastated Hiroshima on 6 August 1945, killing about 150 000 people.
How Big was the Object?
From the amount of energy released we can calculate that the mass of the object must have been about 300 000 tonnes. How do we get this?

We noted above that 1 kilogram of TNT releases 108/25 = 4 x 106 Joules of energy.

Then 107 tons of TNT = 1010 kilograms of TNT will release 4 x 1016 Joules of energy.

This energy comes from the kinetic energy of motion of the asteroid being released as it is brought to rest on impact. We can then use the well known equation:

Kinetic Energy = 1/2 * Mass * Velocity2.

Rewriting this to solve for the mass gives:

Mass = 2 x 4 x 1016 / 160002 = 3 x 108 kilograms, ie 300 000 tonnes.

This allows us to calculate its size. The object was probably made of rock with a density of about 3 tonnes per cubic meter. This is a typical density - for comparison, water has a density of 1 tonne per cubic metre. Rewriting the equation above to solve for the radius, we can then calculate that the object would have had a diameter of about 60 metres. This sort of object would have been a small asteroid. An asteroid whose orbit intersects that of the Earth - as this one's did - is called a meteoroid. The smallest objects detected telescopically near the Earth are about the size of this one.

OUR VISION

To develop and manage an internationally important meteorite impact site as an ecotourism model of the conservation and use of its natural and cultural features, the enhancement of the quality of life of the host community, and the provision of an enjoyable, unique and affordable experience for visitors

OUR MISSION

The Management of Tswaing is committed to:

* The sustainable conservation of the cultural and natural heritage of the area;
* The development and maintenance of systems for the effective and efficient distribution of benefits to the host community;
* The creation of enjoyable, unique and authentic experiences for visitors, based on their needs and expectations as well as resource constraints;
* The creation of an infrastructure to meet the needs of visitors;
* The creation of viable opportunities for private sector involvement.
* The achievement of the mission is based on participation democracy, access and equity, innovation and inspiration, resourcefulness, capacity building, co-operation and partnerships, accountability, integrated planning and Batho Pele (People First).

OUR KEY OBJECTIVES

To attain our vision and mission, our key objectives are:

* The conservation and preservation of Tswaing's biodiversity, biophysical processes, landscape and historical and archaeological resources;
* The sustainable use of Tswaing's natural and cultural heritage resources to the socio-economic benefit of the host community, as well as to international, national, provincial and metropolitan stakeholders;
* An effective Management Plan.