ASTEROID DAY

ASTEROID DAY

Dr N T Jiwaji

ntjiwaji @ yahoo dot com

Asteroid Day, marked on June 30 each year, is an international event founded in 2014 and endorsed by the United Nations in 2016 as a “global day of education to raise awareness about asteroids”, to highlight the impact of asteroids on Earth.

On 30 June, 1908, just over a century ago, Earth’s largest asteroid impact in recorded history struck Tunguska in Siberia, Russia. The asteroid was about 50 metres, the size of half a football stadium, and devastated over 2,000 sq. km of forest, the size of a major city. Recently the Chelyabinsk meteor, about 20 metres across, that streaked fierily over the Russian city on 15 February 2013, caused a series of shock waves that shattered windows, damaged approximately 7,200 buildings and left 1,500 people injured. In 1972, a 1000-tonne object struck the Earth’s atmosphere horizontally and bounced back into space like a stone that bounces off when thrown along the surface of water.

A 10-km size asteroid impact 65 million years ago caused mass extinctions of quarter of living things, including the dinosaurs. The Mbozi meteorite that fell at Mbozi near Mbeya, perhaps a thousand years ago, has left a large 16 tonne lump of hard metal, which is preserved at the site. Many other signs of asteroid falls are visible around the Earth, with the largest 60 tonne meteorite, Hoba in Namibia, as well as remains of many large impact craters, the largest being the 200 kilometres wide crater in South Africa. Even our Moon is thought to have been formed by impact of Earth by a planet sized body during the formation of the Solar System billions of years ago.

The Earth is struck by many other objects from space when it comes sufficiently close, within the range of Earth’s gravitation pull. Though we do not notice it, each day nearly 100 tonnes of dust falls on Earth from space. At night, this shows up as shooting stars, which are trails of light formed when bigger grains of dust about 2 millimetres are heated and burn up due to friction with the atmosphere. Such material produces thin clouds at very high altitudes and have an impact on climate change.

Bigger pieces that do not burn up completely fall on Earth as meteorites. Small pieces are found by meteorite hunters in in undisturbed open, semi-arid areas, and on ice sheets in the Arctic and Antarctic. Bigger pieces strike Earth with high energy and form impact craters and leave behind large meteorites such as the Mbozi meteorite. Large asteroids such as the 50 metre Tunguska asteroid, are pulled towards Earth at extremely high speeds of around 50,000 km/h. The asteroid strikes Earth’s atmosphere like a wall and the heats up and explodes even before it reaches ground. Its shock wave can destroy a much larger area. Asteroids that are completely rocky can disintegrate completely leaving only tiny pieces scattered on the ground as was the case in the Tunguska and Chelyabinsk strikes. Asteroids with metallic cores will burn up the rocky material and melt part of the metal which falls to the surface and create impact craters.

The main source of asteroids is the Main Asteroid Belt, which is a collection of hundreds of thousands of pieces of rocks of all sizes and shapes, spread out and orbiting the Sun in the space between the orbits of Mars and Jupiter, which are the 4th and the 5th planets from Sun. The rocks are remains of material that failed to form a planet due to the enormous pull of gravity of the largest planet, Jupiter. Four of the asteroids are very large, more than 500 km across. Ceres, is the largest is 1000 km across and is classified as a dwarf planet. The remaining rocks are smaller but, as the Tunguska asteroid and the Chelyabinsk meteor show, even 100 or 50 kilometre sized asteroids can cause considerable damage if they were to hit Earth.

Gravitational forces from nearby planets pull some of these rocks out of the Main Asteroid Belt and bring them into the inner solar system where they enter orbits that can bring them close to Earth. Such rocks, when discovered, become known as Near Earth Asteroids (NEA) or Near Earth Objects (NEO), and may occasionally pose a danger of striking Earth directly.

Currently there are estimated to be about 1000 NEOs that are 1 km in size, 20,000 that are 150 metres, and millions that are a few metres across. The frequency of a kilometres-size asteroids that can cause world-wide damage is extremely low about once every 100 million years. But asteroids of several meters such as the Tunguska asteroid can strike one every 500 years or so. Since large parts of Earth are covered by oceans, all large asteroids in the past have fallen on unpopulated areas. Only one person has been recorded to have been hit directly by a small meteorite in the USA in 1954.

Existing telescopes discover about 1,500 NEOs per year, but the problem that we face is that it is still not possible to detect them early enough to give us sufficient warning if any metre-sized asteroids are on direct course to hit Earth. The exact place that an asteroid will fall also cannot be accurately predicted at the moment.

Hence, Asteroid Day is marked to raise awareness of the seriousness of the problem and use available technology to increase discovery efforts for these asteroids by a factor of 100 every year and provide an effective early warning system of a possible direct strike.

Asteroids, being the remnants from the formation of our solar system also provide a clue to our origin, and hence they are subjects of many space missions to study them close up. Missions are also being planned to bring back samples of their materials. Asteroids may in future be targets for mining precious minerals that become scare on Earth. Asteroid deflection studies are also being done to find out how best a collision with Earth could be avoided.

There are many resources available, for example at https://asteroidday.org, to help you bring the attention including videos, power point presentations, exercises, and much more. You can watch live broadcast of the Asteroid Day program from Luxembourg City through the internet at https://asteroidday.org/live/. Use the opportunity to raise awareness of this effort in your community, at your place of work, or at your school or college.

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