TOPIC



Asteroids and Comets
Mountains in the sky





Asteroid Gaspra:

Image of asteroid Gaspra




Asteroid Toutatis:

Image of asteroid Toutatis




Comet:

Image of a comet

(AIS 97/04/25)

There are mountains larger than the Mount Everest. These mountains are barely noticeable because most of them have a dark color and are drifting in comparatively great distances through the space of our solar system. They are called asteroids, stone and metallic bodies that are left during the development of planets in the solar system. Asteroids have sizes between the size of a football and a few hundred kilometers (100 km = 62 miles). Asteroids are not known for a long time. The first asteroid was discovered 1801. This was possible due the progress in development of telescopes and the spreading engagement with space. In contrast to asteroids are comets known for a long time to all cultures. These bodies of ice and stone with their long shinning tails are easy to detect even when they are far away. Often their appearance has been interpreted as a sign of heaven and the account has been passed on from generation to generation.

The asteroid in contrast, even one of considerable size, would have been detected only in the past if it flew burning brightly through the higher layers of the atmosphere or if it suddenly hit the ground. But even then, people would not have known what it was. Suddenly an earthquake, extensive forest fires, a spring tide, that would have been only some of the many inexplicable hazards people have to deal with in the past.

But we know today that asteroids of various sizes have struck the Earth. An asteroid with a diameter of around 12 km struck the Earth 65 million years ago at a point that is now the Yucatan peninsula of Mexico. Half of the Earth's species were extinguished on account of this event and its consequences on the Earth ecosystem and climate system. Today there is a 180 km (112 miles) wide crater. Another crater is the Ries crater in Bavaria/Germany with a diameter of around 25 km caused by an 1.5 km asteroid or comet. Well known is the 'Meteor Crater' in Northern Arizona. It is estimated that a 50 meter metallic asteroid has hit there around 50.000 years ago, leaving a crater 1.2 km wide and 200 m (650 ft.) deep. More than 100 impact crater on Earth are known today. Often these craters are difficult to find due erosion, but are recognizable because of the shock marks on the stones and the minerals in that area.

When we look up to the moon we can see it spotted with big craters. The absence of an atmosphere and the resulting lack of erosion preserves its craters. Also other planets and moons of the solar system show the marks of asteroid impacts. Most asteroids are located in the main belt, a ring of asteroids between the orbits of Mars and Jupiter with the Sun in the middle of the ring. The main belt is around 2 astronomical units away from the Sun and around 2 astronomical units broad (Astronomical Unit = Distance Earth - Sun = 150.000.000 km = 93.000.000 miles). The nine planets of the solar system and the main belt are not wildly distributed in space. They move all more or less in a plane and in the same direction, like runners on parallel tracks around the stadium. Occasionally asteroids collide within the main belt or their orbits are disturbed by gravitational forces from Jupiter which bring them on a new course. It is likely that such a body remains in the plane of the solar system and is pulled toward the Sun by its strong gravity field. Such an asteroid might cross the orbit of the Earth. Beside the main belt asteroids there are many others with orbits closer to Earth and also quite a lot are crossing the Earth's orbit. Asteroids called Amors cross Mars' orbit but do not reach the orbit of the Earth. Apollos are asteroids that cross Earth's orbits in periods more than a year. Atens are asteroids which need for their Earth crossing orbits less than a year.

The danger of a large asteroid hitting the Earth is not part of our daily consciousness of dangers to be dealt with. And that is reasonable to some point, because this danger is beyond our common experience and the chances for an individual to die by a car accident or disease are much higher. But the consequences if Earth is hit by a big asteroid or comet are grave. The effects will not only be a misfortune for some individuals but it will affect whole cultures. Not only some persons, but also all their relatives will be gone at the same time. Whole countries will probably vanish, their languages, achievements and cultural peculiarities may disappear forever in a matter of some moments.

A supposed spherical asteroid with a diameter of 10 km (6.2 miles) and a consistence of three parts stone and one part iron with a rather moderate entry velocity of 20 km/sec (12.4 miles/sec) would have a kinetic impact energy on the whole earth system (atmosphere and globe) of approximately 8.378 E23 Joule. That is an energy equivalent of around 9.500.000.000 hiroshima bombs or 200.000.000 megatons TNT.
The asteroid may break up in several pieces on entry in the atmosphere because of gravitational forces and air resistance, vibrations and internal heating due friction. Thus the destruction will cover an even wider area. The energy will be transformed in some unpleasant effects. Shock waves of extremely heated air will be racing around the globe, igniting and destroying big parts of the ecosystem on Earth's surface. The surface on the impact site will be deformed, parts of the surface will be accelerated at high speed, meaning further danger in a wide circumference. Some of the energy will be transformed in vibrations and oscillations of the terrestrial globe, with earthquakes and volcanic eruptions as a possible result. Huge masses of earth and dust will be ejected in the atmosphere blocking the sunlight and alter world's climate for a longer time. The photosynthesis of plants is likely to be forestalled. When the object impacts in the sea, whole continental coastlines are likely to suffer spring tides which will travel far inside the countries.
It is estimated that there are around 2000 earth crossing objects with diameters of 1 km and above.

Occurrences like the comet fragments chain 'Shoemaker-Levy 9' discovered 1993 and hit Jupiter 1994 show us that the orbits of many bodies in the solar system have not settled to stable orbits in the last million years.

Comet Shoemaker-Levy 9

Some of the fragments of the comet Shoemaker-Levy 9, of estimated sizes between 500 m (1600 ft.) and 2 km (1.25 miles), have punched holes up to Earth's diameter in Jupiter's upper atmospheric layers.

Jupiter


A big explosion in the year 1908 in the barely populated taiga of Tunguska/Siberia is said to be caused by a little fragment of a comet that exploded high above the ground, devastating forest and vegetation in an area of around 2000 sq km.
Comet Hyakutake discovered 1996 and comet Hale-Bopp discovered 1995 have been unknown to us until recently.
Many long period comets have orbits of several thousand years. They come from the disk shaped 'Kuiper belt' 30 to 100 astronomical units away or even from the spherical 'Oort cloud' of comets that encloses our solar system 30.000 to 60.000 astronomical units away. Therefore we have no accurate tradition on how many comets have passed Earth and will come again. These comets are exposed to various gravitational influences on their long journeys which might alter their course. In addition the irregular stream of ejected gases from the comet's nuclei has a difficult to predict influence on their course.

Asteroid Toutatis, size 4.6x2.4x1.9 km (2.85x1.5x1.2 miles) and discovered 1989, passed the Earth 1992 in a distance of around 3.200.000 km (2.000.000 miles). Toutatis has an orbit which is difficult to predict because he has a tumbling rotation, but he will pass the Earth in the year 2004 in a distance four times the distance Earth-Moon, which is pretty near on a space scale.
1989 a 400 meter (1300 ft.) asteroid passed earth in 640.000 km (400.000 miles) distance.

There is a growing awareness of the possible danger of a collision with an asteroid or comet. Several programs like the Near-Earth Asteroid Tracking program or the Spacewatch project (link check, exchanged with working page) try to catalog all asteroids and find out their orbit data. Imagine you put at night a transparent sheet on the window and then you mark very fast all spots on the night sky. Half an hour later you take another sheet of transparent film and repeat the whole procedure. Then you put both sheets on each other in a way that the many spots of the stars on sheet one align with their counterparts on sheet two. This is possible because the distance to the stars is so big, that their movement is not visible in a short period. But some spots will not align with another spot. These objects have moved visibly in the last half hour since they are much closer than the stars. First there are the nine planets of our solar system. But there are more, much fainter spots. These are some of the many asteroids in our solar system.
Instead of transparent sheets today's research use very light sensitive CCD sensors similar to those in video cameras. A sensor is an array of 2048x2048 pixel or even 4096x4096 pixel and is mounted on a telescope. They can quickly take an image of a part of the night sky. Half an hour later another image is taken of the same part. A computer compares both images and marks the differences as possibly new asteroids or comets.
Other objects related to space are:
Meteoroids = these are metallic or stony bodies, too small to be called an asteroid. They have sizes between a few millimeter and the size of a football.
Meteors = when a Meteoroid enters the earth atmosphere it is heated by friction with the air molecules.
The Meteoroid begins to burn out and dissolves on his way through the atmosphere. The shinning trail is called a Meteor (shooting star).
Meteorite = the most time small remaining of a Meteoroid which makes its way to the earth surface. Meteorites look like stones or pieces of ore.

There are several efforts to estimate the options on how to deal with an approaching asteroid , like altering its course when it is still far away from earth. In such an event, only a comparable small amount of energy is needed. A nuclear explosion may change the course of an asteroid or break it up in more manageable pieces. Another option is to move a smaller asteroid against a threading asteroid, deflecting it like a billiard ball pushed against another.

For the first time in the history of mankind we have the technological basics to do something against this kind of danger. Independently of what one may believe, think or say about the collision probabilities, it will hardly affect the actual course of these mountains in the sky. The race against the time begins at the moment when one has the ability to do something.



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