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Johannesburg Centre,
Astronomical Society of Southern Africa
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SPACEGUARD SURVEY In the article on The Minor Planets (November) it was stated that the minor planet Hermes which had been lost since 1937, had probably crashed into the Sun. In the same Canopus Brian Fraser wrote that Brian A. Skiff of Lowell Observatory had rediscovered Hermes. It had also been found by Jean-Luc Margot of the University of California that Hermes consists of two almost equal pieces, each about 2 km in size - a rather rare occurrence. There must be many thousands of bodies smaller than one kilometre which cross the Earth's orbit or come close to the Earth. The smallest bodies of only a few centimetres in size vaporise when they enter the Earth's atmosphere and leave glowing trails. These are called bolides. Larger bodies, of size up to about 60 metres explode when they come down to heights of about six kilometres. The body which exploded over Tunguska in Siberia on 6 June 1908 came down to a height of about 8'/2 kilometres above the surface. Its explosion liberated a force equal to 10 megatons of TNT, flattening trees up to a distance of 30 km. This body was probably the nucleus of a comet because no metallic remnants have been found there. Bodies larger than 1 kilometre would penetrate right through the atmosphere and strike the Earth's surface with a force equal to that liberated by 100 000 megatons of TNT. It appears that every 5 000 years at least one body of that size is slated to strike the Earth. If the point of impact happens to house a city the damage and loss of life would be appalling. Has the time not now arrived that man should take the necessary steps to obviate such a catastrophe? It certainly has and the technology also exists to carry out the necessary measures of prevention. This can be accomplished by launching a rocket to go and push the near-earth-object (n.e.o.) into an orbit which will miss the Earth. The sooner this can be tried out on a "sample body" the better. The vast majority of minor planets revolve in orbits which are direct (counter-clockwise as seen from the north) so they revolve around the Sun in orbits which run more or less parallel to the Earth's orbit. In such cases sufficient time will be available to carry out the rescue mission. However, there are a few cases of orbits which are retrograde, so that these bodies move in retrograde orbits in a direction opposite to the Earth's direction of motion. Jupiter could deflect a minor planet, moving direct, into a retrograde orbit. So this danger really exists. In these cases there will not be sufficient time to go and push the body away into a safe orbit. In desperation, man will have to launch a rocket to such a body, carrying a nuclear bomb which will be made to explode when it comes into contact with the rogue asteroid. The asteroid will be broken up by the explosion into many thousands of pieces of varying sizes, thus minimising the threat of a single crash, but vastly increasing the area of somewhat lesser damage. But blowing the n.e.o. to smithereens could be a last resort. Let us deal with the easiest case, that of a directly orbiting asteroid. In Diag. (i) the asteroid A is shown, headed for a crash with the Earth. In Diag. (ii) the body B has been pushed forward in its orbit so that it will pass the Earth and cross Earth's orbit ahead of the Earth at B'. In Diag (iii) the n.e.o. has been slowed down so that it will pass behind the Earth and cross the Earth's orbit at C'.
The rocket which is to be launched will go and attach itself to the minor planet and then continuously fire out very high velocity ions. Newton's third law states that to every action there is an opposite but equal reaction. In this way the body can gradually be moved. NASA has already started work on what is known as the Prometheus project in which NASA is designing a nuclear reactor that could provide the power to blast high-speed ions to propel a spacecraft to Jupiter's moons, Callisto, Ganymede and Europe. (In 10 years' time!). At its orbital speed of 29,785 km per sec, the Earth takes 430 seconds to move a distance equal to its diameter (12 756 km). So if the n.e.o. can be speeded up or slowed down by 430 seconds it will miss the Earth by one Earth-diameter. This means that the n.e.o. will have its speed changed by 2 cm per sec. A rocky n.e.o. of diameter 1 kilometre will have a mass of about 50 million tons, so to bring about a very small change of speed, will require pushing it for several years. If it ever becomes possible to guide a minor planet so that it describes an orbit around the Earth, it will be possible for man to mine the minerals on the minor planet. During March 2002 astronomers observed a 70 metre long rock 461 000 km from the Earth four days after its closest approach to Earth. Since then an international effort has come on stream to track down such objects. It is called the Spaceguard Survey. This is just the beginning. The scope of the survey will have to be continually increased. Amateur astronomers are welcome to join in the survey. Since 1998 NASA has pursued the goal of detecting at least 90% of the suspected 1 100 n.e.o.'s larger than 1 km in size. After 5 years of searching 660 such objects have been tracked as well as 1 800 smaller bodies. 99% of the bodies so far spotted do not pose a threat to the Earth for the next 100 years. It must be borne in mind that objects as small as 200 metres could destroy a city. Spaceguard Survey needs all the assistance it can get. Jan Eben van Zyl |
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