Using Lasers To Deflect
Earth-Bound Asteroids
By Roy Silson

The following contribution continues the debate in Institute publications about the dangers of human life on earth coming to an end as a result of the planet being struck by an asteroid.
In past discussions of the risk of asteroids hitting the Earth, I have expressed major doubts as to whether we could prevent their impact.
Recently I have seen a report which suggests a plausible solution. Nonetheless we are still left with several serious snags.
This idea proposes the use of a laser beam to heat the surface of an approaching asteroid. For icy comets and many asteroids such heat would boil off gases. The thrust of these gases would change - slightly - the orbit of the asteroid.
Effective action would require warning well in advance of potential impact and gas ejection continued over a long period. With appropriate calculations and sufficient accuracy, the orbital change might become sufficient to remove the danger.
On the other hand, inadequate skills might increase the danger and turn a close miss into an actual impact.
Note however that comets heated by the sun seem to retain their orbits. Perhaps to and from effects cancel. I know very little detail re the constancy of comet orbits - small changes would attract little notice.
They must however vary at least slightly since mutual gravity changes even planetary orbits from simple two body - sun and planet - theoretical expectation. In the past, unknown outer planets were brought to notice by their perturbation of orbits of known planets.
Jupiter and other large planets produce major effects on space objects passing near them.
On the other hand planetary and other orbits tend to approximate to the same plane with possibilities of orbits crossing at some point. The problem arises when two bodies reach the crossing at the same time. This is obviously rare but has a finite probability.
Additional difficulties
Laser beams are nominally parallel but may not be sufficiently so over tens of millions of miles. Spreading reduces heat concentration - effective results would require heating at or near optimum points.
Accurate aiming at an optimum point would be important. Long distance aiming at a point on a small target - perhaps one mile across or less - could be very difficult. Small asteroids differ widely from the spherical shape associated with big ones - the resultant thrust might be largely unpredictable.
Generalised thrust from the whole of any visible surface - even for spherical bodies - will have a large cancellation component.
The source of the laser beam presents a major difficulty. High power beams heat the air. The resultant changes in air density defocuses the beam. This was a major technical problem in Star Wars proposals.
A laser source in orbit avoids this problem but requires a multi-megawatt power supply - also in orbit. It would be immensely expensive and a major engineering problem to place a large atomic power station in orbit and also to keep it working safely. A high power laser also implies a massive structure and precision aiming - to much less than a second of an arc - in each of two dimensions.
This aiming angle will change continually - even for objects following the same orbit. However, remote objects in earth orbit would tend to stay at a constant distance. The danger comes from objects whose orbit crosses that of the Earth at the same time as the Earth is crossing that point.
A secondary problem is that a high power laser beam presents a major danger to life on earth. Even low power beams produce permanent blindness. Great care would be needed to ensure that the beam's orbital sweep - following the asteroid - always missed the Earth and any other satellites in orbit at all times.
All Earth satellites - including lasers - have orbital planes passing through the centre of the Earth. Moreover since the Earth orbits the Sun the plane of this tilted orbit will rotate around the Earth during the solar year. The beam angle to the orbital plane - following the target - continually changes and needs careful checking.
I wonder if this interesting proposal really has a potential. It avoids the space rocket timing problem and could be on permanent standby for any number of space objects. On the other hand, large bodies - especially those with a low content of low boiling point materials - would still present problems of adequate jet thrust.
Current large rockets carry several hundred tons - half their own weight - in fuel. A large asteroid weighs many millions tons. Could we send the energy equivalent of even one million tons of fuel across millions of miles of space in the form of a narrow laser beam?
Perhaps small amounts might be sufficient. A very small initial deflection - if far enough away - could grow into an adequate orbital difference at the time of passing.
R G Silson, Near Station, Tring, Herts, England. HP23 5QX (tel 01442 82 3281).
How safe is Earth from asteroids? And can it be made safer? Information summarised from a number of articles in the Guardian, New Scientist, The Sunday Times, The Times, The Economist and The Daily Telegraph.
In July 1994, fragments from the comet Shoemaker-Levy 9 collided with Jupiter, under close observation by astronomers, watched in turn by an eager press. The impact clearly registered how enormous the devastation wrought by relatively small asteroids-travelling at 72 000 km per hour-could be, and prompted a flurry of reassessments of the Earth's vulnerability to such cosmic vagaries. It also provided the immediate spur for the Pentagon to launch a multi-million dollar feasibility study of asteroid defence systems.
It is estimated that about 2,000 asteroids with diameters greater than one kilometre exist which cross the orbit of the Earth, making collision a possibility-albeit a very remote one. Comets also pose a threat: When very large ones from the edge of the Solar System get drawn further in they are generally pulled towards the Sun, which breaks them into a stream of debris. This, in turn, gathers as dust around our atmosphere, and causes Ice Ages. Collision with the nucleus of a smaller comet is, if anything, more cataclysmic. Both events. mercifully, generally occur only once every 200,000 years or so. However, as the Jupiter collision showed, much smaller bodies can wreak catastrophic damage: and there are believed to be some 300,000 of these crossing the Earth's orbit, each of them at least ten times as destructive as the Hiroshima bomb.
The risk of this kind of catastrophe happening today or tomorrow is minute. However, the likelihood that something of this kind will occur one day is much stronger, and is underlined by explanations of a range of historical and prehistoric cataclysms as the results of asteroid impacts. In 1996, archaeologist Dr Richard Kroon produced what is considered definitive proof that dinosaurs were made extinct by an asteroid collision. Core samples drilled from 300 ft beneath the Atlantic produced 'outstanding' results: "With that knowledge much of the mystery of the dinosaurs' collapse evaporated...It is clear that a major catastrophe wiped out life on Earth at the time dinosaurs lived." Kroon is the latest in a line of scientists who have produced similar, more speculative, accounts. The Biblical flood (9600 BC), has been ascribed by geology Professor Alexander Tollmann to cometary fragments crashing into the ocean and causing 'almighty' tidal waves. Professor Victor Clube of Oxford University has suggested that cometary debris was responsible for the most recent Ice Age, 100,000 years ago. He has identified a particular stream, linked with the Taurid meteor stream (responsible for shooting stars in November skies), through which our planet passes every 3000 years or so, most recently in 500 A.D. This cycle corroborates with geological evidence, and also anecdotal evidence which survives from the Dark Ages reporting mass migrations and the kind of firestorms associated with cosmic impacts. If this cycle continues, it gives us about 1000 years to get ready for the next collision. (Worries had been expressed elsewhere that Comet Swift-Tuttle was due to be coming within periously close range of Earth as soon as 2124. More recent recalculations have written off the risk).
If such millennial events seem remote, a smaller but altogether more recent event provides a good example of the threat presented even by pretty small cometary fragments. In 1908, a cosmic body, thought to have been no more than 60 metres in diameter, exploded over the remote Tunguska region of Siberia, laying waste to about 2,000 square kilometres of forest land and taking a still unknown number of human lives. Such an impact over a more populous area would be unimaginably devastating.
Such scary thoughts have provided the immediate background to the American government's approval of a multi-million dollar project researching the feasibility of asteroid deflection. Clementine 2, as the project is called, will launch a satellite into a distant orbit, equipped with high speed probes which can "lock on" to asteroids, and strike them at up to 40,000 mph. Each probe will be equipped with sophisticated camera equipment which will record the deflective effect of the impacts. The technology is strikingly similar to the Star Wars programme, whose fantastic budget produced some advances in missile technology but very little-as experts had always predicted-in the way of an effective missile shield. Star Wars was abandoned, partly as a result of its gigantic budget, partly because of the disappearance of the 'Soviet threat'. Sceptics have suggested that the asteroid threat is merely a convenient premise for the notoriously powerful 'military-industrial complex' to continue hoovering up vast subsidy and further refining the instruments of terrestrial cataclysm.
The alternative view, proposed in a 1993 Economist editorial, argues that the long-term risk is significant and that only myopia keeps Western governments from funding research into collision prevention: "A report to America's Congress in 1992 by a panel of experts said that if there were a [major asteroidal collision] a quarter of the world's population would die. If the annual risk is one in half a million, that gives an annual risk of dying of one in 2m, and a lifetime risk of dying one in 30,000. The one in 2m risk can be turned into an equivalent annual death rate of 2,700 worldwide, 390 in the rich countries. The British government reckons that it's worth $1.2m to save a life through increased road safety; from that, you might expect the developed world to pay $470m a year to deal with asteroids. At present, in fact, precisely nothing is spent on dealing with this risk."
A compromise strategy may be available, however. If the currently popular initiatives are all too alarmingly compatible with terrestrial weapons development, researchers in Russia and America have recently proposed a rather more pacific strategy for asteroid deflection. Involving launching a mirrored aluminium sail on an orbit tracking an asteroid's, it is essentially a much-enlarged refinement of the schoolboy technique of setting light to things using a magnifying glass to focus the sun's rays. Here, sunlight focused to a temperature of some 2000 degrees centigrade would be reflected onto a point on the side of an asteroid, vaporising ice and rock on the body's surface which would in turn translate into a vapour jet of sufficient velocity to deflect it from collision with Earth. Professors Jay Melosh and Ivan Nemchinov concede that their technology would rely on long range advance information on collision courses-10-15 years ahead for larger bodies. Whilst this degree of forewarning would not be possible for every cosmic vagary, the two argue, with sufficiently well-developed monitoring it would be enough to avert over ninety-per cent of potential disasters.
Given that even such distinguished observers as the Jodrell Bank pioneer, astronomer Sir Bernard Lovell, is appealing for governments to pay more attention to this "very real risk"' it may be that the time has come to put some of these apparently remote contingency plans into action.
Telescoping the problem? Information summarised from a number of articles and letters in the New Scientist and The Times, Summer 2000.
A government panel of experts, the Near Earth Objects Task Force, have recently stated that urgent international action is needed to decrease the risk of a large asteroid hitting the Earth. Their report concluded that global co-operation to track potentially dangerous asteroids and comets, and research into methods of deflecting them, is the only answer to the problem.
The panel proposed building a 9ft telescope in the southern hemisphere to track medium-sized asteroids. This telescope would aim to complement the efforts of US-based initiatives which focus on objects larger than 1 kilometre across in the northern hemisphere. Further to this, a second European telescope would track any objects found by either project.
Professor Colin Kirk (Professor Emeritus of Astronautics and Space Engineering at the College of Aeronautics, Cranfield University) agreed in a letter to The Times (Sept 2000) that tracking and detecting would be enhanced by the addition of such telescopes. He went on to point out, though, the problems with firing a nuclear missile at an asteroid, stating that chances of success were small due to complexities of missile tracking, debris after a strike, and the speed of the intended targets.
His conclusion was that early warning was of 'questionable value' as the impact site would only be able to be estimated just days before the event. Therefore, the sole benefit of early warning might only be to enable us to enjoy our last few days.
In the US, Robert Gold, of John Hopkins Applied Physics Laboratory in Maryland recently proposed a three-part defence system, involving similar ideas. Firstly, a set of orbital telescopes (similar to the Hubble telescope) would identify and track threatening objects. Secondly, a set of spacecraft which could be deployed to intercept the incoming object. Then, finally, an Earth-based control centre to oversee the whole system.
Gold also believes that exploding a nuclear bomb somewhere close to the object's surface would be the best method of interception, with the blast designed to "kick" the asteroid or comet off course. These intercepting craft could be launched, Gold suggests, from orbiting around Venus. Whatever the solution, the scientists are agreed that the impact of such an object poses the greatest (natural) threat to the long-term survival of mankind on Earth, and that a detection and interception system is urgently needed.
The report of the Task Force on Potentially Hazardous Near Earth Objects can be read at
A date for a collision is predicted
Summarised from an article by Robin McKie, entitled '21 September 2030: the date scientists predict an asteroid will hit the Earth', in The Observer (5th November 2000).
Scientists have now put a specific date on when Earth may be hit by an asteroid: 21st September 2030. Nasa's Jet Propulsion Laboratory were the first to notice the risk, and their conclusions were soon verified by a group of experts from the International Astronomical Union. The risk of impact is one in 500, which is something of a first for the scientists involved in the field of impact prediction. As David Morrison, a space scientist at Nasa's Ames Research Center, says, "We have never before had a prediction at this high level of probability. In the past we have talked about one in 10,000 or one in a million."
The object in question, called 2000 SG344, is believed to be an asteroid with a diameter of between 100 and 230 feet. It would hit Earth with a force 100 times greater than Hiroshima, assuming that it is made of stone and iron. There is a possibility, though, that it could be more loosely made up of stones and gravel and thus disintegrate when it enters the atmosphere. The scientists will have to wait till 2028 before the asteroid comes back into their range of observation in order to verify the asteroid's make-up and orbit, though. And that would leave little time for any sort of deflection or interception effort.
For more information on this asteroid, see
The quest for a solution continues Summarised from an article by Hazel Muir, entitled 'Target Earth', in the New Scientist (March 3rd 2001; New Scientist subs £97 or $140, tel +44 [0]1622 778000).
The astronomic world continues to be preoccupied by the prospect of an asteroid hitting the earth, and what (if anything) we should try and do about it. Following on from the report from the British government's Near Earth Objects Task Force (see above), other countries are stepping up their experiments and research in the area. A Japanese mission named MUSES-C will send a spacecraft next year to an asteroid called 1998 SF36. Four years on from launch, the spacecraft will survey the asteroid in question and skip across its surface to take samples from three different sites, before returning to earth in 2007. The European Space Agency is sending a mission called Rosetta to look at Comet Wirtanen in 2003, in the hope of revealing aspects of these potentially hazardous objects with which we are unfamiliar - and thus allowing us to gauge the outcome of impacts better. Meanwhile, NASA's Deep Impact mission is due to launch in 2004, with the aim of studying Comet Tempel 1. Its method will be to drop a half-tonne copper cylinder onto the comet to create a crater the size of a football pitch on the surface. As the dust settles, the craft will then use cameras to study the insides of the comet, and astronomers at home will measure any deviations in course as a result of the impact.
The most important thing now is to convene these international efforts, and to make sure that some sort of international agency for monitoring and warning is set up. As Brian Marsden of the Minor Planet Center in Cambridge, Massachusetts points out, there are no official channels or procedures to follow at the moment should an impact seem likely. "We at the Minor Planet Center are likely to get the information first, but nobody's ever told me who to call if we find something. If any nation is going to do anything about it, it's probably the US. So do I contact someone in the State Department, or in the Defense Department? It has never been made clear to me." When all the different research projects have returned, and all the data been analysed, the real quest for a co-ordinated solution will begin. .

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