Deep Impact

ARE we "cleverer" than the dinosaurs? One day, about 65 million years ago, a fireball streaked across the sky above what is now Central America, and impacted the ground in the region of the Yucatan peninsular. The object, traveling at huge speed, was an asteroid about 10 km (6 miles) across, and the enormous energy released by the impact produced global devastation and played havoc with Earth's climate. As a consequence of this meeting of the celestial with the terrestrial, many scientists believe that the 160-million-year reign of the dinosaurs was brought to an end. Could this happen again, with people this time being the victims of potential extinction?

The answer, disquietingly, is yes. It will happen again. But impacts of the magnitude of the Yucatan event fortunately do not happen often—about once every 100 million years. So it is probably something we do not need to worry about for a long time. However, as we develop the technology to look out into space, we are beginning to realize that there are a large number of objects in orbit around the Sun that potentially could collide with Earth. These objects are called near-Earth objects (NEOs). A NEO is an asteroid or comet in an orbit that crosses or comes close to Earth's orbit around the Sun, and so represents an impact threat. There is intense activity at present to detect and catalogue the NEO population, and so far we have estimated that there are about 1000 objects on the order of 1 km (0.6 miles) in diameter or bigger. The detection of smaller objects becomes more difficult, so it is not known how many smaller objects there are, but we do know that the number of objects increases as the size decreases. Current estimates of the number of objects bigger than 100 m (330 feet) is about 100,000. As a consequence, we are never quite sure when one of these objects will be discovered to be on a collision course with Earth.

The last significant impact event took place in 1908 at Tunguska, Siberia, and this object was estimated to be about 50 m (165 feet) across. Fortunately, the impact site was uninhabited, but the explosion flattened about 2000 square kilometers (770 square miles) of forest. If the object had landed in

G. Swinerd, How Spacecraft Fly: Spaceflight Without Formulae, DOI: 10.1007/978-0-387-76572-3_11, © Praxis Publishing, Ltd. 2008

central London, for example, the area of devastation would correspond approximately to everything within the M25 orbital motorway. An impactor of this size can be expected about once every few hundred years. Consequently, a NEO impact is a fairly rare event, but nevertheless there is a probability we will have to face one in the not-too-distant future! National governments, charged with the responsibility of looking after their citizens, are now at least considering this type of event as a natural disaster, alongside things like earthquakes and hurricanes. Hollywood has also done its bit to raise awareness with films such as Armageddon and Deep Impact, which with the aid of computer-generated images give a graphic depiction of some of the devastating impact-generated effects. For land impacts, these include the effects of blast, heat, and ejecta from the impact site, and the generation of seismic disturbances. However, since the majority of the Earth's surface is water, it is more likely that such an object will fall into the ocean. For this type of event, the main impact-generated effect is a large tsumani wave, which propagates at high speed across the ocean. Tsumanis are very effective at transporting the energy of the impact to distant shores, bringing devastation to coastal cities where most of the world's population is concentrated.

So what can be done? Unlike the dinosaurs, we are at least in a position to see something coming, and to do something to deflect its path to avoid a collision. The technology needed to do this is available now, but the usual roadblock is funding. Budgets available for NEO detection surveys are relatively small, and agency budgets for the development of spacecraft that could be used to deflect a threatening NEO are similarly inadequate. If a 500-m object suddenly came over the horizon today on an impact trajectory, threatening devastation on a continental scale, the funding situation would dramatically change. But would there be enough time to develop and test the require space hardware to be assured of success? Can we afford to take the chance? I would suggest not.

There are a number of ways of deflecting the path of a threatening NEO. They all have one feature in common: they depend on there being sufficient warning of an impact, so that the missions can be launched several years in advance. This is because, generally, the methods are only able to produce small changes in the trajectory of the NEO. Such a small change is able to deflect the object successfully if it is done a long time in advance, as the effects of a small change build up over time to avert disaster. However, if the time to impact is short, then much larger changes are required, which are effectively beyond our current capabilities. This is why surveys designed to detect threatening NEOs well in advance are so important. The following list of deflection techniques is not exhaustive, but it does give a flavour of some of the ideas that are being proposed.

The use of nuclear weapons: This is invariably the Hollywood solution, as it makes for good cinema! The strategy is to launch one, or a number of nuclear warheads against the incoming object to blow it off course. The problem here is that no one really knows how the nuclear blast will affect the object's motion, and tests need to be done to find out. In the vacuum of space, the detonation produces a blast wave that is very much less powerful than it would be in Earth's atmosphere, so that the effect of the explosion on the asteroid's motion may be inadequate. However, there is another deflection mechanism that may be effective if the nuclear weapon is detonated in close proximity to the asteroid's surface. Then the surface layer may possibly be heated sufficiently to vaporize the asteroid and blast it at high speed into space. On the basis of Newton's laws (Chapter 1), this ejection of material would produce a thrust on the asteroid, a bit like a rocket engine, which may be sufficient to deflect the object's path from a collision course. There are many unanswered questions about this technique, which emphasizes the need for flight tests to ascertain how nuclear explosions influence the motion of a NEO in its orbit around the Sun. The other issue with the use of nuclear detonations is the risk that the NEO may be fragmented, resulting in a cloud of smaller but still potentially lethal impactors on their way to Earth. In this case, the situation may have been worsened, with a multitude of Tunguska-type impacts bringing worldwide devastation.

The use of an impactor: This is an intuitive idea—crashing an impactor spacecraft onto the surface of the object to deflect its trajectory, like a billiard ball changing its path after the impact from another ball. But the thing to note about the billiard balls is that a significant change is produced because the two balls are of the same size. Obviously, we are unable to launch an impactor spacecraft with the same mass as, say, a 200-m asteroid, so the amount of deflection is tiny. However, if the deflection is done sufficiently far in advance, then a small change will be adequate to avoid a collision with the Earth. The use of a gravity tractor: This is a rather less intuitive idea, but one of the most effective ways to achieve a controlled deflection of a threatening NEO such as an asteroid, without needing to know anything about its physical characteristics, such as the nature of its surface, or its state of rotation. The idea is a relatively recent one, being proposed in 2005 by Ed Lu and Stan Love of the National Aeronautics and Space Administration's (NASA) Johnson Space Center. The gravity tractor is an unmanned spacecraft that is launched to rendezvous with the asteroid. On arrival, the tractor positions itself a small distance from the object, and then uses small rockets to hold this position above the surface, as shown in Figure 11.1. As the spacecraft hovers above the surface, a force is exerted on the asteroid equal to the mutual gravitation force between them. Effectively, the tractor is using gravity as an invisible tether with which to tow the asteroid off it collision course with Earth.

To get an idea of the numbers, let's suppose the asteroid is 200 m across. Then a quick calculation gives its mass as about 10,000,000 metric tonnes. If the tractor spacecraft's mass is about 5 metric tonnes and it is stationed 50 m above the asteroid's surface, then it will require a thrust of only about 0.2 Newtons to remain stationary above the asteroid. This small force is equal to the mutual gravitational force between the tractor and the asteroid, and it is this force that produces the acceleration to shift the asteroid from its collision course. The application of such a small force to such a huge mass produces a tiny acceleration. However, the saving grace is that this tiny acceleration can be applied for a long period of time to allow a useful change in speed of the asteroid to build up. If the mission is performed a sufficiently long time in advance of the predicted Earth impact, then the required change in the asteroid's orbit to avert disaster is similarly small, and easily accommodated by the technique. The operation to successfully divert the asteroid in this case can be achieved in about 10 days, but much longer periods can be realized if required.

Figure 11.1: An artist's impression of a gravity tractor spacecraft on-station above an asteroid on collision course with Earth. (Image courtesy of Dan Durda, B612 Foundation/FIAAA.)

In terms of the spacecraft, the propulsion requirements can be achieved by two ion drives, each with a thrust of 0.1 N. The electrical power required for these would be about 4 kW, and the fuel mass used in this case is approximately 4 kg.

As well as the technical issues discussed above, there is also a political dimension to the issue of NEO deflection. Once a potentially threatening NEO has been identified, the first task to be undertaken is to determine its orbit around the Sun so that the likelihood of an impact can be estimated. If the object is indeed on a collision course, the site of the impact on the Earth's surface can be estimated, although if the impact is some years away there are going to be significant errors in this process. However, let's suppose that such a process has led to a likelihood that the object will fall somewhere on the North American continent. In this situation, the United States would be eager to launch a deflection mission as soon as possible to avoid the devastating consequences of such an impact on its territory. However, once the spacecraft has reached the object and the deflection process is underway, say, with a gravity tractor, then the resulting changes in the NEO's orbit will progressively change the location of the impact site. In this situation, should the impact site be moved out over the Pacific Ocean where significant tsunami damage to the west coast of the U.S. can result? Or should it be moved east toward the continents of Europe and Africa? Of course the objective is to move the impact site so that the object misses Earth entirely, but what if the gravity tractor spacecraft fails before its mission is completed? In such a time of crisis, the international community may be called upon to make some monumental decisions that will affect the lives of millions of people. But as yet there is no agreed international mechanism through which such decisions can be made.

As we have seen, the probability of an asteroid impact with Earth in the next few decades is small, but nevertheless if one were to occur the consequences are horrible to contemplate. The prospect of such an event provides a good case for developing our space-faring capabilities. I think it is time that we grasped the nettle and showed that we are indeed "cleverer" than the dinosaurs!

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