On Earth, traces of past collisions have been identified by thorough investigations - together a fair amount of luck, given the oceanic, atmospheric and biological phenomena that have since modified the terrestrial soil. Nevertheless, there is firm evidence of a number of collisions, some very remote and some very recent. It is thought that dinosaurs died out 65 million years ago due to the impact of an asteroid which hit the Gulf of Mexico. Twenty-three thousand years ago an asteroid estimated to be about 100 metres in diameter fell near the state of New Jersey, probably causing a tsunami that engulfed what is now New York. According to the story told in a Sumerian clay tablet - backed up by the discovery of a crater near the city of Köfels - a meteorite fell in Austria in about 3000 BC.
More recently, in 1908 a giant explosion due to a minor body impact destroyed the forest of Tunguska in Siberia. More than 2000 square kilometres of forest was affected, the blast being comparable to 1000 Hiroshima-size atomic bombs and noticed from 1000 kilometres away. Almost a century later, in October 2008 an asteroid of 5 metres diameter broke into several fragments in the sky over Sudan. This was the first case of the impact of a minor body with the atmosphere being predicted in advance.
Today, we are aware of the dangerous consequences of asteroid collisions and are equipped with sophisticated tools and techniques to prevent such catastrophes. All over the world there are observatories capable of detecting minor bodies. Systematic monitoring is performed to identify all the objects large enough to represent a hazard to life on the Earth. NEODyS provides a complete and user-friendly database of Near Earth Objects.The international community is now analysing different approaches that can be adopted to avoid an impact, or to mitigate its effects. In April 2009, experts from around the world met in Granada, Spain, at the 1st International Academy of Astronautics Planetary Defence Conference to debate these issues.
Detection of an asteroid is only the first step. Once identified, one then has to be able to predict its behaviour for a reasonable period of time - perhaps thirty to a hundred years. This requires a good model of the forces acting on the body to be used with mathematical methods to allow its trajectory to be followed and predicted - that is, to build a model that can describe the position and velocity of the asteroid over time. In developing such models, we must consider the available observations of the asteroid and their precision, the time between the prediction and the most recent observation, and the accuracy of the method used to solve the equations of motion, as well as the model adopted.
Technology Is Limited
The asteroid Apophis - which shares its name with an Egyptian demon of chaos - will come near the Earth on April 13, 2029, and again on April 13, 2036. If a body of this size - around 200 metres in diameter - collided with the Earth, it would provoke a devastating tsunami if the impact took place in the ocean, or destroy a region as big as the Netherlands if it struck a landmass. At the time of discovery, December 2004, Apophis drew much attention because the probability of impact in 2029 was thought to be significant.
The motion of Apophis is affected by the gravitational attraction of all the bodies in the Solar System. In addition, we must take into account relativistic effects, solar radiation pressure and the thermal radiation effect. Since the asteroid is relatively small and is now moving far from the Earth, the precision of current technology in determining its position is limited. Observations have ruled out the risk of a collision in 2029, but the uncertainties in the trajectory are amplified when the asteroid comes close to a major body, so its passage close to the Earth in 2029 may dramatically affect the events in 2036.
A Low Probability Of Collision in 2036
The challenge is to determine Apophis’s trajectory accurately, taking into account the initial uncertainty in the observations. One can analyze the behaviour of many trajectories, departing from slightly different initial conditions, but this is computationally very expensive. Instead, the Dynamical Systems Group of Barcelona is working on the “jet transport” technique, which represents the final solution as a mathematical function of the initial conditions. This is a very efficient method applicable to a variety of situations, based on mathematical techniques developed by Prof. M. Berz and collaborators at Michigan State University. Using this method, we can compute the probability of passage at different distances to the Earth.
Based on existing observations, we cannot rule out a low probability of collision in 2036, but we can examine how much more precisely the current trajectory needs to be measured in order to do so - in this case, the uncertainties need to be about four times smaller. With new measurements - likely to be available in the near future - the data will be precise enough either to guarantee that Apophis will not hit the Earth, or to allow suitable actions to be developed to deflect it. Nevertheless, Apophis is not the only existing threat - according to NEODyS around 240 possible impactors exist. The techniques that we are developing might help to understand their behaviour.