The Earth in the face of asteroids

No, it is not the scenario of a science fiction film. It concerns the Earth, moving around its orbit at an average speed of 29.79 kilometres per second. Although superheroes only exist in the movies, there is a very real threat of a projectile colliding with our planet. To help us to prepare for this threat, the European Space Agency (ESA) has developed the Don Quijote mission, which is intended to perform an asteroid-deflection test before a collision actually occurs.

Dans le programme Don Quichotte, en préparation, le vaisseau Hidalgo repère un astéroïde avec lequel il entrera en collision. Cet impact sera observé par le vaisseau Sancho, qui pourra ainsi rendre compte des modifications de trajectoire de l’astéroïde. © ESA – AOES Medialab Under the Don Quijote programme, which is still at the study stage, the Hidalgo spacecraft will pinpoint an asteroid with which it will intentionally collide. The Sancho spacecraft will observe the impact to monitor any changes in the asteroid’s trajectory. © ESA – AOES Medialab

The near-Earth asteroid 99942 Apophis, which was discovered in 2004, has caused a lot of ink to flow in the press and sweat to bead the foreheads of scientists, who initially predicted a high probability of collision with the Earth in 2029. Although new observations have allayed the worst fears, the asteroid is still expected to miss us by only 32,000 kilometres, the closest shave on record. Asteroid 99942 Apophis is visible to the naked eye and its trajectory is expected to cross geosynchronous orbit, where most meteorological and telecommunication satellites operate.

A “very real and tangible” risk

“The risk of an asteroid crossing the Earth’s trajectory is very real and tangible,” says Willy Benz, astrophysicist at the University of Berne in Switzerland and member of ESA’s Space Science Advisory Committee. “The questions that arise are how frequent such encounters will be and how large the object is,” he adds. Although dust enters the Earth’s atmosphere every day, it disintegrates as a result of friction with the atmosphere… “So it is harmless. The same goes for the few objects of up to 50 centimetres in diameter that skim off the Earth every year.” (1)

At the other extreme, large solids can cause huge damage. Fortunately they are much rarer and there is hope of detecting them at an early stage. “Remember that no impact has extinguished life on Earth since it first began around 3.8 billion years ago. So we can safely assume that there is no danger to life as we know it.”

However, some species are more vulnerable than others. “The more a species has evolved, the more fragile it is, and humans are particularly threatened, given our current energy, food and social dependency.” According to some calculations, an asteroid 10 kilometres in diameter would suffice to bring the reign of humans to an end. The statistical likelihood of such a disaster happening is around once in every 100 million years.

The latest and most famous collision with Earth so far, caused by a similar-sized projectile, is thought to have resulted in the extinction of the largest dinosaurs, 65 million years ago… A disaster that was to destroy only part of the planet could prove just as catastrophic.

Medium-sized asteroids of a few hundred metres in diameter jeopardise the Earth’s order. “These objects are not only large enough to cause considerable damage, they are also more numerous. Worst of all, they are too small to be detected easily.” To gain an idea of probabilities, an object of 200 metres in diameter (large enough to destroy an entire country) could strike us every 50 000 years, whilst one 500 metres in diameter (able to annihilate an entire continent) could hit every 200 000 years.


The first stage in our defence is to detect these projectiles in advance. The process is fairly inexpensive compared with active defence measures because the observations are made from Earth (although there are plans in the pipeline to launch a satellite to observe certain objects that cannot be detected from Earth, such as those coming from the direction of the sun). There are now programmes for cataloguing the most dangerous objects. NASA’s programme, called LINEAR (Lincoln Near Earth Asteroid Research) has contributed much to the explosion of knowledge on detection in the past decade.

The earlier we detect a near-Earth asteroid before its likely impact, the more time we have to take defensive action. Although Apophis is already in our sights for 2029, asteroid 2002MN, which sped past the Earth at a distance of only 120,000 kilometres in June 2002 (the closest approach ever recorded being 105,000 kilometres) was discovered only three days after traversing Earth’s trajectory! There still seems to be no reliable measurement in this area. “Detection relies chiefly on our ability to define orbits. Orbits change over the long term in accordance with a host of parameters that are difficult to gauge, such as solar radiation.”

Attack is the best defence

“The amount of advance warning also determines the choice of response system, as we have a wide range of methods for deflecting an object. However, none of them has yet been tested, so for the time being they remain at the conceptual stage.” It is here that ESA’s Don Quijote asteroid deflection precursor mission comes in, designed to assess and validate the technology that could one day be used to deflect an asteroid threatening the Earth. The current scenario envisages two spacecraft in separate interplanetary trajectories. Like the famous character, Don Quijote, in the novel by Spanish author Miguel de Cervantes, the Impactor spacecraft, Hidalgo, will deliberately collide with a near-Earth asteroid much larger than itself – in the hope of achieving more success than the character in the novel. The Orbiter spacecraft, Sancho, will monitor the impact from a safe distance, like its namesake in the novel, Don Quijote’s sidekick Sancho Panza. Sancho will be the first to arrive at the collision site and will orbit the asteroid to observe it before, during and after its collision with Hidalgo. “In this way we hope to calculate any changes in the asteroid’s orbit (deflection) or rotation. Neither of the two measurements has ever been computed before.”

“The venture began with an industry-wide call for ideas in 2004. An ESA panel of experts called the Near Earth Objects Mission Advisory Panel (NEOMAP) selected the Don Quijote project, the only one to truly test a deflection measurement, going beyond a simple study or exploratory project.”

“However, the mission has not progressed beyond the paper stage,” points out Andrés Gálves, a representative of ESA’s Advanced Concepts Team (ACT), which is studying the mission in detail. “What is more, we are not yet talking of a mission, only a study. The concept is ready and the tests are conclusive but, as is so often the case, it takes time to bring a long-term project to fruition. The idea is feasible and the interest is there, so all we need now is for the political will to emerge,” adds Gálves.

Before departing for space, some major technological challenges will also need to be overcome. The target chosen for the first tests is intentionally remote from the Earth’s orbit, to avoid creating the very hazards that we wish to learn to avoid. Such a large distance makes it difficult to manoeuvre the observer spacecraft from Earth because it would take too much time to control at that distance. The spacecraft must therefore fly at high speed towards a small, dark object using an automatic pilot system. This means that it must detect the asteroid, correct the spacecraft’s trajectory and strike the asteroid as close as possible to its centre to have maximum impact.

This prolonged and meticulous preparation work may seem insignificant compared with the size of the universe and the unpredictable threats it harbours. However, the shield, tested by Don Quijote, could, as announced, already be in service by 2029, or else by 2036, another of the most likely dates for Apophis to approach the Earth in the next 100 years (although the probabilities are sure to change in line with future observations).

Delphine d’Hoop

  1. All unattributed quotes are from Willy Benz.


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Asteroids, comets and meteorites

The distinction between these space objects is not always obvious. Here is a quick recap for the uninitiated: an asteroid is an object orbiting around the sun, which varies in size from one metre to several hundred kilometres. The largest asteroid, Ceres, is about 1 000 kilometres in diameter. Composed mainly of rock and/or metals, asteroids originated from the innermost regions of the solar system – between the orbits of Mars and Jupiter.

Comets are made from ice and solid matter. They are commonly described as ‘dirty snowballs’. Unlike asteroids, they come from the outermost regions of the solar system. When a disturbance alters their trajectory, they enter the innermost regions and become visible from Earth. A meteorite is any object that enters the Earth’s atmosphere and lands on our planet.


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