Academic Reading — Test 73

Reading passage

For most of human history, the threat posed by asteroids was treated as a matter for storytellers rather than scientists. The discovery that a large impact had contributed to the disappearance of the dinosaurs, however, gradually changed the way researchers regarded these wandering fragments of rock and metal. Today, a field known as planetary defence is devoted to finding the objects that orbit near the Earth, calculating their paths, and developing methods to alter those paths should a collision ever appear likely. The work is unusual among scientific endeavours because its central aim is to ensure that a particular event never happens at all. The first task of planetary defence is detection. Astronomers use ground-based telescopes to scan the sky night after night, recording faint points of light that shift position against the fixed background of stars. When a new object is found, its brightness and motion are measured, and its orbit is calculated and refined over subsequent observations. Most of the objects catalogued in this way are harmless, passing at vast distances, yet a small proportion are classified as near-Earth objects because their orbits bring them relatively close to our planet. Smaller bodies are far more numerous than large ones, and because they reflect little light they are correspondingly harder to spot. A rocky object only a few tens of metres across could still flatten a city, which is one reason why surveys aim to catalogue objects well below the size associated with global catastrophe. Knowing where a dangerous asteroid is, of course, is of limited value unless something can be done about it. Several techniques have been proposed for changing an asteroid's course, and most share a common principle: a small change applied early is far more effective than a large change applied late. Because an asteroid travels enormous distances between any warning and a potential impact, even a tiny adjustment to its speed, made years in advance, can cause it to miss the Earth by a wide margin. The most direct approach is the kinetic impactor, in which a spacecraft is deliberately crashed into the asteroid at high speed. The collision transfers momentum to the body and shifts its orbit by a small but measurable amount. In 2022 this idea was tested for the first time by the Double Asteroid Redirection Test, known as DART. The mission's target was Dimorphos, a small moonlet circling a larger asteroid called Didymos. This pairing was chosen deliberately, because the time Dimorphos took to travel once around its companion could be measured precisely from the Earth, and any change in that period would reveal how much the impact had altered its motion. Neither body posed any danger to our planet; the encounter was a controlled experiment rather than an emergency response. The spacecraft struck Dimorphos at roughly six kilometres per second, and subsequent observations showed that its orbital period had been shortened by about half an hour, a far greater effect than many researchers had predicted. The unexpectedly large result is thought to be explained by the debris thrown out by the collision. When the spacecraft struck, it ejected a plume of rock and dust into space, and the recoil from this material pushed the asteroid more strongly than the impact of the spacecraft alone would have done. The size of this effect depends heavily on the composition and structure of the target, factors which vary considerably from one asteroid to another. A loosely bound pile of rubble behaves quite differently from a solid lump of rock, and this uncertainty means that the outcome of any future mission cannot be predicted with complete confidence. For this reason a follow-up European spacecraft was later sent to study the same system in detail, examining the crater and measuring the mass of Dimorphos so that the experiment could be properly understood. DART demonstrated that a kinetic impactor can work, but it is not the only tool that has been considered. A so-called gravity tractor would station a spacecraft near an asteroid and use the faint mutual pull between the two to tow it slowly off course, an extremely gentle method that would require many years to take effect. Other proposals, more speculative and controversial, involve detonating a nuclear device close to the surface to vaporise part of it and drive the remainder aside. Whichever method might one day be used, all of them depend on the same thing: sufficient warning. The longer the interval between detection and a possible impact, the smaller the nudge that is needed, and the better the chance of steering the danger away. Planetary defence is therefore less about dramatic intervention than about patient vigilance, watching the sky so that, if a threat ever does emerge, there will be time enough to respond.