Academic Reading — Test 17

Reading passage

For most of human history, the region of space just above the atmosphere was empty and untouched. That changed in 1957, when the first artificial satellite was launched, and it has changed far more dramatically in the decades since. Today, low-Earth orbit, the band of space stretching from roughly 200 to 2,000 kilometres above the planet, has become an unexpectedly congested place. It is home to thousands of working satellites that support communications, navigation, weather forecasting and scientific research. Yet alongside these useful machines travels a vast and growing population of rubbish, known to engineers as orbital debris or, more casually, space junk. This debris takes many forms. Some of it consists of entire satellites that have stopped functioning and now drift uncontrolled. Other pieces are spent rocket stages, abandoned after delivering their payloads. Far more numerous, however, are smaller fragments: flecks of paint, shards of metal, bolts and lens covers, and the broken remains of objects that have collided or exploded. The European Space Agency estimates that there are well over one hundred million fragments larger than one millimetre orbiting the Earth, although only the largest of these can currently be tracked from the ground. The smallest pieces are effectively invisible, yet they are far from harmless. The danger lies not in the size of the debris but in its speed. In low-Earth orbit, objects travel at velocities of around 28,000 kilometres per hour, fast enough that even a fragment the size of a marble carries the destructive energy of a small bomb. A collision at such speeds can disable a satellite instantly or puncture the hull of a crewed spacecraft. The International Space Station has occasionally been forced to adjust its orbit to avoid approaching objects, and its crew have on rare occasions sheltered in their return capsules as a precaution. Each collision, moreover, creates yet more fragments, which can in turn strike other objects. This self-perpetuating process has a name. In 1978, an American scientist named Donald Kessler described a scenario in which the density of objects in orbit becomes so high that collisions trigger a cascade, with each impact generating debris that causes further impacts. The result, now widely called the Kessler syndrome, could in theory render certain orbits unusable for generations. Although a runaway cascade has not yet occurred, the launch of large commercial constellations, sometimes numbering thousands of small satellites, has sharply increased the number of objects that must share the same crowded region. Some analysts warn that without coordinated action, the problem will only worsen. A range of measures is being developed to address the threat, and they fall broadly into two categories: prevention and removal. Prevention focuses on stopping new debris from being created. International guidelines now recommend that satellites be designed to move out of busy orbits at the end of their working lives, either by descending into the atmosphere to burn up or by climbing to a higher, less-used "graveyard" orbit. Operators are also encouraged to vent leftover fuel from rocket stages, since trapped fuel has caused explosions in the past. These guidelines, however, are largely voluntary, and compliance has historically been incomplete. Removal is technically harder and remains in its early stages. Several experimental missions have tested methods for capturing and de-orbiting dead satellites. Proposed techniques include nets, harpoons, robotic arms and even ground-based lasers intended to nudge fragments onto a path towards the atmosphere. One European mission, planned for the coming years, aims to capture a discarded piece of a rocket and drag it downwards so that it burns up harmlessly. Such missions are expensive, and a single one can typically deal with only one object at a time, so they are unlikely to clear the existing debris quickly. Nevertheless, engineers argue that demonstrating the technology is an essential first step. Ultimately, the challenge is as much political and economic as it is technical. Space has no single owner, and the nations and companies that launch satellites are not always willing to bear the cost of cleaning up after themselves. Some experts have suggested charging operators a fee for the orbital space they occupy, much as factories may be charged for the pollution they release. Others call for binding international rules to replace today's voluntary codes. What is clear is that low-Earth orbit is a shared resource of growing value, and protecting it will require cooperation on a scale that space exploration has rarely demanded before.