Academic Reading — Test 156

Reading passage

For centuries, comets were regarded as little more than spectacular but mysterious visitors, streaks of light that crossed the night sky and then vanished for generations. Modern astronomy has reframed them as ancient relics: clumps of ice, dust and frozen gases that have survived almost unchanged since the birth of the Solar System more than four and a half billion years ago. Because they formed in the cold outer regions of the early Solar System and have spent most of their existence far from the warmth of the Sun, comets preserve a chemical record of the raw materials from which the planets were assembled. Studying their composition is therefore less an exercise in observing a passing curiosity than an attempt to read a frozen archive of our own origins. The European Space Agency's Rosetta mission was conceived to examine one such object at unprecedented close range. Launched in 2004, the spacecraft did not travel directly to its target. Instead it followed a long and indirect route, swinging past the Earth and Mars several times so that the gravity of those planets could accelerate it and adjust its path, a technique known as a gravity assist. The journey lasted ten years and covered several billion kilometres before Rosetta finally arrived at its destination, a comet officially designated 67P/Churyumov-Gerasimenko, in 2014. For roughly thirty months of that voyage the spacecraft was placed in hibernation to conserve power, with almost all of its instruments switched off, before being woken by a pre-programmed signal. Comet 67P proved to be a stranger object than many scientists had anticipated. Rather than a single rounded lump, it has an irregular two-lobed form, often compared to a rubber duck, with a narrow neck joining the larger and smaller sections. The comet measures only about four kilometres across at its widest, yet its surface displays a surprising variety of terrain, including cliffs, pits, boulders and broad dust-covered plains. Its density is very low, far lower than that of solid rock, which indicates that the body is highly porous and contains a great deal of empty space within it. Such fragility suggests that the comet is a loosely bound aggregate rather than a compacted mass, a finding that has shaped later thinking about how such objects first came together. A central scientific goal of the mission concerned water. Water is abundant on Earth, but its ultimate source has long been debated, and one popular idea held that much of it was delivered by comets striking the young planet. To test this, instruments aboard Rosetta measured the ratio of ordinary hydrogen to a heavier form called deuterium in the water vapour escaping from the comet. This ratio acts as a chemical fingerprint, because it varies depending on where and how the water originally formed. The measurement showed that the water on 67P contained considerably more deuterium than the water in Earth's oceans. The mismatch implied that comets of this kind were unlikely to have been the principal source of terrestrial water, and it strengthened the rival view that asteroids played a larger part. The composition of the comet extended well beyond ice. As 67P approached the Sun and warmed, frozen material turned directly into gas in a process called sublimation, releasing a faint atmosphere of gas and dust around the nucleus. Within this haze the spacecraft detected a rich mixture of organic molecules, including compounds containing carbon that are regarded as the chemical building blocks of life. Among the substances identified was glycine, a simple amino acid, alongside phosphorus and a range of other carbon-bearing molecules. The presence of such material did not prove that comets seeded life on Earth, but it lent support to the broader idea that the ingredients for life are widespread in space rather than unique to our planet. Rosetta was not alone. It carried a small lander named Philae, which separated from the main spacecraft and descended to the comet's surface in November 2014, the first time any craft had achieved a controlled landing on a comet. The descent did not go entirely as planned. Harpoons designed to anchor the lander failed to fire, and Philae bounced twice before coming to rest in a shaded location where its solar panels received too little sunlight. With limited power, it gathered data for around sixty hours before falling silent, yet in that brief window it returned valuable measurements from the surface itself. The mission as a whole continued until September 2016, when the orbiter was deliberately brought down onto the comet, ending more than two years of close observation. The data it returned continue to be analysed, and Rosetta remains one of the most ambitious attempts yet made to understand the frozen remnants of the Solar System's formation.