Academic Reading — Test 18

Reading passage

Reaching the cold, distant reaches of the outer Solar System presents engineers with a stubborn problem of energy. A spacecraft bound for Jupiter, Saturn or beyond must climb continuously against the gravitational pull of the Sun, and the further out it travels, the more velocity it needs to acquire near the start of its journey. Carrying enough propellant to supply this velocity directly would make a probe impossibly heavy, since every additional kilogram of fuel demands still more fuel to lift it. For several decades, mission planners have solved this dilemma not by burning more propellant but by exploiting the motion of the planets themselves, in a technique known as the gravity assist or, more vividly, the slingshot manoeuvre. The principle rests on a careful exchange of momentum between a small spacecraft and a vast planet. As a probe approaches a planet, it falls into that body's gravitational field, accelerates, swings around it along a curved path, and then departs. Viewed from the planet, the spacecraft leaves with exactly the same speed at which it arrived, so no energy appears to have been gained. The crucial point, however, is that the planet is not stationary; it is orbiting the Sun at considerable speed. When the encounter is arranged so that the probe emerges travelling in roughly the same direction as the planet's orbital motion, the spacecraft borrows a tiny fraction of the planet's orbital momentum and leaves the encounter moving faster relative to the Sun. The planet, in turn, is slowed by an utterly negligible amount, because its mass is so enormous that the change cannot be measured. A common misunderstanding is that the spacecraft somehow steals energy from nothing. In reality, the manoeuvre conserves both energy and momentum across the whole system; the gain in the probe's speed is precisely balanced by a microscopic loss in the planet's. Engineers can also run the process in reverse. By passing behind a planet relative to its direction of travel, a spacecraft can deliberately give up speed, which is useful when a mission needs to slow down in order to settle into orbit around a target or to drop closer to the Sun. The same physics therefore serves to accelerate, to decelerate, or simply to bend a trajectory onto a new heading without expending any fuel at all. The history of space exploration is rich with examples of the technique. The two Voyager probes, launched in 1977, took advantage of a rare alignment of the outer planets that occurs only about once every 176 years. Voyager 2 visited Jupiter, Saturn, Uranus and Neptune in succession, using each planet to fling it onward to the next, an itinerary that would have been wholly unaffordable by rocket power alone. The Cassini mission to Saturn was even more elaborate, gathering speed from Venus twice, then from Earth and finally from Jupiter before arriving at its destination. Such tours demand extraordinary precision, for the spacecraft must arrive at each planet at exactly the right moment, having travelled for years across hundreds of millions of kilometres. This precision is, in fact, the principal cost of the method. A gravity assist saves propellant, but it does not save time, and it constrains a mission to launch within a narrow window when the planets are suitably placed. If that window is missed, planners may have to wait months or years for the geometry to repeat. Trajectory designers must also accept longer and more circuitous routes; a probe may be sent first towards the inner Solar System, looping past Venus or Earth, before it is finally hurled outwards. The calculations involved are formidable, since the gravitational influence of every relevant body must be modelled, and a small error early in the flight can grow into a large deviation by the time the probe reaches the outer planets. Despite these constraints, the gravity assist remains indispensable for ambitious exploration. The New Horizons probe, which reached Pluto in 2015, used a single encounter with Jupiter to shorten its journey by several years. Without such assistance, the most distant worlds would lie effectively beyond reach, demanding rockets larger and costlier than any space agency could justify. By treating the planets as free and reusable sources of momentum, engineers have turned the very gravity that makes the outer Solar System so hard to attain into the means of attaining it.