阅读理解
NASA will crash a spacecraft into an asteroid (小行星) to try to change its orbit, attempting to prevent humans going the same way as the dinosaurs.
Earth is constantly being disturbed by small pieces of debris (碎片), but they usually burn up or break up long before they hit the ground. Once in a while, however, something large enough to do significant damage makes impact. About 66 million years ago, one such crash is thought to have wiped out the dinosaurs. Someday, something similar could end human beings—unless we can find a way to tackle it.
NASA's Double Asteroid Redirection Test (Dart) mission is the first attempt to test if such asteroid redirection is a realistic strategy: investigating whether a spacecraft can autonomously reach a target asteroid and intentionally crash into it, as well as measuring the amount of redirection. "If it works, it would be a big deal, because it would prove that we have the technical capability of protecting ourselves," said Jay Tate, the director of the National Near Earth Object Information Center.
The 610kg Dart spacecraft is scheduled to be launched at the target—the Didymos system—a harmless pair of asteroids consisting of a 163-metre "moonlet" asteroid called Dimorphos that orbits a larger 780-metre asteroid called Didymos (Greek for "twin"). The plan is to crash the spacecraft into Dimorphos when the asteroid system is at its closest to Earth—about 6.8 million miles away.
About 10 days before impact, a miniaturized satellite called LiciaCube will separate from the main spacecraft, enabling images of the impact to be relayed back to Earth. Combined with observations from ground-based telescopes, and an onboard camera that will record the final moments before the crash, these recordings will enable scientists to calculate the degree to which the impact has changed Dimorphos's orbit. The expectation is that it will change the speed of the smaller asteroid by approximately 1% and reduce its orbit around the larger asteroid.
Then, in November 2024, the European Space Agency's Hera spacecraft will visit the Didymos system and conduct a further close-up analysis of the consequences of this snooker (斯诺克) game, recording details such as the precise makeup and internal structure of Dimorphos, and the size and shape of the hole left by Dart. Such details are vital for transforming asteroid redirection into a repeatable technique.
Even then, it is impossible that any single redirection strategy would be enough. "The problem is that no two asteroids or comets are alike, and how you redirect one depends on a huge number of variables.There is no silver bullet in this game. What you need is a whole folder of different redirection methods for different types of targets," said Tate.
So, while this may be one small step towards planetary protection, many more are likely to be necessary to avoid destruction.
