科技英语新闻：美国国家航空和宇宙航行局的下一个目标：火星，土卫六或彗星？

As for the Comet Hopper, if it gets the green light, it will be visiting an object that's like a time capsule from the earliest days of the solar system. The planets and the major moons have all been squeezed and heated by gravity and pummeled by giant impacts, erasing much of their history. Comets, by contrast, are relatively pristine — at least, below their outer crust. As 46P/Wirtanen is warmed by the sun, some of that subsurface material heats up and spews out. By being there to sample it — and especially by sampling it at different points on the surface — the Hopper will be taking a whiff of the solar system's original material.
Now, says Paul Hertz, chief scientist at NASA's Science Directorate in Washington, D.C., comes the hard part. "All three missions are absolutely worth doing, all of them have very good plans," he says. Or at least, they have good concepts. "The teams have only done a little bit of work so far," he says. (Many of NASA's million or so Twitter followers have already begun tweeting their own votes, but these, unsurprisingly, won't count.)

The next round of proposals, which NASA is funding to the tune of $3 million each, and which are due in nine months or so, have to be a lot more detailed. Each will have to pass muster with two different panels of experts: a group of scientists who will take a hard look at the potential payoffs being touted by each team, and a group of engineers and project managers who will look with professionally skeptical eyes at the proposed budgets. "We'll be paying a lot of attention to bringing these in on budget," says Hertz. "In the end, it will probably come down to how believable their plans are."

Whoever wins gets the grand prize: NASA's blessing and, more important, $425 million (in 2010 dollars) to complete the mission. But each of the finalists already has Hertz's respect. "If you give the science community a challenge," he says, "they're really creative and clever about doing great science."