未来需要对行星撞地球的预警

在美国的弗吉尼亚州，一颗行星就这样撞上一名医生办公室的屋顶，幸运的是没人受伤。这给我们留下无尽遐想……如果在不久后的某天，我们的的地球被庞大的未知行星撞上，这将会是怎样的未来？

MADELEINE BRAND, host:

A meteorite crashed through the roof of a doctor's office in Virginia earlier this week. No one was hurt. Still, it's a reminder of the space rocks that bombard our planet. They are usually really tiny. A few years ago, Congress told NASA to conduct a search for the big ones that might cause serious damage if they hit Earth.

But as we hear from NPR's Nell Greenfieldboyce, a new report says there is no way NASA will meet its deadline.

NELL GREENFIELDBOYCE: Congress wants NASA to be looking for icy comets and rocky asteroids close to earth that are bigger than about 450 feet across -bigger than, say, a football field. NASA is supposed to find 90 percent of them by the year 2020, but here is the problem.

Professor MICHAEL A'HEARN (University of Maryland): No matter what we do, it's now too late to meet the 2020 deadline that Congress set four or five years ago.

GREENFIELDBOYCE: Michael A'Hearn is an expert on comets and asteroids at the University of Maryland. He served on a National Research Council committee that's just put out a new report called, Defending Planet Earth. He says Congress didn't give NASA any special funding for the search, and the agency didn't ask for any.

Prof. A'HEARN: If you've set a deadline and then don't fund it, it's not going to happen.

(Soundbite of laughter)

GREENFIELDBOYCE: He says NASA could finish the job close to the deadline if it built and launched a new space telescope. Or, it could construct a new ground-based telescope and reach the goal by 2030. But either way, this would cost money.

Prof. A'HEARN: You're talking about a billion dollars over 20 years. That's $50 million a year.

GREENFIELDBOYCE: Far more than the $4 million a year that NASA currently spends. He says how much the government wants to devote to these searches is fundamentally a political decision. A huge impact that could wipe out humanity comes along only once in a hundred million years. But a strike that could potentially flatten, say, a city might occur as often as once every few centuries. A NASA official said the agency has just received the new report and it will take a couple of months to study it and respond to the committee.

Nell Greenfieldboyce, NPR News.