I am holding something remarkably old. It is older than any huam artifact, older than life on Earth, older than thecontinents大陆and the oceans between them. This was formed over four billion years ago in the earliest days of the solar system while the plants were still forming. Thisrusty荒废、生锈的lump ofnickel镍and iron may not appear special, but when it is cut open...you can see that it is different from earthly metals. Thispattern模式、方式、模型reveals metallic crystals that can only form out in space wheremolten溶化的metal金属can coolextremely非常、极端slowly, a few degrees every million years. This was once part of a much larger object, one of millions left over after the plants formed. We call these objectsasteroids小行星.

 Asteroids are our oldest and most numerouscosmic宇宙的neighbors. Thisgraphic图形、图样的shows near-Earth asteroids orbiting around the Sun, shown in yellow, and swinging close to the Earth's orbit, shown in blue. The sizes of the Earth, Sun and asteroids have been greatly exaggerated夸大 so you can see them clearly. Teams of scientists across the globe are searching for these objects. discovering new ones every day,steadily稳定地、坚固地 mapping绘制...的地图 near-Earth space. Much of this work is funded by NASA. I think of the search for these asteroids as a giantpublic works project, but instead of building a highway, we're charting outer space, building anarchive档案 that will last for generations.

These are the 1,556 near-Earth asteroids discovered just last year. And these are all of the known near-Earth asteroids, which at last count was 13,733. Each one has been imaged, cataloged and had its path around the Sun determinded. Although it varies from asteroid to asteroid, the paths of most asteroids can be predicted for dozens of years. And the paths of some asteroids can be predicted with incredibleprecision精确、准确. For example, scientists at the JetPropulsion 推进Laboratory predicted where the asteroid Toutatis小行星图塔蒂斯 was going to be four years in advance to within 30 kilometers. In those four years, Toutatis traveled 8.5 billion kilometers. That'sfractional很小的、很少的，微不足道的precision of 0.000000004.

Now, the reason I have this beautiful asteroid fragment is because, like all neighbors, asteroids sometimes drop by unexpectedly.

Three years ago today, a small asteroidexploded爆炸、over the city ofChelyabinsk

俄罗斯车里雅宾斯克，Russia.That object was about 19 meters across, or about as big as a convenience store. Objects of the size hit the Earth every 50 years or so. 66 million years ago, a much larger object hit the Earth, causing a massive extinction. 75 percent of plant and animal species were lost, including, sadly, the dinosaurs. That object was about 10 kilometers across, and 10 kilometers is roughly thecruising巡航、漫游altitude of a 747 jet. So the next time you're in an airplane,snag抓住、被构筑a window seat, look out and imagine a rock so enormous that resting on the ground, it justgrazes擦过yourwingtip翼梢.It's so wide that it takes your plane one full mimute to fly past it. That's the size of the asteroid that hit the Earth.

It has only been within my lifetime that asteroids have been considered a credible threat to our planet. And since then, there's been a focused effort underway to discover and catalog these objects. I am lucky enough to be part of this effort. I'm part of a team of scientists the use NASA's NEOWISE telescope. Now, NEOWISE was not designed to find asteroids. It was designed to orbit the eat                 h and look far beyond our solar system to seek out the coldest stars and the mostluminous明亮的，发光的galaxies. And it did that very well for its designed lifetime of seven month. But today,six years later, it's still going. We'verepurposed稍加修改it to discover and study asteroids. And although it's a wonderful little space robot, these days it's kind of like  a used car. Thecryogen制冷剂【化】that used torefrigerate冷藏itssensors传感器is long gone, so we joke that its air-conditioning is broken. It's got 920 million miles on theodometer里程表,but it still runs great and reliably takes a photograph of the sky every 11 seconds. It's taken 23 photos sice I began speaking to you.

One of the reasons NEOWISE is so valuable is that it sees the sky in thethermal infrared热红外波段.That means that instead of seeing the sunlight that asteroids reflect,NEOWISE sees the heat that theyemit发出. This is a vital capability since some asteroids are as drak as coal and can be diffcult or impossible tospot注意到with other telescopes. But all asteroids, light or dark, shine brightly for NEOWISE. Astronomers are using every technique at    their disposal to discover and study asteroids. In 2010, a historicmilestone里程碑was reached. The community, together, discocered over 90 percent of asteroids bigger than one kilometer across-objects capable of massive destruction to Earth. But the job's not done yet. An object 140 meters or bigger coulddecimate严重破坏a medium-sized country. So far, we've only found 25 percent of those.

We must keep searching the sky for neat-Earth asteroids. We are the only species able to understandcalculus微积分or build telescopes. We know how to find these objects. This is our responsibility. If we found ahazardous危险的、有害的asteroid with significant early warning, we couldnudge轻推it out of the way. Unlike earthquakes, hurricanes or volcaniceruptions喷发,an asteroidimpact撞击can beprecisely精确地predicted and prevented. What we need to do now is map near-Earth space. We must keep searching the sky.

Thank you.