英语听力：探索发现 2014-06-04 BBC 地平线：陨石的真相-10

Heset out to investigate a hundred-year-old theory that said asteroids werepowered by the Sun itself. It was called the Yarkovsky effect. The Yarkovskyeffect is a very small acceleration and acts on asteroids and what it is is ifyou take a model, the

英语听力：探索发现 2014-06-06 BBC 地平线：陨石的真相-11

Weknew from the radar measurements where Golevka was within a few tens of metres.And yet it was actually 12 or 15 kilometres away from where it was predicted tobe without Yarkovsky effect. So these very precise radar observations allowedus to see the

英语听力：探索发现 2014-06-08 BBC 地平线：陨石的真相-12

But there is a downside to Jupiter. It can also deflect asteroids into orbits that cross the Earths path. The Chelyabinsk meteor appears to be one of these typical Earth-crossing events. The likelihood is that it was thrown out of its regular orbit b

英语听力：探索发现 2014-07-13 BBC 地平线：陨石的真相-27

On 6th October 2008, asteroid hunter Richard Kowalski saw something that would change the assessment of threats presented by asteroid impacts. The night was proceeding normally and up on the screen came another asteroid. As I continued to make observ

英语听力：探索发现 2014-07-11 BBC 地平线：陨石的真相-26

Tim has developed a map to visualize their location. And on that map, the most important are the Near-Earth asteroids, the ones closest to the planet. On the screen here is a map of the solar system. And Ive got the Sun in the centre and the third pl

英语听力：探索发现 2014-07-09 BBC 地平线：陨石的真相-25

Luckily, the very biggest asteroids are few and far between. But there are still plenty of rocks out there that represent a significant danger to us. So, at the summit of an extinct Hawaiian volcano, Professor Nick Kaiser and his colleagues are searc

英语听力：探索发现 2014-07-06 BBC 地平线：陨石的真相-24

The meteorite was 15 kilometres across, enough to cause utter devastation across the whole planet. It exploded with a force of 100 million million tones of TNT. The blast sent a giant plume of vaporized rock out into space. A crater was punched 30 ki

英语听力：探索发现 2014-07-04 BBC 地平线：陨石的真相-23

As the divers have explored further, they discovered the Siluteis are actually part of a huge complex of tounels and caves.In fact, when you look from above, you can see Senluties scattered across hundreds of kilometers.And when they were mapped, it

英语听力：探索发现 2014-07-02 BBC 地平线：陨石的真相-22

Look at the size of this! This is magnificent! That is beautiful. This cave may be stunning, but it provides the evidence for one of the greatest catastrophes in the Earths history. And that water, its so clear! Lower the gear, please! Theres actuall

英语听力：探索发现 2014-06-27 BBC 地平线：陨石的真相-20

Oh, perfect, perfect, perfect. Now were seeing the fireball come in, its brighter than the Sun and then, Kapow, it hits the surface. Jeez! This whole region, downrange, would have been incinerated. It would have been incinerated just by this plasma,

英语听力：探索发现 2014-06-25 BBC 地平线：陨石的真相-19

So this was serial number one. It was built during the Apollo time. I guess because they thought there would be several of them made, but this is the first one and the last one and is the only one like it in the world. This is NASAs Vertical Gun Rang

英语听力：探索发现 2014-06-22 BBC 地平线：陨石的真相-18

Down on the ground, the scale of impact is even more breathtaking.The crater is more than 1000 meters across and nearly 200 meters deep.The forces here were enoumous.The impact on this solid rock into this pulverized mochi,just bustled in your hand.Y

英语听力：探索发现 2014-06-20 BBC 地平线：陨石的真相-17

Really, its a balance between the size of the object, its speed into the atmosphere and critically the altitude at which it explodes. Too high, if its too small and it explodes too high, the shockwave has little effect on the ground. If its quite low

英语听力：探索发现 2014-06-18 BBC 地平线：陨石的真相-16

Most of the damage from an explosion like this is actually the blast wave. Its the very high winds. Mark created a simulation to see what size an asteroid would need to be to be able to generate such destructive power. In this simulation I include mo

英语听力：探索发现 2014-06-15 BBC 地平线：陨石的真相-15

On June 30th, 1908, a huge explosion tore through the forest of Tunguska, Siberia. It was 20 years before the Russians mounted an expedition to the site. What they found astonished them. 60 million trees across an area the size of London had been lev

英语听力：探索发现 2014-06-13 BBC 地平线：陨石的真相-14

So was this a near miss for us? If the asteroid had been in a different part of its orbit, so it didnt hit this year but it hit next year, it would have still hit us on February 15th. But instead of coming in over Russia, it would have come in over t

英语听力：探索发现 2014-06-11 BBC 地平线：陨石的真相-13

There it is! Yep. Its about 90 kilometres up, at that stage, travelling at 17.5 kilometres per second. Using the different camera positions, scientists have pinpointed the exact position at which the meteor entered the atmosphere. And by tracking the

英语听力：探索发现 2014-06-29 BBC 地平线：陨石的真相-21

Comparing the effects of an airburst with a ground strike, it seems the Chelyabinsk got away lightly. Its estimated that the largest piece to hit the ground weighed 500 kilos, a fraction of the asteroids original mass of 7,000 tonnes. Now if a piece

英语听力：探索发现 2014-07-16 BBC 地平线：陨石的真相-28

An asteroid strike would create a huge explosion. NASA feared this might even be mistaken for a nuclear bomb. We wanted folks to know this was a natural event by Mother Nature rather than some sort of man-made event like a missile or something dreadf

英语听力：探索发现 2014-07-18 BBC 地平线：陨石的真相-29

Approaching from beneath the planet, asteroid 2012 DA14 passed inside the orbit of our geostationary satellites before heading off to the north. This asteroid had been successfully tracked for a year. And despite its proximity, scientists knew that i