It is one of the world's largest radio telescopes, with a dish 300 metres across.

And I had seen pictures and video of this before I came here,

but, honestly, the sheer scale is impossible to get across on camera.

It is massive.

For 50 years, it's been listening to the sky, and one day,

it might just save the world.

Arecibo's been part of discovering the large-scale structure of the universe.

We've discovered pre-biotic molecules in distant galaxies,

the first millisecond pulsar, the first exoplanets.

The big one, which everyone knows about,

is the Nobel Prize for the discovery of the binary pulsar

and the way that its orbit decayed exactly as predicted by general relativity.

There's 50 years of discoveries here,

I could talk for 50 years on discoveries of Arecibo.

One of the most incredible things about the observatory is this,

the suspended platform that I'm standing on right now.

This entire colossal structure is suspended in the air on cables,

like a suspension bridge, there are no pillars underneath

holding this up and getting in the way.

All this weight is being carried by the three concrete towers around the dish.

And this platform isn't static: big parts of it move.

The telescope, by going a short way north and south,

can actually see about a third of the total sky from here.

The main bowl of this dish is spherical.

The complication, though, is:

a spherical mirror doesn't have a point of focus, like a hyperbolic mirror,

which we're used to with big optical telescopes and most big radio telescopes.

And so, in order to get the signal to a focus,

the Gregorian dome contains two smaller reflectors.

These smaller reflectors actually bring everything to a point focus.

That means, in the Gregorian dome, we can have much wider band receivers

and cover from 0.3GHz to 10GHz in frequency.

Arecibo can do something that most radio telescopes can't: it can transmit.

Famously, in 1974, it beamed the Arecibo message out to the stars,

but there are also some more practical uses

for sending big pulses of energy into space: radar.

Arecibo can bounce radar signals off planets, asteroids and, well,

anything nearby in the solar system.

And it can build up clear pictures of what's out there in the dark.

The advantage of using the radar imaging is

you can actually see the shape much better compared to optical imaging.

We can actually see boulders on the surface if they are close enough,

if it looks like a bone or if it looks like a... sweet potato!

The S-band transmitter is a high-powered transmitter

that works at 2380MHz of frequency, that's the S-band.

The whole transmitter's a megawatt of power.

This transmitter produces a beam of power,

and if you put your hand right through it you could get burned.

We mostly observe near-Earth asteroids.

When new asteroids are found their orbit is not very well defined yet,

the main thing that we can do is really enhance the orbit information.

You transmit a signal and when it hits a moving object

the wavelength of the signal changes depending on

if the asteroid is moving towards the observer or away from the observer.

In that way we can actually get an estimate, to millimetres per second,

of the velocity of the asteroid.

When new asteroids are found the distance can be determined

to hundreds of metres, even tens of metres.

We can get very accurate information of where the asteroid will be

from now to a decade forward, or even a century forward.

Asteroid Apophis, the preliminary orbiting information showed

that it actually might hit the Earth.

After it was observed with radar they could determine it will come super-close,

it will probably disturb some of the satellites that are orbiting Earth,

but it doesn't hit the Earth.

That's very important to know, because now

we know we don't have to send Bruce Willis to deflect the asteroid(!)

Of all natural catastrophes,

impacts are the one thing that we can actually avoid.

Earthquakes, volcanoes, they just happen and we can't actually help that.

But if we can observe an asteroid early enough

we have an opportunity to deflect it.

Thank you so much to everyone at the Arecibo Observatory.

If you'd like to know more about them and their discoveries

pull down the description for lots of links,

and if you want to see behind the scenes on this video

then Matt, who's on camera, has been taking a load of footage

and has put it together for the second channel.