What's space made up of?

克里斯·史密斯问朗帝国理工学院don's Toby Wiseman.

Toby - Space is pretty empty compared to what we're used to on Earth, the atmosphere and rocks.

Chris - Has anyone actually measured it then? If I took a metre cubed of air on the Earth's surface, how many atoms and molecules would be in that compared to if I repeated that experiment out beyond the moon?

Toby - I think it is approximately one neutron or proton per metre cubed in space.

Andrew - That's roughly what I recollect, it being as thin as that.

Toby - But I'm not completely sure it's uniformly distributed in that way, it tends to amass into clouds. So there'll be less in some regions and more in others, but our solar system essentially has an enormous bomb at its centre, a massive nuclear fusion reaction - that's the sun. And it is pumping out all sorts of stuff: it's pumping out radiation, it's pumping out charged particles, the solar wind. And so whilst that isn't very dense compared to the densities we are used to here on Earth, there is certainly a lot of stuff in the solar system. And in fact, interestingly, I remember a couple of years ago there was that wonderful story where the Voyager spacecraft actually left what's called the heliopause, which is actually the point where the solar wind bangs into the interstellar medium, the stuff that's actually out there in deep space and forms the edge of our solar system. And so there certainly is stuff, there's not very much, but there's definitely stuff there.

Chris - Voyager very much in the news recently because NASA sent an incorrect command which had Voyager deflect its dish a bit which put it off of that alignment with the Earth's signals. I think the Australians were able to use an extremely big dish to send some signals and they said they've detected the heartbeat of Voyager 2. So they think they've got it back, don't they?

Toby - How awful would you feel if that was you.

Chris - NASA obviously realised that humans make mistakes when they launch Voyager because it resets itself on axis automatically every x number of months. So they said it's all right because, come October, it will be back anyway. But that would be a while to wait, wouldn't it? You'd be nervous. You'd think, 'I was the guy who killed Voyager.'

Toby - It's been going for a while, though.