Sky at Night’s Chris Lintott on why a space probe the size of a washing machine is visiting Pluto

As the 750th edition of The Sky at Night zooms in on the dwarf planet, the presenter explains the mission of the New Horizons space probe

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Tell us about the New Horizons probe.

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It’s about the size of a washing machine, with a radio dish attached that’s a little bit bigger than I am. The thing that limits what the probe can do is its power source.

Out by Pluto [about five billion km from Earth] there’s no hope of using solar power, because the Sun is too faint, so it carries a plutonium reactor.

That means it doesn’t have a huge amount of power, but it has cameras and a spectrometer, which will tell us what the surface of Pluto is made up of.

What’s the significance of the mission?

What’s really exciting is that it’s our first glimpse of a whole new part of the solar system.

We’ve never even tried to visit anything this far away before. We have had to build a probe that will still work after more than ten years in interplanetary space.

The science that we get back will be unprecedented, but we are relying on the engineering skill of those who put New Horizons together.

What could go wrong?

While the probe has been on its way, we’ve discovered new moons of Pluto.

We now know there are five, so it’s a lot more crowded than we thought it would be out there. Because the spacecraft is going so quickly, hitting something even the size of a grain of sand could be fatal.So the big hazard is whether it can navigate through this crowded part of space unscathed.

The other thing is that the probe won’t talk to Earth during the crucial part of the flyby, because it will be concentrating on photographing Pluto. So it’s only after the most perilous part of the mission that it will be able to talk back to us.

That will be a scary moment – waiting to see if it has carried out its mission.

Why isn’t the probe trying to land on Pluto?

This is the fastest vehicle we’ve ever sent to the outer solar system, travelling at 36,000mph
– 100 times faster than a normal plane – and it’s got up to that speed by slingshotting round Jupiter.

It would take a huge amount of fuel to slow it down from that speed, so that’s why this is just a brief flyby.

How much time will New Horizons have to photograph Pluto?

It’s already sending back images that are better than anything we get from telescopes, but the close-up encounter will last about a day or so.

What are you looking out for?

We know that Pluto is icy, but we don’t know much more than that. We know there is a complex mix of chemicals. What we will be looking out for is whether there is any activity, like ice volcanos. It may be that Pluto is

an active world. The reason we care is that Pluto is a relic from the very early days of the solar system, so we’re getting to see
a frozen piece of history.

How long will it take for the data to be beamed back to Earth?

Because it’s so far away, so small and so relatively low-powered, it will take quite a long time. We’ll get the data back over the next year. The team have made sure that the high-priority data will be sent back over the first few days. So we will have the best images early on.

Could we reinstate Pluto as a planet?

Whenever you go somewhere new, you find the completely unexpected.

There will be things that shock and surprise us. The New Horizons team is very keen that Pluto should be a planet – after all, their probe set off when it was still classified as a planet.

It doesn’t really matter to astronomers; it’s more about getting the first window on this remarkable little world. 

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The Sky at Night: Pluto Revealed is on Monday 20th July on BBC4 at 10pm