# How satellites right themselves

Science demo superstar Dave Ansell explains how satellites and spacecraft orient themselves...
31 March 2020

## Interview with

Dave Ansell, Sciansell; David Rothery, Open University

## SATELLITE

Science demo superstar Dave Ansell explains to Adam Murphy how satellites and spacecraft orient themselves...

Adam - So, navigating in space seems to be a bit of a trick. So we moved to our next demo, which appears to be a stool sitting on a DJ turntable. Dave, what have we got going on here?

Dave - I thought I would pretend to be a spacecraft.

Adam - You're going to be what kind of spacecraft?

Dave - It doesn't really matter.

David - Be BepiColombo!

Adam - Okay. BepiColombo. What have we got going on here?

Dave - I have a problem. If I'm standing up, normally I want to turn around. All I do is I push on the floor. Floor gives an equal and opposite reaction to me, and I turn around. Dead easy. The problem is, if you're in space, you've got nothing to push against.

Dave - So this spinny stool is a good model of a spacecraft in one particular way, and that is I can't apply any force to the outside world. And so I can try and turn around and all I do is just kind of wiggle.

Adam - I assume spacecraft don't actually wiggle.

Dave - No, because engineers have worked out a solution to this problem.

Adam - So what is that solution?

Dave - What they have is a wheel.

Dave - Probably not a bike wheel, something more expensive and with better engineering in it. So what I can do now, is if I want to turn around, I can apply a force to the bike wheel. So if I push one way, the bike wheel applies a force to me in the opposite direction and I turn around. If I stop the wheel, I stop. If I spin the way the other way, I go the other way by doing this you can certainly turn round the spacecraft and then stop.

Adam - Is this what spacecraft are doing to turn around?

Dave - Exactly. They have, normally, at least three wheels on different axes. They can spin them up and slow them down and turn round, and there's probably some horrific gyroscopic effects, which I don't want to think about, but luckily someone else's engineering problem.

Dave - Indeed. The problem is if there's a force on you from solar wind, or even just light, light can apply a force to you, you'll start moving a bit. You can spin up, start moving, you can spin your wheel up and kind of slow yourself down. But the problem is at some point, you get to a point where you can't spin the wheel fast enough to stop moving, at which point you have a problem. They solve it for a while by throwing stuff out.

Adam - These highly scientific bags of rice?

Dave - My model of a rocket is to have a bag of rice. So if I'm turning this way and I throw a bag of rice out I can slow myself down a bit. And instead of throwing bags of rice, obviously you use hot gases in a rocket, but it's the same principle. You push something that way, it pushes you back, you stop moving so you can slow yourself down and you can slow your gyroscopes down. And it goes great until the forces build up and you've run out of fuel. At which point your spacecraft's dead. Cause if the spacecraft doesn't know where it's pointing and can't point at things, it's almost useless. You can't point your antenna back to home. You can't use your rockets to point in the right direction. Your spacecraft's dead.

Adam - I suppose if you want your telescope taking photos of stars, you don't want it pointed at your house.

Dave - Or spinning round very fast. It would be even worse.

Adam - Nice pictures though. So Dave, you mentioned BepiColombo is going to split in half as it goes around. How is it going to control doing that?

David - It's built in two parts. The Japanese one will be spun up and set loose, and the European one will be concentrating on looking at the surface. It has reaction wheels to flip it round. All spacecraft that need to point in different directions, use these reaction wheels, because you can get electricity from the Sun to drive them and you can use those to steer you without having to vent rocket fuel all the time. Yes. Excellent demonstration of how spacecraft are steered.

Now where's Dan G who's aged 11 because Dan's got a question for you, David Rothery about volcanoes. Hello.

Dan G - How many volcanoes are in the solar system?

David - Okay, Dan, do you mean volcanoes that are still erupting or that are extinct?

Dan G - That are still erupting.

David - Still erupting. Okay. That's easier. Thank you. On Earth, that have erupted in the past 10,000 years and that might still erupt, there are several hundreds. On Jupiter's moon Io, there are again, hundreds which are erupting and these are both erupting molten rock. There are volcanoes, if you want to call them that, on Saturn's moon Enceladus, which is venting ice particles into space like an explosive eruption. They're called cryovolcanoes, icy volcanoes. And there are several fissures near the South pole that are venting those. Those are the active places for volcanism and cryovolcanism.

And then Mercury has got a hundred or so of these explosive volcanoes that I mentioned just now, but they're extinct. They're not operating today and unlikely to erupt ever again. Almost every rocky surface, Venus, we think might have active volcanoes today, there's a big push to get another mission back to Venus, which can nail that, but the whole surface of Venus, 90% of it is made of volcanic rocks. So they've been erupted from volcanoes. So volcanoes are everywhere on icy moons, on rocky moons and on rocky planets. It's a very important process in forming the crust of the planet to begin with.

Chris - David Rothery. Thank you very much.