Finding our way through space
If you want to get from A to B on Earth you can pull out yourphone, turn on the GPS, pull up my favourite "map app" and hey presto; but what about navigating in space? Even in our own solar system the distances are measured in millions or even billions of kilometres, journeys take years to complete and there's no GPS out there. Nick James works on space navigation for BAE systems, and he spoke to Chris Smith about the challenges spacecraft are facing...
Nick - If you imagine we've got a spacecraft that's going say between the Earth and Mars, it's primarily being affected by the Sun's gravity. We also have to deal with other gravitational forces as well so all the planets in the solar system will exert a gravitational force on that spacecraft to a greater or lesser extent. The Sun is usually the dominant one but other planets, like Jupiter, have got significant gravitational force and, if you're going from Earth to Mars, when you're close to Earth or close to Mars, they do as well.
Chris - Presumably we know where the planets are going to be at any given time, so clever mathematicians can work out roughly what sort of gravity you're going to feel from the planets across your journey?
Nick - That's right. We know the positions of the planets in the inner solar system, probably with an accuracy of a few hundred metres at any particular time. And even the larger planets in the far solar system we know to a kilometre level accuracy. So what we can do in terms of the forces on the spacecraft, the gravitational forces, is that we know where the planets are, and we have an estimate of where the spacecraft is, so we can work out those gravitational forces on the spacecraft.
We do though then have other forces which act on the spacecraft which are not gravitational forces and they're called, not surprisingly, non-gravitational forces. And those forces range from things like solar radiation pressure where, essentially, just sunlight falling on the spacecraft exerts a small acceleration on the spacecraft.
Chris - How?
Nick - It's basically momentum recoil. So you have photons coming from the Sun. If you imagine you've got a spacecraft with solar panels and Rosetta's a good example of this. It's got huge solar panels because it had to go a long way out in the solar system. It's got an area of about 64 square metres. And all the solar photons that fall on those solar panels panels, some of them get absorbed, but some of them will get reflected and, essentially, a photon has momentum and that reflection transfers momentum to the spacecraft. So, essentially, there is a very small pressure exerted on those solar panels just by the sunlight that's falling on them.
Chris - How big is it?
Nick - Tiny.
Chris - If I like laid a human hair on a tabletop, is it sort of equivalent to that, or more, or less?
Nick - In units it's about nine micronewtons per square metre.
Chris - So that's a lot of zeros after the decimal point before you get to a big real number?
Nick - Yes.
Chris - So it's a tiny force, but your point is that because these missions are seven to ten years in the making -they're flying for that long to reach their target - a tiny, tiny force, but over a really long period of time, means that it could turn into a really quite large error by the time you get where you're going?
Nick - Yes, that's right. And it's particularly important because unlike if you're navigating with GPs where, essentially, every moment you get a precise measurement of where you are - your position, when we're navigating through space we don't have that.
Basically, we have three different kinds of measurements. We have doppler, we have a ranging measurement which allow us measure the distance to the spacecraft. So both of those are radial - just in one direction. And we have another measurement called deltadore which, actually, gives us an angle to the spacecraft. But all of those don't directly give us the position of the spacecraft.
So what we have to do to determine the position of the spacecraft is we have to make an estimate of where it was. We then integrate that forward using an orbit estimator, which essentially takes into account the forces acting on the spacecraft and then produces a new set of positions. When we do that with a particular model of the orbit, we can then predict what the observations should be, so we can then predict what the range and doppler and the deltadore should be. Using that model we can then compare that with what we actually measure and see if they compare and, hopefully, if we've got the orbit right - they do.
Chris - Now thinking science fiction hundreds of years hence when there are spacecraft zipping all over the solar system, will there be the equivalent of GPS for our solar system? How would you if you were designing the nav system for the spacefarers around the solar system of tomorrow, how would you set it up?
Nick - Well there are all sorts of possibilities and one of the most interesting ones is to use a natural GPS system called pulsars. Pulsars are objects which are neutron stars, the stars that have formed at the end of a star's life. Essentially, they are very, very accurate clocks. If you have an X ray detector you can detect those clocks. And, essentially, because these pulsars are in all sorts of different directions, but they have very well known positions, if you had a suitable detector on board your spacecraft you could use these natural objects as a kind of GPS system. It's very similar to GPS, you're measuring arrival times and determining your position from that. The big problem there is that the detector, so designing a detector for a spacecraft that could detect enough pulsars to make it practical. But if we're talking a hundred year's time, I'm sure that's a possibility.