Posted January 11, 2016

A thruster to achieve perfect rest

ESA's recently launched LISA Pathfinder spacecraft uses special thrusters to make sure the craft experiences no forces at all.

You probably never flew on a parabolic flight, as most people haven't. But I did, way back in 1991. On such a flight a plane goes into free fall, exposing people and stuff inside to microgravity. I've done some fun things in my time but this was certainly one of the high points of my career. I have the pictures to prove it.

The word 'microgravity' is important here. At the time I was told that to make the plane fall as freely als possible, the pilot has to work quite hard. There is air resistance and turbulence for instance, and the forces involved are dependent on many factors such as the speed and orientation of the plane. If the plane gets slowed down just a bit by air resistance, everything inside tends to drift to the front. There is no free fall, and the pilot needs to work his controls to simulate one. So there is no 'weightlessness' either, just almost that, and it's called 'microgravity.' Doing nothing would only lead to a first approximation of free fall and for training and testing purposes in the space business that's not good enough.

Even in a space station experts maintain there is no weightlessness, no zero gravity. There are people and machines at work, causing vibrations - sorry. So where do you have that precious zero gravitational force? Out in the solar system, on orbit around the Sun? Not even there, as the case of LISA Pathfinder shows.

Pathfinder is some sort of test flight itself. It tests a technology to measure gravitational waves. These waves are elusive things, very difficult to measure. Any force or unintended movement will drown them out. In the vacuum of space, in a supposedly undisturbed orbit there is still a disturbing force at work: radiation pressure from the Sun. It's the same force that propels solar sails, which were featured on this blog before.

The solution is very much like the pilot of a parabolic flight working his controls to compensate for air resistance and such: ESA has installed thrusters to compensate for radiation pressure. These thrusters are called colloid micronewton thrusters and they were developed at NASA's Jet Propulsion Laboratory. They work by accelerating minute charged droplets in an electric field until they get spit out of the thruster's nozzle at high speed. It's not unlike the ion engines that were used to accelerate and brake Dawn, the craft that is currently orbiting and photographing the dwarf planet Ceres. Only in that case ions - charged atoms - are used as a propellant instead of small droplets. (Story continued after video.)

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Pathfinders's thrusters can generate 30 micronewtons - equivalent to about the weight of a mosquito. This should suffice to control the position of the probe to within a millionth of a millimetre. If the test succeeds, a future mission to detect gravity waves 'for real' may consist of three similar spacecraft, each equipped with colloid micronewton thrusters to ensure perfect zero gravity.


About Herbert Blankesteijn

Herbert Blankesteijn is a technology journalist from the Netherlands who has written for many prominent Dutch newspapers. He presented and directed television and radio programmes and has 10 books to his name. Herbert is interested in nascent fields such as 3D printing, drones, robotics and the private space business.