Engineering news
The spacecraft for the first UK led European Space Agency science mission in 30 years is leaving the Airbus Defence and Space facility in Stevenage for final testing before its launch in September.
The Lisa (Laser Interferometric Space Antenna) Pathfinder spacecraft will demonstrate key technologies needed to find and measure gravitational waves in space. These space ripples are predicted by Einstein's theory of relativity, but their detection is difficult from Earth.
The mission has two main objectives, to show that an interferometer can be flown in space, and that the movement of the spacecraft will not effect the readings from the interferometer.
After launch from French Guiana, Lisa Pathfinder will travel 1.5 million kilometres over 10 weeks to the first Earth-Sun Lagrange point. This location has the minimal amount of gravity, magnetic, atmospheric and thermal disturbances needed to enable the testing of the equipment.
Lisa Pathfinder is equipped with several high precision systems, including micro-propulsion thrusters and a drag-free control system. The spacecraft contains a specially-designed inertial sensor, that consists of two electrostatically-suspended 4.5cm wide gold blocks connected by a laser interferometer. The sensor works by measuring the movement of the two gold “test masses” relative to each other. Gold is used because of its low magnetic properties and relative heaviness.
Andy Wealthy, systems engineer on the project, said: “This is as quiet a platform as possible. It's the first time we've flown an interferometer. We need to know that the acceleration on the test masses is low enough, so that when we use the laser interferometer we know the system is measuring gravitational waves and not the acceleration of the spacecraft.”
The systems onboard Lisa Pathfinder are designed to keep the test masses as stable as possible. Data about their position is used to compensate for movement of the spacecraft – even solar wind is compensated for. Displacement of the test mass is also measured using a high precision metrology system.
The project, which started in 2001, has been subject to delays because of funding problems. Engineers started building Lisa Pathfinder at the Airbus Defence and Space facility at Stevenage in 2008. The spacecraft has been built in two parts, a propulsion module and a module that contains the scientific instruments. The modules were integrated today before being sent to IABG (Industrie Anlagen Betriebs Gesellschaft) near Munich in Germany, where the spacecraft will undergo thermal, EMC, mechanical and acoustic tests.
The spacecraft's micro-thrusters use two sets of six “cold gas” micro thrusters that run off nitrogen. The thrusters are so small 4000 of them would be required to lift a sheet of paper on Earth. However, they are accurate enough to position the spacecraft to within one 10,000th of a hair's breadth while in space.
The spacecraft also carries a propulsion technology being tested for Nasa's Jet Propulsion Laboratory. It's Disturbance Reduction System uses collodial thrusters. These work by ionising the collodial liquid so that molecules are fired from the thrusters to emit a force even smaller than the micro-thrusters.
Once these technologies are demonstrated, the final Lisa mission to detect and measure gravitational waves is scheduled to launch in the early 2030s.
