Space

A pathfinder

Print edition : January 22, 2016

The LISA Technology Package measures and controls the near-perfect free fall of two test masses in their vacuum chambers. Photo: ESA/ATG Medialab

AFTER a technical glitch on the scheduled launch date on December 2, 2015, the European Space Agency (ESA) launched its LISA Pathfinder (LPF) satellite on December 3 by a Vega rocket from the European Spaceport at Kourou, French Guiana. LISA Pathfinder will demonstrate novel technologies for the planned gravitational-wave observatory called the Evolved Laser Interferometer Space Antenna (eLISA), which scientists hope will one day capture the sound of the universe. The project has taken more than 10 years of R&D. Karsten Danzmann, director at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute), a leading partner in the mission, says the Pathfinder will demonstrate crucial technologies for eLISA and other future missions and will be one large step closer to the detection of gravitational waves from space. It was on December 2, 1915, that Einstein published his theory of gravitation, the General Theory of Relativity (GTR), which predicts the existence of such waves. 2015 marked the centenary year of the GTR, probably the grandest visionary theory ever.

Once the LPF is in its prescribed orbit around Lagrange point L1, 1.5 million kilometres from the earth towards the Sun, its main task will be to release two test masses in perfect free fall and measure and control their positions with unprecedented precision. This is achieved through state-of-the-art technology comprising inertial sensors, a laser metrology system, a drag-free control system and an ultra-precise micro-propulsion system.

The scientific payload includes two separate vacuum tanks, which house each of the two identical 2 kg cubic test masses. They will fall freely inside the satellite, protected from all inner and outer disturbances and will demonstrate the precise measurement and control of force-free motion.

A laser interferometer will measure the position and orientation of the two test masses relative to the spacecraft and to each other with a precision of approximately 10 picometres (one hundred millionth of an mm). In addition, there are less precise capacitive inertial sensors that also help determine their positions.

The main scientific mission of LPF begins on March 1, 2016, and will last for at least six months. During this time, the researchers will conduct several series of consecutive experiments, depending on one another. These experiments will measure the near-perfect free fall and look by non-gravitational parasitic accelerations, identifying important sources of disturbances and further minimising those if necessary.

The two test masses will be free-floating in their own vacuum canisters for the duration of the mission. They will be almost free of all internal and external disturbances and will thus allow the demonstration of the precise measurement of free-falling masses in space.

A special gold-platinum alloy has been used for the masses to eliminate any influence from magnetic forces. Using ultraviolet radiation, a contact-free discharge system prevents electrostatic charge build-up on the test masses.

The caging and grabbing mechanism—responsible for protecting the test masses from intense vibrations during launch, releasing them in a highly controlled setting, and capturing them as necessary—is a particular challenge in this context. The positional data is used by a Drag-Free Attitude Control System (DFACS) to control the spacecraft and ensure it always remains centred on one test mass. The actual position of the satellite is controlled through cold gas micronewton (equal to the weight of a grain of sand on the earth) thrusters, which have the capability of delivering propulsion in extremely fine and uniform amounts.

LISA Pathfinder paves the way for eLISA, a large space observatory for the direct observation of gravitational waves, one of the most elusive astronomical phenomena.

Space observatories such as eLISA will measure gravitational waves in the millihertz range that are emitted, say by pairs of supermassive black holes or binary white dwarf systems. They will complement ground-based detectors which observe less massive objects at higher frequencies in the audio range.

R. Ramachandran

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