LISA Pathfinder approaches its operational orbit

While LISA Pathfinder is en route to its operational orbit, the science and engineering teams are testing the systems on the spacecraft. This week, they will begin to switch on elements of the science payload, including the laser that will be used to monitor the most precise free-fall motion ever obtained in space.

LISA Pathfinder (LPF) is the test mission to demonstrate new technologies necessary for the planned gravitational wave observatory eLISA, a large space observatory for the direct observation of one of the most elusive astronomical phenomena – gravitational waves, tiny fluctuations in the fabric of space-time predicted by Albert Einstein’s Theory of General Relativity.

Launched on 3 December 2015, LISA Pathfinder has used its propulsion module to raise its orbit six times and embark on the path to its operational orbit around the Lagrange point L1, 1.5 million km from Earth in the direction of the Sun. After releasing the propulsion module on 22 January the science module will enter its final orbit. Science operations will then begin on 1 March.

At the centre of the science module lies the LISA Technology Package, the scientific heart of the mission, housing the two test masses. Once the spacecraft is in a stable orbit this pair of identical gold-platinum cubes will be set free to move under the influence of gravity. A laser interferometer will track their motions and measure the minuscule changes caused by forces other than gravity to assess how much they deviate from actual free fall.

Even in space, isolating a test mass from all the non-gravitational forces acting on it is a daunting endeavour. Demonstrating that such an accurate free-fall can be obtained is a crucial condition for future space-borne observatories of gravitational waves.

Physicists at the University of Birmingham were involved in the design and build of the Phasemeter – an electronic unit, which measures the separation between two test masses linked by laser beams. It will allow scientists to search for any spurious influences on the measurements between the masses that might hinder the future detection of gravitational waves by eLISA.

During LISA Pathfinder's cruise to L1, teams from ESA and Airbus Defence and Space have been commissioning the spacecraft, verifying that all systems, subsystems and instruments function as expected. Just before Christmas the cold gas micro-newton thrusters, which will be used to accurately adjust the spacecraft position by tiny shifts during the operations phase were first activated. Last week the colloidal micro-newton thrusters provided by NASA-JPL as part of the Disturbance Reduction System were switched on and tested.

Professor Mike Cruise from the School of Physics and Astronomy at the University of Birmingham said: ‘The scientific payload on LISA Pathfinder will be switched on in stages over the coming weeks in preparation for science operations which start in March. Scientists and engineers from research groups across Europe will be studying the early data to check that all is working correctly.’

This week the teams will start commissioning the LISA Technology Package, switching on the payload computer and other electronics, testing the control unit of the caging mechanism that holds the test masses secure during launch and cruise, and verifying that the radiation monitor works well.

On 13 January they will switch on the laser, the first scientific component of the LISA Technology Package to be activated. During science operations the laser will be used to feed two light beams to the interferometer, which will measure the position and attitude of the free-falling test masses to unprecedented accuracy.

Over the next couple of days the teams will perform a number of tests to examine how the laser, which was calibrated on the ground, works in space. In particular they will verify the laser performance under many different combinations of temperature and power output from the pump diode module that provides energy to the laser crystal, collecting data that will be crucial to run the laser during science operations.

On 22 January the propulsion module will be separate from the science module in order to reduce the self-induced gravitational disturbances, and the science module will proceed on its journey to L1.

The commissioning of the spacecraft and instruments will continue until the end of February. During this period, the test masses will be released, in a two-step process, from the mechanisms that have been holding them secure in position during the launch and cruise phase.

For further information, please contact:
Kate Chapple, Press Office, University of Birmingham, tel 0121 414 2772 or 07789 921164

Images and videos

LISA Pathfinder in space (artist's impression)

The LISA Technology Package reference laser unit

Video Caption
LISA Pathfinder commissioning simulation

This timelapse video shows teams from ESA and Airbus Defence and Space (the prime contractor for LISA Pathfinder) carrying out a simulation of the commissioning of the LISA Technology Package, the scientific heart of the mission. The video spans two hours in the morning of 23 September 2015.

The scientists and engineers are grouped to represent the different spacecraft subsystems. In the foreground are Paul McNamara, LISA Pathfinder ESA Project Scientist, and Stefano Vitale, LISA Pathfinder Principal Investigator. At the same desk, to their left, is the representative of the Science and Technology Operations Centre (STOC) station, and further to the left the following subsystems are represented: Optical Metrology System (OMS), UV Light Unit (ULU), Data Management Unit (DMU), and Inertial sensor front end electronics (ISS FEE).

On the right side of the frame are the representatives of the ESA project team, Airbus Defence and Space Ltd, and Airbus Defence and Space GmbH (from top to bottom, respectively). At the centre of the room is the 'hot desk' space, where the team members can discuss and work on specific issues.

Credit: ESA