Freefall achieved on LISA Pathfinder

LISA Pathfinder feb24-operating in space node full image 2

Title LISA Pathfinder operating in space
Released 24/02/2016 12:00 pm
Copyright ESA/ATG medialab

LISA Pathfinder operates 1.5 million km from Earth towards the Sun, orbiting the first Sun–Earth ‘Lagrangian point’, L1.

At the core of the spacecraft are the two test masses: a pair of identical 46 mm gold–platinum cubes, floating freely, several millimetres from the walls of their housings. The cubes are separated by 38 cm and linked only by laser beams to measure their position continuously.

During the science operations phase, microthrusters will make minuscule shifts in order to keep the spacecraft centred on one of the masses. This will isolate the two cubes from all external and internal forces except gravity, placing them in the most precise freefall ever obtained, in order to test technologies for future observations of gravitational waves from space.
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24 February 2016

On Monday, the two cubes housed in the core of ESA’s LISA Pathfinder were left to move under the effect of gravity alone – another milestone towards demonstrating technologies to observe gravitational waves from space.

It has been an intense couple of months for LISA Pathfinder. After launch on 3 December and six burns to raise the orbit, it finally reached its work site – 1.5 million km from Earth towards the Sun – in January, and the team of engineers and scientists started to switch on and test its systems.

One of the most delicate operations entailed releasing the two test masses from the mechanisms that kept them in place during ground handling, launch and cruise.

First, the eight locking ‘fingers’ pressing on the corners of the identical gold–platinum cubes were retracted on 3 February. The cubes were then being held in position only by two rods, softly pushing on opposite faces.

LISA Pathfinder exploded view node full image 2

Title LISA Pathfinder exploded view
Released 01/09/2015 11:00 am
Copyright ESA/ATG medialab

This exploded view shows the LISA Pathfinder spacecraft in its entirety.

The solar array provides power to the payload and support systems, and shields its sensitive payload from the Sun.

Beneath the array is the white- and gold-hued science module, which carries the payload with the test masses and their electrode housings, the optical bench interferometer, and vacuum enclosure. All of these components fit neatly within the central cylinder, which in turn slots into the centre of the science module.

The science module also contains support systems for LISA Pathfinder’s scientific experiments, and carries micronewton thrusters on its outer panels to adjust the position of the spacecraft.

The propulsion module at the bottom of the exploded view will help LISA Pathfinder reach its final orbital location.
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These rods were retracted from the first test mass on 15 February, and from the second on the following day, leaving the cubes floating freely several millimetres from the walls of their housings.

The successful release of the two cubes, floating in space 1.5 million km away, left the team members thrilled and delighted.

Over the following days, minute electrostatic forces were applied to manoeuvre the cubes and make them track the spacecraft’s motion through space as it is slightly disturbed by external forces such as the pressure from sunlight.

This enabled the team to run further tests on the instruments, including the system used to measure the electrical charge of each cube and the procedures used to monitor their position and orientation.

Then the team aligned the two cubes with the laser beams that link them, and checked that the laser measurements agreed with those from the electrostatic sensors.

After verifying that everything was working as planned, the intensity of the electrostatic forces was gradually reduced until none was being applied along the sensitive axes of the masses. This resulted in a brief test of drag-free motion, on 19 February.

Finally, on 22 February, the team tackled the greatest challenge: setting the two cubes completely free, letting them move under the effect of gravity alone and actively manoeuvring the spacecraft around them.

To do this, LISA Pathfinder measures the position and orientation of each cube, and corrects its movement by firing microthrusters to keep it centred on one cube.

“This is a historic achievement: we are demonstrating the most precise freefall that has ever been obtained in space,” says Paul McNamara, LISA Pathfinder project scientist.

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