The first results of the LISAPathfinder mission were presented this Tuesday June 7 in a press conference organized by ESA and published in Physical Review Letters. And they are even better than was anticipated.
This mission tests one key aspect of the future space gravitational wave observatory, known as LISA: it measures the variation, due to a passing gravitational wave, of distances between test masses which are in free fall i.e. which follow purely gravitational trajectories. To protect the masses from any perturbation, there is a clever set up, known as “drag free”. It consists in using the satellite for protecting the test mass placed in its centre from any perturbation.
To understand how it works, imagine that a micrometeorite hits the satellite: the satellite moves sideways, the test mass is thus no longer in its centre, the satellite detects this anomaly through sensors, it ignites some micro thrusters to reposition itself around the test mass.
Of course, such a device is never perfect, and the test mass feels some tiny perturbations, but the goal of the LISAPathfinder is to show that the perturbations are small enough that they still allow to have confidence in the detection of gravitational waves.
Quantitatively, the goal is to minimize the stray forces acting on the test masses. But a force induces an acceleration and the goal of LISAPathfinder is to minimize the stray acceleration (one talks of an acceleration noise). The initial goal of the mission was to reach over periods of 1000 seconds a stray acceleration which is smaller than 10-13 times g, the local acceleration due to the Earth gravity.
How to realize this? In the future LISA mission, the test masses are placed at the centre of each satellite, 5 million kilometers apart from one another. Laser beams connect the three satellites, thus forming a huge triangle. Relative variations of distance are measured by interferometry, just as for ground detectors, such as LIGO.
In LISAPathfinder, one arm of the future LISA mission is reduced to 38 cm in order to locate two test masses into a single spacecraft. The distance between these two masses is monitored by laser beams that form an interferometer very similar to the one on board LISA (apart from the distance covered by the beams). You can find below a video provided by ESA that explains the main characteristics of the mission.
It is not possible to have simultaneously the two masses in free fall, because their orbits are very similar but not exactly identical. This is why one uses one of the two masses as a reference, whereas the other one is left free. It is on this second mass that one checks that it is in free fall, at least in the limits required for the acceleration noise.
“The measurements have exceeded our most optimistic expectations,” says Paul McNamara, LISA Pathfinder Project Scientist. “We reached the level of precision originally required for LISA Pathfinder within the first day, and so we spent the following weeks improving the results a factor of five better.”
Indeed, as shown on the following plot which appears in the published paper, the acceleration noise reached is 5 times smaller than what was required, basically it is already what is needed for the LISA mission, and even better for high frequencies.
“Not only do we see the test masses as almost motionless, but we have identified, with unprecedented precision, most of the remaining tiny forces disturbing them,” explains Stefano Vitale, the scientist in charge of the mission (Principal Investigator).
This success is obviously a green light for the gravitational wave observatory, the third large mission (L3) of the European Space Agency, known as LISA. This mission was originally identified for a launch in 2034 but this success, and the historic discovery of gravitational waves by the LIGO detector, offer strong arguments to advance significantly the schedule.