LISA Pathfinder to successfully achieve its mission

We have exceeded not only the requirements set for LISA Pathfinder, but also the accuracy required for LISA at all frequencies: we are definitely ready to take the next step.” Karsten Danzmann*

We started our LISA Pathfinder Saga on June 18th 2015, and followed with you and the researchers involved, this exciting mission for exactly two years. The goal of LISA Pathfinder was to validate the technology that will be used for the future LISA mission. That is, having two test masses placed in each of the three spacecraft of the LISA constellation, which is designed to detect gravitational waves in space.

Nonetheless the LISA Pathfinder mission exceeded its objectives, and the LISA mission has just been selected as the third large-class mission in ESA’s Science Programme. Along with a third detection of gravitational waves emitted from the coalescence of two black holes, announced last month by the Advanced LIGO collaboration, we are definitively starting the era of Gravitational Astronomy.

The LISA Pathfinder mission performed its final tests and will receive its last command on July 18th 2017.


You can read everything about the achievements of the LISA Pathfinder mission on the ESA website:


This has been a splendid journey that we enjoyed following with you. And we can certainly expect more like this in the coming years.


*Director at the Max Planck Institute for Gravitational Physics, director of the Institute for Gravitational Physics of Leibniz Universität Hannover, Germany, Co-Principal Investigator of the LISA Technology Package, and lead proposer of LISA.

Image: LISA Pathfinder in space. Credit: ESA/C. Carreau

Thursday June 16: Gravity! hangout from Stanford on black holes and gravitational waves

Pierre Binétruy and George Smoot invite you to participate to the final hangout of the second session of the Gravity! course. This hangout will focus on black holes and gravitational waves. It will be broadcasted this Thursday June 16 at 19h00 UTC (20h00 London, 21h00 Paris, 12h00 California), live from the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at SLAC, Stanford University.

KIPAC_logoThe Google Hangout will be streamed live on Google Hangouts and Youtube for approximately 60 minutes, where you can follow the questions and answers live. If you are not registered in this session of Gravity! you may ask your questions below or on Twitter using#FLGravity.

Two highlights for this hangout will be the recent publication of the first results by the LISAPathfinder mission, as well as the exciting new result of the LIGO-Virgo collaboration announced on June 15.

Our guests for this event will be:


Tom Abel is the director of the Kavli Institute for Particle Astrophysics and Cosmology, joint laboratory of the SLAC National Laboratory and Stanford University. His group explores the first billion years of cosmic history using ab initio supercomputer calculations. He has shown from first principles that the very first luminous objects are very massive stars and has developed novel numerical algorithms using adaptive-mesh-refinement simulations that capture over 14 orders of magnitude in length and time scales.  Most recently he is pioneering novel numerical algorithms to study collisionless fluids such as dark matter.



Roger Blandford, a native of England, held a faculty position at Caltech since 1976 when, in 2003, he moved to Stanford University to become the first Director of the Kavli Institute of Particle Astrophysics and Cosmology. He is a world-renrecognized expert in black hole astrophysics, cosmology, gravitational lensing, cosmic ray physics and compact stars.



michael_landryMichael Landry is Detection Lead Scientist with the LIGO Hanford Observatory in Washington state, and a physicist with the California Institute of Technology. Michael began work in the field of gravitational wave physics as a postdoc with Caltech in 2000, stationed at the LIGO Hanford Observatory, and has remained there as a scientist since that time. From 2010 to 2015, he led the installation of the Advanced LIGO detector at Hanford. This collaborative work, done by the LIGO Scientific and Virgo Collaborations totaling a thousand people, culminated in the first direct detection of gravitational waves from a binary black hole merger, announced Feb 11, 2016.



Stefano Vitale is the Principal Investigator (P.I.) of the LISAPathfinder mission. He is professor at the University of Trento in Italy and is a key figure of the gravitational wave community in Europe. He worked on the cryogenic acoustic detector AURIGA before joining the LISA mission where he is leading the Italian effort. He has developed in Trento a laboratory which contributed the inertial sensor onboard the LISAPathfinder mission.

LISAPathfinder exciting first results: green light for the LISA mission

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 technological mission was launched on December 3 from Kourou (see the launch  and meet the technical team who participated in a Gravity! hangout  a few hours before launch).

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.


LISAPathfinder test masses released : a major step on the road

The gravitational skies seem to be auspicious these days. While the LIGO collaboration was announcing the discovery of gravitational waves, the LISAPathfinder team was going through a very delicate process : the release of the two test masses which was completed successfully this morning.


Behind the polished press release of ESA, let me explain what was at stake, and why everyone in the eLISA project is relieved and very joyful today. Indeed, you can see in the tweet below the reaction of Stefano Vitale, the scientist in charge of the mission, and César García, the project manager (we had met him in Kourou last November a few hours before launch, see the video of the hangout).


LISAPathfinder is testing the basic principle behind the eLISA mission which is to be able to measure variation of distances between two test masses which are only submitted to gravitation, in other words which are freely floating in the cosmos. These test masses are small cubes of gold-platinum of 46 mmm side.


Test mass ©CGS SpA

During the experimental phase, these masses are floating in a cage, called electrode housing. Thanks to these electrodes, the satellite is constantly monitoring the position of the test mass, and operates its external microthrusters in order to change its own position and reposition itself in such a way that the test mass stays at the centre of its cage. In this way, the satellite protects the test mass from external perturbations.

But there is one difficulty: whereas the test mass is floating once on site, it has to be tightly locked during launch: otherwise, the strong vibrations would shake it within its housing, which would provoke irreparable damage.


(c) University of Trento

Electrode housing © University of Trento



And here is the tricky engineering problem, which gave nightmares to ESA (and, in an earlier stage, NASA) teams: this was known to everyone in the mission as the “infamous caging mechanism”. How do you release the test mass once you have tightly locked it? The difficulty is that, once tightly pressed, the test mass sticks to the metal finger that presses on it. But one has to release the mass very softly, because only very small forces can be acted upon.


The solution that was finally adopted relies on a two-stage process.


Throughout LISAPathfinder’s launch, , and the six-week cruise to its work site, each cube was held firmly in place by eight ‘fingers’ pressing on its corners. On 3 February, the locking fingers were retracted and a valve was opened to allow any residual gas molecules around the cubes to vent to space. Each cube remained in the centre of its housing held by a pair of rods softly pushing on two opposite sides.



The rods were finally released from one test mass yesterday and from the other this morning, leaving the cubes floating freely, with no mechanical contact with the spacecraft.

Congratulations to the project manager, César García, and all the technical teams involved in this success !

It will be another week before the cubes are left completely at the mercy of gravity, with no other forces acting on them. Before then, minute electrostatic forces are being applied to move them around and make them follow the spacecraft as its flight through space is slightly perturbed by outside forces such as pressure from sunlight.


On 23 February, the team will switch LISA Pathfinder to science mode for the first time, and the opposite will become true: the cubes will be in free fall and the spacecraft will start sensing any motion towards them owing to external forces. Microthrusters will make minuscule shifts in order to keep the craft centred on one mass.


The final word to Stefano Vitale: “Releasing LISA Pathfinder’s test masses is another step forward in gravitational wave astronomy within this memorable month: the test masses are, for the first time, suspended in orbit and subject to measurements”.


Pierre Binétruy

LISAPathfinder arrives at its destination

After a six-week journey, LISA Pathfinder arrived today at its destination, the Lagrange point L1, a point on the virtual line joining the Earth to the Sun, some 1.5 million km from Earth, where the gravitational effects are balanced by the centrifugal force.

LISA Pathfinder’s arrival came after a final thruster burn using the spacecraft’s propulsion module on 20 January. The small, 64-second firing was designed to slightly change its speed and just barely tip the craft onto its new orbit about L1. “We had planned two burns to get us into final orbit at L1, but only one was needed,” says Ian Harrison, Spacecraft Operations Manager at ESA’s ESOC operation centre in Darmstadt, Germany, where the mission control and science teams are located.

LISAPathfinder journey from the Earth to L1

LISAPathfinder journey from the Earth to L1

Since launch, the propulsion module raised the orbit around Earth six times, the last of which kicked it towards L1.


The propulsion module separated from the science section at 11:30 GMT (12:30 CET) after the combination was set spinning for stability.

“Heat and vibration from the regular, hot thrusters on the propulsion module would cause too much disturbance during the spacecraft’s delicate technology demonstration mission,” notes Ian. “Primary propulsion during the rest of the mission will be provided by microthrusters to keep us at L1.”

These small thrusters were used in the hours after separation to kill the spin and stabilise the spacecraft.

Next week, LISA Pathfinder’s trajectory will be fine-tuned with a series of three micro-thruster bursts, taking it onto its final orbit, a 500 000 km × 800 000 km orbit around L1.

The next delicate step will be the final release of the test masses on 15 and 16 of February.


After one week orbiting Earth, heading to L1


Since LISAPathfinder launch, on Thursday December 3rd 2015, teams from the European Space Observations Centre (ESOC) in Darmstadt are working day and night to ensure the orbital operating steps. As illustrated in the figure, these maneuvers consist in correcting the path and increasing by successive pushes the  altitude of the apogee, that is to say, the point on the orbit which is farthest from the Earth . Each time the engine of the propulsion module is activated for several tens of minutes.

Just after the launch the apogee was at an altitude of 1,540km. The first push took place Saturday 07/12/2015 at 5:02 with an increase in speed of 393 m/s which rose the apogee at 3,392km. Then the same day another push at 17:20 rose the apogee at 7.105 km (+ 552 m/s). Then, Tuesday 08/12/2015, two new pushes allowed to reach orbits with apogees at 14,488 km (+ 603 m/s) and then 44,526 km (+ 807 m/s). And the fifth push was operated on 10/12/2015 at 0:34 with an increase in speed of 398 m/s which rose the apogee to 129,122 km.

Finally after running tests on its performance, the engine was activated  on Saturday 12 at 5:18 during 6 minutes to make the final push (+ 234 m/s). This allowed to eject LISAPathfinder outside the Earth’s orbit and to put the satellite on its trajectory towards the Lagrangian point L1. We will have now nearly fifty days to cover the 1.5 million kilometers from the Earth to L1.

Have a good journey LISAPathfinder, arrival late January 2016!

Antoine Petiteau


Successful launch for LISAPathfinder

Liftoff of Vega carrying LISAPathfinder - ESA

Liftoff of Vega carrying LISAPathfinder – ESA

We had intense moments of emotion last night in Kourou.

All the lights were green when the Vega rocket beautifully took off at an impressive speed. Stress was there with each operating step that took place in a silence only punctuated by the brief announcements.

It took almost two hours, a little more than for LISAPathfinder to circle the Earth, to have the confirmation that we were on the right orbit and finally to let our joy explode.

Soon it will be the task of the scientific teams, including the APC Laboratory, to make a success out of this mission!

Kourou, December 3, 2015

Antoine Petiteau


Watch with us the launch of LISAPathfinder: launch rescheduled Dec. 3 at 4:04 GMT

After a great hangout with the LISApathfinder technical team, join us here on Gravity! to watch live the images of the launch of the mission from Kourou.

The launch is rescheduled on December 3 at 4:04 GMT (5:04 am Paris time, 1:04 am Kourou time). 

We will start broadcasting at 3.45 GMT (4.45 Paris time). You may watch the event on Arianespace TV:

Good morning. Follow the event here and comment below.

Lift off.

First stage is now separated.

Altitude 97 km

Fairing separated. One can see the satellite now!

Altitude 300 km from Earth

Altitude 370 km, 4400 km from Earth: everything nominal

Altitude 400 km, first ballistic phase

Next important phase at 5:45 GMT! TV comments restart on at 5.42 GMT.

Separation performed.

First signal from LISAPathfinder satellite separated.

Separation confirmed! Everybody applauds and congratulates in Kourou. Launch is a success. LISAPathfinder is on its way to Lagrange point L1.

See the video of the launch (final countdown at minute 13:00: you may recognize Jon Harr who participated to the hangout on Monday; the video is also commented by Paul McNamara, LISAPathfinder project scientist, who was our host in Kourou for the hangout; and you will recognize also Cesar Garcia, project manager, very busy at this time, e.g. at minute 22:20):



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