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The LISAPathfinder mission

The LISAPathfinder mission of ESA is described by Stefano Vitale, the scientist in charge of the mission, and other members of the collaboration.
Stefano Vitale, you are the head of the LISAPathfinder mission. As we say, the Principal Investigator, or PI. So the first question is, why LISAPathfinder? Why LISAPathfinder– that’s a good question. Well, you know these gravitational waves move bodies very far apart. They give very tiny push to bodies very far apart, and that’s the way we plan to detect them, observe them. And if you ask how much force is needed to move the particles, how much the equivalent force from a gravitational wave. Well, you can think like that– if you have one of these masses in your hand, which is two kilos, if you add a bacterium, that’s the force of a gravitational wave.
And there’s no way you can test this and demonstrate that it is going to work in a ground-based laboratory. The gravity of the Earth is too strong. And so in the early days, when we were promoting the concept of flying a gravitational wave observatory, everybody says this is a fantastic idea, that’s beautiful technology. But how do you prove it? And so very soon, it was clear that we had to go into space to prove that this technique of detecting gravitational wave may work. The technique is, as I said, is that you take two bodies. We take two small cubes of gold platinum– they are very shiny and nice to see. We put them 5 million kilometres away.
And they are to be so quiet that if a ripple in space-time passes by, you see the motion. And everything else can move your test masses. And so, to try and see if all this is possible, we give away the 5 million kilometres, we put these two test masses in a satellite, and we look if we can make them so quiet, so still, one relative to the other, that we’re going to see gravitational waves once we build LISA.
So when did we get the idea? In the late ’90s, we proposed LISA to ESA, to the European Space Agency.
And very soon, we could see their insistance at flying something that is orders and orders of magnitude better than anything that has been done before. So in the late ’90s, we started to conceive what was a sufficient test that could convince the agency that this can work without too much risk. And so, we started in the late ’90s. We were very naive. We weren’t space people, so it took some time for us to understand how to do a mission like this. And it took some years, also for agencies and industry to understand what a mission like this is because it’s brand new. It’s a lot of new technologies– I would say ideas, right?
A lot of new ideas, a lot of new knowledge that you have to learn before you make something sensible. Not everything worked out as planned. And actually, we had some suffering. But I would say we’re now– I would say now, it was worth it. It was all worth it.
Well, you know, roughly a month and a half ago, this baby, the satellite, and its propulsion module– the little rocket used to send it into planetary orbit– was shipped from Europe to Kourou, to the spaceport we are in.
That was risky, right? You take 15 years of work, put it in the truck, then put it in an airplane, and move elsewhere. But everything worked well. And so, in the last months, what the team has done here is put the satellite on the rocket– I mean, built up the rocket, put the satellite in the rocket. And this is the work which is still going on and will go on until the fatidic night of the launch, which is coming soon.
So the idea behind LISAPathfinder is the following. You take two test masses, call them 1 and 2, and put them within a satellite. And what you want to do is to measure the distance between the two satellites. You need to put them within a satellite. Because if they’re floating in space, you would have an influence of the solar pressure, for example, that would push these masses. And this solar pressure will not correspond to gravitational forces. So if there is solar pressure, this acts on the satellite, which acts as a shield. And this will modify the distance between the satellite and test mass 1.
And when this difference in the position is modified, then we act on microthrusters in order to put back the satellite in the proper position so that test mass 1 is centred in its housing. Once this is done, then we can proceed with a measurement of distance d12, and this has to be kept to within a precision of the order of one picometer. And this is what LISAPathfinder wants to measure.
Once all the equipment have been integrated within the satellite, a final step is to test that the satellite is behaving correctly, and that the performances are at the level expected by all the scientists and engineers. The first test is to check that all the equipment have the adequate temperature for a proper function. Actually, in space, the temperature is not cold or hot. It is a subtile equilibrium between the different heat sources that are in space, namely, the Sun, the Earth, the dark sky, and the heat generated by the satellite itself.
In order to test that under these conditions, the different equipments have the proper temperature, the satellite is put in a big vacuum chamber– vacuum thermal chamber– where it is possible to lit the satellite with an artificial cell. With this equipment, it is possible to check that all the equipments will have the proper temperature, and that they will function correctly in space. Once this test has been done and completed successfully, the next step is to put the whole satellite on a vibrating column. This test will check that the satellite will withstand the very strong vibrations and accelerations that will occur during the launch of the satellite.
This test is very impressive, very spectacular, and sometimes can lead to the destruction of the satellite if it was not properly designed and constructed. Once all these tests have been performed successfully, then the satellite is ready for launch. And for LISAPathfinder, that’s a moment we are all expecting, and that should occur within a few months.
As for all science space missions, data analysis is a crucial point in the mission. It’s particularly important in LISAPathfinder and eLISA because we want to achieve a very high level of precision. The LISAPathfinder spacecraft will be located in the Lagrange L1 point at 1.5 million kilometres away from the Earth. And the data are emitted every day during five hours at the rate of seven kilobytes per second. To give an idea, this rate corresponds to downloading a full movie during one week without stop.
The data are received by the antenna of the ESA networks, and then sent to the ESOC– the European Space Operational Centre in Darmstadt, that will distribute the data to the other data processing centres as the Francois Arago Centre in Paris. And both ESOC, and Francois Arago Centre will analyse the data. The goal of this analysis is to measure with which precision we succeed to leave a proof mass in freefall in space. And the other goal of this data analysis is to measure the parameters of the system, and to measure as much as as possible parameters to understand how they behave, and to be able to transfer this technology to the eLISA mission.
To be ready for the data analysis, we are doing since several years, each three months, a series of exercises where we are simulating the data. We simulate data as close as possible to the real data and we are testing all data analysis procedures on it.
The LISAPathfinder mission is going to be launched by a Vega rocket. This new type of launcher from the European Space Agency has been designed for small payload. By small, I’m talking about two tonnes of scientific instruments that can be put in a low Earth orbit. As a comparison, the Ariane 5 launcher has a capacity of 20 tonnes. So we want to bring our satellite that has the weight of a small truck to the L1 Lagrangian point at 1.5 million kilometres from Earth. Two questions– what is the Lagrange L1 point and how do we get there? Lagrangian points are five gravitational-equivalent places between two massive objects that we usually call L1 to 5.
In our case, we’re talking about the Earth-Sun system and the L1 point is located on the line between them. It has two advantages for us– a fixed distance between the Earth and Sun, so we get an almost constant illuminaton on the solar panel, and it is a relatively calm gravitational zone, especially in terms of tidal effects. The equilibrium around this point is unstable, but by using small trajectory correction, we can maintain our satellite in this post during the whole mission. Of course, you cannot reach 1.5 million kilometres distance from Earth in one shot. The cost in energy would be too important.
So first, Vega will place the spacecraft in an elliptical orbit around the Earth at distances between 200 and 1600 kilometres. Then, successive pushes by the spacecraft propulsion system will increase the size of the elliptical orbit and its speed. When it reaches a sufficiently high speed, a last push will bring the spacecraft to our L1 point. The propulsion system will be separated from the scientific instrument and the science programme will begin. So the liftoff is scheduled for October 2015 and we’re all waiting for that moment.
What are your feelings as the PI a few days, a few hours before launch now? How does it feel?
Tense, right? We are risking 15 years of work in one night. And the future mission. And the future mission. So this is a very critical moment. But I think if mankind doesn’t take risks, we’re not going to get anywhere, right? So each of us has to take his own part of the risk. He must manage risk very carefully, do his best. But if we don’t do a risky thing, we don’t open new fields and new knowledge for mankind, so I think we did our best. Let’s see how it works.
The LISAPathfinder mission is a technological mission that tests the main technological challenges of the future gravitational wave observatory LISA.
The launch took place on December 3, 2015 from the Kourou basis in French Guyana. Meet the head of the mission, Stefano Vitale, a few days before, and other members of the team, to understand what are the main challenges of this mission. You can even meet the project team live from Kuru a few hours before launch, during a dedicated hangout which was organized especially for learners on the first run of the Gravity! course. A great occasion to get a close-up at a space mission, at a key moment.
We thought that it was a good illustration of what a space mission is from an insider’s point of view: a long preparation and a lot of stress when all this work is lifted off into space. Then, of course, once the satellite has reached its planned location in space, one has to wake all the instruments up and perform the scientific work. LISAPathfinder released its first results in June 2016, and they are spectacular: they exceed the mission requirement by a factor of 10, even 100 at some frequencies! See here on our website for details. This has given an enormous push to the full LISA mission and the European Space Agency decided to start immediately with the implementation of the mission. Thanks LISAPathfinder!
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