Remember LISA

    Now that the whole world is talking about the detection of gravitational waves, I do not want to write about anything other than astronomy. Moreover, the project of the space laboratory for observing gravitational waves has been under development for many years, and the satellite - a technological demonstrator is already in space and will soon begin observations. Its launch went without much fanfare, but now, it seems to me, it's time to think about the LISA Pathfinder probe.

    Launch, flight and latest news

    Starting November 12, the probe was assembled at the installation and testing complex at the Kourou Space Center in French Guiana and was launched on December 3:

    There are no optical telescopes on the probe, this is a purely service image from a star sensor obtained as part of a system check.

    After putting the support orbit into the reference orbit, the acceleration unit lifted the tip of the probe for two weeks and, after the sixth pulse, LISA went on a six-week journey to the Lagrange point L 1 Sun-Earth systems:

    On the way, the acceleration block was dropped, and the probe corrected the orbit in order to enter the trajectory around point L 1 :

    LISA Pathfinder entered the target orbit on January 22. The latest news is that on February 3, the first clamps were removed, holding two cubes of an alloy of gold and platinum, the movement of which will determine the effect of gravity. In mid-February, the second clamps will be removed and, as expected, the probe will begin observations on February 23. Well, to understand how these cubes will measure gravity, we will tell you more about the design of the device.


    Structurally, the LISA Pathfinder probe (Laser Interferometer Space Antenna) is a hexagon made of composite panels with a total mass of 1900 kg:

    From top to bottom: solar panels, the central element of the LTP instrument in disassembled form, the housing, the accelerating unit

    The main scientific tool is the installation called Lisa Test Package Core Assembly:

    On the sides are two 46 mm gold and platinum alloy cubes in individual vacuum containers. The cubes are polished and serve as mirrors for measuring the distance between them.

    One of the cubes. Semicircular recesses are the attachment points of the first fixation system (filmed on February 3), the pyramidal recess in the center is the attachment point of the second fixation system and, at the same time, is a point for measuring distance.

    In the center is a laser interferometer with 22 mirrors and beam dividers, which can measure the movement of cubes by 0.01 nanometers.

    The total weight of the LTP Core Assembly is approximately 120 kg.

    A separate complex technical problem that the developers had to solve was creating a system that can withstand shaking and vibration when put into orbit without losing the accuracy of the measurements. Therefore, the process of releasing cubes takes place in three stages. First, the first stage latches are removed (Caging Mechanism). Then, much more accurate second stage latches (GPRM) will be removed with an accuracy of 200 micrometers along all axes, telling the cubes a speed of no more than 5 micrometers per second. At this speed, the cubes will be able to get closer to the walls in less than half an hour. If necessary, the GPRM locks will be able to lock the cubes again. Simultaneously with the release of the second stage clamps, an electrostatic field will turn on, which will suspend the cubes, preventing them from touching the walls. And finally, the electrostatic field will be gradually removed,

    However, hanging cubes is only half the task. The fact is that random forces will affect the satellite. And even a surge of the solar wind from a flash on the Sun is enough for the probe to start moving relative to the cubes. It is necessary, firstly, to fix this displacement, and secondly, to compensate for it with jet micromotors. To do this, Lisa Pathfinder has as many as two systems: LTP FEEP and DRS.

    FEEP (Field Effect Electric Propulsion) is an electric motion system using a field effect. Since the system is experimental, it tests two types of engines - slot and needle:

    Needle motors on the left, slot engines - on the right

    The slit engine uses field emission of cesium heated to the melting point (≈ 29ºC). Needle motors use molten indium (≈ 156ºC). Both engines produce traction, measured in micrograms, and their total traction is comparable to the weight of a mosquito.

    DRS (Disturbance Reduction System) - this system went to the probe from NASA. Due to NASA's budgetary constraints, it does not have its ultra-precise accelerometers and will use LTP data. Engines are comparable in traction to FEEP, but use ionic fluid as their working fluid.

    DRS engines

    According to the flight plan, the satellite should work 180 days - 90 days on the LTP FEEP, 60 days on the DRS (on its less accurate sensors) and 30 days of collaboration when the LTP will control the DRS engines. However, the reserves of the working fluid of the engines will have to extend the active life.

    All this complex and most accurate technique is needed in order to fix the movement of cubes by nanometers, while isolating them as much as possible from all possible interference. As you have probably heard, gravitational waves are recorded precisely by such a small displacement of the masses.


    LISA Pathfinder is a technology demonstrator. A full-fledged detector, called Evolved LISA (eLISA), will have to consist of three satellites and, in addition to ultra-precise measurement of the test mass displacement, it will also have to measure the distance between satellites with very high accuracy:

    But in space there will be no problem to ensure a large distance between individual satellites. For this project, it is estimated from 1 to 5 million kilometers, which, obviously, cannot be achieved on Earth. Moving in orbit, the triangle of probes will change its plane and will be able to fix gravitational waves from different directions:

    Such a detector will be able to detect the rotation of compact binary stars, the fall of stars into black holes, the rotation of double black holes from neighboring galaxies, and, possibly, it will allow you to look into the young Universe when matter has not passed light yet (according to modern concepts, the Universe has become transparent for photons 380 thousand years after the Big Bang). There is even speculation that he will be able to confirm string theory.

    A separate advantage of the eLISA detector is that it will operate at a different frequency than the ground-based LIGO detector, which will complement its observations.

    One problem - according to current plans, the deployment of the constellation eLISA satellites is planned for 2034. One can only hope that the recent success in detecting gravitational waves will increase funding and start the detector earlier.

    PS A small announcement for Ufa residents - my next lecture will be at the Ufa planetarium on February 26th. The theme is the outer solar system, from the asteroid belt to the mysterious ninth planet.

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