Remember LIZA

Now, when the whole world talks about the detection of gravitational waves, I don’t want to write about anything other than astronomy. Moreover, the project of a space laboratory for observing gravitational waves has been developed for many years, and the satellite - a technology demonstrator is already in space and will soon begin to observe. Its launch took place without a special pump, but now, as it seems to me, it's time to recall the LISA Pathfinder probe.

Launch, Flight and Latest News

Starting from November 12, the probe was assembled at the assembly and test facility at the Kourou cosmodrome 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 the system check.

After launching to the reference orbit, the accelerating unit lifted the probe apogee for two weeks and, after the sixth pulse, LISA set off on a six-week journey to the Lagrange point L ₁ of the Sun-Earth system:

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


LISA Pathfinder went into target orbit on January 22. Last news - on February 3, the first fixers 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 fixators 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 more about the design of the device.

Design

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


From top to bottom: solar panels, the central element of the LTP disassembled instrument, the body, the upper stage

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


Along the sides are two 46-mm cubes of an alloy of gold and platinum in individual vacuum containers. The cubes are ground and serve as mirrors to measure the distance between them.


One of the cubes. Semicircular grooves - attachment points of the first fixation system (removed February 3), pyramidal recess in the center - attachment point of the second fixation system and, simultaneously, a point for measuring the distance

In the center is a laser interferometer with 22 mirrors and beam splitters, which is capable of measuring the movement of cubes per 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 to create a system that can withstand shaking and vibration when placed into orbit without losing measurement accuracy. Therefore, the process of freeing cubes takes place in three stages. First, the first stage fixers are removed (Caging Mechanism). Then, much more accurate latches of the second stage (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. With such a speed, the cubes will be able to get closer to the walls in no less than half an hour. If necessary, GPRM latches will be able to fix the cubes again. Simultaneously with the release of the second stage fixers, the 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 so that the cubes hang in the maximum ideal free fall.

However, hanging cubes is only half the task. The fact is that random forces will affect the satellite. And even a burst of solar wind from a flare at the Sun is enough for the probe to start moving relative to the cubes. It is necessary, firstly, to fix this shift, and secondly, to compensate it with reactive micromotors. For this, there are two systems on Lisa Pathfinder: LTP FEEP and DRS.

FEEP (Field Effect Electric Propulsion) is an electrical 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, slotted - on the right

The slit engine uses field emission of cesium heated to the melting point (≈ 29ºC). Molded indium is used in needle engines (≈ 156ºC). Both engines produce thrust measured in micrograms and their total thrust is comparable to the weight of a mosquito.

DRS (Disturbance Reduction System) - this system went to the probe from NASA. Due to NASA's budget constraints, it does not have its own ultra-precise accelerometers and will use LTP data. Engines are comparable to FEEP, but use ionic liquid as a 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 joint work, when the LTP will control the DRS engines. However, the stocks of the working fluid of the engines will have to allow the extension of the active life.

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

eLISA

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 the ultra-precise measurement of the displacement of test masses, will also have to measure the distance between the 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, a triangle of probes will change its plane and will be able to capture gravitational waves from different directions:



Such a detector will be able to detect the rotation of compact double stars, the falling of stars into black holes, the rotation of double black holes from neighboring galaxies, and it may also allow you to look into the young Universe when matter has not yet transmitted light (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 will be 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 of eLISA satellites is scheduled for 2034. One can only hope that the recent success in the detection of gravitational waves will increase funding and start the detector earlier.

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