The gravitational waves that LISA is designed to observe have typical timescales of hours. So long as the distance between the satellites is smoothly changing over these time scales, the gravitational waves can be observed as an additional modulation on top of this smooth change. Each satellite is in an independent Keplerian orbit around the Sun with the plane of the triangle inclined at 60 degrees to the plane of the ecliptic. Over the course of the mission, the nominal 2.5 million kilometer distance between each satellite will vary by hundreds of thousands of kilometers. LISA will be able to measure the absolute distance between the satellites to a few centimeters and will measure hour-scale fluctuations at the level of several picometers (1pm = 1 trillionth of a meter), the level required to detect gravitational waves.
What is Time Delay Interferometry (TDI) and how does it work?
Interferometry is a technique that uses the interference of waves to make precise measurements. The wavelength of the interfering waves acts like the tick marks on a ruler for measuring distance. Optical interferometers can make very precise measurements because the wavelength of the light waves they use is small — around one micron for instruments like LIGO and LISA. A fundamental limitation of interferometry is that precision of the measurement is limited by the stability of the waves used in the interferometer. For an optical interferometer, if the wavelength of the light fluctuates, a spurious signal will be generated that mimics physical motion. One way to mitigate the effect of a fluctuating source is to compare pairs of distances using a common light source. This is the underlying concept of the Michelson interferometer that was used by Albert Michelson and Edward Morely to search for the “luminferous aether” in the late 19th century. LIGO uses the same concept in its interferometers over a century later. In order for this technique to work, the lengths of the light paths must be precisely matched. While LISA’s orbits produce approximately-equal arms, they differ by up to a percent and fluctuate by almost the same amount over long time periods due to orbital mechanics. Time Delay Interferometry (TDI) is a technique that was developed in the late 1990s and early 2000s to allow LISA to take advantage of the “common mode rejection” effect despite having unequal arms. TDI takes advantage of the fact that LISA measures the interference in each one-way laser link individually. While each of these signals is dominated by fluctuations in the LISA laser wavelength, those same fluctuations are measured at multiple points in the LISA constellation with varying time delays. By combining these individual measurements and correcting for the time delays, and adding in some rough knowledge of the constellation geometry, a significant amount of suppression of laser wavelength noise can be achieved. The ability to suppress laser wavelength noise through TDI is primarily determined by the precision of the individual interference measurements and the accuracy of the estimates of the LISA arm lengths. TDI has been extensively examined in analytic studies, numerical simulations, and experimental analogues and has been demonstrated to work as expected. The LISA team continues to refine our understanding of this important technique to ensure that it will provide the sensitivity that LISA requires to achieve its science goals.
How are the three LISA spacecraft able to point at one another?
The orbits of the LISA spacecraft are set up in such a way that the constellation maintains a nearly perfect equilateral triangular shape that is inclined by roughly 60 deg with respect to the ecliptic plane. Once each spacecraft is inserted into its predetermined orbit, tracking from the ground will be used to precisely locate them and determine their relative positions. The spacecraft will then undergo a “constellation acquisition” procedure which begins with one spacecraft turning on its laser while its partner spacecraft scans the sky. At some point during the scan, an acquisition sensor on the partner spacecraft will detect the laser and record its position. The spacecraft will then orient towards that position and turn on its own laser. Once a two-way laser “link” is established, precision interferometric measurements can be used to align the beams. This same procedure is repeated to establish the remaining links in the constellation. This procedure has been verified in simulations and will continue to be refined as the LISA design matures. A variant of this procedure was used to establish the laser link between widely-separated spacecraft on the GRACE-FO mission which launched in 2018.
What are LISA´s key features?
Key features of LISA are interferometric measurement of distances, million-km long baselines, drag free spacecraft based on inertial sensors, and the familiar “cartwheel”-orbits. Unique are the free-falling test masses inside each spacecraft. The test masses will be undisturbed by forces other than gravitation. A new technology, the so-called “drag-free” operation, allows the spacecraft to follow the test masses, all the while shielding the test masses from spurious forces.
Why are the LISA spacecraft sometimes called Sciencecraft?
The usual structural and thermal analysis of the spacecraft has therefore been extended to include gravitational effects as well to ensure that the requirements on gravity gradient at the position of the test masses is fully met. In addition, the payload controls the position of the spacecraft during science operations, rendering the spacecraft effectively a part of the instrument. The importance of the co-design and the co-operation of spacecraft and payload is captured in the term “sciencecraft”.
How can LISA measure the distance between its spacecraft?
LISA’s distance measuring system is a continuous interferometric laser ranging scheme, similar to systems used for radar-tracking of spacecraft. But for LISA, the direct reflection of laser light, such as in a normal Michelson interferometer, is not feasible due to the large distance of million km between the spacecraft: Diffraction expands the laser beam so much that for each Watt of laserpower sent, only about 250 pW are received. Direct reflection would thus result in an attenuation factor of about 6.25 x 10-20, yielding about one photon in every three days.
What are the test masses made of?
The test masses are 46 mm cubes, made from a dense non-magnetic Au-Pt alloy and shielded by the Gravitational Reference Sensor (GRS). The GRS core is a housing of electrodes, at several mm separation from the test mass, used for precision capacitive sensing and electrostatic force actuation in all non-interferometric degrees of freedom. The GRS also includes fibers for UV light injection for photoeletric discharge of the test mass and a caging mechanism for protecting the test mass during launch and then releasing it in orbit. The GRS technology is a direct heritage from LISA Pathfinder.
What is LISA´s payload?
LISA´s payload consists of two identical units on each spacecraft. Each unit contains a Gravitational Reference Sensor (GRS) with an embedded free-falling test mass that acts both as end point of the optical length measurement and as geodesic reference test particle. A telescope transmits the laser light along the arm and also receives the weak light (few hundred pico-Watts!) from the other end. Laser interferometry is performed on an optical bench in between the telescope and the GRS.