LISA uses free-flying test masses as reference points from which to measure passing gravitational waves. A key requirement for LISA's performance is that these test masses are not affected by other forces. Electrostatic forces are one such force that arise when the test mass gains an electric charge due to the impact of cosmic rays on the spacecraft. If left unchecked, this charge would build up to the point where electrostatic forces would overwhelm the gravitational wave signal.
LISA will use ultraviolet light to control charge on the test masses through the photoelectric effect. This technology was successfully demonstrated first on NASA’s Gravity Probe B mission, then on LISA Pathfinder. NASA and its partners at the University of Florida are developing an improved charge management system based on UV LEDs that are smaller, lighter, less power-hungry, and more robust than the mercury-vapor lamps used on LISA Pathfinder.
LISA uses laser light to make measurements of the distance between pairs of spacecraft. In order to reach the required precision of picometers (1pm = 10-12m), the LISA lasers must carefully control both their intensity and their wavelength. In addition, the lasers have to operate for an extended period in the harsh environment of space.
NASA is developing a prototype LISA laser system to meet these challenging requirements. The LISA laser utilizes the same non-planar ring oscillator (NPRO) technology employed by ground-based gravitational wave detectors like LIGO but in a customized package that is optimized for spaceflight. The laser is stabilized to a frequency reference cavity similar to the one which flew on the NASA-German GRACE-FO mission.
LISA utilizes an innovative technique called "drag-free flight" to isolate its test masses from external disturbances. Once on orbit, the test masses are released from the spacecraft and allowed to drift freely inside a small cavity within the spacecraft. Sensors monitor the position and orientation of the spacecraft relative to the test mass and command thrusters on the spacecraft to maintain proper separation. This requires an extremely precise and stable propulsion system.
While the total required thrusts are small, on the order of micronewtons, the required precision of tenths of micronewtons is challenging to achieve. NASA has developed an innovative electric micropropulsion system called a colloidal micronewton thruster (CMNT). CMNT thrusters are less massive, more efficient, and lower noise than cold gas thrusters. The NASA CMNT thruster design was demonstrated on the ESA-NASA LISA Pathfinder mission. NASA is continuing development of the CMNT thrusters for LISA as well as for applications in other missions.
Interferometry, the measurement technique employed by LISA to measure the distance between pairs of test masses, takes advantage of the wave nature of light to make precise distance measurements. When beams of light that have travelled over different paths are overlapped, they will either constructively or destructively interfere depending on small differences between the path lengths. The ability to precisely measure this interference is a key driver of the sensitivity of an interferometer.
For LISA to reach a precision of picometers, it must measure relative shifts of a millionth of a wavelength for the 1 micron near-infrared light produced by the LISA laser. Moreover, LISA is a "heterodyne" interferometer where the interfered beams of light come from multiple independent light sources resulting in an interference pattern that is rapidly changing with time.
The task of the LISA Phase Measurement System or "phasemeter" is to track and measure these interference patterns with the required precision to later extract the gravitational wave signals. NASA has worked to develop LISA phasemeters for over a decade, building on technologies used in advanced GPS receivers. A variant of the LISA phasemeter technology was flown on the NASA-German GRACE-FO mission and demonstrated many of the essential features in flight. NASA is now working to incorporate lessons from that experience into further developments of LISA phasemeters.
LISA will provide the first view of the Universe as measured in millihertz-band gravitational waves. In this band we expect to find many astrophysical sources, including some we have seen in other ways, and others not yet detected. These include black hole binaries like those LIGO detects, but years before LIGO would detect them, mergers of two massive black holes, inspirals of stellar-scale black holes into massive black holes and also thousands of ultracompact stellar binaries in our galaxy. LISA will also measure stochastic signals from the combination of countless sources too weak to measure individually and perhaps also more exotic or even unanticipated signals.
NASA is working to understand more precisely what information we expect LISA to measure about these systems and how scientists can apply this information together with other astronomical observations and astrophysical theory. NASA is developing the data analysis algorithms which will be needed to extract this information from the LISA data stream and beginning to develop the tools that scientists will need to work with this information.
One of the most promising gravitational wave signals for LISA is the inspiral and merger of two supermassive black holes (SMBHs). Theory predicts that these systems will often be surrounded by hot gas, typically in the form of an accretion disk. The SMBH binary will impart a powerful time-varying influence on this gas, leading to a gap in the inner part of the disk surrounding the binary, along with individual "mini-disks" around each black hole (see image).
To fully understand the behavior and predict the observational properties of these circumbinary accretion disks, we must perform sophisticated computer simulations, including a wide range of astrophysics such as general relativity, plasma physics, fluid dynamics, and radiation transport. The state-of-the-art codes used for this purpose are run on massive supercomputers and produce immense datasets, which are carefully analyzed in order to understand the unique properties that will allow us to find and characterize the electromagnetic signals associated with the gravitational waves detected by LISA.
A prime target of LISA is the merger of comparable-mass black holes (SMBHs) in the range ~105 and 107 solar masses. This is briefly the most luminous phenomenon in the Universe, releasing gravitational-wave (GW) energy at a peak rate of ~ 1056 erg/second. Such massive objects will typically be surrounded by significant matter, in the form of accretion disks. The merger of the central holes will disrupt this matter violently, likely triggering unique electromagnetic (EM) emission that can serve as an additional signal of the merger, carrying information about the local environment not present in the gravitational waves.
Calculating the simultaneous GW and EM signals of SMBH mergers requires the use of supercomputer resources to solve Einstein's equations of general relativity that describe the warping of spacetime around the rapidly converging holes, as well as the relativistic magnetohydrodynamics equations that describe the behavior of the surrounding plasma in this strong-field, dynamic region. Converting these twisting EM fields into an ensemble of high-energy photons will allow us to build a simultaneous GW signature and EM light curve for these events.
LISA is a complex instrument that is distributed across multiple spacecraft and which is being developed through a partnership of multiple agencies and institutions. Thorough and disciplined systems engineering is an essential part of making such a mission successful. NASA systems engineers are contributing to this effort through management of technologies under development at NASA as well as by supporting systems engineering activities in Europe.
The distances between the LISA spacecraft are so vast that it is necessary to efficiently transmit light from one spacecraft to another. LISA uses optical telescopes to simultaneously transmit and receive the laser light between widely-spaced pairs of spacecraft in order to deliver enough light to make the interferometric distance measurement at the required precision.
The telescopes are designed to function as afocal beam expanders with pupil relays optimized to minimize the cross-coupling of angular jitter into pathlength. Each pair of telescopes is in series with the interferometric measurement path between the LISA test masses and therefore the optical pathlength through the telescope must be extremely stable so as not to mask gravitational wave signals with the distortions of the telescope. This requires careful selection of materials and a design that is insensitive to environmental disturbances. With a primary mirror diameter of roughly 30cm (~1 foot), it is not the largest telescope NASA has ever developed, but meeting the requirements presents some unique challenges.
The LISA Ambassadors are current graduate students whose research interests lie in black hole physics, understanding the properties of supermassive black holes, gravitational wave astronomy, or related subjects. They receive in-depth training on how to communicate with high school students and the general public about the Laser Interferometer Space Antenna (LISA) mission and the importance of gravitational wave astrophysics. Ambassadors play an essential role in generating public interest in this exciting European Space Agency (ESA) and NASA mission.